Astronomy

Is there a term for asteroseismology as applied to giant planets?

Is there a term for asteroseismology as applied to giant planets?

Giant planets such as Jupiter have oscillations which enable analyses using the techniques of asteroseismology, for example Gaulme et al. (2011) detected global modes on Jupiter via radial velocity measurements. Is there a specific term for this technique as applied to giant planets instead of stars?


Is there a specific term for this technique (asteroseismology) as applied to giant planets instead of stars?

No it seems there is not.

Instead, people just use normal literal constructions.

  1. "Seismology of Giant Planets""
  2. "Jovian Seismology" and "giant planet seismology" or (your planet here) seismology in general
  3. "Planetary normal-mode seismology"

A good example of the Fourier imaging technique is SYMPA:

JOVIAL is next-gen implementation, also from Earth:

  • JOVIAL; Jovian Oscillations through radial Velocimetry ImAging observations at several Longitudes.
  • Jupiter's interior: from gravimetry to seismology

note: after posting I realized that one of the linked papers is the same as in the OP"s question. I'll leave it here for completeness.


Yes. Although there is not much point looking for them on the surfaces of the giant planets, as those are too dynamic, unfortunately.

However, the rings of Saturn can give us some insight into internal oscillation modes, this area of research is then dubbed Kronoseismology.

According to my (very limited) view of the developments in this subfield of planetary science, results were coming in only slowly and with low accuracy.

But since Cassinis grand finale hit us, we have close up observations of moving density waves in the rings, together with high-precision orbital data, so it seems research is picking up speed in recent years.


TESS Asteroseismology of the Known Red-giant Host Stars HD 212771 and HD 203949

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1 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal [email protected]

2 Departamento de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, s/n, 4169-007 Porto, Portugal

3 Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106-4030, USA

4 INAF—Osservatorio Astrofisico di Catania, via S. Sofia 78, I-95123 Catania, Italy

5 Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark

6 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France

7 Institute of Space Sciences (ICE, CSIC) Campus UAB, Carrer de Can Magrans, s/n, E-08193, Bellaterra, Spain

8 Institut d'Estudis Espacials de Catalunya (IEEC), C/Gran Capità, 2-4, E-08034, Barcelona, Spain

9 Centre for Exoplanets and Habitability, University of Warwick, Coventry CV4 7AL, UK

10 Department of Physics, University of Warwick, Coventry CV4 7AL, UK

11 Tata Institute of Fundamental Research, Mumbai, India

12 School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

13 Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA

14 Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, Sydney, NSW 2006, Australia

15 IRFU, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France

16 AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France

17 School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia

18 Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA

19 Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA

20 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany

21 Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain

22 Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain

23 Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, France

24 Center for Space Science, NYUAD Institute, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, UAE

25 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA

26 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, D-69117 Heidelberg, Germany

27 Vanderbilt University, Department of Physics and Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA

28 Vanderbilt Initiative in Data-intensive Astrophysics (VIDA), 6301 Stevenson Center Ln., Nashville, TN 37235, USA

29 Department of Astronomy and Space Sciences, Science Faculty, Ege University, 35100, Bornova, İzmir, Turkey

30 Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Dr., Honolulu, HI 96822, USA

31 Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA

32 Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio av. 3, 10257 Vilnius, Lithuania

33 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA

34 SETI Institute, Carl Sagan Center for the Study of Life in the Universe, Off-Site: 2801 Shefford Drive, Madison, WI 53719, USA

35 STFC Ernest Rutherford Fellow.

Received 2019 July 31
Revised 2019 September 10
Accepted 2019 September 12
Published 2019 October 29

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Contents

Spacecraft design Edit

The CoRoT optical design minimized stray light coming from the Earth and provided a field of view of 2.7° by 3.05°. The CoRoT optical path consisted of a 27 cm (10.6 in) diameter off-axis afocal telescope housed in a two-stage opaque baffle specifically designed to block sunlight reflected by the Earth and a camera consisting of a dioptric objective and a focal box. Inside the focal box was an array of four CCD detectors protected against radiation by aluminum shielding 10mm thick. The asteroseismology CCDs are defocused by 760μm toward the dioptric objective to avoid saturation of the brightest stars. A prism in front of the planet detection CCDs gives a small spectrum designed to disperse more strongly in the blue wavelengths. [14]

The four CCD detectors are model 4280 CCDs provided by E2V Technologies. These CCDs are frame-transfer, thinned, back-illuminated designs in a 2,048 by 2,048 pixel array. Each pixel is 13.5 μm × 13.5 μm in size which corresponds to an angular pixel size of 2.32 arcsec. The CCDs are cooled to −40 °C (233.2 K −40.0 °F). These detectors are arranged in a square pattern with two each dedicated to the planetary detection and asteroseismology. The data output stream from the CCDs are connected in two chains. Each chain has one planetary detection CCD and one asteroseismology CCD. The field of view for planetary detection is 3.5°. [14] The satellite, built in the Cannes Mandelieu Space Center, had a launch mass of 630 kg, was 4.10 m long, 1.984 m in diameter and was powered by two solar panels. [10]

Mission design Edit

The satellite observed perpendicular to its orbital plane, meaning there were no Earth occultations, allowing up to 150 days of continuous observation. These observation sessions, called "Long Runs", allowed detection of smaller and long-period planets. During the remaining 30 days between the two main observation periods, CoRoT observed other patches of sky for a few weeks long "Short Runs", in order to analyze a larger number of stars for the asteroseismic program. After the loss of half the field of view due to failure of Data Processing Unit No. 1 in March 2009, the observation strategy changed to 3 months observing runs, in order to optimize the number of observed stars and detection efficiency.

In order to avoid the Sun entering in its field of view, during the northern summer CoRoT observed in an area around Serpens Cauda, toward the galactic center, and during the winter it observed in Monoceros, in the Galactic anticenter. Both these "eyes" of CoRoT have been studied in preliminary observations carried out between 1998 and 2005, [15] allowing the creation of a database, called CoRoTsky, [16] with data about the stars located in these two patches of sky. This allowed selecting the best fields for observation: the exoplanet research program requires a large number of dwarf stars to be monitored, and to avoid giant stars, for which planetary transits are too shallow to be detectable. The asteroseismic program required stars brighter than magnitude 9, and to cover as many different types of stars as possible. In addition, in order to optimize the observations, the fields shouldn't be too sparse – fewer targets observed – or too crowded – too many stars overlapping. Several fields have been observed during the mission: [17]

  • IRa01, from 18 January 2007 to 3 April 2007 – 9,879 stars observed
  • SRc01, from 3 April 2007 to 9 May 2007 – 6,975 stars observed
  • LRc01, from 9 May 2007 to 15 October 2007 – 11,408 stars observed
  • LRa01, from 15 October 2007 to 3 March 2008 – 11,408 stars observed
  • SRa01, from 3 March 2008 to 31 March 2008 – 8,150 stars observed
  • LRc02, from 31 March 2008 to 8 September 2008 – 11,408 stars observed
  • SRc02, from 8 September 2008 to 6 October 2008 – 11,408 stars observed
  • SRa02, from 6 October 2008 to 12 November 2008 – 10,265 stars observed
  • LRa02, from 12 November 2008 to 30 March 2009 – 11,408 stars observed
  • LRc03, from 30 March 2009 to 2 July 2009 – 5,661 stars observed
  • LRc04, from 2 July 2009 to 30 September 2009 – 5,716 stars observed
  • LRa03, from 30 September 2009 to 1 March 2010 – 5,289 stars observed
  • SRa03, from 1 March 2010 to 2 April 2010
  • LRc05, from 2 April 2010 to 5 July 2010
  • LRc06, from 5 July 2010 to 27 September 2010
  • LRa04, from 27 September 2010 to 16 December 2010
  • LRa05, from 16 December 2010 to 5 April 2011
  • LRc07, from 5 April 2011 to 30 June 2011
  • SRc03, from 1 July 2011 to 5 July 2011 – a run made to reobserve the transit of CoRoT-9b
  • LRc08, from 6 July 2011 to 30 September 2011
  • SRa04, from 30 September 2011 to 28 November 2011
  • SRa05, from 29 November 2011 to 9 January 2012
  • LRa06, from 10 January 2012 to 29 March 2012 – a run dedicated to reobservation of CoRoT-7b
  • LRc09, from 10 April 2012 to 5 July 2012
  • LRc10, from 6 July 2012 to 1 November 2012 - interrupted by the fatal failure which ended the mission.

The spacecraft monitored the brightness of stars over time, searching for the slight dimming that happens in regular intervals when planets transit their host star. In every field, CoRoT recorded the brightness of thousands stars in the V-magnitude range from 11 to 16 for the extrasolar planet study. In fact, stellar targets brighter than 11 saturated the exoplanets CCD detectors, yielding inaccurate data, whilst stars dimmer than 16 don't deliver enough photons to allow planetary detections. CoRoT was sensitive enough to detect rocky planets with a radius two times larger than Earth, orbiting stars brighter than 14 [18] it is also expected to discover new gas giants in the whole magnitude range. [19]

CoRoT also studied asteroseismology. It can detect luminosity variations associated with acoustic pulsations of stars. This phenomenon allows calculation of a star's precise mass, age and chemical composition and will aid in comparisons between the sun and other stars. For this program, in each field of view there was one main target star for asteroseismology as well as up to nine other targets. The number of observed targets have dropped to half after the loss of Data Processing Unit No. 1.

The mission began on 27 December 2006 when a Russian Soyuz 2-1b rocket lifted the satellite into a circular polar orbit with an altitude of 827 km . The first scientific observation campaign started on 3 February 2007. [20]

Until March 2013, the mission's cost will amount to €170 million, of which 75% is paid by the French space agency CNES and 25% is contributed by Austria, Belgium, Germany, Spain, Brazil and the European Space Agency ESA. [21]

The primary contractor for the construction of the CoRoT vehicle was CNES, [22] to which individual components were delivered for vehicle assembly. The CoRoT equipment bay, which houses the data acquisition and pre-processing electronics, was constructed by the LESIA Laboratory at the Paris Observatory and took 60 person-years to complete. [22] The design and building of the instruments were done by the Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA) de l'Observatoire de Paris, the Laboratoire d'Astrophysique de Marseille, the Institut d'Astrophysique Spatiale (IAS) from Orsay, the Centre spatial de Liège (CSL) in Belgium, the IWF in Austria, the DLR (Berlin) in Germany and the ESA Research and Science Support Department. The 30 cm afocal telescope Corotel has been realized by Alcatel Alenia Space in the Centre spatial de Cannes Mandelieu.

Before the beginning of the mission, the team stated with caution that CoRoT would only be able to detect planets few times larger than Earth or greater, and that it was not specifically designed to detect habitable planets. According to the press release announcing the first results, CoRoT's instruments are performing with higher precision than had been predicted, and may be able to find planets down to the size of Earth with short orbits around small stars. [9] The transit method requires the detection of at least two transits, hence the planets detected will mostly have an orbital period under 75-day. Candidates that show only one transit have been found, but uncertainty remains about their exact orbital period.

CoRoT should be assumed to detect a small percentage of planets within the observed star fields, due to the low percentage of exoplanets that would transit from the angle of observation of the Solar System. The chances of seeing a planet transiting its host star is inversely proportional to the diameter of the planet's orbit, thus close in planets detections will outnumber outer planets ones. The transit method is also biased toward large planets, since their very depth transits are more easily detected than the shallows eclipses induced by terrestrial planets.

On 8 March 2009 the satellite suffered a loss of communication with Data Processing Unit No. 1, processing data from one of the two photo-detector chains on the spacecraft. Science operations resumed early April with Data Processing Unit No. 1 offline while Data Processing Unit No. 2 operating normally. The loss of photo-detector chain number 1 results in the loss of one CCD dedicated to asteroseismology and one CCD dedicated to planet detection. The field of view of the satellite is thus reduced by 50%, but without any degradation of the quality of the observations. The loss of channel 1 appears to be permanent. [23]

The rate of discoveries of transiting planets is dictated by the need of ground-based, follow-up observations, needed to verify the planetary nature of the transit candidates. Candidate detections have been obtained for about 2.3% of all CoRoT targets, but finding periodic transit events isn't enough to claim a planet discovery, since several configurations could mimic a transiting planet, such as stellar binaries, or an eclipsing fainter star very close to the target star, whose light, blended in the light curve, can reproduce transit-like events. A first screening is executed on the light curves, searching hints of secondary eclipses or a rather V-shaped transit, indicative of a stellar nature of the transits. For the brighter targets, the prism in front of the exoplanets CCDs provides photometry in 3 different colors, enabling to reject planet candidates that have different transit depths in the three channels, a behaviour typical of binary stars. These tests allow to discard 83% of the candidate detections, [24] whilst the remaining 17% are screened with photometric and radial velocity follow-up from a network of telescopes around the world. Photometric observations, required to rule out a possible contamination by a diluted eclipsing binary in close vicinity of the target, [25] is performed on several 1 m-class instruments, but also employs the 2 m Tautenburg telescope in Germany and the 3,6 m CFHT/Megacam in Hawaii. The radial velocity follow-up allows to discard binaries or even multiple star system and, given enough observations, provide the mass of the exoplanets found. Radial velocity follow-up is performed with high-precision spectrographs, namely SOPHIE, HARPS and HIRES. [26] Once the planetary nature of the candidate is established, high-resolution spectroscopy is performed on the host star, in order to accurately determine the stellar parameters, from which further exoplanet characteristics can be derived. Such work is done with large aperture telescopes, as the UVES spectrograph or HIRES.

Interesting transiting planets could be further followed-up with the infrared Spitzer Space Telescope, to give an independent confirmation at a different wavelength and possibly detect reflected light from the planet or the atmospheric compositions. CoRoT-7b and CoRoT-9b have already been observed by Spitzer.

Papers presenting the results of follow-up operations of planetary candidates in the IRa01, [27] LRc01, [28] LRa01, [29] SRc01 [30] fields have been published. In April 2019, a summary of the exoplanet search results have been published, [31] with 37 planets and brown dwarves confirmed, and a further one hundred planet candidates still to be verified. Sometimes the faintness of the target star or its characteristics, such as a high rotational velocity or strong stellar activity, do not allow to determine unambiguously the nature or the mass of the planetary candidate.

Asteroseismology and stellar physics Edit

Stars vibrate according to many different pulsation modes in much the same way that musical instruments emit a variety of sounds. Listening to an air on the guitar does not leave any doubt as to the nature of the instrument, and an experienced musician can even deduce the cords' material and tension. Similarly, stellar pulsation modes are characteristic of global stellar properties and of the internal physical conditions. Analyzing these modes is thus a way of probing stellar interiors to infer stellar chemical composition, rotation profiles and internal physical properties such as temperatures and densities. Asteroseismology is the science which studies the vibration modes of a star. Each of these modes can be mathematically represented by a spherical harmonic of degree l and azimuthal order m. Some examples are presented here below with a color scheme in which blue (red) indicates contracting (expanding) material. The pulsation amplitudes are highly exaggerated.

When applied to the Sun, this science is called helioseismology and has been ongoing for a few decades by now. The solar surface helium abundance was derived very accurately for the first time, which has definitely shown the importance of microscopic diffusion in the solar structure. Helioseismology analyses have also unveiled the solar internal rotational profile, the precise extent of the convective envelope and the location of the helium ionization zone. Despite enormous technical challenges, it was thus tempting to apply similar analyses to stars. From the ground this was only possible for stars close to the Sun such as α Centauri, Procyon, β Virginis. The goal is to detect extremely small light variations (down to 1 ppm) and to extract the frequencies responsible for these brightness fluctuations. This produces a frequency spectrum typical of the star under scrutiny. Oscillation periods vary from a few minutes to several hours depending of the type of star and its evolutionary state. To reach such performances, long observing times devoid of day/night alternations are required. Space is thus the ideal asteroseismic laboratory. By revealing their microvariability, measuring their oscillations at the ppm level, CoRoT has provided a new vision of stars, never reached before by any ground-based observation.

At the beginning of the mission, two out of four CCDs were assigned to asteroseismic observations of bright stars (apparent magnitude 6 to 9) in the so-called seismo field while the other CCDs were reserved for exoplanet hunting in the so-called exo field. Albeit with a lower signal to noise ratio, interesting science on stars was also obtained from the exoplanets channel data, where the probe records several thousands of light curves from every observed field. Stellar activity, rotation periods, star spot evolution, star–planet interactions, multiple star systems are nice extras in addition to the main asteroseismic program. This exo field also turned out to be of incalculable richness in asteroseismic discoveries. During the first six years of its mission, CoRoT has observed about 150 bright stars in the seismo field and more than 150 000 weak stars in the exo field. The figure shows where most of them are located in the Hertzsprung–Russell diagram together with some others observed from the ground.

Discoveries were numerous. [32] Let us cite the first detection of solar-like oscillations in stars other than the Sun, [33] the first detection of non-radial oscillations in red giant stars, [34] the detection of solar-like oscillations in massive stars [35] · , [36] the discovery of hundreds of frequencies in δ Scuti stars, [37] the spectacular time evolution of the frequency spectrum of a Be (emission lines B) star during an outburst, [38] the first detection of a deviation from a constant period spacing in gravity modes in an SPB (Slowly Pulsating B) star. [39] Interpreting those results opened new horizons in our vision of stars and galaxies. In October 2009 the CoRoT mission was the subject of a special issue of Astronomy and Astrophysics, dedicated to the early results of the probe. [40] Below are some examples of breakthrough contributions to stellar astrophysics, based on CoRoT's data:

Extension of the chemically mixed zone in main sequence stars Edit

Above the convective core where mixing of chemicals is instantaneous and efficient, some layers can be affected by partial or total mixing during the main sequence phase of evolution. The extent of this extra mixed zone as well as the mixing efficiency are, however, difficult to assess. This additional mixing has very important consequences since it involves longer time scales for nuclear burning phases and may in particular affect the value of the stellar mass at the transition between those stars which end up their life as white dwarfs and those which face a final supernova explosion. The impact on the chemical evolution of the galaxy is obvious. Physical reasons for this extra-mixing are various, either a mixing induced by internal rotation or a mixing resulting from convective bubbles crossing the convective core boundary to enter the radiative zone where they finally lose their identity (overshooting), or even some other poorly known processes.

  1. Solar-like stars: The solar-like star HD 49933 is illustrative of this extra-mixing problem. [41] Its convective envelope is responsible for the presence of solar-like oscillations. Comparing the observed frequency spectrum with that obtained from theoretical models of 1.19 Mʘ computed with and without additional mixing clearly excludes a model without extra mixing.
  2. Sub-giant stars: Such an additional mixing also affects the structure of more evolved sub-giant stars since the mass extension of the helium core formed during core hydrogen burning is increased. The sub-giant star HD 49385 of 1.3 Mʘ was submitted to CoRoT scrutiny and although not fully conclusive, new constraints were brought to the modeling of such stars. [42]
  3. SPB stars: More massive SPB (Slowly Pulsating B) stars show a frequency spectrum dominated by high order gravity modes excited by the κ mechanism at work in layers where ionizations of iron group elements produces an opacity peak. In such stars, the convective core is surrounded by a region of varying chemical composition, the so-called μ-gradient region, left by the progressive withdrawal of the convective core as hydrogen is transformed into helium. This region is rather thin and constitutes a sharp transition region, which induces a very subtle signature in the gravity modes frequency spectrum. Instead of a constant period spacing found in a homogeneous stellar model, periodic deviations from this constant value are expected in models affected by a sharp transition region. Moreover, the period of the deviations is directly related to the precise location of the sharp transition. [43] This phenomenon has been detected in two hybrid B stars (showing at the same time acoustic β Cephei and gravity SPB modes): (1) HD 50230 [39] where an extra-mixing with a somewhat smooth shape is clearly required in the modeling and (2) HD 43317. [44]

Structure of the superficial stellar layers Edit

  1. Transition layers in stellar envelopes: Transition layers such as the helium ionization region or the lower boundary of the convective envelope in low mass and red giant stars also affect frequency spectra. In a structure devoid of such discontinuities, high order acoustic modes obey some regularities in their frequency distribution (large frequency separation, second difference. ). Transition zones introduce periodic deviations with respect to these regularities and the periods of the deviations are directly related to the precise location of the transition zones. These deviations were predicted by theory and were first observed in the Sun. [45] Thanks to CoRoT they were also detected in the solar-like star HD 49933 [46] and also in the red giant star HD 181907. [47] In both cases the location of the helium ionization zone could be accurately derived.
  2. Amplitudes and line widths in solar-like oscillation spectra: One of the major successes of the CoRoT space mission has definitely been the detection of solar-like oscillations in stars slightly hotter than the Sun. [33] As was previously done for the Sun, measurements of amplitudes and line widths in their frequency spectra resulted in new constraints in the modeling of stochastic excitations of acoustic modes by turbulent convection. The frequency spectrum of HD 49933 [48] was confronted to the stochastic excitation model developed by Samadi et al. [49][50] Except at high frequencies, a good agreement can be reached by adopting a metallicity ten times smaller than the solar metallicity. With the solar value on the contrary, disagreements in amplitudes can reach a factor 2 at low frequencies.
  3. Granulation: The presence of granulation was detected in the frequency spectrum of HD 49933. Analyses have been done with 3D hydrodynamical model atmospheres computed at solar and ten times smaller than solar metallicities. [51] Here again the model with the lowest metallicity shows up to be closer to the observations although significant disagreements still remain.

Red giants and chemical evolution of our galaxy Edit

Following exhaustion of hydrogen in the core, the overall stellar structure drastically changes. Hydrogen burning now takes place in a narrow shell surrounding the newly processed helium core. While the helium core quickly contracts and heats up, the layers above the hydrogen-burning shell undergo important expansion and cooling. The star becomes a red giant whose radius and luminosity increase in time. These stars are now located on the so-called red giant branch of the Hertzsprung–Russell diagram they are commonly named RGB stars. Once their central temperature reaches 100 10 6 K, helium starts burning in the core. For stellar masses smaller than about 2 Mʘ, this new combustion takes place in a highly degenerate matter and proceeds through a helium flash. The readjustment following the flash brings the red giant to the so-called red clump (RC) in the Hertzsprung-Russell diagram.

Whether RGB or RC, these stars all have an extended convective envelope favorable to the excitation of solar-like oscillations. A major success of CoRoT has been the discovery of radial and long-lived non-radial oscillations in thousands of red giants in the exo field. [34] For each of them, the frequency at maximum power νmax in the frequency spectrum as well as the large frequency separation between consecutive modes Δν could be measured, [52] [53] defining a sort of individual seismic passport.

  1. Red giant population in our galaxy: Introducing these seismic signatures, together with an estimation of the effective temperature, in the scaling laws relating them to the global stellar properties, [54]gravities (seismic gravities), masses and radii can be estimated and luminosities and distances immediately follow for those thousands of red giants. Histograms could then be drawn and a totally unexpected and spectacular result came out when comparing these CoRoT histograms with theoretical ones obtained from theoretical synthetic populations of red giants in our galaxy. Such theoretical populations were computed from stellar evolution models, with adopting various hypotheses to describe the successive generations of stars along the time evolution of our galaxy. [55]Andrea Miglio and collaborators noticed that both types of histograms were spitting images of one another, [56] as can be seen in the histograms picture. Moreover, adding the knowledge of the distances of these thousands of stars to their galactic coordinates, a 3D map of our galaxy was drawn. This is illustrated in the figure where different colors relate to different CoRoT runs and to Kepler observations (green points).
  2. Age-metallicity relation in our galaxy: The age of a red giant is closely related to its former main sequence lifetime, which is in turn determined by its mass and metallicity. Knowing the mass of a red giant amounts to knowing its age. If the metallicity is known the uncertainty in age does not exceed 15%! Observational missions such as APOGEE(Apache Point Observatoty Galactic Evolution Environment) whose goal is to measure metallicities for 100 000 red giants in our galaxy, GALAH(Galactic Archaeology with HERMES) and GAIA(Global Astrometric Interferometer for Astrophysics) could of course widely benefit from these seismic gravities with the ultimate output of establishing the age-metallicity relation in our galaxy. Asteroseismology has crossed the doorstep of the structure and chemical evolution of our galaxy. [57]
  3. Seismic signatures and extension of mixed zones during central hydrogen and helium burning: Increasing even further the scrutiny in analyzing the CoRoT [58] and Kepler[59] frequency spectra of red giants brought new important discoveries. Small and subtle differences in seismic signatures allow us to distinguish RGB from RC stars notwithstanding their similar luminosities. This is now theoretically confirmed thanks to elaborate red giant modeling. [60] The period spacings of gravity-dominated modes are expected to be especially meaningful. Their detection for a large number of red giants could give us clues to establishing the extent of the extra-mixed region above the convective core during core hydrogen burning, but also the extent of the extra-mixed region during core helium burning, both mixing processes being a priori totally unrelated. [61]

Massive stars Edit

Massive variable main sequence stars have frequency spectra dominated by acoustic modes excited by the κ mechanism at work in layers where partial ionization of iron group elements produce a peak in opacity. In addition the most advanced of these stars present mixed modes i.e. modes with a g-character in deep layers and p-character in the envelope. Hydrogen burning takes place in a convective core surrounded by a region of varying chemical composition and an envelope mostly radiative except for tiny convective layers related to partial ionization of helium and/or iron group elements. As in lower mass stars the extent of the fully or partially mixed region located just above the convective core (extra-mixed zone) is one of the main uncertainties affecting theoretical modeling.

  1. β Cephei stars: Seismic analyses of β Cephei stars show that it is not obvious to derive a one-to-one extent of this extra-mixed zone. [62] A rather large extent seems to be required to model θ Ophiuchi [63] while a much smaller one is favored for HD 129929, [64][65] for β Canis Majoris, [66] for δ Ceti, [67] and for 12 Lacertae. [68][69] This extra-mixed zone could even be absent in the structure of V1449 Aquilae (HD 180642) [70] and ν Eridani. [71][72] It would be extremely interesting to establish a relation between the extent of this zone and the rotation velocity and/or the magnetic field of the star. Seismic analysis of V2052 Ophiuchi [73] shows that this star although rapidly rotating, which would favor extra-mixing, could be devoid of such a region. The magnetic field detected in this star could be the reason of this lack of extra-mixing.
  2. Be stars: Late Be type stars HD 181231 and HD 175869 are very rapid rotators, about 20 times more rapid than the Sun. Their seismic analysis seems to require a centrally mixed zone about 20% larger than what is expected from convection only. [74] Another Be star, HD 49330, had a very exciting surprise in store. Observed by CoRoT during an outburst of matter towards its circumstellar disk, which is typical of such stars, its frequency spectrum suffered drastic changes. Firstly dominated by acoustic modes the spectrum showed the appearance of gravity modes with amplitudes strictly in line with the outburst. [75] Such a link between the nature of the excited modes and a dynamical phenomenon is of course a gold mine in our quest for the internal structure of Be stars.
  3. O stars: A bunch of O stars have been observed by CoRoT. Among them HD 46150 and HD 46223 (members of the galactic cluster NGC 2264) and HD 46966 (member of the OB association Mon OB2) do not seem to pulsate, which is in agreement with stellar modeling of stars with similar global parameters. [76] The frequency spectrum of the Plaskett's star HD 47129 on the contrary shows a peak with six harmonics in the frequency range expected from theoretical modeling. [77]

Another unexpected CoRoT discovery was the presence of solar-like oscillations in massive stars. The small convective shell related to the opacity peak resulting from the ionization of iron group elements at about 200 000 K (iron opacity peak) could indeed be responsible for the stochastic excitation of acoustic modes like those observed in our Sun.

  1. V1449 Aquilae (HD 180642): This CoRoT target is a β Cephei star whose frequency spectrum reveals high frequency and very small amplitude acoustic modes. A careful analysis has shown that they were solar-like oscillations excited by turbulent bubbles origination from this convective iron opacity peak zone or even from the convective core. [35] This is indeed a major discovery since it was the first time that pulsations excited by the κ mechanism acting in the iron opacity peak zone were present side by side in the same star with pulsations stochastically excited by this very same zone. This is the reason why Kevin Belkacem, main discoverer of these solar-like oscillations in V1449 Aquilae, added a new baptismal certificate to this β Cephei star and named it Chimera. The figure illustrates the behavior of the frequency versus time for two modes in the frequency spectrum of Chimera, a solar-like mode (top) and a β Cephei mode (bottom). The stochastic nature of the solar-like mode reveals itself in the instability of its frequency as time goes on and in the spread in frequency on several μHz. The contrast with the stability in frequency and the narrow frequency range of the β Cephei mode is striking.
  2. HD 46149: Later on solar-like oscillations were even discovered in a more massive O star member of the binary system HD 46149. [36] Constraints coming from the binary nature of the system coupled with seismic constraints led to the determination of the orbital parameters of the system as well as to the global properties of its members.

The open cluster NGC 2264 Edit

During a 23-day observing run in March 2008, CoRoT observed 636 members of the young open cluster NGC 2264. The so-called Christmas tree cluster, is located in the constellation Monoceros relatively close to us at a distance of about 1800 light years. Its age is estimated to be between 3 and 8 million years. At such a young age, the cluster is an ideal target to investigate many different scientific questions connected to the formation of stars and early stellar evolution. The CoRoT data of stars in NGC 2264 allow us to study the interaction of recently formed stars with their surrounding matter, the rotation and activity of cluster members as well as their distribution, the interiors of young stars by using asteroseismology, and planetary and stellar eclipses.

The stellar births and the stars' childhoods remain mostly hidden from us in the optical light because the early stars are deeply embedded in the dense molecular cloud from which they are born. Observations in the infrared or X-ray enable us to look deeper into the cloud, and learn more about these earliest phases in stellar evolution. Therefore, in December 2011 and January 2012, CoRoT was part of a large international observing campaign involving four space telescopes and several ground-based observatories. All instruments observed about 4000 stars in the young cluster NGC 2264 simultaneously for about one month at different wavelengths. The Canadian space mission MOST targeted the brightest stars in the cluster in the optical light, while CoRoT observed the fainter members. MOST and CoRoT observed NGC 2264 continuously for 39 days. [78] The NASA satellites Spitzer and Chandra measured at the same time the stars in the infrared (for 30 days) and the X-ray domains (for 300 kiloseconds). Ground-based observations were taken also at the same time, for example, with the ESO Very Large Telescope in Chile, the Canadian-French-Hawaiian Telescope in Hawaii, the McDonald Observatory in Texas, or the Calar Alto Observatory in Spain.

The CoRoT observations led to the discovery of about a dozen pulsating pre-main sequence (PMS) δ Scuti stars and the confirmation of the existence of γ Doradus pulsations in PMS stars. [79] Also the presence of hybrid δ Scuti/γ Doradus pulsations was confirmed in members of NGC 2264. The CoRoT observations included also the well known pre-main sequence pulsators, V 588 Mon and V 589 Mon, which were the first discovered members of this group of stars. The precision attained in the CoRoT light curves also revealed the important role of granulation in pre-main sequence stars. [80]

The investigation of T Tauri stars and their interaction with their circumstellar matter using CoRoT data revealed the existence of a new class, the AA Tauri type objects. [81] Previously to the CoRoT observations, T Tauri stars were known to either show sinusoidal light variations that are caused by spots on the stellar surface, or completely irregular variability that is caused by the gas and dust disks surrounding the young stars. AA Tauri type objects show periodically occurring minima that are different in depth and width, hence are semi-regular variables. With the CoRoT observations this class of objects could be established. [82] Exciting insights into the earliest phases of stellar evolution also come from the comparison of the variability present in the optical light to that in the infrared and the X-ray regime.

Binary systems Edit

A large number of binary systems with non-radially pulsating members were observed by CoRoT. [83] Some of them, which were eclipsing binaries with members of γ Doradus type, were discovered during CoRoT runs. [84] The eclipse phenomenon plays a key role since global parameters can immediately follow, bringing invaluable constraints, in addition to the seismic ones, to stellar modeling.

    AU Monocerotis: This semi-detached binary system contains a Be star interacting with its G star companion. Its observation by CoRoT provided an extremely high quality lightcurve. Global parameters could then be improved and new ephemeris for the orbital motion as well as for another long term variation were derived. This long period variation seems to originate from a periodic light attenuation by circumstellar dust. [85]

Exoplanets Edit

To find extra solar planets, CoRoT uses the method of transits detection. The primary transit is the occultation of a fraction of the light from a star when a celestial object, such as a planet, passes between the star and the observer. Its detection is made possible by the sensitivity of CCD to very small changes in light flux. Corot is capable of detecting changes in brightness of about 1/10,000. Scientists can thus hope finding planets with a size of approximately 2 times that of the Earth with this method, a class of planet called Super-Earth detection of Corot-7b, whose radius is 1.7 times that of the Earth has shown that these predictions were correct. CoRoT takes an exposure of 32 seconds duration, each 32 seconds, but the image is not fully transmitted to Earth because the data flow would be too large. The onboard computer performs an important work of data reduction: the field around each target star, previously selected by the exoplanets team, is defined on a certain number of pixels described by a particular mask, the sum all pixels within the mask is then performed and several exposures are added (usually 16, which amounts to an integration time of about 8 minutes) before sending this information to the ground. For some stars, considered particularly of interest, data of each exposure is transmitted every 32 seconds. Such a sampling of 32s or 512s is well suited to the detection of a planetary transit that lasts from a little less than an hour to several hours. A feature of this method is that it requires to detect at least three successive transits separated by two equal time intervals before one can consider a target as a serious candidate. A planet of orbital period T should at least be observed for a time interval between 2T and 3T to have a chance to detect three transits. The distance of the planet to the star ( which is characterized by a the semi-major axis of the elliptical orbit ) is linked to its orbital period by the second law of Kepler / Newton a 3 = T 2 Mstar, using respectively as units for a, M and T: the distance from the Earth to the Sun (150 million km), the mass of the Sun, the orbital period of the Earth (1 year) this implies that if the observing time is less a year, for example, the orbits of the detectable planets will be significantly smaller than that of the Earth. So, for CoRoT, due to the maximum duration of 6 months of observation for each star field, only planets closer to their stars than 0.3 Astronomic Units (less than the distance between the Sun and Mercury) can be detected, therefore generally not in the so-called habitable zone. The Kepler mission (NASA) has continuously observed the same field for many years and thus had the ability to detect Earth sized planets located farther from their stars.

The moderate number of exoplanets discovered by CoRoT (34 during the 6 years of operation), is explained by the fact that a confirmation should absolutely be provided by ground-based telescopes, before any announcement is made. Indeed, in the vast majority of cases, the detection of several transits does not mean the detection of a planet, but rather that of a binary star system, either one that corresponds to a grazing occultation of a star by the other, or that the system is close enough to a bright star (the CoRoT target) and the effect of transit is diluted by the light of this star in both cases the decrease in brightness is low enough to be compatible with that of a planet passing in front of the stellar disk. To eliminate these cases, one performs observations from the ground using two methods: radial velocity spectroscopy and imaging photometry with a CCD camera. In the first case, the mass of the binary stars is immediately detected and in the second case one can expect to identify in the field the binary system near the target star responsible for the alert: the relative decline of brightness will be greater than the one seen by CoRoT which adds all the light in the mask defining the field of measurement. In consequence, the CoRoT exoplanet science team has decided to publish confirmed and fully characterized planets only and not simple candidate lists. This strategy, different from the one pursued by the Kepler mission, where the candidates are regularly updated and made available to the public, is quite lengthy. On the other hand, the approach also increases the scientific return of the mission, as the set of published CoRoT discoveries constitute some of the best exoplanetary studies carried out so far.

Timeline of planetary discoveries Edit

CoRoT discovered its first two planets in 2007: the hot Jupiters CoRoT-1b and CoRoT-2b. [9] [92] Results on asteroseismology were published in the same year. [93]

In May 2008, two new exoplanets of Jupiter size, CoRoT-4b and CoRoT-5b, as well as an unknown massive celestial object, CoRoT-3b, were announced by ESA.

In February 2009, during the First CoRoT Symposium, the super-earth CoRoT-7b was announced, which at the time was the smallest exoplanet to have its diameter confirmed, at 1.58 Earth diameters. The discoveries of a second non-transiting planet in the same system, CoRoT-7c, and of a new Hot Jupiter, CoRoT-6b, were also announced at the Symposium.

In March 2010 CoRoT-9b was announced. It's a long period planet (95.3 days) in an orbit close to that of Mercury. [94]

In June 2010 the CoRoT team announced [95] six new planets, CoRoT-8b, CoRoT-10b, CoRoT-11b, CoRoT-12b, CoRoT-13b, CoRoT-14b, and a brown dwarf, CoRoT-15b. [96] All the planets announced are Jupiter sized, except CoRoT-8b, which appears to be somewhat between Saturn and Neptune. The probe was also able to tentatively detect the reflected light at optical wavelengths of HD46375 b, a non-transiting planet. [97]

In June 2011, during the Second CoRoT Symposium, the probe added ten new objects to the Exoplanet catalogue: [98] CoRoT-16b, CoRoT-17b, CoRoT-18b, CoRoT-19b, CoRoT-20b, CoRoT-21b, CoRoT-22b, CoRoT-23b, CoRoT-24b, CoRoT-24c.

As of November 2011, around 600 additional candidate exoplanets are being screened for confirmation. [99]

Main results Edit

Among the exoplanets CoRoT detected, one can highlight a subset with the most original features :

  • CoRot-1b, the first planet detected by CoRoT is a hot Jupiter. By further analysis, CoRoT-1b became the first exoplanets to have its secondary eclipse detected in the optical, [100] thanks to the high precision lightcurve delivered by CoRoT.
  • CoRoT-3b, with a mass of 22 MJup, it appears to be "something between a brown dwarf and a planet." According to the definition of planet proposed by the owners of the exoplanet.eu database [101] three years later, CoRoT-3b, being less massive than 25 Jupiter masses, is classified as an exoplanet. In an August 2010 paper, CoRoT detected the ellipsoidal and the relativistic beaming effects in the CoRoT-3 lightcurve. [102]
  • CoRot-7b, with a radius of 1.7 REarth and a mass of 7.3 MEarth, was the first confirmed rocky planet, with a density and composition which are close to those of the Earth.

List of planets discovered Edit

The following transiting planets have been announced by the mission.

Light green rows indicate that the planet orbits one of the stars in a binary star system.

Star Constellation Right
ascension
Declination App.
mag.
Distance (ly) Spectral
type
Planet Mass
(MJ)
Radius
(RJ)
Orbital
period
(d)
Semi-major
axis
(AU)
Orbital
eccentricity
Inclination
(°)
Discovery
year
Ref
CoRoT-1 Monoceros 06 h 48 m 19 s −03° 06′ 08″ 13.6 1,560 G0V b 1.03 1.49 1.5089557 0.0254 0 85.1 2007 [103]
CoRoT-2 Aquila 19 h 27 m 07 s +01° 23′ 02″ 12.57 930 G7V b 3.31 1.465 1.7429964 0.0281 0 87.84 2007 [104]
CoRoT-3 Aquila 19 h 28 m 13.265 s +00° 07′ 18.62″ 13.3 2,200 F3V b 21.66 1.01 4.25680 0.057 0 85.9 2008 [105]
CoRoT-4 Monoceros 06 h 48 m 47 s −00° 40′ 22″ 13.7 F0V b 0.72 1.19 9.20205 0.090 0 90 2008 [106]
CoRoT-5 Monoceros 06 h 45m m 07s s +00° 48′ 55″ 14 1,304 F9V b 0.459 1.28 4.0384 0.04947 0.09 85.83 2008 [107]
CoRoT-6 Ophiuchus 18 h 44 m 17.42 s +06° 39′ 47.95″ 13.9 F5V b 3.3 1.16 8.89 0.0855 < 0.1 89.07 2009 [108]
CoRoT-7 Monoceros 06 h 43 m 49.0 s −01° 03′ 46.0″ 11.668 489 G9V b 0.0151 0.150 0.853585 0.0172 0 80.1 2009 [109]
CoRoT-8 Aquila 19 h 26 m 21 s +01° 25′ 36″ 14.8 1,239 K1V b 0.22 0.57 6.21229 0.063 0 88.4 2010 [110]
CoRoT-9 Serpens 18 h 43 m 09 s +06° 12′ 15″ 13.7 1,500 G3V b 0.84 1.05 95.2738 0.407 0.11 >89.9 2010 [111]
CoRoT-10 Aquila 19 h 24 m 15 s +00° 44 ′ 46″ 15.22 1,125 K1V b 2.75 0.97 13.2406 0.1055 0.53 88.55 2010 [112]
CoRoT-11 Serpens 18 h 42 m 45 s +05° 56′ 16″ 12.94 1,826 F6V b 2.33 1.43 2.99433 0.0436 0 83.17 2010 [113]
CoRoT-12 Monoceros 06 h 43 m 04 s −01° 17′ 47″ 15.52 3,750 G2V b 0.917 1.44 2.828042 0.04016 0.07 85.48 2010 [114]
CoRoT-13 Monoceros 06 h 50 m 53 s −05° 05′ 11″ 15.04 4,272 G0V b 1.308 0.885 4.03519 0.051 0 88.02 2010 [115]
CoRoT-14 Monoceros 06 h 53 m 42 s −05° 32′ 10″ 16.03 4,370 F9V b 7.58 1.09 1.51215 0.027 0 79.6 2010 [116]
CoRoT-16 Scutum 18 h 34 m 06 s −06° 00′ 09″ 15.63 2,740 G5V b 0.535 1.17 5.3523 0.0618 0.33 85.01 2011 [117]
CoRoT-17 Scutum 18 h 34 m 47 s −06° 36′ 44 ″ 15.46 3,001 G2V b 2.43 1.02 3.768125 0.0461 0 88.34 2011 [118]
CoRoT-18 Monoceros 06 h 32 m 41 s −00° 01′ 54″ 14.99 2,838 G9 b 3.47 1.31 1.9000693 0.0295 <0.08 86.5 2011 [119]
CoRoT-19 Monoceros 06 h 28 m 08 s −00° 01′ 01″ 14.78 2,510 F9V b 1.11 1.45 3.89713 0.0518 0.047 87.61 2011 [120]
CoRoT-20 Monoceros 06 h 30 m 53 s +00° 13′ 37″ 14.66 4,012 G2V b 4.24 0.84 9.24 0.0902 0.562 88.21 2011 [121]
CoRoT-21 Monoceros 16 F8IV b 2.26 1.30 2.72474 0.0417 0 86.8 2,011 [122]
CoRoT-22 Serpens 18 h 42 m 40 s +06° 13′ 08″ 11.93 2,052 G0IV b < 0.15 0.52 9.7566 0.094 < 0.6 89.4 2011
CoRoT-23 Serpens 18 h 39 m 08 s +04° 21′ 28″ 15.63 1,956 G0V b 2.8 1.05 3.6314 0.0477 0.16 85.7 2011 [123]
CoRoT-24 Monoceros 06 h 47 m 41 s −03° 43′ 09″ 4,413 b < 0.1 0.236 5.1134 2011
CoRoT-24 Monoceros 06 h 47 m 41 s −03° 43′ 09″ 4,413 c 0.173 0.38 11.749 2011
CoRoT-25 Ophiuchus 18 h 42 m 31.120 s +06° 30′ 49.74″ 15.02 3,711 F9V b 0.27 1.08 4.86 0.0578 84.5 2011
CoRoT-26 Ophiuchus 18 h 39 m 00.0 s +06° 58′ 12.00″ 15.76 5,446 G8IV b 0.5 1.26 4.204 0.0526 0 86.8 2012
CoRoT-27 4413 G2 b 10.39±0.55 1.01±0.04 3.58 0.048 <0.065 2013 [124] [125]
CoRoT-28 Ophiuchus 18 h 34 m 45.0 s +05° 34′ 26″ 13.47 1826 G8/9IV b 0.484±0.087 0.9550±0.0660
CoRoT-29 b
CoRoT-30 15.65 G3V b 0.84 (± 0.34) 1.02 (± 0.08) 9.06005 (± 0.00024) 0.084 (± 0.001) 0.007 (+0.031 -0.007) 90.0 (± 0.56) 2017 [126]
CoRoT-31 15.7 G2IV b 2.84 (± 0.22) 1.46 (± 0.3) 4.62941 (± 0.00075) 1.46 (± 0.3) 0.02 (+0.16 -0.02) 83.2 (± 2.3) 2017 [127]
CoRoT-33 b

Other discoveries Edit

The following table illustrates brown dwarf detected by CoRoT as well as non-transiting planets detected in the follow-up program:

Star Constellation Right
ascension
Declination App.
mag.
Distance (ly) Spectral
type
Object Type Mass
(MJ)
Radius
(RJ)
Orbital
period
(d)
Semi-major
axis
(AU)
Orbital
eccentricity
Inclination
(°)
Discovery
year
Ref
CoRoT-7 Monoceros 06 h 43 m 49.0 s −01° 03′ 46.0″ 11.668 489 G9V c planet 0.0264 3.69 0.046 0 2009 [128]
CoRoT-15 Monoceros 06 h 28 m 27.82 s +06° 11′ 10.47″ 16 4,140 F7V b brown dwarf 63.3 1.12 3.06 0.045 0 86.7 2010 [129]

Global properties of the exoplanets discovered by CoRoT Edit

All CoRoT planets were detected during long runs i.e. of at least 70 days. The detection team found on average between 200 and 300 cases of periodic events for each run, corresponding to 2–3% of the stars monitored. Of these, only 530 in total were selected as candidate planets (223 in the direction of the galactic anti-center and 307 towards the center ). Only 30 of them were finally found to be true planets, i.e. about 6%, other cases being eclipsing binaries ( 46%) or unresolved cases (48%). [130]

The detection capabilities of Corot are illustrated by the figure D showing the depth of the transits measured for all candidates, depending on the period and the brightness of the star: there is indeed a better ability to detect small planets (up to 1.5 R Earth ) for short periods (less than 5 days) and bright stars.

The CoRoT planets cover the wide range of properties and features found in the disparate family of exoplanets: for instance, the masses of CoRoT planets cover a range of almost four orders of magnitude, as shown on Figure.

Tracing the mass of the planet versus the mass of the star (Figure), one finds that the CoRoT data set, with its lower scatter than other experiments, indicates a clear trend that massive planets tend to orbit massive stars, which is consistent with the most commonly accepted models of planetary formation.

    – A robotic optical telescope searching for extrasolar planets – A 2007 European study concept of an array of space observatories – Tenth mission of the Discovery program optical space telescope for exoplanetology – Wikimedia list article – Cancelled NASA space telescope – A NASA concept study of an array of space telescopes
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The latest version of the GIZMO code is made available by its author, P. Hopkins, at http://www.tapir.caltech.edu/

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Pebble accretion

The biggest challenge to core accretion is time &mdash building massive gas giants fast enough to grab the lighter components of their atmosphere. Recent research on how smaller, pebble-sized objects fused together to build giant planets up to 1000 times faster than earlier studies.

"This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see," study lead author Harold Levison, an astronomer at the Southwest Research Institute (SwRI) in Colorado, told Space.com in 2015.

In 2012, researchers Michiel Lambrechts and Anders Johansen from Lund University in Sweden proposed that tiny pebbles, which were once written off, held the key to rapidly building giant planets.

"They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem," Levison said.

Levison and his team built on that research to model more precisely how the tiny pebbles could form planets seen in the galaxy today. While previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, Levison's simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a far faster rate.

"The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk," study co-author Katherine Kretke, also from SwRI, told Space.com. "The bigger guy basically bullies the smaller one so they can eat all the pebbles themselves, and they can continue to grow up to form the cores of the giant planets."


Wobbly, Sunlike Star Being Pulled by Giant Alien Planet

By analyzing sonic vibrations in a distant sunlike star, astronomers might have calculated exactly how fast the star spins and how much a nearby giant alien planet weighs.

Stars, including the sun, experience sound waves that zip around inside them and cause tiny rhythmic fluctuations in their brightness. By studying these variations, scientists can better understand the interiors of stars— an emerging scientific field known as asteroseismology that is akin to seismology on Earth, which helps geologists yield insights into the innards of this planet.

Scientists used the COROT satellite to analyze the sunlike star HD 52265, located more than 90 light-years from Earth in the constellation Monoceros, the Unicorn. The star, which has a mass about 1.2 times that of the sun and a diameter 1.3 times greater than the sun's, is about 2.1 billion to 2.7 billion years old. [The Strangest Alien Planets (Gallery)]

Repeated wobbles in the movements of HD 52265 suggested a giant planet's gravitational pull was tugging on it, which astronomers dubbed HD 52265b. The magnitude of the wobbles suggested the planet had a mass at least 1.09 times that of Jupiter — scientists could not give a more precise figure based on the wobbles alone.

The oscillations in brightness that the researchers investigated are linked to ripples in that star that are, in turn, based in part on its rate of rotation. The scientists calculated HD 52265's interior completes a revolution every 12 days, meaning it revolves some 2.3 times faster than the sun.

"Knowing the rotation of stars is important to understand stellar activity cycles," said Laurent Gizon, an astrophysicist at the Max Planck Institute for Solar System Research in Germany and the study's lead author. "Magnetic fields in stars like the sun are maintained by rotation and convection."

Discovering the manner in which the star HD 52265 rotates also provides clues about how the planet HD 52265b is oriented toward it, assuming the star's equator is lined up with the planet's, as is typically the case in Earth's solar system. When these data are combined with the information about the magnitude of the wobbles the planet exerts on its star, the mass of world is about 1.85 times the mass of Jupiter, the researchers calculated.

"Asteroseismology is a very powerful technique to fully characterize exoplanets," Gizon said.

In the future, the European Space Agency's PLATO mission could use asteroseismology to analyze a multitude of stars and planets.

"The decision on the selection of the mission is expected at the beginning of 2014," Gizon said.

The scientists detailed their findings online July 29 in the journal Proceedings of the National Academy.


Far out: A giant exoplanet where none has been seen before

Timeline of where planets are expected to be found as their parent stars evolve when forming in a protoplanetary disk. The star GU Psc is part of the AB Doradus Moving Group, with an estimated age between 70-130 million years. As such, GU Psc b would be expected to be within 100 AU of GU Psc. Instead, it is 2000 AU away, suggesting a different formation mechanism. Credit: Shu et al. 1987, Courtesy of the James Webb Telescope and NASA

Humans have an eye for the familiar: for people, for civilizations, for planets and planetary systems that match what we have seen in the past. For this reason, as well as a few others, we rarely find something truly unique in the universe. When we do, it's often by happenstance.

Finding a planet ten times the mass of Jupiter orbiting 40 times further out than Pluto was in some ways unexpected.

"It's not a type of exoplanet we had previously found and it's certainly not one that the theorists were expecting," said paper co-author PhD student Marie-Eve Naud at the University of Montréal, "especially not around low-mass stars like GU Psc (

Last June, Astrobio.net ran the surprising story of a planet 5-10 times the mass of Jupiter that formed 80 AU from its parent star. In yet another twist in the continuing saga of planetary treasure-hunting, researchers in Naud's group at the University of Montréal announced the discovery of a planet many times the mass of Jupiter at an incredible distance from its star: 2000 AU.

This planet-GU Psc b-is so far from its star that it could be photographed without the aid of adaptive optics, appearing as an independent point of infrared light from its star GU Psc.

Unlike GU Psc b, most of the 1700 plus exoplanets in NASA's current catalog have not been directly imaged, or photographed directly, in any wavelength. Their gravitational effects on their parent stars have been measured. Sometimes the dimming effect as they occlude the light from their parent stars have been recorded. Occasionally, existence of planet is inferred by the presence of a gap in dust disks rotating around a star.

At present, the Exoplanet catalog maintained by the Paris Observatory lists only 46 planets that have been directly imaged, of which GU Psc b is unique in a number of ways.

Of the exoplanets that have been photographed, GU Psc b at 9-13 times the mass of Jupiter is one the largest that is probably not a failed star, or brown dwarf. At thirteen times the mass of Jupiter, a gas giant attains the minimum mass to begin some nuclear fusion early in its life. Many of the photographed companions might in fact have a mass greater than 13 times that of Jupiter, and may be more like brown dwarfs than planets in nature. GU Psc b is also one of the few so far out from its companion. Perhaps most interestingly, of all the far-out planets discovered so far by any means, this new planet is only giant, distant one at all that rotates around a star like our own.

"GU Psc b is certainly the farthest planetary-mass companion known around a regular, main-sequence star," said Naud.

In their recent paper, Naud and her co-advisor at the University of Montreal Dr. Étienne Artigau describe how they applied an old technique in a new way. Seeing giant planets that are relatively close to their stars, many light years away, requires state-of-the art adaptive optics (AO) and the Gemini Observatory. Naud and Artigau had the idea of looking for farther out planets using a "standard" camera. To find distant planets without specialized instruments, they exploited certain color, or wavelengths, that distinguish planets from stars or galaxies in the background. They surveyed dozens of stars with this technique before finding GU Psc b.

"My idea with this project was to explore the entire 'sphere' where a planet could physically stay for an astrophysically significant amount of time," said Artigau, "regardless of predictions relative to formation mechanisms."

Many models predict that planets should not be found beyond 100 AU. Those same models also predict that if planets are present within about 5000 AU of a parent star, they should stay there for billions of years. For astronomers, that means there is a wide swath of sky that can potentially contain a planet. Regardless, many recent searches have focused on finding planets very near young stars. Naud and her colleagues took a slightly broader view.

Credit: ©Stellarium

"This does not allow us to find planets as close as the ones found by specialized

instruments, but it allowed us to detect GU Psc b, located 42 arc seconds from its star," said Naud, "GU Psc b is precious to validate our models of similar objects, and we will be able to use the information we obtain on this object to better understand other, closer-in and harder to study planetary-mass companions.

Because GU Psc b is so far from its parent star, astronomers can learn a tremendous amount about it. In just one hour on the Gemini North scope, Naud and her team obtained a spectrum of GU Psc b that allowed them to estimate important physical properties of the planet, like mass and temperature. These are usually difficult or impossible to observe because of the nearby star.

As a very distant companion, GU Psc b is a diamond in the rough. Not only does it provide us a very clear view of the planet itself with its mass and temperature, but it also provides us with an impetus to look for more planets like it, in places we would have never looked before.

"This type of object exists, and we can quite easily find them," said Naud, "so we should definitely search for them."


Meteor magnets in outer space

Astronomers believe planets like Jupiter shield us from space objects that would otherwise slam into Earth. Now they’re closer to learning whether giant planets act as guardians of solar systems elsewhere in the galaxy.

A UCR-led team has discovered two Jupiter-sized planets about 150 light years away from Earth that could reveal whether life is likely on the smaller planets in other solar systems.

“We believe planets like Jupiter have profoundly impacted the progression of life on Earth. Without them, humans might not be here to have this conversation,” said Stephen Kane, lead study author and UCR associate professor of planetary astrophysics. “Understanding how many other stars have planets like Jupiter could be very important for learning about the habitability of planets in those systems.”

Along with liquid water oceans, Kane said astronomers believe such planets have the ability to act as ‘slingshots,’ pulling objects like meteors, comets, and asteroids out of their trajectories en route to impact with small, rocky planets.

Many larger planets have been found close to their stars. However, those aren’t as useful for learning about the architecture of our own solar system, where the giant planets including Saturn, Uranus and Neptune are all farther from the sun. Big planets far from their stars have, until now, been harder to find.

A study recently published in the Astronomical Journal details how Kane’s team found success in a novel approach combining traditional detection methods with the latest technologies.

One popular method of searching for exoplanets — planets in other solar systems — involves monitoring stars for “wobble,” in which a star moves toward and away from Earth. The wobble is likely caused by the gravitational pull a nearby planet is exerting on it. When a star wobbles, it’s a clue there may be an exoplanet nearby.

When the planet is far from its star, the gravitational pull is weaker, making the wobble smaller and harder to detect. The other problem with using the wobble detection method, Kane said, is that it just takes a long time. Earth only takes a year to orbit the sun. Jupiter takes 12, Saturn takes 30, and Neptune takes an astonishing 164 years.

The larger exoplanets also take many years to circle their stars, which means observing a complete orbit could engulf an astronomer’s entire career. To accelerate the process, Kane and his team combined the wobble method with direct imaging. This way, if the team thought a planet might be causing wobble, they could confirm it by sight.

Obtaining a direct image of a planet quadrillions of miles away is no simple task. It requires the largest possible telescope, one that is at least 32 feet long and highly sensitive. Even from this distance, the light of the stars can overexpose the image, obscuring the target planets.

The team overcame this challenge by learning to recognize and eliminate the patterns in their images created by starlight. Removing the starlight allowed Kane’s team to see what remained.

“Direct imaging has come a long way both in terms of understanding the patterns we find, and in terms of the instruments used to create the images, which are much higher resolution than they’ve ever been,” Kane said. “You see this every time a new smartphone is released — the camera detectors are always being improved and that’s true in astronomy as well.”

In this project, the team applied the combination of wobble and imaging method to 20 stars. In addition to the two being orbited by giant Jupiter-like planets that had not been previously discovered, the team also detected a third, previously observed star with a giant planet in its system.

Going forward, the team will continue to monitor 10 of the stars where planetary companions could not be ruled out. In addition, Kane is planning a new project to measure how long it takes these exoplanets to complete rotations toward and away from their stars, which cannot currently be measured.

Kane’s team is international, with members at the Australian Astronomical Observatory, University of Southern Queensland, University of New South Wales and Macquarie University in Australia, as well as at the University of Hertfordshire in the United Kingdom. They are also spread across the U.S. at the National Optical Astronomy Observatory in Tucson, AZ, Southern Connecticut State University, NASA Ames Research Center and Stanford University in California and the Carnegie Institution of Washington in D.C.

“This discovery is an important piece of the puzzle because it helps us understand the factors that make a planet habitable and whether that’s common or not,” said Kane. “We are converging rapidly on answers to this question that the past 3,000 recorded years of history could only wish they had available to them.”


Alphard is situated at around 177 light-years / 54 parsecs away from the Sun. It is so bright that it can be seen with the naked eye.

Since Alphard has evolved away from the main-sequence and became a giant star, its radius has expanded considerably. Alphard’s radius has been estimated at around 50.5 solar radii or 5050% times the Sun’s radius.

The angular diameter of Alphard is also impressive, being beaten only by stars such as Betelgeuse or R Doradus. In regards to its mass, Alphard has around 3.03 solar masses or 303% of the Sun’s mass.


Ultrabass Sounds of the Giant Star xi Hya: First Observations of Solar-type Oscillations in a Star Very Different from the Sun

About 30 years ago, astronomers realised that the Sun resonates like a giant musical instrument with well-defined periods (frequencies). It forms a sort of large, spherical organ pipe. The energy that excites these sound waves comes from the turbulent region just below the Sun`s visible surface.

Observations of the solar sound waves (known as “helioseismology”) have resulted in enormous progress in the exploration of the interior of the Sun, otherwise hidden from view. As is the case on Earth, seismic techniques can be applied and the detailed interpretation of the observed oscillation periods has provided quite accurate information about the structure and motions inside the Sun, our central star.

It has now also become possible to apply this technique to some solar-type stars. The first observations concerned the northern star eta Bootis (cf. ESO PR 16/94). Last year, extensive and much more accurate observations with the 1.2-m Swiss telescope at the ESO La Silla Observatory proved that Alpha Centauri, a solar “twin”, behaves very much like the Sun (cf. ESO PR 15/01), and that some of the periods are quite similar to those in the Sun.

These new observational data were of a superb quality, and that study marked a true break-through in the new research field of “asteroseismology” (seismology of the stars) for solar-type stars. But what about other types of stars, for instance those that are much larger than the Sun?

Based on an extremely intensive observing project with the same telescope, an international group of astronomers [1] has found that the giant star xi Hya (“xi” is the small greek letter [2] “Hya” is an abbreviation of “Hydrae”) behaves like a giant sub-ultra-bass instrument. This star is located in the constellation Hydra (the Water-Monster) at a distance of 130 light-years, it has a radius about 10 times that of the Sun and its luminosity is about 60 times larger.

The new observations demonstrate that xi Hya oscillates with several periods of around 3 hours. xi Hya is now approaching the end of its life – it is about to expand its outer envelope and to become a “red giant star”. It is quite different from stars like the Sun, which are only halfway through their active life. xi Hya is considerably more massive than any other star in which solar-like oscillations have so far been detected.

This observational feat allows to study for the first time with seismic techniques the interior of such a highly evolved star. It paves the way for similar studies of different types of stars. A new chapter of stellar astrophysics is now opening as asteroseismology establishes itself as an ingenious method that is able to revolutionise our detailed understanding of stellar interiors and the overall evolution of stars.

The difficult art of asteroseismology

Helioseismology (seismology of the Sun) is based on measurements of the changing radial velocity of the solar upper atmospheric layers (the “surface”) by means of the well-known Doppler effect, as this surface moves up and down during acoustic oscillations. The corresponding amplitudes are very small, with velocities of up to 15 – 20 cm/sec, and the typical period is around 5 minutes. Therefore the phenomenon was first known as the “five-minute oscillations”.

Intensity measurements have also been tried, but the noise level is larger than for velocity data due to the presence of “granulation” (moving cells of hot gas) on the solar surface.

In the case of larger and brighter stars like the giant stars, the corresponding amplitudes and periods increase. For instance, theoretical predictions for the giant star xi Hya have indicated that velocity amplitudes of about 7 m/sec and periods of the order of 3 – 4 hours could be expected.

Observations of such oscillations are much more difficult, because the demands on the performance of the spectrograph increase dramatically, as this timescale is similar to that of variations of conditions in the Earth`s atmosphere during the observing night.

Spurious instrumental effects, like mechanical flexure, would be detrimental to such demanding observations. However, the experience from the search for exoplanets orbiting other stars – by observing the periodic change in velocity of the parent star due to the weak pull of the orbiting planet over even longer timescales – has proven to be very useful. Indeed, asteroseismology has benefitted greatly from the development of accurate techniques now employed in the search for exoplanets.

The observations of the giant star xi Hya

An international team of astronomers [1] observed xi Hya with the Swiss 1.2-m Euler telescope at the ESO La Silla Observatory (Chile). They used the CORALIE spectrograph, which is well known for numerous discoveries of exoplanets (cf. PR 07/01), and recently for the detection of 7-min acoustic oscillations in the solar-twin star Alpha Centauri A (cf. PR 15/01).

The same technique that delivered superb observations of Alpha Centauri A was employed to investigate the oscillations of xi Hya. The sound waves make the surface of the star oscillate periodically in and out, and the CORALIE spectrograph measures the velocities of the up-down motion.

As xi Hya is a giant, these waves need more time to propagate through the stellar interior up to the stellar surface than they do in a solar-like star. Thus, the generated oscillations of the surface are slower.

An observing campaign lasting no less than one full month, taking about two measurements every hour was necessary to detect the tiny movements of the surface of xi Hya.

The detected oscillations have periods of about 3 hours, and have speeds of only up to 2 metres per second. This is somewhat smaller than expected, but the predictions for these amplitudes were very uncertain as the conditions in xi Hya are so very different from those in the Sun.

First results for xi Hya

PR Photo 13a/02 shows the frequency spectrum of xi Hya, based on these extensive observations. The “power peaks” indicate the frequencies of the oscillation of the stellar atmosphere. The broad distribution means that several different sound waves are clearly present. This is the first time such a spectrum has ever been obtained for a giant star.

A first analysis showed the presence of about one dozen significant frequencies and correspondingly, periods. Among those, four have amplitudes above 1 metre per second. In addition to these twelve frequencies, others appear to have been detected as well, but with less certainty and their reality must be confirmed by a subsequent, more detailed study.

The “sound of xi Hya” has been synthesized in PR Audio Clip 01/02.

A good model of the star is necessary before the observed oscillation frequencies (periods) can be properly interpreted. Current models of the Sun are accurate and represent a typical main-sequence star at midlife, and the oscillations are well understood. The sound spectrum corresponding to the full disk – i.e., what we would observe if the Sun were as distant as other stars and we would therefore see it as a light point in the sky – shows a regular pattern in which the observed frequencies are separated by two different and constant intervals, the “large” and the “small” separations.

It is much more difficult to “model” the interior of a giant star as the core has changed a lot during the evolution of the star. The nuclear fuel has been exhausted, the stellar core has contracted and the envelope has expanded substantially [3]. The resulting sound spectrum has therefore also changed considerably. Now there is only a small group of oscillating modes that display the same regular pattern as seen in the Sun. They are the radial modes, pressure modes that correspond to a radial expansion and contraction of the star (up and down motion of the surface).

The modes in the Sun are sound waves for which most of the oscillation energy is concentrated in the outer parts of the Sun. In stars as highly evolved as xi Hya, they partly take on the character of gravity modes in the interior of the star.

Gravity modes are oscillations that move matter up and down in the gravity field, under the influence of buoyancy, with only small changes of the pressure. This is the same effect that makes an air-filled ball pop to the surface when released under water. Gravity modes are normally trapped in the stable interior inside the upper (convective) envelope of a star.

So far gravity modes have not been detected in the Sun. In a giant star, however, there is a chance to see some, because some of the oscillations have a mixed character: they behave like gravity modes in the interior and like sound waves in the envelope.

The nature of the oscillations observed in xi Hya

The high-resolution spectra of xi Hya were also used to determine improved values of the fundamental parameters of this star: its temperature is 4950 +/- 100 K, the mass is 3.31 +/- 0.17 times that of the Sun, and the age is 276 +/- 21 million years [3]. These values may be refined in a subsequent, more extensive analysis.

With this improved model for xi Hya, the astronomers calculated the frequencies of all oscillations likely to be observed. As in the Sun, the radial modes are expected to be the dominating ones. In fact, three out of the four modes actually observed in xi Hya coincide within the errors with the predicted radial modes. The fourth mode seems not to be radial, but agrees with a non-radial mode with 2 or 3 wave peaks and valleys over the surface. PR Photo 13b/02 provides a graphical illustration of this in the case of a star seen almost equator-on.

Some of the observed lower-amplitude modes must be mixed non-radial modes, since more modes are detected than can be accounted for by the radial modes of the models alone.

Moving directly from stars of about one solar mass to the giant star xi Hya is a rather great leap. With the CORALIE and HARPS instruments (the latter soon to be installed on the ESO 3.6-m telescope at La Silla), an entire sequence of stars at different evolutionary stages will be observed next: from newly born to middle-aged stars like the Sun, and also old ones that are near retirement.

The new observations of xi Hya show that this is now technically feasible. Once more stars have been observed, changes in the interior structure and composition can be followed and current theories of the internal stellar structure can be verified and improved. Clearly, asteroseismology is bound to have a major impact on the understanding of stellar evolution.

The detection of oscillations in the giant star xi Hya also has implications for the target selection of several space missions aiming at seismic measurements: the Canadian MOST mission, the French-led European COROT mission (with launch expected in 2005), and some that are still under consideration, as the Danish Roemer mission (now in the detailed design phase) and the ESA Eddington mission. The present observations have proven that these space missions will be able to observe oscillations in a wide range of stars, and thus will constitute a major new source of detailed information about the interior of stars, not accessible from the ground.

[1]: The team consists of Conny Aerts and Thomas Maas (Dept. of
Physics and Astronomy, Catholic University of Leuven, Belgium), Fabien
Carrier, Michel Burnet, Jose de Medeiros and Francois Bouchy (Geneva
Observatory, Switzerland), Soeren Frandsen, Dennis Stello, Hans
Kjeldsen, Teresa C. Teixeira, Frank Pijpers, Joergen
Christensen-Dalsgaard and Hans Bruntt (Dept. of Physics and
Astronomy, Aarhus University and Theoretical Astrophysics Center,
Aarhus University, Denmark).

[2]: Some HTML-browsers support character entities for greek letters – “xi”
is then represented by “?”.

[3]: In astrophysical terms, xi Hya is currently in the core-He-burning
phase, having left the main sequence some time ago and now near the
sub-giant/giant border.


Watch the video: Δημιουργία και εξέλιξη της Γης (December 2021).