Astronomy

Why is Proxima Centauri called Proxima Centauri?

Why is Proxima Centauri called Proxima Centauri?



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It seems very logical, to call the star, closest to earth, "Proxima" ("Proximus" is Latin for "close" and the word for star, "stella", is feminin, hence "Proxima"), but there's one thing I don't understand: in order to call a star ±"The closest one", you must be sure that in future, no closer star will be found.
This is not so simple: lots of very close stars, like Barnard's star, Kapteyn's,… , are very weak and even not visible to the naked eye.
So, how can we be that sure that Proxima is indeed the nearest one? Have astronomers indeed examined the complete sky in order to prove this?


So, how can we be that sure that Proxima is indeed the nearest one? Have astronomers indeed examined the complete sky in order to prove this?

The minimum absolute magnitude for a star is, according to this well-known diagram about +16. Proxima Centuari is at the very low end of this scale with an absolute magnitude of +15.60. Still, it has an apparent magnitude of +11.13. That means that if a star is closer to us, it needs to be +11.5 or brighter. Every object of that magnitude or brighter has been categorized, and a hypothetical close star would've stood out due to its large parallax. So it's safe to say there are no closer stars… at this moment.

you must be sure that in future, no closer star will be found.

Stars move (they rotate around the galaxy center), so in the (far) future, there might be a star closer to use than Proxima Centauri, just like sometimes Venus is the planet closest to Earth, and sometimes Mars. So it's possible that Proxima might not be the nearest star in the far future. This is not something we have to worry in our lifetimes, though :)


A 1928 paper mentions this very point:

Iɴɴᴇs has suggested the name Proxima Centauri for this star. Since it is the nearest of the three components of the system, the name is most appropriate and may well be retained even though some star nearer the Sun be subsequently discovered in the same constellation.

There are a couple of points to note here. "Proxima Centauri" can be taken to mean "close one of the Centaur". So a more close star in a different constellation would not invalidate the name.

Innes, the discover of the star, was hopeful that his discovery was important. He attempted to make a distance measurement by parallax (seeing how far the star appears to move as the Earth orbits the sun.) The value for the distance he published was slightly smaller than the accepted distance of Alpha Centauri, but the error bars in he measurement meant that in truth he did not know if it was slightly nearer, or slightly further from the Sun. Nevertheless he suggested:

If this small star had a name it would be convenient-it is therefore suggested that it should be referred to as Proxima Centaurus. (quoted)

This is basically grandstanding. Innes hoped that his star might turn out to be the very closest star and suggested that name. In 1928, it was established to be the closest star in the Alpha centauri system, the name was set and seen to be appropriate, even if some other star was later discovered to be closer.

In the future other stars will come closer. A star called Ross 248 will briefly be the closest star in about 30000 years time.


Proxima Centauri

Proxima Centauri in infrared. Image by 2-Micron All Sky Survey.

Proxima Centauri (also known as Gliese 551) is the nearest star to the Sun, just 4.24 light-years away (268,000 AU or 40 trillion miles), but you can't see it without a telescope. That's because it's a red dwarf – a star that's much smaller, cooler, and fainter than the Sun – more than 10,000 times fainter. It's part of the Alpha Centauri system, going around the two main stars, A and B, in a huge orbit, at a distance of more than a fifth of a light-year, which that takes it more than half a million years to complete. Proxima Centauri was discovered in 1915 by Robert Innes.

Because of its nearness, both to the Sun and its Alpha Cen neighbors, Proxima has well-determined physical properties, including a mass of 0.11 solar masses, a radius of about 15% that of the Sun, and an age of 5 to 6 billion yr. Despite its considerable age, Proxima has an active chromosphere and is also a flare star (variable-star designation V645 Centauri), capable of brightening a magnitude or more in minutes. Observations of its chromosphere at ultraviolet wavelengths suggest a rotation period of about 31 days. Claims made in the mid-1990s, based on data from the Hubble Space Telescope, that Proxima may be orbited by a large planet or a brown dwarf, have not been substantiated.

distance 4.24 light-years (1.30 pc)
spectral type M5.5Ve
apparent magnitude 11.09
absolute magnitude 15.53
luminosity (Sun = 1) 0.0017
temperature 3,040 K
mass (Sun = 1) 0.12
radius (Sun = 1) 0.14
other designations &alpha Centauri C, V645 Centauri,
GCTP 3278.00, GJ 551,
LHS 49, LFT 1110,
LTT 5721, HIP 70890


Why is Proxima Centauri called Proxima Centauri? - Astronomy

For Release: October 11, 2016


An artist's illustration depicts the interior of a low-mass star. Such stars have different interior structures than our Sun, so they are not expected to show magnetic activity cycles. However, astronomers have discovered that the nearby star Proxima Centauri defies that expectation and shows signs of a 7-year activity cycle.
Credit: NASA/CXC/M.Weiss

Cambridge, MA - In August astronomers announced that the nearby star Proxima Centauri hosts an Earth-sized planet (called Proxima b) in its habitable zone. At first glance, Proxima Centauri seems nothing like our Sun. It’s a small, cool, red dwarf star only one-tenth as massive and one-thousandth as luminous as the Sun. However, new research shows that it is sunlike in one surprising way: it has a regular cycle of starspots.

Starspots (like sunspots) are dark blotches on a star’s surface where the temperature is a little cooler than the surrounding area. They are driven by magnetic fields. A star is made of ionized gases called plasma. Magnetic fields can restrict the plasma’s flow and create spots. Changes to a star’s magnetic field can affect the number and distribution of starspots.

Our Sun experiences an 11-year activity cycle. At the solar minimum, the Sun is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun’s surface on average.

The new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak. However, its cycle is much more dramatic. At least a full one-fifth of the star’s surface is covered in spots at once. Also, some of those spots are much bigger relative to the star’s size than the spots on our Sun.

“If intelligent aliens were living on Proxima b, they would have a very dramatic view,” says lead author Brad Wargelin of the Harvard-Smithsonian Center for Astrophysics (CfA).

Astronomers were surprised to detect a stellar activity cycle in Proxima Centauri because its interior is expected to be very different from the Sun’s. The outer third of the Sun experiences a roiling motion called convection, similar to water boiling in a pot, while the Sun’s interior remains relatively still. There is a difference in the speed of rotation between these two regions. Many astronomers think the shear arising from this difference is responsible for generating the Sun’s magnetic activity cycle.

In contrast, the interior of a small red dwarf like Proxima Centauri should be convective all the way into the star’s core. As a result, it shouldn’t experience a regular cycle of activity.

“The existence of a cycle in Proxima Centauri shows that we don’t understand how stars’ magnetic fields are generated as well as we thought we did,” says Smithsonian co-author Jeremy Drake.

The study does not address whether Proxima Centauri’s activity cycle would affect the potential habitability of the planet Proxima b. Theory suggests that flares or a stellar wind, both of which are driven by magnetic fields, could scour the planet and strip away any atmosphere. In that case, Proxima b might be like Earth’s Moon – located in the habitable zone, but not at all friendly to life.

“Direct observations of Proxima b won’t happen for a long time. Until then, our best bet is to study the star and then plug that information into theories about star-planet interactions,” says co-author Steve Saar.

The team detected the activity cycle using ground-based observations from the All Sky Automated Survey combined with space-based X-ray measurements by several missions, including Swift, Chandra, and XMM-Newton. Their results have been accepted for publication in the Monthly Notices of the Royal Astronomical Society and appear online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.


Contents

In 1915, the Scottish astronomer Robert Innes, director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri. [23] [24] [25] [26] He suggested that it be named Proxima Centauri [27] (actually Proxima Centaurus). [28] In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax at 0.755″ ± 0.028″ and determined that Proxima Centauri was approximately the same distance from the Sun as Alpha Centauri. It was also found to be the lowest-luminosity star known at the time. [29] An equally accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of 0.783″ ± 0.005″ . [24] [27]

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known. [30] [31] The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995. [32] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra. [33]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. [34] The WGSN approved the name Proxima Centauri for this star on August 21, 2016 and it is now so included in the List of IAU approved Star Names. [35]

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N. [nb 4] Red dwarfs such as Proxima Centauri are too faint to be seen with the naked eye. Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star. [36] [37] It has apparent visual magnitude 11, so a telescope with an aperture of at least 8 cm (3.1 in) is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon. [38]

In 2018, a superflare was observed from Proxima Centauri, the strongest flare ever seen. The optical brightness increased by a factor of 68× to approximately magnitude 6.8. It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before. [39]

On 2020 April 22 and 23, the New Horizons spacecraft took images of two of the nearest stars, Proxima Centauri and Wolf 359. When compared with Earth-based images, a very large parallax effect was easily visible. However, this was mostly useful for illustrative purposes and did not improve on previous distance measurements. [40] [41]

Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. M5.5 means that it falls in the low-mass end of M-type dwarf stars. [16] Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs (33 ly), is 15.5. [42] Its total luminosity over all wavelengths is 0.17% that of the Sun, [10] although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun. [43] More than 85% of its radiated power is at infrared wavelengths. [44]

In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri is 1.02 ± 0.08 mas . Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. The star's mass, estimated from stellar theory, is 12.2% M , or 129 Jupiter masses ( M J). [45] The mass has been calculated directly, although with less precision, from observations of microlensing events to be 0.150 +0.062
−0.051 M . [46]

Lower mass main-sequence stars have higher mean density than higher mass ones, [47] and Proxima Centauri is no exception: it has a mean density of 47.1 × 10 3 kg/m 3 (47.1 g/cm 3 ), compared with the Sun's mean density of 1.411 × 10 3 kg/m 3 (1.411 g/cm 3 ). [nb 5] The measured surface gravity of Proxima Centauri, given as the base-10 logarithm of the acceleration in units of cgs, is 5.20. [11] This is 162 times the surface gravity on Earth. [nb 6]

A 1998 study of photometric variations indicates that Proxima Centauri rotates once every 83.5 days. [48] A subsequent time series analysis of chromospheric indicators in 2002 suggests a longer rotation period of 116.6 ± 0.7 days. [49] This was subsequently ruled out in favor of a rotation period of 82.6 ± 0.1 days. [15]

Because of its low mass, the interior of the star is completely convective, [50] causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end after about 4 trillion years. [51]

Convection is associated with the generation and persistence of a magnetic field. The magnetic energy from this field is released at the surface through stellar flares that briefly (as short as per ten seconds) [52] increase the overall luminosity of the star. On May 1, 2019, an extreme flare event briefly became the brightest ever detected, with a far ultraviolet emission of 2 × 10 30 erg . [53] These flares can grow as large as the star and reach temperatures measured as high as 27 million K [33] —hot enough to radiate X-rays. [54] Proxima Centauri's quiescent X-ray luminosity, approximately (4–16) × 10 26 erg/s ((4–16) × 10 19 W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 10 28 erg/s (10 21 W). [33]

Proxima Centauri's chromosphere is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. [55] About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona, [56] and its total X-ray emission is comparable to the sun's. [57] Proxima Centauri's overall activity level is considered low compared to other red dwarfs, [57] which is consistent with the star's estimated age of 4.85 × 10 9 years, [16] since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases. [58] The activity level also appears to vary [59] with a period of roughly 442 days, which is shorter than the solar cycle of 11 years. [60]

Proxima Centauri has a relatively weak stellar wind, no more than 20% of the mass loss rate of the solar wind. Because the star is much smaller than the Sun, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface. [61]

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming into a so-called "blue dwarf". Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity ( L ) and warming up any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy. [51]

Distance and motion Edit

Based on a parallax of 768.0665 ± 0.0499 mas , published in 2020 in Gaia Data Release 3, Proxima Centauri is 4.2465 light-years (1.3020 pc 268,550 AU) from the Sun. [8] Previously published parallaxes include: 768.5 ± 0.2 mas in 2018 by Gaia DR2, 768.13 ± 1.04 mas , in 2014 by the Research Consortium On Nearby Stars [62] 772.33 ± 2.42 mas , in the original Hipparcos Catalogue, in 1997 [63] 771.64 ± 2.60 mas in the Hipparcos New Reduction, in 2007 [2] and 768.77 ± 0.37 mas using the Hubble Space Telescope 's fine guidance sensors, in 1999. [9] From Earth's vantage point, Proxima Centauri is separated from Alpha Centauri by 2.18 degrees, [64] or four times the angular diameter of the full Moon. [65] Proxima Centauri also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. [66] It has a radial velocity toward the Sun of 22.2 km/s. [7]

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun. In 2001, J. García-Sánchez et al. predicted that Proxima Centauri will make its closest approach to the Sun in approximately 26,700 years, coming within 3.11 ly (0.95 pc). [67] A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly (0.89 pc) in about 27,400 years, [68] followed by a 2014 study by C. A. L. Bailer-Jones predicting a perihelion approach of 3.07 ly (0.94 pc) in roughly 26,710 years. [69] Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Centre that varies from 27 to 31 kly (8.3 to 9.5 kpc), with an orbital eccentricity of 0.07. [70]

Ever since the discovery of Proxima Centauri, it has been suspected to be a true companion of the Alpha Centauri binary star system. Data from the Hipparcos satellite, combined with ground-based observations, were consistent with the hypothesis that the three stars are a bound system. For this reason, Proxima Centauri is sometimes referred to as Alpha Centauri C. Kervella et al. (2017) used high-precision radial velocity measurements to determine with a high degree of confidence that Proxima and Alpha Centauri are gravitationally bound. [7] Proxima Centauri's orbital period around the Alpha Centauri AB barycenter is 547 000 +6600
−4000 years with an eccentricity of 0.5 ± 0.08 it approaches Alpha Centauri to 4300 +1100
−900 AU at periastron and retreats to 13 000 +300
−100 AU at apastron. [7] At present, Proxima Centauri is 12,947 ± 260 AU (1.94 ± 0.04 trillion km) from the Alpha Centauri AB barycenter, nearly to the farthest point in its orbit. [7]

Such a triple system can form naturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 M within their embedded star cluster before the cluster disperses. [71] However, more accurate measurements of the radial velocity are needed to confirm this hypothesis. [72] If Proxima Centauri was bound to the Alpha Centauri system during its formation, the stars are likely to share the same elemental composition. The gravitational influence of Proxima might also have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions, so possibly enriching any terrestrial planets in the system with this material. [72] Alternatively, Proxima Centauri may have been captured at a later date during an encounter, resulting in a highly eccentric orbit that was then stabilized by the galactic tide and additional stellar encounters. Such a scenario may mean that Proxima Centauri's planetary companions have had a much lower chance for orbital disruption by Alpha Centauri. [14]

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, [73] such as in a star cluster.

The Proxima Centauri planetary system [74] [75] [19] [76] [18] [20]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
Eccentricity Inclination Radius
d (unconfirmed) ≥ 0.29 ± 0.08 M 0.028 95 ± 0.000 22 5.168 +0.051
−0.069
b 1.60 +0.46
−0.36 M
0.048 57 +0.000 29
−0.000 29
11.184 18 +0.000 68
−0.000 74
0.109 +0.076
−0.068
1.30 +1.20
−0.62 R
c 7 ± 1 M 1.489 ± 0.049 1928 ± 20 0.04 ± 0.01 133 ± 1 °
RV-derived upper mass limits of potential companions [77]
Orbital
period
(days)
Separation
(AU)
Maximum
mass [nb 7]
(M)
3.6–13.8 0.022–0.054 2–3
< 100 < 0.21 8.5
< 1000 < 1 16

So far, as of 2021, two planets have been confirmed to orbit around Proxima Centauri, with one being close to Earth’s size and within the habitable zone (b) and another which may be a gas dwarf that orbits much father out (c). There are signs a third, even smaller planet could be orbiting closer than both of the planets, however this has not been confirmed yet.

Ever since the first exoplanets were discovered, there has been a hunt for exoplanets in the Alpha Centauri system. Multiple measurements of the star's radial velocity constrained the maximum mass that a detectable companion to Proxima Centauri could possess. [9] [78] The activity level of the star adds noise to the radial velocity measurements, complicating detection of a companion using this method. [79] In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the Hubble Space Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU. [80] A subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions. [81] Astrometric measurements at the Cerro Tololo Inter-American Observatory appear to rule out a Jupiter-sized planet with an orbital period of 2−12 years. [82]

Planet b Edit

Proxima Centauri b, or Alpha Centauri Cb, orbits the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.17 times that of the Earth. [83] Moreover, the equilibrium temperature of Proxima Centauri b is estimated to be within the range where water could exist as liquid on its surface thus, placing it within the habitable zone of Proxima Centauri. [74] [84] [85]

The first indications of the exoplanet Proxima Centauri b were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data. [86] [87] To confirm the possible discovery, a team of astronomers launched the Pale Red Dot [nb 8] project in January 2016. [88] On August 24, 2016, the team of 31 scientists from all around the world, [89] led by Guillem Anglada-Escudé of Queen Mary University of London, confirmed the existence of Proxima Centauri b [90] through a peer-reviewed article published in Nature. [74] [91] The measurements were performed using two spectrographs: HARPS on the ESO 3.6 m Telescope at La Silla Observatory and UVES on the 8 m Very Large Telescope at Paranal Observatory. [74] Several attempts to detect a transit of this planet across the face of Proxima Centauri have been made. A transit-like signal appearing on September 8, 2016 was tentatively identified, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica. [92]

Planet c Edit

Proxima Centauri c is a super-Earth or gas dwarf about 7 Earth masses orbiting at roughly 1.5 astronomical units (220,000,000 km) every 1,900 days (5.2 yr). [93] If Proxima Centauri b were the star's Earth, Proxima Centauri c would be equivalent to Neptune. Due to its large distance from Proxima Centauri, it is unlikely to be habitable, with a low equilibrium temperature of around 39 K. [94] The planet was first reported by Italian astrophysicist Mario Damasso and his colleagues in April 2019. [94] [93] Damasso's team had noticed minor movements of Proxima Centauri in the radial velocity data from the ESO's HARPS instrument, indicating a possible additional planet orbiting Proxima Centauri. [94] In 2020, the planet's existence was confirmed by Hubble astrometry data from c. 1995. [95] A possible direct imaging counterpart was detected in the infrared with the SPHERE, but the authors admit that they "did not obtain a clear detection." If their candidate source is in fact Proxima Centauri c, it is too bright for a planet of its mass and age, implying that the planet may have a ring system with a radius of around 5 R J. [96] If this direct imaging detection is confirmed, Proxima Centauri c will be the closest exoplanet ever directly imaged.

Other discoveries Edit

In 2016, in a paper that helped to confirm Proxima Centauri b's existence, a second signal in the range of 60 to 500 days was also detected. However, its nature is still unclear due to stellar activity and inadequate sampling. [74]

In 2017, a team of astronomers using the Atacama Large Millimeter/submillimeter Array reported detecting a belt of cold dust orbiting Proxima Centauri at a range of 1−4 AU from the star. This dust has a temperature of around 40 K and has a total estimated mass of 1% of the planet Earth. They also tentatively detected two additional features: a cold belt with a temperature of 10 K orbiting around 30 AU and a compact emission source about 1.2 arcseconds from the star. There was also a hint at an additional warm dust belt at a distance of 0.4 AU from the star. [97] However, upon further analysis, these emissions were determined to be most likely the result of a large flare emitted by the star in March 2017. The presence of dust is not needed to model the observations. [98] [99]

In 2019, a team of astronomers revisited the data from ESPRESSO about Proxima Centauri b to refine its mass. While doing so, the team found another radial velocity spike with a periodicity of 5.15 days. They estimated that if it were a planetary companion, it would be no less than 0.29 Earth masses. [18] The discovery was released in 2020.

Habitability Edit

Prior to the discovery of Proxima Centauri b, the TV documentary Alien Worlds hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarfs. Such a planet would lie within the habitable zone of Proxima Centauri, about 0.023–0.054 AU (3.4–8.1 million km) from the star, and would have an orbital period of 3.6–14 days. [100] A planet orbiting within this zone may experience tidal locking to the star. If the orbital eccentricity of this hypothetical planet is low, Proxima Centauri would move little in the planet's sky, and most of the surface would experience either day or night perpetually. The presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet. [101]

Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but the documentary's scientists thought that this obstacle could be overcome. Gibor Basri of the University of California, Berkeley, mentioned that "no one [has] found any showstoppers to habitability". For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet. If the planet had a strong magnetic field, the field would deflect the particles from the atmosphere even the slow rotation of a tidally locked planet that spins once for every time it orbits its star would be enough to generate a magnetic field, as long as part of the planet's interior remained molten. [102]

Other scientists, especially proponents of the rare-Earth hypothesis, [103] disagree that red dwarfs can sustain life. Any exoplanet in this star's habitable zone would likely be tidally locked, resulting in a relatively weak planetary magnetic moment, leading to strong atmospheric erosion by coronal mass ejections from Proxima Centauri. [104]

Because of the star's proximity to Earth, Proxima Centauri has been proposed as a flyby destination for interstellar travel. [105] Proxima Centauri currently moves toward Earth at a rate of 22.2 km/s. [7] (Barycenter of system moving closer, while rotation around AB is away from Sun, i.e. prograde). After 26,700 years, when it will come within 3.11 light-years, it will begin to move farther away. [67]

If non-nuclear, conventional propulsion technologies are used, the flight of a spacecraft to Proxima Centauri and its planets would probably require thousands of years. [106] For example, Voyager 1, which is now travelling 17 km/s (38,000 mph) [107] relative to the Sun, would reach Proxima Centauri in 73,775 years, were the spacecraft travelling in the direction of that star. A slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance. [108]

Nuclear pulse propulsion might enable such interstellar travel with a trip timescale of a century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot. [108]

Project Breakthrough Starshot aims to reach the Alpha Centauri system within the first half of the 21st century, with microprobes travelling at 20% of the speed of light propelled by around 100 gigawatts of Earth-based lasers. [109] The probes would perform a fly-by of Proxima Centauri to take photos and collect data of its planets' atmospheric compositions. It would take 4.25 years for the information collected to be sent back to Earth. [110]

From Proxima Centauri, the Sun would appear as a bright 0.4-magnitude star in the constellation Cassiopeia, similar to that of Achernar from Earth. [nb 9]

In December 2020, a candidate SETI radio signal BLC-1 was announced as potentially coming from the star. [111]


Contents

In 1915, the Scottish astronomer Robert Innes, director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri. [23] [24] [25] [26] He suggested that it be named Proxima Centauri [27] (actually Proxima Centaurus). [28] In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax at 0.755″ ± 0.028″ and determined that Proxima Centauri was approximately the same distance from the Sun as Alpha Centauri. It was also found to be the lowest-luminosity star known at the time. [29] An equally accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of 0.783″ ± 0.005″ . [24] [27]

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known. [30] [31] The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995. [32] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra. [33]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. [34] The WGSN approved the name Proxima Centauri for this star on August 21, 2016 and it is now so included in the List of IAU approved Star Names. [35]

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N. [nb 4] Red dwarfs such as Proxima Centauri are too faint to be seen with the naked eye. Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star. [36] [37] It has apparent visual magnitude 11, so a telescope with an aperture of at least 8 cm (3.1 in) is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon. [38]

In 2018, a superflare was observed from Proxima Centauri, the strongest flare ever seen. The optical brightness increased by a factor of 68× to approximately magnitude 6.8. It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before. [39]

On 2020 April 22 and 23, the New Horizons spacecraft took images of two of the nearest stars, Proxima Centauri and Wolf 359. When compared with Earth-based images, a very large parallax effect was easily visible. However, this was mostly useful for illustrative purposes and did not improve on previous distance measurements. [40] [41]

Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. M5.5 means that it falls in the low-mass end of M-type dwarf stars. [16] Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs (33 ly), is 15.5. [42] Its total luminosity over all wavelengths is 0.17% that of the Sun, [10] although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun. [43] More than 85% of its radiated power is at infrared wavelengths. [44]

In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri is 1.02 ± 0.08 mas . Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. The star's mass, estimated from stellar theory, is 12.2% M , or 129 Jupiter masses ( M J). [45] The mass has been calculated directly, although with less precision, from observations of microlensing events to be 0.150 +0.062
−0.051 M . [46]

Lower mass main-sequence stars have higher mean density than higher mass ones, [47] and Proxima Centauri is no exception: it has a mean density of 47.1 × 10 3 kg/m 3 (47.1 g/cm 3 ), compared with the Sun's mean density of 1.411 × 10 3 kg/m 3 (1.411 g/cm 3 ). [nb 5] The measured surface gravity of Proxima Centauri, given as the base-10 logarithm of the acceleration in units of cgs, is 5.20. [11] This is 162 times the surface gravity on Earth. [nb 6]

A 1998 study of photometric variations indicates that Proxima Centauri rotates once every 83.5 days. [48] A subsequent time series analysis of chromospheric indicators in 2002 suggests a longer rotation period of 116.6 ± 0.7 days. [49] This was subsequently ruled out in favor of a rotation period of 82.6 ± 0.1 days. [15]

Because of its low mass, the interior of the star is completely convective, [50] causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end after about 4 trillion years. [51]

Convection is associated with the generation and persistence of a magnetic field. The magnetic energy from this field is released at the surface through stellar flares that briefly (as short as per ten seconds) [52] increase the overall luminosity of the star. On May 1, 2019, an extreme flare event briefly became the brightest ever detected, with a far ultraviolet emission of 2 × 10 30 erg . [53] These flares can grow as large as the star and reach temperatures measured as high as 27 million K [33] —hot enough to radiate X-rays. [54] Proxima Centauri's quiescent X-ray luminosity, approximately (4–16) × 10 26 erg/s ((4–16) × 10 19 W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 10 28 erg/s (10 21 W). [33]

Proxima Centauri's chromosphere is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. [55] About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona, [56] and its total X-ray emission is comparable to the sun's. [57] Proxima Centauri's overall activity level is considered low compared to other red dwarfs, [57] which is consistent with the star's estimated age of 4.85 × 10 9 years, [16] since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases. [58] The activity level also appears to vary [59] with a period of roughly 442 days, which is shorter than the solar cycle of 11 years. [60]

Proxima Centauri has a relatively weak stellar wind, no more than 20% of the mass loss rate of the solar wind. Because the star is much smaller than the Sun, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface. [61]

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming into a so-called "blue dwarf". Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity ( L ) and warming up any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy. [51]

Distance and motion Edit

Based on a parallax of 768.0665 ± 0.0499 mas , published in 2020 in Gaia Data Release 3, Proxima Centauri is 4.2465 light-years (1.3020 pc 268,550 AU) from the Sun. [8] Previously published parallaxes include: 768.5 ± 0.2 mas in 2018 by Gaia DR2, 768.13 ± 1.04 mas , in 2014 by the Research Consortium On Nearby Stars [62] 772.33 ± 2.42 mas , in the original Hipparcos Catalogue, in 1997 [63] 771.64 ± 2.60 mas in the Hipparcos New Reduction, in 2007 [2] and 768.77 ± 0.37 mas using the Hubble Space Telescope 's fine guidance sensors, in 1999. [9] From Earth's vantage point, Proxima Centauri is separated from Alpha Centauri by 2.18 degrees, [64] or four times the angular diameter of the full Moon. [65] Proxima Centauri also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. [66] It has a radial velocity toward the Sun of 22.2 km/s. [7]

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun. In 2001, J. García-Sánchez et al. predicted that Proxima Centauri will make its closest approach to the Sun in approximately 26,700 years, coming within 3.11 ly (0.95 pc). [67] A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly (0.89 pc) in about 27,400 years, [68] followed by a 2014 study by C. A. L. Bailer-Jones predicting a perihelion approach of 3.07 ly (0.94 pc) in roughly 26,710 years. [69] Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Centre that varies from 27 to 31 kly (8.3 to 9.5 kpc), with an orbital eccentricity of 0.07. [70]

Ever since the discovery of Proxima Centauri, it has been suspected to be a true companion of the Alpha Centauri binary star system. Data from the Hipparcos satellite, combined with ground-based observations, were consistent with the hypothesis that the three stars are a bound system. For this reason, Proxima Centauri is sometimes referred to as Alpha Centauri C. Kervella et al. (2017) used high-precision radial velocity measurements to determine with a high degree of confidence that Proxima and Alpha Centauri are gravitationally bound. [7] Proxima Centauri's orbital period around the Alpha Centauri AB barycenter is 547 000 +6600
−4000 years with an eccentricity of 0.5 ± 0.08 it approaches Alpha Centauri to 4300 +1100
−900 AU at periastron and retreats to 13 000 +300
−100 AU at apastron. [7] At present, Proxima Centauri is 12,947 ± 260 AU (1.94 ± 0.04 trillion km) from the Alpha Centauri AB barycenter, nearly to the farthest point in its orbit. [7]

Such a triple system can form naturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 M within their embedded star cluster before the cluster disperses. [71] However, more accurate measurements of the radial velocity are needed to confirm this hypothesis. [72] If Proxima Centauri was bound to the Alpha Centauri system during its formation, the stars are likely to share the same elemental composition. The gravitational influence of Proxima might also have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions, so possibly enriching any terrestrial planets in the system with this material. [72] Alternatively, Proxima Centauri may have been captured at a later date during an encounter, resulting in a highly eccentric orbit that was then stabilized by the galactic tide and additional stellar encounters. Such a scenario may mean that Proxima Centauri's planetary companions have had a much lower chance for orbital disruption by Alpha Centauri. [14]

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, [73] such as in a star cluster.

The Proxima Centauri planetary system [74] [75] [19] [76] [18] [20]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
Eccentricity Inclination Radius
d (unconfirmed) ≥ 0.29 ± 0.08 M 0.028 95 ± 0.000 22 5.168 +0.051
−0.069
b 1.60 +0.46
−0.36 M
0.048 57 +0.000 29
−0.000 29
11.184 18 +0.000 68
−0.000 74
0.109 +0.076
−0.068
1.30 +1.20
−0.62 R
c 7 ± 1 M 1.489 ± 0.049 1928 ± 20 0.04 ± 0.01 133 ± 1 °
RV-derived upper mass limits of potential companions [77]
Orbital
period
(days)
Separation
(AU)
Maximum
mass [nb 7]
(M)
3.6–13.8 0.022–0.054 2–3
< 100 < 0.21 8.5
< 1000 < 1 16

So far, as of 2021, two planets have been confirmed to orbit around Proxima Centauri, with one being close to Earth’s size and within the habitable zone (b) and another which may be a gas dwarf that orbits much father out (c). There are signs a third, even smaller planet could be orbiting closer than both of the planets, however this has not been confirmed yet.

Ever since the first exoplanets were discovered, there has been a hunt for exoplanets in the Alpha Centauri system. Multiple measurements of the star's radial velocity constrained the maximum mass that a detectable companion to Proxima Centauri could possess. [9] [78] The activity level of the star adds noise to the radial velocity measurements, complicating detection of a companion using this method. [79] In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the Hubble Space Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU. [80] A subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions. [81] Astrometric measurements at the Cerro Tololo Inter-American Observatory appear to rule out a Jupiter-sized planet with an orbital period of 2−12 years. [82]

Planet b Edit

Proxima Centauri b, or Alpha Centauri Cb, orbits the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.17 times that of the Earth. [83] Moreover, the equilibrium temperature of Proxima Centauri b is estimated to be within the range where water could exist as liquid on its surface thus, placing it within the habitable zone of Proxima Centauri. [74] [84] [85]

The first indications of the exoplanet Proxima Centauri b were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data. [86] [87] To confirm the possible discovery, a team of astronomers launched the Pale Red Dot [nb 8] project in January 2016. [88] On August 24, 2016, the team of 31 scientists from all around the world, [89] led by Guillem Anglada-Escudé of Queen Mary University of London, confirmed the existence of Proxima Centauri b [90] through a peer-reviewed article published in Nature. [74] [91] The measurements were performed using two spectrographs: HARPS on the ESO 3.6 m Telescope at La Silla Observatory and UVES on the 8 m Very Large Telescope at Paranal Observatory. [74] Several attempts to detect a transit of this planet across the face of Proxima Centauri have been made. A transit-like signal appearing on September 8, 2016 was tentatively identified, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica. [92]

Planet c Edit

Proxima Centauri c is a super-Earth or gas dwarf about 7 Earth masses orbiting at roughly 1.5 astronomical units (220,000,000 km) every 1,900 days (5.2 yr). [93] If Proxima Centauri b were the star's Earth, Proxima Centauri c would be equivalent to Neptune. Due to its large distance from Proxima Centauri, it is unlikely to be habitable, with a low equilibrium temperature of around 39 K. [94] The planet was first reported by Italian astrophysicist Mario Damasso and his colleagues in April 2019. [94] [93] Damasso's team had noticed minor movements of Proxima Centauri in the radial velocity data from the ESO's HARPS instrument, indicating a possible additional planet orbiting Proxima Centauri. [94] In 2020, the planet's existence was confirmed by Hubble astrometry data from c. 1995. [95] A possible direct imaging counterpart was detected in the infrared with the SPHERE, but the authors admit that they "did not obtain a clear detection." If their candidate source is in fact Proxima Centauri c, it is too bright for a planet of its mass and age, implying that the planet may have a ring system with a radius of around 5 R J. [96] If this direct imaging detection is confirmed, Proxima Centauri c will be the closest exoplanet ever directly imaged.

Other discoveries Edit

In 2016, in a paper that helped to confirm Proxima Centauri b's existence, a second signal in the range of 60 to 500 days was also detected. However, its nature is still unclear due to stellar activity and inadequate sampling. [74]

In 2017, a team of astronomers using the Atacama Large Millimeter/submillimeter Array reported detecting a belt of cold dust orbiting Proxima Centauri at a range of 1−4 AU from the star. This dust has a temperature of around 40 K and has a total estimated mass of 1% of the planet Earth. They also tentatively detected two additional features: a cold belt with a temperature of 10 K orbiting around 30 AU and a compact emission source about 1.2 arcseconds from the star. There was also a hint at an additional warm dust belt at a distance of 0.4 AU from the star. [97] However, upon further analysis, these emissions were determined to be most likely the result of a large flare emitted by the star in March 2017. The presence of dust is not needed to model the observations. [98] [99]

In 2019, a team of astronomers revisited the data from ESPRESSO about Proxima Centauri b to refine its mass. While doing so, the team found another radial velocity spike with a periodicity of 5.15 days. They estimated that if it were a planetary companion, it would be no less than 0.29 Earth masses. [18] The discovery was released in 2020.

Habitability Edit

Prior to the discovery of Proxima Centauri b, the TV documentary Alien Worlds hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarfs. Such a planet would lie within the habitable zone of Proxima Centauri, about 0.023–0.054 AU (3.4–8.1 million km) from the star, and would have an orbital period of 3.6–14 days. [100] A planet orbiting within this zone may experience tidal locking to the star. If the orbital eccentricity of this hypothetical planet is low, Proxima Centauri would move little in the planet's sky, and most of the surface would experience either day or night perpetually. The presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet. [101]

Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but the documentary's scientists thought that this obstacle could be overcome. Gibor Basri of the University of California, Berkeley, mentioned that "no one [has] found any showstoppers to habitability". For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet. If the planet had a strong magnetic field, the field would deflect the particles from the atmosphere even the slow rotation of a tidally locked planet that spins once for every time it orbits its star would be enough to generate a magnetic field, as long as part of the planet's interior remained molten. [102]

Other scientists, especially proponents of the rare-Earth hypothesis, [103] disagree that red dwarfs can sustain life. Any exoplanet in this star's habitable zone would likely be tidally locked, resulting in a relatively weak planetary magnetic moment, leading to strong atmospheric erosion by coronal mass ejections from Proxima Centauri. [104]

Because of the star's proximity to Earth, Proxima Centauri has been proposed as a flyby destination for interstellar travel. [105] Proxima Centauri currently moves toward Earth at a rate of 22.2 km/s. [7] (Barycenter of system moving closer, while rotation around AB is away from Sun, i.e. prograde). After 26,700 years, when it will come within 3.11 light-years, it will begin to move farther away. [67]

If non-nuclear, conventional propulsion technologies are used, the flight of a spacecraft to Proxima Centauri and its planets would probably require thousands of years. [106] For example, Voyager 1, which is now travelling 17 km/s (38,000 mph) [107] relative to the Sun, would reach Proxima Centauri in 73,775 years, were the spacecraft travelling in the direction of that star. A slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance. [108]

Nuclear pulse propulsion might enable such interstellar travel with a trip timescale of a century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot. [108]

Project Breakthrough Starshot aims to reach the Alpha Centauri system within the first half of the 21st century, with microprobes travelling at 20% of the speed of light propelled by around 100 gigawatts of Earth-based lasers. [109] The probes would perform a fly-by of Proxima Centauri to take photos and collect data of its planets' atmospheric compositions. It would take 4.25 years for the information collected to be sent back to Earth. [110]

From Proxima Centauri, the Sun would appear as a bright 0.4-magnitude star in the constellation Cassiopeia, similar to that of Achernar from Earth. [nb 9]

In December 2020, a candidate SETI radio signal BLC-1 was announced as potentially coming from the star. [111]


Super-Earth Proxima Centauri c

To find the new super-Earth, scientists used the HARPS spectrograph at the La Silla Observatory and the UVES spectrograph on the Very Large Telescope, both in Chile. Mario Damasso (Astrophysical Observatory of Torino, Italy) and colleagues analyzed data collected between 2000 and 2017, looking for the signature “wobble” in Proxima’s Centauri’s light spectrum that would indicate the presence of a planet, the same technique that enabled scientists to confirm the Proxima b’s presence in 2016.

“Stars like Proxima Centauri are rather restless and continuously present eruptions and spots on their surface, which make the detection of a planetary-induced oscillation very complicated,” says coauthor Fabio Del Sordo (University of Crete and Foundation for Research and Technology-Hellas in Heraklion, Greece).

Because the observations span almost two decades, the scientists have confidently ruled out those sources of noise, but they caution that follow-up observations are needed to confirm that the signal comes from a planet. “The problem with planets in distant orbits,” says Yiannis Tsapras (Astronomical Calculation Institute, Germany), “is that their orbital periods span several years to decades, which means that any observing campaigns need to be of comparable length, or even longer if the aim is to verify the signal over multiple orbits.” Tsapras was part of the team that discovered Proxima b but was not involved with the current study.

Proxima c is about 1.5 astronomical units (a.u.) away from its star — just a little farther out than Earth is from the Sun, but about 30 times farther out than Proxima b. Because of this large distance, even if the planet were rocky, it would be too cold to host life as we know it.


This artist&rsquos impression shows the exoplanet Proxima b, which orbits the red dwarf star Proxima Centauri. The double star Alpha Centauri AB appears in the image between the exoplanet and its star. Proxima b appears to be at least 1.3 times the mass of Earth, making it slightly larger than our home planet.

The planet Proxima b was discovered by scientists using a telescope at the European Southern Observatory in Chile. The top panel in this image offers a view of the southern skies over the ESO 3.6-meter telescope at the La Silla Observatory in Chile. The lower panel shows real images of the stars Proxima Centauri (lower right) and the double star Alpha Centauri AB (lower left), taken with the NASA/ESA Hubble Space Telescope.


New tricks from old data: Astronomer uses 25-year-old Hubble data to confirm planet Proxima Centauri c

Benedict, G. Fritz. Credit: University of Texas McDonald Observatory

Fritz Benedict has used data he took over two decades ago with Hubble Space Telescope to confirm the existence of another planet around the Sun's nearest neighbor, Proxima Centauri, and to pin down the planet's orbit and mass. Benedict, an emeritus Senior Research Scientist with McDonald Observatory at The University of Texas at Austin, will present his findings today in a scientific session and then in a press conference at a meeting of the American Astronomical Society.

Proxima Centauri has been in the news frequently since 2016, when scientists including McDonald Observatory's Michael Endl found its first planet, Proxima Centauri b. The discovery incited speculation on the types of in-depth studies that could done on an extrasolar planet so close to our own solar system.

Adding to the excitement, earlier this year a group led by Mario Damasso of Italy's National Institute for Astrophysics (INAF) announced they might have found another planet orbiting Proxima Centauri farther out. This group used radial velocity observations, that is, measurements of the star's motion on the sky toward and away from Earth, to deduce the possible planet (dubbed Proxima Centauri c) orbits the star every 1,907 days at distance of 1.5 AU (that is, 1.5 times the distance at which Earth orbits the Sun).

Still, the existence of planet c was far from certain. Thus Benedict decided to re-visit his studies of Proxima Centauri from the 1990s made with Hubble Space Telescope. For that study, he had used Hubble's Fine Guidance Sensors (FGS).

Though their primary role is to ensure accurate pointing of the telescope, Benedict and others routinely used FGS for a type of research called astrometry: the precise measurement of the positions and motions of celestial bodies. In this case, he used FGS to search for Proxima Centauri's motion on sky caused by tugging from its surrounding—and unseen—planets.

When Benedict and research partner Barbara MacArthur originally studied Proxima Centauri in the 1990s, he said, they only checked for planets with orbital periods of 1,000 Earth days or fewer. They found none. He now revisited that data to check for signs of a planet with a longer orbital period.

Indeed, Benedict found a planet with an orbital period of about 1,907 days buried in the 25-year-old Hubble data. This was an independent confirmation of the existence of Proxima Centauri c.

Shortly afterward, a team led by Raffaele Gratton of INAF published images of the planet at several points along its orbit that they had made with the SPHERE instrument on the Very Large Telescope in Chile.

Benedict then combined the findings of all three studies: his own Hubble astrometry, Damasso's radial velocity studies, and Gratton's images to greatly refine the mass of Proxima Centauri c. He found that the planet is about 7 times as massive as Earth.

This analysis shows the power of combining several independent methods of studying an exoplanet. Each approach has its strengths and weaknesses, but together they serve to confirm the existence of Proxima Centauri c.

"Basically, this is a story of how old data can be very useful when you get new information," Benedict said. "It's also a story of how hard it is to retire if you're an astronomer, because this is fun stuff to do!"


Sunscreen anyone? Proxima Centauri just spit out its biggest solar flare ever

In an eruption of amazing magnitude, the volatile star Proxima Centauri (our Sun's closest neighbor) has just belched out its biggest solar flare ever recorded and the implications may mean that if alien life existed on its pair of orbiting planets, it would look very different from Earth's.

Led by a team of researchers at CU Boulder in Colorado, a study of the fantastic flare was published this week in the online journal, The Astrophysical Journal Letters. Officially categorized as a Red Dwarf, Proxima Centauri is about one-eighth the mass of our own star and lies four light-years away.

More Proxima Centauri

In a new press release, university astrophysicist Meredith MacGregor indicated that Proxima Centauri might be a small, dim star but don't let its diminutive size and feeble radiance fool you. Existing 20 trillion miles beyond our own sun, it's home to at least two planets, Proxima Centauri a and Proxima Centauri b.

MacGregor is an assistant professor at the Center for Astrophysics and Space Astronomy (CASA) and Department of Astrophysical and Planetary Sciences (APS) at CU Boulder in Colorado.

She and her team watched Proxima Centauri for roughly 40 hours using an array of nine ground telescopes and one orbital observatory. During the study, Proxima Centauri shot out an intense radiation plume that instantly elevated it to one of the largest ever observed in the Milky Way.

“The star went from normal to 14,000 times brighter when seen in ultraviolet wavelengths over the span of a few seconds,” said MacGregor. “If there was life on the planet nearest to Proxima Centauri, it would have to look very different than anything on Earth. A human being on this planet would have a bad time.”

Proxima Centauri has been in the crosshairs of astronomers for decades as a potential spot for alien life outside our own planetary neighborhood. One of its exoplanets, Proxima Centauri b, is Earth-like and exists in a “habitable zone” whose temperature range might be conducive to the formation of liquid water.

“A lot of the exoplanets that we’ve found so far are around these types of stars,” she explained. “But the catch is that they’re way more active than our sun. They flare much more frequently and intensely.”

To record this rare event, MacGregor and her crew aimed their astronomical devices at Proxima Centauri for a total of 40 hours over several months back in 2019. The instruments included the Hubble Space Telescope, the Atacama Large Millimeter Array (ALMA), and NASA’s Transiting Exoplanet Survey Satellite (TESS). Five of the nine were fortunate enough to capture the incredible flare and its accompanying broad spectrum of radiation.

“It’s the first time we’ve ever had this kind of multi-wavelength coverage of a stellar flare,” noted MacGregor. “Usually, you’re lucky if you can get two instruments.”

By using this layered process, the CU Boulder team was able to capture one of the most detailed solar flares ever obtained in the Milky Way. The record-setting, 7-second occurrence was seen on May 1, 2019. It did not generate a significant level of visible light, but was instead characterized by waves of both ultraviolet and radio, also called “millimeter” radiation.

“In the past, we didn’t know that stars could flare in the millimeter range, so this is the first time we have gone looking for millimeter flares,” she said.

The star's observed flare was nearly 100 times stronger than any similar flare witnessed from Earth’s sun. This type of energy can ravage a planet’s atmosphere and potentially blanket life forms with lethal deadly radiation.

“Proxima Centauri’s planets are getting hit by something like this not once in a century, but at least once a day if not several times a day," MacGregor concluded. “There will probably be even more weird types of flares that demonstrate different types of physics that we haven’t thought about before.”