# Why the light curve goes down when the planet is behind the star?

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There is a video explaing the transiting exoplanet light curve - https://www.youtube.com/watch?v=RrusIZaWDW8

It is clear to my why the curve goes down when the planet is between the observer and the star, but I don't understand why the curve goes a little bit down when the the planet is behind the star. I was expected that this should not change the light curve at all.

Think of when the planet is at the "side" - there's a little bit of light from the planet (ie, reflecting off the planet) shining towards us.

Could it be due to that little bit of light - when the planet is behind the star, it no longer reflects towards us?

Maybe that's the effect you have in mind?

## Astronomer discusses the science behind that mysterious star you've heard so much about

Credit: NASA / JPL-Caltech

A Yale-led team of astronomers has had its eye on a particularly bright star whose dramatic dips in energy output may be the result of fragmented, extra-solar comets buzzing past the star.

The star in question, KIC 8462852, was a source of much speculation during the course of NASA's Kepler space mission. Kepler's primary goal is to find Earth-like planets located outside of our solar system. But the speculation intensified greatly in recent months, as news stories pondered whether the phenomenon had an extra-terrestrial explanation. Now, additional data from NASA's Spitzer Space Telescope supports the "swarm of comets" theory.

YaleNews recently met with Yale postdoctoral researcher Tabetha Boyajian, first author of the original study on KIC 8462852, to talk about what her team actually found.

Also of note—Boyajian's colleagues have taken to calling it "Tabby's star."

How far away is Tabby's star and why did it attract your interest?

This star is about 1,400 light years away. For comparison, the Earth is only about 8½ light minutes away from the Sun. Proxima Centauri, the closest star to the Sun, is 4.22 light years away. The Kepler light curve for this star (KIC 8462852) piques everyone's interest—it is truly unique!

How does this relate to your work with Planet Hunters, the citizen science program?

Planet Hunters is a citizen science project started here at Yale (the principal investigator is astronomy professor Debra Fischer) in 2010. It is a web-based interface designed for users to view light curves from the Kepler mission and identify signals of transiting planets.

I am a science team member of Planet Hunters, which means I am responsible for the organization, data analysis, and publication of Planet Hunter discoveries. In addition, I lead a "guest observer" program, which basically manages projects not directly related to transiting exoplanets in the Kepler data. Such "guest observer" projects typically come out of the back-end utility of the Planet Hunters site called Talk. Talk is where users can discuss any aspects of a light curve further. It was through the Talk interface that KIC 8462852 was first identified by our volunteers. Within Talk, KIC 8462852's light curve was a very popular discussion, and this was brought to the science team's attention as a possibly interesting object to follow up.

Why do you believe it is most likely that comets are the cause of the unusual data from KIC 8462852? Are comets large enough to cause such large-scale dimming?

Exocomets are the most promising scenario that fits all the data we have in hand. Comets themselves are small, so at first it seems unlikely to have such large dips in an object's brightness (down by 22%). The scenario we propose invokes a swarm of exocomet fragments, broken up in a recent collision, that are blocking the starlight. Aside from the true size of the object causing the dimming, there are many variables, including the opacity (transparency) of the orbiting material, and geometric orientation of the orbit at our viewing angle—all of which will influence the depths and shapes of the dips.

At this point, however, no detailed modeling has been done to try to reproduce the unique dips in the Kepler light curve, and we are currently seeking more data on the exocomet possibility.

What theories were you able to rule out?

The paper discussed many scenarios that were not consistent with the data we have for this star. This includes scenarios that the variability is caused by something intrinsic to the star, such as star spots rotating in and out of view, as well as extrinsic to the star, such as something in orbit around the star getting in the way and intermittently blocking the star's light. The latter scenario is elaborated upon in detail in the paper, with several examples such as an asteroid belt, planetary collisions, and the presence of a transiting object with rings.

What has been the biggest challenge in deciphering the data?

The biggest challenge was developing a single hypothesis that could explain all the observed dimming events. Many scenarios discussed would match one set of features observed, but not any of the others.

On a personal level, how exciting is it to try to find answers to questions such as the ones posed by KIC 8462852?

Science is addictive. To be the first person on Earth to say something about any cosmic question is a rush!

## Finding Exoplanets

Exoplanets are some of the most difficult objects in the Universe to find. They are tiny compared to their host stars, and they are dark, not giving off any visible light of their own. Looking for exoplanets around distant stars is a little bit like trying to photograph a mosquito next to a lighthouse.

Never passing up a good challenge, astronomers have found several clever techniques for finding those mosquitoes. Most of them involve looking for hints of planets by seeing how they affect the light from stars. While astronomers have used at least a half dozen techniques, the vast majority of confirmed exoplanets have been found by either of two different methods that detect how an orbiting planet can affect the light of its star.

### Wobble Method

The technical term for this is the radial-velocity method.

The first exoplanets orbiting normal stars were found by looking for tiny wobbles in the star's motion. Newton's law of gravity teaches us that for every gravitational action between a star and an orbiting planet, there is an equal and opposite reaction on the star. So as a star's gravity tugs on a planet, pulling it into an orbit around the star, the planet's gravity also tugs on the star, causing it to wobble back and forth as the planet goes around it.

Astrononomers look for these tiny stellar wobbles by examining the spectrum of the star and looking for slight shifts in features in the spectrum. These shifts indicate changes in the motion of the star, revealing the presence of orbiting planets. From these measurements we can learn the length of the planet's orbit and get a general idea of its mass.

### Transit Method

The transit method reveals the physical size of a planet as well as the length of its orbit. It does not reveal the planet's mass, however.

Scientists now think that most of the stars in our galaxy have planets orbiting them, but the orientations of these systems are random. If you imagine that a star is at the center of a dinner plate, and a planet is orbiting in a circle at the edge of that plate, we see some of the systems face-on, as if looking at the plate from above, while others look edge-on to us, like holding up your plate and looking at the edge. Most systems are somewhere in between, which is like angling your plate somewhere between face-on and edge-on. The orientation of a planetary system compared to Earth is called its inclination, and this can have a big effect on whether we see a planet or not.

A small fraction of the planetary systems out there are oriented edge-on, which means that as the planet orbits its star, it passes between the star and us, blocking some of the light from the star as it does so. For these systems, every time the planets pass in front of their star&mdashwhat astronomers call a transit&mdasha telescope can can detect this tiny drop in the amount of light we see from the star.

By carefully observing a star's brightness over a long period of time, astronomers can create a graph called a light curve. When an orbiting planet transits the star during its orbit it creates a tiny dip. This dip occurs every time the planet orbits its star, so looks like a repeating pattern in the light curve. The size of this dip tells us the size of the planet (in proportion to the size of the star). Since planets are generally tiny compared to their stars, this drop is usually only a few tenths of a percent. Jupiter-sized worlds are easiest to detect, because of their large size, but much smaller Earth-sized planets are especially difficut to see.

Most of the known transiting exoplanets have been found by NASA's Kepler mission. Kepler has stared at a single patch of sky for years, observing the brightness of around 100,000 stars looking for transits.

The tiny changes in the light curve caused by the glow of the planet are miniscule compared to the transit, but in rare cases have been seen&mdashin infrared light.

The animation below shows a simulated light curve and how it relates to the motion of the planet around its star. In addition to the dip in light caused by the planet's transit across the star, there is also a much smaller dip when the planet moves behind the star. A slight ripple in the overall brightness reflects the changing brightness of the planet itself, as we see different views of the day and night sides of the planet as it orbits.

Once a planetary candidate has been detected by the transit method, infrared telescopes like NASA's Spitzer Space Telescope and the Keck Observatory swing into action, taking further observations at their longer wavelengths to confirm the detection. By working together, these telescopes and their different wavelengths of light have been able to confirm over a hundred planets so far. Astronomers believe that this is just the tip of the iceberg, and now think that most of the stars in our galaxy could host planets, meaning that there are billions of them out there, just waiting to be found.

## Statistical Methods for Physical Science

### 12.3.3 Protheroe Test

If the light curve has a symmetric, double-peaked stmcture the first harmonic Fourier amplitude, g s, may be quite small. For instance if the two peaks in Fig. 2(a) were of equal height, g s would be 0 and the Rayleigh test would indicate no signal. The Protheroe test [ 10 , 11 ] does not have this weakness and it is also particularly appropriate for light curves with sharp stmcture (narrow peaks) and is free of the binning uncertainties inherent in epoch folding. Its disadvantage is that it is computationally very intensive and therefore appropriate only for data with very few events (usually less than 200). It is used most extensively in ultra-high-energy gamma-ray astronomy.

The basic idea goes as follows. For a uniform distribution of phases the phasors are spread roughly equally over a unit circle as illustrated in Fig. 1 . The Rayleigh test picks out the tendency for phasors to point in the same direction. The Protheroe test tends to pick out phases that are correlated pairwise with other phases regardless of where they lie on the circle, i.e., clumpy phase distributions. The deviation, Δ i,j, between any two phases is defined as

and the Protheroe statistic is

A large value of YN indicates a signal.

The time required to compute YN is thus proportional to the square of the number of data points. The distribution of YN for a uniform phase distribution must in general be found from Monte Carlos simulations.

## Astronomy Ingram - Test 2

The Earth is small relative to other planets, so it has a low escape velocity
• Earth is close to the sun, so gases' velocities are faster because of higher temperatures
• Earth can capture slower moving gases, but not fast-moving gases that exceeds the earth's escape velocity

There are more dwarf sized planets than sun-like planets

A small transitioning planet has a higher gravitational pull on the star making it easier to measure the density of the planet

Extreme tides, effectively "locking" one side of the world in darkness while the other faces the star

Proxima Centauri's continual flares are harsher than the Sun's

Object moving away is red shifted, low energy, long wave

w/sound you have high pitch toward you and low past you. Light is too fast to detect with eyes

Amount of shift is proportional to radial velocity

Radial velocity: distance is radial sphere in comparison to earth. Depends on if it is breaking its radial line

## Why the light curve goes down when the planet is behind the star? - Astronomy

All but a few stars appear as mere pinpoints in even the largest telescopes. They are much too far away to derive their diameters from measuring their angular diameters and distances. Eclipsing binaries are used to determine indirectly the diameters of stars. These are two stars orbiting each other in a plane that is parallel to your line of sight so you see their orbits edge-on. This means that one star will periodically cover up the other star. During the eclipse the total brightness measured for the binary will decrease. The amount of the dip in brightness depends on the luminosity and relative size of the two stars.

A star's diameter is found from speed = (distance travelled)/(time it takes). The speed comes from the doppler shift and the time is the length of the eclipse. The distance travelled during the eclipse is equal to the diameter of the star = 2 × radius. The light curve---plot of brightness vs. time---is used to derive the star diameters. Here is an example of two stars orbiting each other in circular orbits seen edge-on with one star small and hot and the other large and cool:

When the small star moves from position 1 to position 2 (or from position 3 to position 4), it has moved a distance equal to its diameter. When the small star moves from position 1 to position 3 (or from position 2 to position 4), it has moved a distance equal to the diameter of the large star.

Star sizes can also be found (less accurately) from the luminosity and the flux. Recall from the magnitude section above that the luminosity = [4 p ×(star radius) 2 ] × [×(star's surface temperature) 4 ], where is the Stefan-Boltzmann constant. If you compare the star with the Sun, you can cancel out the constants to get (star's radius)/(Sun's radius) = (Sun's temperature/star's temperature) 2 × Sqrt[star's luminosity/Sun's luminosity]. See the How do you do that?'' box below for an example. The sizes of different types of stars are summarized in the Main Sequence Star Properties table below.

### How do you do that?

Try other scenarios of the star sizes and star masses with the UNL Astronomy Education program's Eclipsing Binary Simulator (link will appear in a new window). You can adjust the star masses, sizes, temperatures and separations and orbit inclination and eccentricity to see how the light curve changes.

## Contents

The names "Tabby's Star" and "Boyajian's Star" refer to American astronomer Tabetha S. Boyajian, who was the lead author of the scientific paper that announced the discovery of the star's irregular light fluctuations in 2015. [28] [29] The nickname "WTF Star" is a reference to the paper's subtitle "where's the flux?", which highlights the observed dips in the star's radiative flux. [30] [31] [32] [33] The nickname is a double entendre for the colloquial expression of disbelief, "what the fuck", or "WTF". [34] The star has also been given the nickname "LGM-2" – a homage to the first pulsar discovered, PSR B1919+21, which was given the nickname "LGM-1" when it was originally theorized to be a transmission from an extraterrestrial civilization. [35] Alternate designations in various star catalogues have been given to Tabby's Star. In the Kepler Input Catalog, a collection of astronomical objects catalogued by the Kepler space telescope, Tabby's Star is known as KIC 8462852 . [1] In the Tycho-2 Catalogue, an enhanced collection of stars catalogued by Hipparcos, the star is known as TYC 3162-665-1 . [1] In the infrared Two Micron All-Sky Survey (2MASS), the star is identified as 2MASS J20061546+4427248 . [1]

Tabby's Star in the constellation Cygnus is roughly halfway between the bright stars Deneb and Delta Cygni as part of the Northern Cross. [37] [38] Tabby's Star is situated south of 31 Cygni, and northeast of the star cluster NGC 6866. [38] While only a few arcminutes away from the cluster, it is unrelated and closer to the Sun than it is to the star cluster.

With an apparent magnitude of 11.7, the star cannot be seen by the naked eye, but is visible with a 5-inch (130 mm) telescope [39] in a dark sky with little light pollution.

Tabby's Star was observed as early as the year 1890. [40] [41] [42] The star was cataloged in the Tycho, 2MASS, UCAC4, and WISE astronomical catalogs [43] (published in 1997, 2003, 2009, and 2012, respectively). [44] [45] [46] [47]

The main source of information about the luminosity fluctuations of Tabby's Star is the Kepler space observatory. During its primary and extended mission from 2009 to 2013 it continuously monitored the light curves of over 100,000 stars in a patch of sky in the constellations Cygnus and Lyra. [48]

Observations of the luminosity of the star by the Kepler space telescope show small, frequent, non-periodic dips in brightness, along with two large recorded dips in brightness two years apart. The amplitude of the changes in the star's brightness, and the aperiodicity of the changes, mean that this star is of particular interest for astronomers. [16] The star's changes in brightness are consistent with many small masses orbiting the star in "tight formation". [17]

The first major dip, on 5 March 2011, reduced the star's brightness by up to 15%, and the next 726 days later (on 28 February 2013) by up to 22%. (A third dimming, around 8%, occurred 48 days later.) In comparison, a planet the size of Jupiter would only obscure a star of this size by 1%, indicating that whatever is blocking light during the star's major dips is not a planet, but rather something covering up to half the width of the star. [16] Due to the failure of two of Kepler 's reaction wheels, the star's predicted 750-day dip around February 2015 was not recorded. [1] [49] The light dips do not exhibit an obvious pattern. [50]

In addition to the day-long dimmings, a study of a century's worth of photographic plates suggests that the star has gradually faded in 100 years (from c. 1890 to c. 1990) by about 20%, which would be unprecedented for any F-type main-sequence star. [40] [41] Teasing accurate magnitudes from long-term photographic archives is a complex procedure, however, requiring adjustment for equipment changes, and is strongly dependent on the choice of comparison stars. Another study, examining the same photographic plates, concluded that the possible century-long dimming was likely a data artifact, and not a real astrophysical event. [42] Another study from plates between 1895 and 1995 found strong evidence that the star has not dimmed, but kept a constant flux within a few percent, except an 8% dip on 24 October 1978, resulting in a period of the putative occulter of 738 days. [51]

A third study, using light measurements by the Kepler observatory over a four-year period, determined that Tabby's Star dimmed at about 0.34% per year before dimming more rapidly by about 2.5% in 200 days. It then returned to its previous slow fade rate. The same technique was used to study 193 stars in its vicinity and 355 stars similar in size and composition to Tabby's Star. None of these stars exhibited such dimming. [52]

In 2018, a possible 1,574-day (4.31-year) periodicity in dimming of the star was reported. [53]

The red dwarf stellar companion at projected separation 880 ± 10 AU from Tabby's Star was confirmed to be comoving in 2021. [2]

Originally, and until Kohler's work of 2017, it was thought that, based on the spectrum and stellar type of Tabby's Star, its changes in brightness could not be attributed to intrinsic variability. [1] Consequently, a few hypotheses have been proposed involving material orbiting the star and blocking its light, although none of these fully fit the observed data. [54]

Some of the proposed explanations involve interstellar dust, a series of giant planets with very large ring structures, [55] [56] a recently captured asteroid field, [1] the system undergoing Late Heavy Bombardment, [13] [18] and an artificial megastructure orbiting the star. [57]

By 2018, the leading hypothesis was that the "missing" heat flux involved in the star's dimming could be stored within the star's interior. Such variations in luminosity might arise from a number of mechanisms affecting the efficiency of heat transport inside the star. [12] [58]

However, in September 2019, astronomers reported that the observed dimmings of Tabby's Star may have been produced by fragments resulting from the disruption of an orphaned exomoon. [22] [23]

### Circumstellar dust ring Edit

The smoking gun: Researchers found less dimming in the infrared light from the star than in its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby's Star.

Meng et al. (2017) suggested that, based on observational data of Tabby's Star from the Swift Gamma-Ray Burst Mission, Spitzer Space Telescope, and Belgian AstroLAB IRIS Observatory, only "microscopic fine-dust screens", originating from "circumstellar material", are able to disperse the starlight in the way detected in their measurements. [6] [7] [8] [59] Based on these studies, on 4 October 2017, NASA reported that the unusual dimming events of Tabby's Star are due to an "uneven ring of dust" orbiting the star. [6] Although the explanation of a significant amount of small particles orbiting the star regards "long-term fading" as noted by Meng, [7] the explanation also seems consistent with the week-long fadings found by amateur astronomer Bruce L. Gary and the Tabby Team, coordinated by astronomer Tabetha S. Boyajian, in more recent dimming events. [9] [10] [60] [61] [62] A related, but more sophisticated, explanation of dimming events, involving a transiting "brown dwarf" in a 1600-day eccentric orbit near Tabby's Star, a "drop feature" in dimness, and predicted intervals of "brightening", has been proposed. [63] [64] [65] [66] Dimming and brightening events of Tabby's Star continue to be monitored related light curves are currently updated and released frequently. [67] [68]

Nonetheless, data similar to that observed for Tabby's Star, along with supporting data from the Chandra X-ray Observatory, were found with dust debris orbiting WD 1145+017, a white dwarf that also has unusual light curve fluctuations. [69] Further, the highly variable star RZ Piscium, which brightens and dims erratically, has been found to emit excessive infrared radiation, suggesting that the star is surrounded by large amounts of gas and dust, possibly resulting from the destruction of local planets. [70] [71]

### A cloud of disintegrating comets Edit

One proposed explanation for the reduction in light is that it is due to a cloud of disintegrating comets orbiting the star elliptically. [1] [13] [15] [72] This scenario would assume that a planetary system around Tabby's Star has something similar to the Oort cloud and that gravity from a nearby star caused comets from said cloud to fall closer into the system, thereby obstructing the spectra of Tabby's Star. Evidence supporting this hypothesis includes an M-type red dwarf within 132 billion kilometers (885 AU) of Tabby's Star. [1] The notion that disturbed comets from such a cloud could exist in high enough numbers to obscure 22% of the star's observed luminosity has been doubted. [16]

Submillimetre-wavelength observations searching for farther-out cold dust in an asteroid belt akin to the Sun's Kuiper Belt suggest that a distant "catastrophic" planetary disruption explanation is unlikely the possibility of a disrupted asteroid belt scattering comets into the inner system is still to be determined. [73]

### Younger star with coalescing material around it Edit

Astronomer Jason T. Wright and others who have studied Tabby's Star have suggested that if the star is younger than its position and speed would suggest, then it may still have coalescing material around it. [30] [33] [74]

A 0.8–4.2-micrometer spectroscopic study of the system using the NASA Infrared Telescope Facility (NASA IRTF) found no evidence for coalescing material within a few astronomical units of the mature central star. [13] [18]

### Planetary debris field Edit

High-resolution spectroscopy and imaging observations have also been made, as well as spectral energy distribution analyses using the Nordic Optical Telescope in Spain. [1] [55] A massive collision scenario would create warm dust that glows in infrared wavelengths, but there is no observed excess infrared energy, ruling out massive planetary collision debris. [16] Other researchers think the planetary debris field explanation is unlikely, given the very low probability that Kepler would ever witness such an event due to the rarity of collisions of such size. [1]

As with the possibility of coalescing material around the star, spectroscopic studies using the NASA IRTF found no evidence for hot close-in dust or circumstellar matter from an evaporating or exploding planet within a few astronomical units of the central star. [13] [18] Similarly, a study of past infrared data from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer found no evidence for an excess of infrared emission from the star, which would have been an indicator of warm dust grains that could have come from catastrophic collisions of meteors or planets in the system. This absence of emission supports the hypothesis that a swarm of cold comets on an unusually eccentric orbit could be responsible for the star's unique light curve, but more studies are needed. [13] [5]

### Consumption of a planet Edit

In December 2016, a team of researchers proposed that Tabby's Star swallowed a planet, causing a temporary and unobserved increase in brightness due to the release of gravitational energy. As the planet fell into its star, it could have been ripped apart or had its moons stripped away, leaving clouds of debris orbiting the star in eccentric orbits. Planetary debris still in orbit around the star would then explain its observed drops in intensity. [75] Additionally, the researchers suggest that the consumed planet could have caused the star to increase in brightness up to 10,000 years ago, and its stellar flux is now returning to the normal state. [75] [76]

### Large planet with oscillating rings Edit

Sucerquia et al. (2017) suggested that a large planet with oscillating rings may help explain the unusual dimmings associated with Tabby's Star. [77] [78]

### Large ringed planet followed by Trojan swarms Edit

Ballesteros et al. (2017) proposed a large, ringed planet trailed by a swarm of Trojan asteroids in its L5 Lagrangian point, and estimated an orbit that predicts another event in early 2021 due to the leading Trojans followed by another transit of the hypothetical planet in 2023. [79] The model suggests a planet with a radius of 4.7 Jupiter radii, large for a planet (unless very young). An early red dwarf of about 0.5 R would be easily seen in infrared. The current radial velocity observations available (four runs at σv ≈ 400 m/s) hardly constrain the model, but new radial velocity measurements would greatly reduce the uncertainty. The model predicts a discrete and short-lived event for the May 2017 dimming episode, corresponding to the secondary eclipse of the planet passing behind KIC 8246852, with about a 3% decrease in the stellar flux with a transit time of about 2 days. If this is the cause of the May 2017 event, the planet's orbital period is more precisely estimated as 12.41 years with a semi-major axis of 5.9 AU. [79] [80]

### Intrinsic luminosity variations Edit

The reddening observed during the deep dimming events of Tabby's Star is consistent with cooling of its photosphere. [81] It does not require obscuration by dust. Such cooling could be produced by a decreased efficiency of heat transport caused e.g. by decreased effectiveness of convection due to the star's strong differential rotation, or by changes in its modes of heat transport if it is near the transition between radiative and convective heat transport. The "missing" heat flux is stored as a small increase of internal and potential energy. [12]

The possible location of this early F star near the boundary between radiative and convective transport seems to be supported by the finding that the star's observed brightness variations appear to fit the "avalanche statistics" known to occur in a system close to a phase-transition. [82] [83] "Avalanche statistics" with a self-similar or power-law spectrum are a universal property of complex dynamical systems operating close to a phase transition or bifurcation point between two different types of dynamical behavior. Such close-to-critical systems are often observed to exhibit behavior that is intermediate between "order" and "chaos". Three other stars in the Kepler Input Catalog likewise exhibit similar "avalanche statistics" in their brightness variations, and all three are known to be magnetically active. It has been conjectured that stellar magnetism may be involved in Tabby's Star. [83]

### An artificial megastructure Edit

Some astronomers have speculated that the objects eclipsing Tabby's Star could be parts of a megastructure made by an alien civilization, such as a Dyson swarm, [17] [30] [57] [72] a hypothetical structure that an advanced civilization might build around a star to intercept some of its light for their energy needs. [84] [85] [86] According to Steinn Sigurðsson, the megastructure hypothesis is implausible and disfavored by Occam's razor and fails to sufficiently explain the dimming. He says that it remains a valid subject for scientific investigation, however, because it is a falsifiable hypothesis. [82] Due to extensive media coverage on this matter, Tabby's Star has been compared by Kepler's Steve Howell to KIC 4150611 , [87] another star with an odd light curve that was shown, after years of research, to be a part of a five-star system. [88] The likelihood of extraterrestrial intelligence being the cause of the dimming is very low [62] however, the star remains an outstanding SETI target because natural explanations have yet to fully explain the dimming phenomenon. [30] [57] The latest results have ruled out explanations involving only opaque objects such as stars, planets, swarms of asteroids, or alien megastructures. [89]

### Exomoons Edit

Two papers published in summer 2019 offered plausible scientific scenarios involving large moons being stripped from their planets. Numeric simulations were performed of the migration of gas giant planets, and their large gaseous moons, during the first few hundred million years after the formation of the planetary system. In approximately 50% of the cases, the results produce a scenario where the moon is freed from its parent planet and its orbit evolves to produce a light curve similar to that of Tabby's Star. [23] [24] [90] [91]

As of 2015 [update] , numerous optical telescopes were monitoring Tabby's Star in anticipation of another multi-day dimming event, with planned follow-up observations of a dimming event using large telescopes equipped with spectrographs to determine if the eclipsing mass is a solid object, or composed of dust or gas. [92] Additional follow-up observations may involve the ground-based Green Bank Telescope, the Very Large Array Radio Telescope, [55] [93] and future orbital telescopes dedicated to exoplanetology such as WFIRST, TESS, and PLATO. [57] [86]

In 2016, a Kickstarter fund-raising campaign was led by Tabetha Boyajian, the lead author of the initial study on the star's anomalous light curve. The project proposed to use the Las Cumbres Observatory Global Telescope Network for continuous monitoring of the star. The campaign raised over US\$100,000 , enough for one year of telescope time. [94] [ needs update ] Furthermore, as of 2016, more than fifty amateur astronomers working under the aegis of the American Association of Variable Star Observers were providing effectively full coverage since AAVSO's alert about the star in October 2015, [95] namely a nearly continuous photometric record. [96] In a study published in January 2018, Boyajian et al. reported that whatever is blocking Tabby's Star filters different wavelengths of light differently, so it cannot be an opaque object. They concluded that it is most likely space dust. [9] [10] [11]

In December 2018, a search for laser light emissions from Tabby's Star was carried out using the Automated Planet Finder (APF), which is sensitive enough to detect a 24 MW laser at this distance. Although a number of candidates were identified, further analysis showed that they are coming from the Earth and not from the star. [97]

### SETI results Edit

In October 2015, the SETI Institute used the Allen Telescope Array to look for radio emissions from possible intelligent extraterrestrial life in the vicinity of the star. [98] [99] After an initial two-week survey, the SETI Institute reported that it found no evidence of technology-related radio signals from the star system. [100] [101] [102] No narrowband radio signals were found at a level of 180–300 Jy in a 1 Hz channel, or medium-band signals above 10 Jy in a 100 kHz channel. [101]

In 2016, the VERITAS gamma-ray observatory was used to search for ultra-fast optical transients from astronomical objects, with astronomers developing an efficient method sensitive to nanosecond pulses with fluxes as low as about one photon per square meter. This technique was applied on archival observations of Tabby's Star from 2009 to 2015, but no emissions were detected. [103] [104]

In May 2017, a related search, based on laser light emissions, was reported, with no evidence found for technology-related signals from Tabby's Star. [105] [106]

In September 2017, some [email protected] workunits were created based on a previous RF survey of the region around this star. [107] This was coupled with a doubling in the size of [email protected] workunits, so the workunits related to this region will probably be the first workunits to have less issues with quantization noise.

### EPIC 204278916 Edit

A star called EPIC 204278916, as well as some other young stellar objects, have been observed [ when? ] to exhibit dips with some similarities to those observed in Tabby's Star. They differ in several respects, however. EPIC 204278916 shows much deeper dips than Tabby's Star, and they are grouped over a shorter period, whereas the dips at Tabby's Star are spread out over several years. Furthermore, EPIC 204278916 is surrounded by a proto-stellar disc, whereas Tabby's Star appears to be a normal F-type star displaying no evidence of a disc. [20]

• 14 May 2017 ("Elsie" 2% dip)
• 11 June ("Celeste" 2% dip)
• 2 August ("Skara Brae" 1% dip)
• 5 September ("Angkor" 2.3% [109] 3% [110] dip)
• 20 November (unnamed 1.25% [111] dip) [10]
• 16 March 2018 ("Caral-Supe" 1% [67] 5% [112] dip)
• 24 March ("Evangeline" 5%+ dip)

On 20 May 2017, Boyajian and her colleagues reported, via The Astronomer's Telegram, on an ongoing dimming event (named "Elsie" [10] [114] ) which possibly began on 14 May 2017. [115] It was detected by the Las Cumbres Observatory Global Telescope Network, specifically by its telescope in Maui (LCO Maui). This was verified by the Fairborn Observatory (part of the N2K Consortium) in Southern Arizona (and later by LCO Canary Islands). [116] [117] [118] Further optical and infrared spectroscopy and photometry were urgently requested, given the short duration of these events, which may be measured in days or weeks. [115] Observations from multiple observers globally were coordinated, including polarimetry. [119] Furthermore, the independent SETI projects Breakthrough Listen and Near-InfraRed Optical SETI (NIROSETI), both at Lick Observatory, continue to monitor the star. [115] [120] [121] [122] By the end of the three-day dimming event, [123] a dozen observatories had taken spectra, with some astronomers having dropped their own projects to provide telescope time and resources. More generally the astronomical community was described as having gone "mildly bananas" over the opportunity to collect data in real-time on the unique star. [124] The 2% dip event was named "Elsie" (a homophone of "LC", in reference to Las Cumbres and light curve). [125]

Initial spectra with FRODOSpec at the two-meter Liverpool Telescope showed no changes visible between a reference spectrum and this dip. [120] [121] [122] Several observatories, however, including the twin Keck telescopes (HIRES) and numerous citizen science observatories, acquired spectra of the star, [115] [121] [122] showing a dimming dip that had a complex shape, and initially had a pattern similar to the one at 759.75 days from the Kepler event 2, epoch 2 data. Observations were taken across the electromagnetic spectrum.

Evidence of a second dimming event (named "Celeste" [114] ) was observed on 13–14 June 2017, which possibly began 11 June, by amateur astronomer Bruce L. Gary. [126] While the light curve on 14–15 June indicated a possible recovery from the dimming event, the dimming continued to increase afterwards, [126] and on 16 June, Boyajian wrote that the event was approaching a 2% dip in brightness. [10] [127]

A third prominent 1% dimming event (named "Skara Brae" [114] ) was detected beginning 2 August 2017, [128] [129] and which recovered by 17 August. [10] [130]

A fourth prominent dimming event (named "Angkor" [114] ) began 5 September 2017, [131] and is, as of 16 September 2017, between 2.3% [109] and 3% [110] dimming event, making it the "deepest dip this year". [10] [132]

Another dimming event, amounting to a 0.3% dip, began around 21 September 2017 and completely recovered by 4 October 2017. [60]

On 10 October 2017, an increasing brightening, lasting about two weeks, of the starlight from KIC 8462852 was noted by Bruce L. Gary of the Hereford Arizona Observatory [63] and Boyajian. [133] A possible explanation, involving a transiting brown dwarf in a 1,600-day eccentric orbit near KIC 8462852, a "drop feature" in dimness and predicted intervals of brightening, to account for the unusual fluctuating starlight events of KIC 8462852, has been proposed. [63] [64] [65]

On about 20 November 2017, a fifth prominent dimming event began and had deepened to a depth of 0.44% as of 16 December 2017, the event recovered, leveled off at dip bottom for 11 days, faded again, to a current total dimming depth of 1.25%, and is now recovering again. [63] [111]

Dimming and brightening events of the star continue to be monitored related light curves are currently updated and released frequently. [67] [134]

The star was too close to the Sun's position in the sky from late December 2017 to mid February 2018 to be seen. Observations resumed in late February. [67] [135] A new series of dips began on 16 March 2018. By 18 March 2018 the star was down by more than 1% in g-band, according to Bruce L. Gary, [67] and about 5% in r-band, making it the deepest dip observed since the Kepler Mission in 2013, according to Tabetha S. Boyajian. [112] [136] [137] A second even deeper dip with a depth of 5%+ started on 24 March 2018, as confirmed by AAVSO observer John Hall. [138] [139] As of 27 March 2018, that second dip is recovering. [140]

No significant dips have been observed since March 2018, but monitoring continues. The 2019 observing season began in mid-March, when the star reappeared after its yearly conjunction with the Sun. [141]

Observations with the Transiting Exoplanet Survey Satellite (TESS) are scheduled for some periods from 18 July to 11 September 2019. During that time, the brightness of the star will be measured at great precision once every 2 minutes. As of 19 July 2019, an observing campaign including both TESS as well as ground based observers is under way. [142] [143]

On 3–4 September 2019, the star's brightness dipped again by 1.4%, as seen by the TESS spacecraft. [144]

Between October 2019 and December 2019, at least seven separate dips were observed, the deepest of which had a depth of 2%. By the end of the observing season in early January 2020, the star had once again recovered in brightness. The total combined depth of the dips in 2019 was 11%, comparable to that seen in 2011 and 2013, but spread over a long time interval. [145] This cluster of dips is roughly centered on the 17 October 2019 date predicted by Sacco et al. [53] for a reappearance, given a 1,574 day period, of orbiting material comprising the original "D800" dip.

Consolidated plot of major (>= 1%) dimmings (3 April 2021)

All light curve data − December 2009 to May 2013, scan days 0066 to 1587 (Kepler)

## Why the light curve goes down when the planet is behind the star? - Astronomy

If you’re looking at a galaxy from the edge, it appears flat, more or less. If you’re looking down on it…well, you see the whole thing spread out in front of you.

You notice that when you see the Milky Way from earth, it’s just a relatively narrow band of stars across the sky, and that’s because you’re looking at it edge-on. If you were looking down on it, you’d see the middle of it and the spiral arms flinging around it.

I BETTER not have just answered a homework question.

Val123 ( 12709 />) “Great Answer” ( 2 />)

@Val123: @Haroot asked for the case of a binary system, not a galaxy. However, the same basic idea applies.

An edge-on binary system is where our observing angle corresponds with the orbital orientation of the stars. In such a system, we see the stars eclipsing one-another at regular intervals. You can record a “light curve” that might look something like this. The valleys are where one star is in front of the other, and then again when they “reverse” position. The high plateaus are where both stars are visible. You would also observe a periodic doppler-shift of the light as the stars move towards and away from us while they circle eachother.

A face-on binary system is where we are looking directly “down” on the plane of the orbit. If our resolution is good enough, we can physically see the stars circling one another.

In reality, most systems are neither perfectly face-on or edge-on, but are inclined at some angle (usually called i). Additionally, you can’t easily tell from direct observation what the inclination angle is, unless the system is perfectly edge-on. This wikipedia article might help. An edge-on system has an inclination angle of 90 degrees, while a face-on system has an inclination of zero degrees.

hannahsugs ( 3238 />) “Great Answer” ( 3 />)

Alright. I think I got it. Thanks.

@Val123 And no, it wasn’t. Just something I was confused about.

Haroot ( 2118 />) “Great Answer” ( 0 />)

Put a lemon and a lime on a table top.

Look down on them: face-on. You’ll see the tops of the two fruits. ˚ ˚

Put your face on the edge of table and look at the fruit edge-on. You’ll see the silhouette of the outline of the two fruits. 0 0

gailcalled ( 54584 />) “Great Answer” ( 0 />)

Fine. I’m posting something I’ve been holding off on, waiting for @hannahsugs to report to me on. Here goes….

Val123 ( 12709 />) “Great Answer” ( 0 />)

OK. My next thought was, take a plate, hold it edge on to your face. What you see then, just the edge of the plate, is very different than if you flipped it up and looked at the plate face on….but then @hannahsugs came in with how it applied to a binary star system (noted in your question…)
I read his/her post (thought about it…) and I came up with this simplistic explanation (and this is what I sent to hannahsugs, and waited to see if it was accurate…no answer so I don’t know….) So, here’s how I perceive it. Take two balls that are trailing tracers of light behind them, revolving around each other, more or less evenly. If you look down on them from above, or directly above their equal trajectories, the tracers form a circle. If you look at them from other angles, (since their trajectories aren’t exactly the same) they form other, varying patterns, such as @hannahsugs‘s graph showed, because they probably aren’t going exact circle around circle around each other. One is going faster, one is going slower, one is going up and down, the other isn’t….it’ll create different “light” patterns, depending on the angle you’re viewing them from.

Val123 ( 12709 />) “Great Answer” ( 0 />)

D’oh! I wrote that post right before signing off my computer for a few hours. oops!

@Val123 has sorta the right idea, as far as if the stars were emitting “tracers” and seeing different patterns over time. However, I’m afraid I might have confused things with the link to the graph. In a binary star system, if the stars were emitting “tracers” of light as they moved, if you looked at them “face-on”, their paths would form too overlapping circles or ellipses. Try this website, changing the mass of the purple planet to

150, and you have an idea of what that would look like.

If you looked at the system edge-on, you’d just see a line, with two bright “dots” moving back and forth along the line as they looked at eachother. As @Val123 and @gailcalled suggested, this is similar to taking a dinner plate or a CD and holding it flat, even with the plane of your eyes.

For a system that is somewhere between edge-on and face-on, we would simply see similar overlapping ellipses as in the face-on case, they would just be “squished” or flattened. Play with the simulation i linked to above, and try these initial inputs:
Body 1: 200 -90 0 -90 0
Body 2: 150 150 0 -80 40
Try to imagine, those could be more circular orbits, but because of an inclination angle, the appear to us to be elliptical.

The graph i showed is something different. That is a “light curve” for an edge-on binary system. It shows the total brightness of the whole system, as perceived by us, as the stars orbit eachother. It assumes that the stars are not of equal brightness. On the plateaus, from our perspective the stars are “next to” one another, so we see the full brightness from both of them. When the brighter star passes behind the dimmer star, we get the first dip in the graph, because we ONLY see the light from the dimmer star. The bright star emerges again, and we see the same brightness level as before, until the dim star passes behind the brighter star. Now we ONLY see the light from the brighter star, so there’s a dip again. Does that make more sense? The graph does NOT show position, it shows overall brightness over time.

Edge-on binary stars are very useful to astronomers. They are the only system where the inclination angle can be truly and surely known, because we can SEE the stars passing in front of one another. With systems that are inclined, we can only make an educated guess as to what the inclination angle is, or if the system is 100% face-on. When a system is edge-on, we can get true orbital velocities of the stars, which means we can get their masses, the radius of the orbit, etc. Unfortunately, as you can probably guess, edge-on or perfectly face-on binaries are rare. Random-inclination binaries are much more common. Luckily, more than ½ the stars in our galaxy seem to be in binary systems. Our sun is one of the odd-ball lonely stars. With 300 billion stars in the galaxy, at least half of them binaries, there’s some edge-on systems for us to study!

hannahsugs ( 3238 />) “Great Answer” ( 0 />)

@hannahsugs THAT is VERY cool! I could spend hours playing! (Be right back)

Oh dear. I set the mass of the purple star to 1000….oops! I hope they didn’t have populated solar systems.

Neat! I made a hydrogen atom!

Oh! I created a four star system and created a traffic jam!

Oh crap! Don’t give purple a mass of 150 and a position of 142, and the yellow a mass of 50 and DON’T make both of their velocities a 10. DON’T DO IT!

@Haroot give it 4 stars. You can see a better example of the wave thing he had going on above.

Shoot…I can’t get them to stop getting into head on collisions. I wanted to check something, but I am God and I’ve screwed up my binary star system. I have the power to keep recreating them, but I can’t figure out how to reset them so they orbit, instead of crash….what would a good default be?

Also, IS there such a thing as a 4 star system.

Val123 ( 12709 />) “Great Answer” ( 0 />)

@hannahsugs I found the original settings, so I could check what I wanted to see.

@Haroot take the link with it’s original settings of:

Yellow: Mass=200 / All three positions at 0/ velocity at -1.

Purple: Mass=10 / Position X=142, next two positions at zero / Velocity=140

Now set the velocity of Purple at 80…you can see how it makes the yellow star wobble, which creates the waves whch the instruments will read as going up and down, or side to side, or towards us and away from us (all which create the dopple shifts)

(Or, set it at 50, tell your girlfriend this is for her, go away for 4 minutes, look again and viola!) It gets better

Val123 ( 12709 />) “Great Answer” ( 0 />)

## Gravity betrays the presence of a planet orbiting a binary star 10,000 light years from Earth

Astronomers have discovered an exoplanet, a planet orbiting an alien star.

That by itself would've been news just a few years ago, but given that we’ve found thousands of exoplanets, there has to be more. And there is! It’s orbiting a binary star, a pair of stars orbiting each other. Since it orbits both, it’s called a circumbinary exoplanet.

But those have been found before too! So what makes this one special? It’s how it was found. And that’s a bit of a tale.

The planet is called — are you ready for this? — OGLE-2007-BLG-349L(AB)b. There’s a lot in that name, and unpacking it tells you how it was found, so bear with me.

OGLE stands for the Optical Gravitational Lensing Experiment. This project is actually designed to look for dark matter. We know dark matter exists, but we’re not sure what it is. On the assumption (now probably outdated) that it might be lots of dark objects like rogue planets, black holes, and dead stars, OGLE stares at regions of the sky thick with stars. If one of these dark objects passes almost exactly between us and one of those stars, a funny thing can happen: The background star gets brighter.

This is because gravity bends space, and that means the path light takes to get here from the background star gets distorted. This is called gravitational lensing, and a lot of very odd effects can come from this, one of which is that the background star can have its light focused by the lensing object, making it appear brighter.

So OGLE looks for this brightening, which can take many days as the motion of the three objects (the distant star, or lensed object the intervening object, or lens and us: Earth) moves them into and then out of alignment. Mind you, any massive object can act as a lens: a black hole, a dead star, or even a perfectly normal star and/or a planet orbiting it. It’s the gravity that does the work.

This video shows gravitational lensing by a galaxy, but the principle is the same. Credit: ESA/Hubble, L. Calçada

In 2007 one such brightening occurred (hence the number in the planet’s name). Astronomers were alerted, and many telescopes were pointed toward the star. The star was located near the center of our Milky Way Galaxy, where there is a swarm of stars forming a spherical bulge — that’s the BLG in the planet’s name. It was the 349 th lens found toward the bulge, so that’s the 349L.

But right away the astronomers saw something odd. There were two brightening events, not one. That meant the lens was actually two objects, perhaps a star and a planet! OGLE had already found several planets orbiting stars this way it’s an added bonus of the project.

The “light curve” of the lensing event, showing how the brightness of the lensed star changed over two weeks or so (top). The various colors are observations from various observatories. The bottom panel shows a zoom in to the central two days, and the bottom panel is a residual, the observations minus a model. The wiggle in the middle of the green and blue observations shows that a third object is needed to fit the data. Credit: Bennett et al.

But when astronomers took the data and applied their physical models to it, basically using the equations of gravitational lenses (based, I’ll note, on Einstein’s relativity physics), they found they don’t quite fit. A small twist in the middle of the light curve (the plot showing the change in brightness over time) showed that this wasn’t a pair of objects, like a star and planet: It was a trio! It was either a star with two planets orbiting it, or a pair of stars orbiting each other with a planet orbiting both.

The models indicated the total mass of the lensing system was about 0.7 times the Sun’s mass — so if it were a single star it’d be cooler and dimmer than the Sun, but if it were two stars sharing that mass then both were much cooler and much dimmer. The problem was, the models couldn’t distinguish between the two scenarios. Both fit the data pretty well. So what to do?

Enter Hubble. Astronomers were able to snag time on the orbiting telescope to observe the lensing event, and caught the tail end of it in 2007 and 2008. The lensed star was still fading in 2007 during the first observation, but by 2008 it was essentially over.

Even Hubble couldn’t spatially separate out the lensing trio from the background lensed star (they were aligned so closely together in the images they look like one object), but it could do something else: measure not just the brightness of the objects but also their colors during and after the event.

That’s critical. A single star with 0.7 times the Sun’s mass would be redder than the Sun, kinda orange, but two stars of, say, 0.35 times the Sun’s mass each would be far redder.

And Hubble found the color of the lens was very red. That meant it was two stars: A binary. Once that was known and put into the models, the other lensing object was found to be a planet orbiting both: a circumbinary exoplanet.

Hubble images of the lensing event in 2007 (left) when it was fading, and a few months later (right) in 2008 when the event was over. Credit: Bennett et al.

And now, finally, we can get some numbers to go with all this. The background (lensed) star is probably located near the center of the galaxy, about 25,000 light-years away. That puts the binary and planet about 10,000 light-years from Earth.

The two stars in the binary are indeed dinky red dwarfs, with about 0.4 and 0.3 times the mass of the Sun. They orbit each other about once every 10 days, and are about 12 million kilometers apart (that’s a decently tight orbit Mercury is roughly 60 million km from the Sun).

The planet has about 80 times the mass of the Earth. That’s nearly the mass of Saturn (which is 95 times the Earth’s mass), so it’s likely this planet is a small gas giant. It orbits the pair very roughly 500 million kilometers out, taking about 7 years to do so … and that’s interesting, too. If a circumbinary planet orbits too closely to the stars, the orbit can become unstable the changing gravity it feels from each star will be enough to disrupt the orbit. Almost all circumbinary planets found orbit very close to this limit. This one, though, orbits pretty far out, well into the stable zone.

Under the usual methods of finding planets — the transit method, for example — planets closer in to their stars are easier to find, so that’s what we see. OGLE doesn’t discriminate that way it finds farther-out planets just as well. So it’s possible that circumbinary planets on wide orbits are common, but it’s just that we don’t see them as easily.

Right away, that makes this planet important. Half the stars in the galaxy are in binary systems, so planets like this could possibly be as common as planets like ours! That’s pretty cool.

One last thing: That letter at the end of the name. In general, exoplanets are given the star’s name plus a lowercase b at the end. In general, the two stars in a binary are called (capital) A and B, in order of their brightness. This planet orbits both, so we get OGLE-2007-BLG-349L(AB)b. Unwieldy, but informative if you can read the code.

However, the discovery paper itself calls the planet c, not b. They say that they did this to avoid an inconsistency in the naming convention for planets in binary systems, by using a unique letter for each object. I have to say, though, I disagree.

Exoplanets are given letter designations in order of their discovery, starting with a lowercase b and moving from there. The lowercase a is skipped, because sometimes that’s used to refer to the star. That’s a bit odd, I’ll admit, but it’s the rule, and it’s consistent.

For our circumbinary planet here, the use of a lowercase b works if the stars are listed in parentheses, making it clear it’s a circumbinary. If it only orbited star A, we’d call the planet OGLE-2007-BLG-349LAb. This can get a little hard to read, but it’s still consistent, and it’s not hard to get used to. Better, it makes it easy to decipher if you break it down.

Calling this planet c is inconsistent with other exoplanet designations, making it seem like this is the second planet found in the system. So I prefer b.

Normally I’m not so concerned about the naming of names, but in reality we’re discovering these planets so rapidly that we need a consistent naming orthodoxy before matters get too messy.

And don't let it overshadow the planet itself! This was a very cool discovery, and it strongly implies we'll be finding more of these using gravitational lensing the more we look. OGLE's been going since 1992, and it looks like it'll keep staring at stars for the foreseeable future.