When listing the major scientific powers, the tiny nation of Qatar is not one that generally comes to mind. However, a Qatar astronomer, partnered with teams from the Harvard-Smithsonian Center for Astrophysics (CfA) as well as other institutions has just discovered a new exoplanet, dubbed Qatar-1b.
The planet itself, is another in the class of hot Jupiters which are massive, gassy planets that orbit their stars extremely closely. It has an orbital period of 1.4 days and is expected to be tidally locked with its parent star, a K type star.
It was discovered by a set of wide angle cameras located in New Mexico which are capable of surveying a large number of stars at a single time. The goal was to find planets that eclipsed the parent star and would thus show regular variations in their light curve. Images taken from this system were then sent to teams working at Universities in St. Andrews, Leicester, and Qatar. These teams processed the images and narrowed the stars down to a list of a few hundred candidates to be studied further.
From there Dr. Khalid Al Subai as well as the Harvard CfA team used the Smithsonian’s Whipple 48-inch telescope to more accurately measure the transits as well as as their 60-inch telescope to make spectroscopic observations to weed out binary star systems. These observations confirmed the existence of the planet.
“The discovery of Qatar-1b is a great achievement — one that further demonstrates Qatar’s commitment to becoming a leader in innovative science and research,” said Al Subai. Indeed, in the past 15 years, Qatar has undergone a large revolution towards science and education. Many universities have begun to open remote campuses, including Carnegie Mellon and Texas A&M. A more comprehensive list of science initiatives can be found here.
“The discovery of Qatar-1b is a wonderful example of how science and modern communications can erase international borders and time zones. No one owns the stars. We can all be inspired by the discovery of distant worlds,” said CfA team member David Latham.
One of the discovery images of the system obtained at the Keck II telescope using adaptive optics system and the NIRC2 Near-Infrared Imager. Image shows all four confirmed planets indicated as b, c, d and e in the labeled image. Planet "b" is a ~5 Jupiter-mass planet orbiting at about ~68 AU, while planets c, d, and e are ~7 Jupiter-mass companions orbiting the star at about 38, 24 and 14.5 AU. Credit: NRC-HIA, C. Marois & Keck Observatory
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Among one of the first exoplanet systems imaged was HR 8799. In 2008, a team led by Christian Marois at the Herzberg Institute of Astrophysics in Canada, took a picture of the system directly imaging three giant planets. The team revisited the system in 2009 – 2010 with the Keck II telescope and discovered a fourth planet in the system.
The new planet, designated HR 8799e, orbits at a distance of 14.5 AU, making it the innermost planet in the system. The other planets all orbit at distances of >25 AU. The images were taken in the near infrared where they are most noticeable because the system is relatively young (<100 Myr) and the planets are still radiating large amounts of heat from their formation.
The youth of these planets is part of what makes them an interesting target for astronomers. There exists a controversy in the community of planetary astronomers on the formation method of large planets. One theory states that planets form from a single, monolithic collapse that creates the entire planet’s mass at one time. Another possibility is that the initial collapse forms small cores early on, but then there is substantial growth later, as the planetesimal sweeps up additional material.
The discovery of the new planet challenges both theories. Marois states, “none of [the theories] can explain the in situ formation of all four planets.” Thus, a combination of both methods may be in use in the system. Several belts of dust are also known in the system which may help astronomers determine what modes of formation were present.
In particular HR 8799e is challenging to an in situ formation because the gravitational perturbations from the parent star should disrupt the formation of large gas planets within 20-40 AU from a single formation. Instead, the new planet would likely have had to been a core collapse with subsequent accretion, or alternatively, moved to its present location via migration.
Schematic representation of the HR 8799 planetary system compared to our solar system (viewed pole on and at the same distance as HR 8799). HR 8799 planet orbits are plotted assuming a pole-on view and circular orbits. A Kuiper Belt-like ring and an asteroid-like belt of dust, suggested by excess infrared light seen by the IRAS and ISO satellites, have been added. The HR 8799 dust disk is one of the heaviest detected by ISO and IRAS. It is thought that HR 8799e and HR 8799b dynamically interact with those dust disks in a way very similar to Jupiter with the asteroid belt and Neptune with the Kuiper Belt. Credit: NRC-HIA & C. Marois
Studying systems such as this may help astronomers better understand the formation of our own solar system. The paper notes that the HR 8799 “does show interesting similarities with the Solar system with all
giant planets located past the system’s estimated snow line (~2.7 AU for the Solar system and ~6 AU for HR 8799)”. Additionally, both have debris disks beyond the outer orbits with similar temperatures.
Different methods of detecting planetary formation necessarily turn up different types of systems. Radial velocity studies detect massive, close-in planets whereas direct imaging most easily finds more distant planets. These two apparent populations represent different modes of planetary formation and for a full understanding, astronomers will need a continuous sampling that merges the two. Marois notes that we are still far from this goal as “[w]e just do not have enough exoplanets detected by direct imaging (~6 so far)” to make any conclusions besides constraints from the non-detections occurring thus far. To truly merge these two populations, astronomers will likely need to wait until more systems are discovered.
Previously, some work has been done to estimate the composition of the atmospheres of the three planets already discovered in the system. These systems have been suggested to have cloudy atmospheres for CH4 and CO. According to Marois, his team is, “planning more observations on e, but it will be hard. We might have to wait for new instruments, like the Gemini Planet Imager to do it properly.” This new instrument “will put a ‘thumb’ on the star (or what we call a coronagraph) to physically block the star light and allow ‘easy’ detection of nearby faint planets.”
While this discovery is a first, it will certainly be one of a long line of exoplanet images. Marois is obviously excited about the ability to directly image planets. I asked him what the single most important thing he wanted readers to get from this research. His response was simple, “That we now have the telescopes and instruments to SEE planets orbiting other stars – that’s really cool! The exoplanet field is still very young and we have so much to learn.”
Trees on an alien world? No, a dune field on Mars with sand flows. Credit: NASA/JPL/U of Arizona
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Excitingly, we’ve been able to detect the composition of atmospheres on a handful of planets orbiting other stars. But if next-generation space observatories go online within the next couple of decades, some scientists propose using a new technique to determine details such as tree-like multicellular life on extrasolar planets.
While previous studies have discussed the likelihood of detecting life on exoplanets through signs of biogenic gases in the atmosphere, or seeing “glints” of light off oceans or lakes, those technique are limited in that, for example, biogenic gases could be signs of either single-celled or multicellular life – not providing much detail — and as we’ve seen from Titan, glints off planetary bodies do not necessarily come from water-filled lakes.
Researchers Christopher Doughty and Adam Wolf from the Carnegie Institution propose using a technique that Earth-orbiting satellites already use to in order to determine types of crops and land cover, as well as cloud detection, atmospheric conditions and other applications.
Called Bidirectional Reflectance Distribution Function (BRDF), this type of remote sensing determines the causes of differing reflectance at different sun- and view-angles. For example, trees cast shadows on the planet, and the large-scale pattern of shadows would make the light reflected off the vegetation to take on specific brightness and color characteristics.
“BRDF arises from the changing visibility of the shadows cast by objects,” the researchers wrote in their paper, “and the presence of tree-like structures is clearly distinguishable from flat ground with the same reflectance spectrum. We examined whether the BRDF could detect the existence of tree-like structures on an extrasolar planet by using changes in planetary albedo as a planet orbits its star.”
BRDF and different light reflection for various planetary sufaces. Credit: Wolfgang Lucht.
They used a computer model to simulate vegetation reflectance at different planetary phase angles and added both simulated and real cloud cover to calculate the planetary albedo for a vegetated and non-vegetated planet with abundant liquid water.
Depending on how accurately planetary cloud cover can be resolved, as well as the sensitivity instruments on proposed missions such as the Terrestrial Planet Finder, this technique could theoretically detect tree-like multicellular life on exoplanets in about 50 nearby stellar systems.
The angles of the spacecraft, the planet and its sun would have to be taken into account but the team says these characteristics would change in predictable ways over time, producing a detectable pattern.
If vegetation on the exoplanet was wide¬spread enough, it would affect the reflective properties of the whole planet.
“We found that even if the entire planetary albedo were rendered to a single pixel, the rate of increase of albedo as a planet approaches full illumination would be comparatively greater on a vegetated planet than on a non-vegetated planet,” they said.
Illustration of WASP-12b in orbit about its host star (Credit: ESA/C Carreau)
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Since its discovery in 2008, WASP-12b has been an unusual planet. This 1.4 Jovian mass, gas giant lies so close to its parent star that gas is being stripped from its atmosphere. But being stripped away isn’t the only odd property of this planet’s atmosphere. A new study has shown that it’s full of carbon.
The discovery was published in today’s issue of Nature was led by Nikku Madhusudhan, a postdoctoral researcher at Princeton University in combination with the Wide Angle Search for Planets (WASP) team that originally discovered the planet. Unlike other recent studies of planetary atmospheres, this study did not employ transit spectroscopy. Instead, the team examined the reflective properties of the planet at four wavelengths, observations of which three came from another study using the Canada-France-Hawaii Telescope in Hawaii.
To determine the composition of the atmosphere, the flux of the planet at each of these wavelengths was then compared to models of planetary atmospheres with differing compositions. The models included compounds such as methane, carbon dioxide, carbon monoxide, water vapor and ammonia as well as the temperature distribution of the planet.
For a typical hot Jupiter, models have most closely fit a ratio of about 0.5 for carbon to oxygen which suggests that oxygen is more prevalent in the atmospheres, often in the form of water vapor, as well as very little methane. For WASP-12b, Madhusudhan’s team found an abundance of more than 100 times that of standard hot Jupiters for methane (CH4). When examining the carbon to oxygen ratio, they discovered a ratio greater than one implying that the planet is unusually carbon rich.
While WASP-12b is certainly not a friendly place for life, the discovery of a planet with so much carbon may hold implications for life elsewhere in the universe. Astronomers expect that the abundance was due to the formation of the planet from rocky materials high in carbon as opposed to icy bodies like comets. This suggests that there may be an entire range of carbon abundances available for planets. With the versatility of carbon for forming organic compounds, this enhanced abundance may lead to other, rocky planets covered in tar like substances rife with organics.
The team speculates that such worlds may exist in the same solar system. Previous studies have shown that WASP-12b’s orbit is not circular and some have suggested that this may indicate the presence of another body which tugs on 12b’s orbit.
With the recent milestone of the discovery of the 500th extra solar planet the future of planetary astronomy is promising. As the number of known planets increases so does our knowledge. With the addition of observations of atmospheres of transiting planets, astronomers are gaining a fuller picture of how planets form and live.
Thus far, the observations of atmospheres have been limited to the “Hot-Jupiter” type of planets which often puff up, extending their atmospheres and making them easier to observe. However, a recent set of observations, to be published in the December 2nd issue of Nature, have pushed the lower limit and extended observations of exoplanetary atmospheres to a super-Earth.
The planet in question, GJ 1214b passes in front of its parent star when viewed from Earth allowing for minor eclipses which help astronomers determine features of the system such as its radius and also its density. Earlier work, published in the Astrophysical Journal in August of this year, noted that the planet had an unusually low density (1.87 g/cm3). This ruled out an entirely rocky or iron based planet as well as even a giant snowball composed entirely of water ice. The conclusion was that the planet was surrounded by a thick gaseous atmosphere and the three possible atmospheres were proposed that could satisfy the observations.
The first was that the atmosphere was accreted directly from the protoplanetary nebula during formation. In this instance, the atmosphere would likely retain much of the primordial composition of hydrogen and helium since the mass would be sufficient to keep it from escaping readily. The second was that the planet itself is composed mostly of ices of water, carbon dioxide, carbon monoxide and other compounds. If such a planet formed, sublimation could result in the formation of an atmosphere that would be unable to escape. Lastly, if a strong component of rocky material formed the planet, outgassings could produce an atmosphere of water steam from geysers, as well as carbon monoxide and carbon dioxide and other gasses.
The challenge for following astronomers would be to match the spectra of the atmosphere to one of these models, or possibly a new one. The new team is composed of Jacob Bean, Eliza Kempton, and Derek Homeier, working from the University of Göttingen and the University of California, Santa Cruz. Their spectra of the planet’s atmosphere was largely featureless, showing no strong absorption lines. This largely rules out the first of the cases in which the atmosphere is mostly hydrogen unless there is a thick layer of clouds obscuring the signal from it. However, the team notes that this finding is consistent with an atmosphere composed largely of vapors from ices. The authors are careful to note that “the planet would not harbor any liquid water due to the high temperatures present throughout its atmosphere.”
These findings don’t conclusively demonstrate that nature of the atmosphere, but narrow down the degeneracy to either a steam filled atmosphere or one with thick clouds and haze. Despite not completely narrowing down the possibilities, Bean notes that the application of transit spectroscopy to a super-Earth has “reached a real milestone on the road toward characterizing these worlds.” For further study, Bean suggests that “[f]ollow-up observations in longer wavelength infrared light are now needed to determine which of these atmospheres exists on GJ 1214b.”
Its often said that the number of grains of sand on Earth equals the number of stars in the Universe. Well it looks like a recent study by astronomers working at the Keck Observatory in Hawaii have found that its more like three times the number of grains of sand on Earth! Working with some of the most sophisticated equipment available, astronomers from Yale University have been counting the number of dim red dwarf stars in nearby galaxies which has led to a dramatic rethink of the number of stars in the Universe.
Red dwarfs are small, faint stars compared to most others and until now, have not been detected in nearby galaxies. Pieter van Dokkum and his team from Yale University studied eight massive elliptical galaxies between 50 and 300 million lights years from us and discovered that these tiny stars are much more bountiful than first thought. “No one knew how many of these stars there were,” said Van Dokkum. “Different theoretical models predicted a wide range of possibilities, so this answers a long standing question about just how abundant these stars are.”
For years astronomers have assumed that the number of red dwarfs in any galaxy was in the same proportion that we find here in the Milky Way but surprisingly the study revealed there are about 20 times more in the target galaxies. According to Charlie Conroy of the Harvard-Smithsonian Center who also worked on the project, “not only does this affect our understanding of the number of stars in the Universe but the discovery could have a major impact on our understanding of galaxy formation and evolution.” Knowing that there are now more stars than previously thought, this lowers the amount of dark matter (a mysterious substance that cannot be directly observed but its presence inferred from its gravitational influence) needed to explain the observed gravitational influence on surrounding space.
Not only has the discovery affected the amount of dark matter we expect to find but it also changes the quantity of planets that may exist in the Universe. Planets have recently been discovered orbiting around other red dwarf stars such as the system orbiting around Gliese 581, one which may harbour life. Now that we know there are a significantly higher number of red dwarfs in the Universe, the potential number of planets in the Universe has increased too. Van Dookum explains “There are possibly trillions of Earth’s orbiting these stars, since the red dwarfs they have discovered are typically more than 10 billion years old, so have been around long enough for complex life to evolve, its one reason why people are interested in this type of star.”
It seems then that this discovery, which on the face of it seems quite humdrum, actually has far reaching consequences that not only affect our view of the number of stars in the Universe but has dramatically changed our understanding of the distribution of matter in the Universe and the number of planets that may harbour intelligent life.
The new findings appear in the Dec. 1st online issue of the journal Nature.
One of the greatest potentials of transiting exoplanets is the ability to monitor the spectra and examine the composition of the planet’s atmosphere. This has been done already for HD 18733b and HD 209458b. In a new article by a team of astronomers at Keele University in the UK, absorption spectroscopy has been applied to the unusual exoplanet WASP-17b, which is known to orbit retrograde.
Not only does the spectra tell astronomers the atmospheric composition, but can also give an understanding of the the composition, but can also be indicative of how the atmosphere absorbs the light from the star and how heat is transferred around the planet. Additionally, since the atmosphere will absorb differently at different wavelengths, this gives differences in the timing of the eclipse and can be used to probe the radius of the planet more tightly as well as potentially examining the layering of the atmosphere.
For their investigation, the team concentrated on the sodium doublet lines at 5889.95 and 5895.92 Å. Observations were taken by the Very Large Telescope in Chile to observe 8 transits of the planet in June of 2009. The planet itself has a short orbit of 3.74 days.
Applying these spectroscopic techniques to WASP-17b, the team discovered the presence of sodium in the atmosphere. Yet the absorption wasn’t as strong as expected based on models using formation mechanisms from a nebula with solar composition and forming a planet with a cloudless atmosphere. Instead, the team describes 17b’s atmosphere as “sodium-depleted” similar to HD 209458b.
An additional observation was that the depth of seeing dropped off when using certain filters with different bandwidths (ranges of allowed wavelengths). The team noted that at bandwidths greater than 3.0 Å, the amount of sodium absorption seen nearly disappeared. Since this property is related to how much atmosphere the light travels through, this allowed the team to speculate that this may be indicative of clouds in the upper layers of the atmosphere.
Lastly, the team speculated as to the reason on the lack of sodium in the atmosphere. They proposed that energy from the star ionizes sodium on the day side. The motion of the atmosphere carrying it to the night side would then allow it to condense and be removed from the atmosphere. Since giant exoplanets in such tight orbits would likely be tidally locked, the sodium would have little chance to return to the day side and be brought back into the atmosphere.
While the examination of extrasolar atmospheres is undoubtedly new and will certainly be revised as the number of explored atmospheres increases, these pioneering studies are among the first that can allow astronomers directly test predictions of planetary atmospheres which, until recently have been solely based on observations of our own solar system. More generally, this will allow us to develop a fuller understanding of how planets evolve.
Artist's impression of a yellowish star being orbited by an extra-solar planet. Credit: ESO/L. Calçada
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While astronomers have detected over 500 extrasolar planets during the past 15 years, this latest one might have the most storied and unusual past. But its future is also of great interest, as it could mirror the way our own solar system might meet its demise. This Jupiter-like planet, called HIP 13044 b, is orbiting a star that used to be in another galaxy but that galaxy was swallowed by the Milky Way. While astronomers have never directly detected an exoplanet in another galaxy, this offers evidence that other galaxies host stars with planets, too. The star is nearing the end of its life and as it expands, could engulf the planet, just as our Sun will likely snuff out our own world. And somehow, this exoplanet has survived the first death throes of the star.
“The star is in the horizontal branch stage and it still has a planet, which is a glimmer of hope for those of us who worry about how our Solar System will look in 5 billion years,” said Markus Poessel, from the Max-Planck-Institut für Astronomie (MPIA) press office.
The star, HIP 13044, lies about 2,000 light-years from Earth in the southern constellation of Fornax (the Furnace). It is part of the so-called Helmi stream, a group of stars that originally belonged to a dwarf galaxy that was devoured by the Milky Way, probably about six to nine billion years ago.
The planet was detected using the radial velocity method — astronomers saw tiny telltale wobbles of the star caused by the gravitational tug of an orbiting companion. The instrument used was FEROS, a high-resolution spectrograph attached to the 2.2-meter MPG/ESO telescope at the La Silla Observatory in Chile.
“This discovery is very exciting,” says Rainer Klement from MPIA, who selected the target stars for this study. “For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin. Because of the great distances involved, there are no confirmed detections of planets in other galaxies. But this cosmic merger has brought an extragalactic planet within our reach.”
This artist’s impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy. Credit: ESO/L. Calçada
HIP 13044 is in the red giant phase of stellar evolution, and this exoplanet must have survived the period when its host star expanded massively after exhausting the hydrogen fuel supply in its core . The star has now contracted again and is burning helium in its core. Until now, these horizontal branch stars have remained largely uncharted territory for planet-hunters.
“This discovery is part of a study where we are systematically searching for exoplanets that orbit stars nearing the end of their lives,” says Johny Setiawan, also from MPIA, who led the research. “This discovery is particularly intriguing when we consider the distant future of our own planetary system, as the Sun is also expected to become a red giant in about five billion years.”
Our sun is going down the same stellar evolutionary path as HIP 13044, so astronomers may be able to determine the fate of our solar system by studying the system.
Setiawan told Universe Today that he and his team will continue to observe HIP 13044 and other stars in the group to search for other planets. “It is of course difficult to follow how this particular star evolves over time,” he said, “but if you just observe other stars with different evolutionary phase, you can also complete the picture without waiting until this one single star evolves.”
How has this planet survived so far?
“The star is rotating relatively quickly for a horizontal branch star,” said Setiawan. “One explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly.”
HIP 13044b probably once orbited much farther away from the star but spiraled inwards as the star began to spin faster.
The star also poses interesting questions about how giant planets form, as the star appears to contain very few elements heavier than hydrogen and helium — fewer than any other star known to host planets, and Setiawan said it is a puzzle how such a star could have formed a planet.
“There is indeed a possibility to form planets around metal-poor stars due to gravitational disk instability, which is an alternative to the core accretion model,” Setiawan said in an email. “But, for such a very metal poor star like HIP 13044, I am also not completely sure if the disk instability model can also explain the whole process. Still, it is probably the best explanation for this particular system.”
At best, the few extrasolar planets we have imaged directly are just points of light. But what can that light tell us about the planet? Maybe more than we thought. As you probably know the, Deep Impact spacecraft flew by comet Hartley 2 today, taking images from only 700 km away. But maneuvering to meet up with the comet is not the only job this spacecraft has been doing. The EPOXI mission also looked for ways to characterize extrasolar planets and the team made a discovery that should help identify distinctive information about extrasolar planets. How did they do it? By using the Deep Impact spacecraft to look at the planets in our very own solar system.
The spacecraft imaged the planetary bodies in our solar system — in particular the Earth, Mars and our Moon — (see here for movies of the Moon transiting Earth) and astronomer Lucy McFadden and UCLA graduate Carolyn Crow compared the reflected red, blue, and green light and grouped the planets according to the similarities they saw. The planets fall into very distinct regions on this plot, where the vertical direction indicates the relative amount of blue light, and the horizontal direction the relative amount of red light.
This suggests that when we do have the technology to gather light from individual exoplanets, astronomers could use color information to identify Earth-like worlds. “Eventually, as telescopes get bigger, there will be the light-gathering power to look at the colors of planets around other stars,” McFadden says. “Their colors will tell us which ones to study in more detail.”
On the plot, the planets cluster into groups based on similarities in the wavelengths of sunlight that their surfaces and atmospheres reflect. The gas giants Jupiter and Saturn huddle in one corner, Uranus and Neptune in a different one. The rocky inner planets Mars, Venus, and Mercury cluster off in their own corner of “color space.”
But Earth really stands out, and its uniqueness comes from two factors. One is the scattering of blue light by the atmosphere, called Rayleigh scattering, after the English scientist who discovered it. The second reason Earth stands out in color is because it does not absorb a lot of infrared light. That’s because our atmosphere is low in infrared-absorbing gases like methane and ammonia, compared to the gas giant planets Jupiter and Saturn.
“It is Earth’s atmosphere that dominates the colors of Earth,” Crow says. “It’s the scattering of light in the ultraviolet and the absence of absorption in the infrared.”
So, this filtering approach could provide a preliminary look at exoplanet surfaces and atmospheres, giving us an inkling of whether the planet is rocky or a gas planet, or what kind of atmosphere it has.
EPOXI is a combination of the names for the two extended mission components for the Deep Impact spacecraft: the first part of the acronym comes from EPOCh, (Extrasolar Planet Observations and Characterization) and the flyby of comet Hartley 2 is called the Deep Impact eXtended Investigation (DIXI).
The white dwarf G29-38. Many stars, including our Sun, end their lives as white dwarfs. Determining the masses of white dwarf stars is key to the new technique of determining a star's age. Image Credit: NASA
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While supernovae are the most dramatic death of stars, 95% of stars will end their lives in a far more quiet fashion, first swelling up to a red giant (perhaps a few times for good measure) before slowly releasing their outer layers into a planetary nebula and fading away as a white dwarf. This is the fate of our own sun which will expand nearly to the orbit of Mars. Mercury, Venus, and Earth will be completely consumed. But what will happen to the rest of the planets in the system?
While many stories have suggested that as the star reaches the red giant phase, even before swallowing the Earth, the inner planets will become inhospitable while the habitable zone will expand to the outer planets, perhaps making the now frozen moons of Jupiter the ideal beach getaway. However, these situations routinely only consider planets with unchanging orbits. As the star loses mass, orbits will change. Those close in will experience drag due to the increased density of released gas. Those further out will be spared but will have orbits that slowly expand as the mass interior to their orbit is shed. Planets at different radii will feel the combination of these effects in different ways causing their orbits to change in ways unrelated to one another.
This general shaking up of the orbital system will result in the system becoming once again, dynamically “young”, with planets migrating and interacting much as they would when the system was first forming. The possible close interactions can potentially crash planets together, fling them out of the system, into looping elliptical orbits, or worse, into the star itself. But can evidence of these planets be found?
A recent review paper explores the possibility. Due to convection in the white dwarf, heavy elements are quickly dragged to lower layers of the star removing traces of elements other than hydrogen and helium in the spectra. Thus, should heavy elements be detected, it would be evidence of ongoing accretion either from the interstellar medium or from a source of circumstellar material. The author of the review lists two early examples of white dwarfs with atmospheres polluted in this respect: van Maanen 2 and G29-38. The spectra of both show strong absorption lines due to calcium while the latter has also had a dust disk detected around the star?
But is this dust disk a remnant of a planet? Not necessarily. Although the material could be larger objects, such as asteroids, smaller dust sized grains would be swept from the solar system due to radiation pressure from the star during the main sequence lifetime. Much like planets, the asteroids orbits would be perturbed and any passing too close to the star could be torn apart tidally and pollute the star as well, albeit on a much smaller scale than a digested planet. Also along these lines is the potential disruption of a potential Oort cloud. Some estimates have predicted that a planet similar to Jupiter may have it’s orbit expanded as much as a thousand times, which would likely scatter many into the star as well.
The key to sorting these sources out may again lie with spectroscopy. While asteroids and comets could certainly contribute to the pollution of the white dwarf, the strength of the spectral lines would be an indirect indicator of the averaged rate of absorption and should be higher for planets. Additionally, the ratio of various elements may help constrain where the consumed body formed in the system. Although astronomers have found numerous gaseous planets in tight orbits around their host stars, it is suspected that these formed further out where temperatures would allow for the gas to condense before being swept away. Objects formed closer in would likely be more rocky in nature and if consumed, their contribution to the spectra would be shifted towards heavier elements.
With the launch of the Spitzer telescope, dust disks indicative of interactions have been found around numerous white dwarfs and improving spectral observations have indicated that a significant number of systems appear polluted. “If one attributes all metal-polluted white dwarfs to rocky debris, then the fraction of terrestrial planetary systems that survive post-main sequence evolution (at least in part) is as high as 20% to 30%”. However, with consideration for other sources of pollution, the number drops to a few percent. Hopefully, as observations progress, astronomers will begin to discover more planets around stars between the main sequence and white dwarf region to better explore this phase of planetary evolution.
