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.”
The theorised evolution of the circumbinary planet PSR B1620?26 b. Credit: NASA.
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Binary star systems can have planets – although these are generally assumed to be circumbinary (where the orbit encircles both stars). As well as the fictional examples of Tatooine and Gallifrey, there are real examples of PSR B1620-26 b and HW Virginis b and c – thought to be cool gas giants with several times the mass of Jupiter, orbiting several astronomical units out from their binary suns.
Planets in circumstellar orbits around a single star within a binary system are traditionally considered to be unlikely due to the mathematical implausibility of maintaining a stable orbit through the ‘forbidden’ zones – which result from gravitational resonances generated by the motion of the binary stars. The orbital dynamics involved should either fling a planet out of the system or send it crashing to its doom into one or other of the stars. However, there may be a number of windows of opportunity available for ‘next generation’ planets to form at later stages in the evolving life of a binary system.
A binary stellar evolution scenario might go something like this:
1) You start with two main sequence stars orbiting their common centre of mass. Circumstellar planets may only achieve stable orbits very close in to either star. If present at all, it’s unlikely these planets would be very large as neither star could sustain a large protoplanetary disk given their close proximity.
2) The more massive of the binaries evolves further to become an Asymptotic Giant Branch star (i.e. red giant) – potentially destroying any planets it may have had. Some mass is lost from the system as the red giant blows off its outer layers – which is likely to increase the separation of the two stars. But this also provides material for a protoplanetary disk to form around the red giant’s binary companion star.
3) The red giant evolves into white dwarf, while the other star (still in main sequence and now with extra fuel and a protoplanetary disk) can develop a system of orbiting ‘second generation’ planets. This new stellar system could remain stable for a billion years or more.
4) The remaining main sequence star eventually goes red giant, potentially destroying its planets and further widening the separation of the two stars – but it also may contribute material to form a protoplanetary disk around the distant white dwarf star, providing the opportunity for third generation planets to form there.
How a binary system might give birth to generations of planets: a) First generation planets - small and close-in - might be possible while both stars are on the main sequence (MS) and in close proximity to each other; b) Eventually one star evolves from the main sequence to the Asymptotic Giant Branch (AGB) - in other words, it goes red giant. c) The two stars spread further apart while stellar material blown off from the red giant builds a protoplanetary disk around the other star and second generation planets form; d) the second star eventually goes red giant giving the first star (now a white dwarf - WD) a protoplanetary disk which could create a third generation of planets. Credit: Perets, H.B.
The development of the third generation planetary system depends on the white dwarf star sustaining a mass below its Chandrasekhar limit (being about 1.4 solar masses – depending on its rate of spin) despite it having received more material from the red giant. If it doesn’t stay below that limit, it will become a Type 1a supernova – potentially lobbing a small proportion of its mass back to the other star again, although by this stage that other star would be a very distant companion.
An interesting feature of this evolutionary story is that each generation of planets is built from stellar material with a sequentially increasing proportion of ‘metals’ (elements heavier than hydrogen and helium) as the material is cooked and re-cooked within each stars’ fusion processes. Under this scenario, it becomes feasible for old stars, even those which formed as low metal binaries, to develop rocky planets later in their lifetimes.
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.”
An artist's impression of a transiting exoplanet. Credit: ESA C Carreau
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It was only a little over a year ago that the 400th extrasolar planet was confirmed, but time flies when you’re discovering exoplanets. The 19th of November 2010 marked the date that over 500 exoplanets had been confirmed on The Extrasolar Planets Encyclopedia.
Though it’s an arbitrary number to celebrate, the fact that we’ve confirmed the existence over 500 exoplanets since their initial discovery 20 years ago is deserving of merriment. Discovery of exoplanets has really ramped up over the last few years, thanks in part to the ESA’s COROT satellite, the Hubble and Spitzer space telescopes, the Keck Interferometer, and the improvement of observational techniques to discover and confirm exoplanets. NASA’s Kepler spacecraft has over 700 candidates for exoplanets. Only 7 planets have been confirmed after being discovered by the Kepler spacecraft so far, though.
Jean Schneider, an astrobiologist at the Paris-Meudon Observatory, keeps up a database of the confirmed exoplanetary discoveries at The Extrasolar Planets Encyclopedia. He posted a warning about how muddle the declaration of “the discovery of the 500th exoplanet” could be. He wrote in the warning:
“The number of exoplanets, recored for instance at http://exoplanet.eu is necessarily subject to some uncertainty for several reasons:
– the mass limit below which a substellar object is called a planet
is somewhat arbitrary
– the mass measurement is always affected by some instrumental inaccuracy
– whatever this mass limit is, the true mass for most planets is subject
to some uncertainty, intrinsic to the detection method (unkown
inclination of the orbit, modelisation of planet atmosphere)
– some planet detections, even published in refereed papers, are sometimes
retracted afterwards
For all these reasons
1/ The boundary between “confirmed”/”unconfirmed” planets is somewhat fuzzy
2/ The number of planet candidates at http://exoplanet.eu ;(collected
in the survey of professional litterature, conferences or websites)
is affected by an uncertainty of a few units.”
In essence, to say that there is a “500th exoplanet” is really not possible, given that there needs to be confirmation of the planet. Even after that confirmation, there could be the possible retraction of the planet from the database. 5 confirmations were posted on the 19th, all of them published in refereed papers and discovered in 2010. This kicked the total over 500. But then another was announced the next day, and it was discovered in 2007 but only recently confirmed. So, putting a number on the 500th extrasolar planet to be confirmed is pretty much impossible, arbitrary at best.
Schneider was interviewed by Scientific American on just why he is the keeper of the encyclopedia, and some of his thoughts on the discoveries made so far and the future of the field. The text of the interview is available here.
Complicating matters even further, there is another running tally of extrasolar planets maintained by the NASA’s Jet Propulsion Laboratory at PlanetQuest. Their count on the 22nd of November was only 497, and today rests at 500. The Extrasolar Planets Encyclopedia now stands at 504.
PlanetQuest has this video that succinctly describes the history of extrasolar planet discovery, for those interested:
Even if it’s arbitrary, you can still have that “500th exoplanet” party if you’d like, complete with Kepler satellite-shaped hats. Nobody will likely stop you; if they do, there will likely be another few dozen planets discovered – or a few retracted – by then anyways, making their point rather moot.
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.
Well, not only may up to 25% of Sun-like stars have Earth-like planets – but if they are in the right temperature zone, apparently they are almost certain to have oceans. Current thinking is that Earth’s oceans formed from the accreted material that built the planet, rather than being delivered by comets at a later time. From this understanding, we can start to model the likelihood of a similar outcome occurring on rocky exoplanets around other stars.
Assuming terrestrial-like planets are indeed common – with a silicate mantle surrounding a metallic core – then we can expect that water may be exuded onto their surface during the final stages of magma cooling – or otherwise out-gassed as steam which then cools to fall back to the surface as rain. From there, if the planet is big enough to gravitationally retain a thick atmosphere and is in the temperature zone where water can remain fluid, then you’ve got yourself an exo-ocean.
We can assume that the dust cloud that became the Solar System had lots of water in it, given how much persists in the left-over ingredients of comets, asteroids and the like. When the Sun ignited some of this water may have been photodissociated – or otherwise blown out of the inner solar system. However, cool rocky materials seem to have a strong propensity to hold water – and in this manner, could have kept water available for planet formation.
Meteorites from differentiated objects (i.e. planets or smaller bodies that have differentiated such that, while in a molten state, their heavy elements have sunk to a core displacing lighter elements upwards) have around 3% water content – while some undifferentiated objects (like carbonaceous asteroids) may have more than 20% water content.
Mush these materials together in a planet formation scenario and materials compressed at the centre become hot, causing outgassing of volatiles like carbon dioxide and water. In the early stages of planet formation much of this outgassing may have been lost to space – but as the object approaches planet size, its gravity can hold the outgassed material in place as an atmosphere. And despite the outgassing, hot magma can still retain water content – only exuding it in the final stages of cooling and solidification to form a planet’s crust.
Mathematical modelling suggests that if planets accrete from materials with 1 to 3% water content, liquid water probably exudes onto their surface in the final stages of planet formation – having progressively moved upwards as the planet’s crust solidified from the bottom up.
Otherwise, and even starting with a water content as low as 0.01%, Earth-like planets would still generate an outgassed steam atmosphere that would later rain down as fluid water upon cooling.
As the Earth formed, water contained in rocky materials either 'outgassed' or just exuded onto the surface - as magma solidified, from the bottom up, to form the Earth's crust. And OK, this is just a nice image of a deep sea volcanic vent - but you get the idea. Credit: Woods Hole Oceanographic Institution.
If this ocean formation model is correct, it can be expected that rocky exoplanets from 0.5 to 5 Earth masses, which form from a roughly equivalent set of ingredients, would be likely to form oceans within 100 millions years of primary accretion.
This model fits well with the finding of zircon crystals in Western Australia – which are dated at 4.4 billion years and are suggestive that liquid water was present that long ago – although this preceded the Late Heavy Bombardment (4.1 to 3.8 billion years ago) which may have sent all that water back into a steam atmosphere again.
Currently it’s not thought that ices from the outer solar system – that might have been transported to Earth as comets – could have contributed more than around 10% of Earth’s current water content – as measurements to date suggest that ices in the outer solar system have significantly higher levels of deuterium (i.e. heavy water) than we see on Earth.
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.”
