December 29, 2010 | 9
On 29th September this year, astronomers announced the discovery of an exoplanet called Gliese 581 g. This planet, they said, was exactly the right distance from its star for water to exist on its surface, with a good chance that it could hold an atmosphere. These two properties are very important when judging whether a planet can support life so, inevitably, some people got excited. But an exoplanet doesn’t have to be capable of supporting life in order to tell us about the universe we live in. In fact, some planets that are very different to our own may be about to turn our theories about planet and solar system formation upside down.
Artists impression of Gliese 581 g and its star. Image credit: Lynette Cook
When Gliese 581 g was discovered the news spread quickly. This was helped in no small way by the lead author of the paper announcing that his "personal feeling is that the chances of life on this planet are 100 percent." While saying life is certain to exist on a planet we’ve not even directly imaged is a bit of a strong claim, Gliese 581 g does seem to be a good candidate for a habitable planet. After all, it is in the so-called Goldilocks zone around its star, indicating that it’s not too cold and not too hot to hold liquid water. Its mass is around three to five Earth masses, which means it is rocky and Earth-like and could hold an atmosphere. So far, so habitable.
But when astronomers call a planet "habitable," they don’t necessarily mean that it’s somewhere we could live. They just mean it’s somewhere that some form of life may be able to survive.
It turns out that Gliese 581 g may not be the wonder planet that some newspaper reports would have us believe. For one thing, it’s very likely that Gliese 581 g is tidally locked to its star, meaning that the same side of the planet faces the star at all times. It’s the same mechanism that means we only ever see one side of our moon, but for a planet has much bigger consequences. One side of the planet is in perpetual day time, and the other in perpetual night — with a huge temperature difference between the two.
Before planning any interstellar excursions there, it may also be a good idea to check whether Gliese 581 g has a magnetic field to protect it from its star’s activity. Like our Sun, Gliese 581 is a hive of magnetic activity. In fact, red dwarf stars like Gliese 581 are known to be more active than main sequence stars like our Sun. Because its star is cooler than the Sun, Gliese 581 g has to huddle in closer to get the warmth required to be in the habitable zone — but being closer means it will be subjected to more hazards. Add to this the fact that tidally locked planets close to their stars are less likely to be able to produce a magnetic field and Gliese 581 g may begin to run into problems.
And last, but by no means least: Gliese 581 g might not even exist. A few weeks after the original paper came out, Francesco Pepe of the Geneva Observatory in Switzerland presented results that suggest Vogt and his colleagues could have got it wrong — the planet may not be there after all. Pepe’s analysis used the same information as Vogt’s did, with some additional data from the same experiment that Vogt and his colleagues didn’t have. So, the jury is still out on Gliese 581 g — the exoplanet encyclopaedia has it listed as "unconfirmed."
However, habitable planets aren’t always the ones that we can learn the most from. Our catalogue (yes, there is an actual catalogue) of exoplanets is rapidly expanding and the vast majority of planets we’ve found are nothing like Earth, but that doesn’t mean they’re not interesting. In fact, these "abnormal" discoveries could actually tell us more about our own solar system than dozens of Earth-like exoplanets ever could.
This hot Jupiter orbits in the opposite direction to the rotation of its star. Image credit: ESO/L. Calçada
One of these weird but revealing finds is the unusual way that some "hot Jupiters" orbit their parent star. Hot Jupiters are exoplanets about the same size as (or bigger than) their namesake that are found very close to their parent star — they are never further away than half the distance between the Earth and the Sun, but can be as close as only one tenth of this distance. It is thought that hot Jupiters must have moved into their positions after they were formed, because there would not have been enough material close to the star for gas giants to have formed there.
But their proximity to their star isn’t the odd thing about this subset of exoplanets. About half of the hot Jupiters discovered in a survey done by SuperWASP (Wide Angle Search for Planets) were orbiting in a different plane of rotation to their star, and six were orbiting in the opposite direction to the rotation of the star. Now, this may not sound like groundbreaking news to most people, but when you compare it to our solar system — where all the major bodies orbit in the same direction the Sun rotates — it starts to look a little odd. The most widely accepted theory about the formation of the solar system is based on this observation, so finding other planetary systems that work in a different way to our own could turn this theory on its head.
Composite image of Beta Pictoris and its planet. Image credit: ESO/A.-M. Lagrange et al.
Beta pictoris is a star just over 63 light years away from Earth. It is less than twice the mass of the Sun, but nearly nine times as bright, and holds a planet that doesn’t quite fit in with our current knowledge of planet formation. The planet is around eight times the mass of Jupiter was confirmed to orbit it in late 2009, after a first sighting in 2003. It is classed as a super-Jupiter because of its size and known as Beta Pictoris b, and was originally detected by scientists using the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in 2003. At that point, they couldn’t verify that it was bound by gravity to the star — there was still a chance that it was simply a star in the background. But new observations six years later confirmed their original suspicion that it was in fact a planet.
At only 10 million years old, Beta Pictoris is a very young star in astronomical terms. This doesn’t leave much time for its planet to have formed. In fact, scientists believe that Beta Pictoris b formed in just 2 million years. This is much quicker than we’d expect, meaning that we might not know as much as we’d like to think about planet formation.
Artists impression of HIP 13044 and its planet. Image credit: ESO/L. Calçada
Another out-of-the-ordinary discovery that could teach us something about planet formation is an exoplanet of extragalactic origin. The planet, that orbits a star known as HIP 13044 two thousand light years away, is the first exoplanet discovered that is believed to have formed in another galaxy. HIP 13044 is in the Helmi stream, which is all that remains of a galaxy that collided with the Milky Way at some point between 6 and 9 billion years ago. The Helmi stream itself was only discovered in 1999 when astronomers noticed that some of the stars in the sky were moving in a different way to the rest. They worked out that these stars must have come from somewhere outside our galaxy — and now the main suspect is a dwarf galaxy that was dragged into the Milky Way when they collided billions of years ago.
The planet formed before the collision, and has had a pretty tough life. Its star HIP 13044 is much further along its life than our Sun and has already passed the red giant stage of evolution, meaning that at some point in the past it will have run out of hydrogen to fuse and swelled up to many times its previous size. As the planet orbits very close to the star, when the star grew it will have probably engulfed the planet in the process. But somehow the planet survived this and is still there today for us to detect.
This is all pretty amazing stuff — but what can a discovery that is so out of the ordinary tell us about the rest of the universe? Well, here comes the conundrum: all the stars in the Helmi Stream are very old and have only small amounts of heavy elements, such as metals. This is because these heavy elements are only created when a star reaches the end of its life and goes supernova, so older stars created in the early universe when there had been fewer supernovae have less of these elements.
Up until now, we have found many more planets around stars with high levels of heavy elements. In fact, the number of exoplanets discovered around a star "rises rapidly" with the level of heavy elements in the star. Our theories of planet formation tell us that heavy elements are needed to form planets. The planet orbiting HIP 13044, and any others we might discover around low metallicity stars, could mean we have to rethink how planets form.
These are just three in a long list of out-of-the-ordinary exoplanet finds. In fact, it may turn out that systems like these turn out to be more normal than they appear. Who’s to say we’re not the odd ones? And even if they do turn out to be anomalies, these discoveries and other non-Earth-like exoplanets waiting to be found have shown that they are anything but dull, and can come in very useful when we need to test our theories. So the next time an all singing, all dancing, potentially life-supporting exoplanet hits the news, spare a thought for the unlikely heroes of the cosmos that are quietly shaping our ideas about how the universe works.
Steven S. Vogt, R. Paul Butler, Eugenio J. Rivera, Nader Haghighipour, Gregory W. Henry, & Michael H. Williamson. (2010) The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. arXiv: 1009.5733v1
Amaury H. M. J. Triaud, Andrew Collier Cameron, Didier Queloz, David R. Anderson, Michaël Gillon, Leslie Hebb, Coel Hellier, Benoît Loeillet, Pierre F. Maxted, Michel Mayor. (2010) Spin-orbit angle measurements for six southern transiting planets; New insights into the dynamical origins of hot Jupiters. arXiv: 1008.2353v1
A.M. Lagrange, M. Bonnefoy, G. Chauvin, D. Apai, D. Ehrenreich, A. Boccaletti, D. Gratadour, D. Rouan, D. Mouillet, S. Lacour. (2010) A giant planet imaged in the disk of the young star Beta Pictoris. arXiv: 1006.3314v1
Johny Setiawan, Rainer J. Klement, Thomas Henning, Hans-Walter Rix, Boyke Rochau, Jens Rodmann, Tim Schulze-Hartung, MPIA Heidelberg, & ESTEC Noordwijk. (2010) A Giant Planet Around a Metal-poor Star of Extragalactic Origin. Science Express, 18 November 2010. arXiv: 1011.6376v1
About the Author: Kelly Oakes is a final year physics student at Imperial College London. When she’s not busy learning about the mysteries of the universe, she writes about them on her blog basic space and at LabSpaces. In her (very small amount of) spare time she enjoys drinking cocktails, cooking up vegetarian feasts and teaching herself how to knit.
The views expressed are those of the author and are not necessarily those of Scientific American.
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