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The odd couple
●
THEY are out there, and they are unlike anything previously discovered in the galaxy. Dubbed “super-Earths” because of their size, these planets are the first predominantly rocky worlds found outside our solar system. Big deal? You bet. The discovery of this new class of planet takes us a significant step closer to finding other Earth-like worlds that might be suitable for life. It also sheds new light on the physics of planetary formation. Indeed, the race to find Earth’s bigger cousins is hotting up: a French-led space telescope called Corot is scheduled to launch later this month, with the promise of detecting more of these mysterious worlds. What is most interesting is not that superEarths are big, but that they are relatively small. Since the discovery in 1995 of a Jupiter-sized planet around the star 51 Pegasi, astronomers have located more than 200 extrasolar planets. Until last year, all had been classified as gas giants like Jupiter, Saturn, Uranus and Neptune – most of them hundreds of times the Earth’s mass. (A few were originally mislabelled super-Earths but have since been judged Neptune-class.) Now, thanks to more sensitive instruments and better analytical techniques, researchers are detecting smaller planets which, crucially, should be more Earth-like. So far we know of just two super-Earths (see Diagram). They are less than half the mass of any previously known extrasolar planet, which is why researchers are confident they have crossed the threshold from gas giants to
rocky worlds. Although no telescope is yet powerful enough to reveal their surfaces, astronomers have plenty of ideas about what they might look like. The first was discovered in early 2005 by a team led by Geoff Marcy at the University of California, Berkeley. “Gliese 876 d” is roughly 7.5 times the Earth’s mass and orbits a small star 15 light years away from us. Researchers detected the planet and determined its mass by measuring the tiny wobble that its gravity induced on its star. It is so close to the star, completing an orbit in just two Earth days, that it is probably hotter than an oven, with a surface dominated by Promethean volcanic activity. Its atmosphere might be a thick blanket, producing a perpetual orange glow. A second super-Earth was found in August 2005 and announced in January this year by a group led by Jean-Philippe Beaulieu of the Institute for Astrophysics of Paris, France. “OGLE-05-BLG-390Lb” is smaller and much farther away, orbiting a star near the centre of our galaxy roughly 22,000 light years away. It was detected when it drifted in front of another more distant star; the planet’s gravitational field focused more of the star’s light towards Earth, thus boosting its brightness – a phenomenon known as microlensing, which gives an estimate of the planet’s mass. This super-Earth orbits its star at such a great distance that it is almost certainly a frozen ball of rock and ice, like a gigantic Pluto. Its atmosphere is probably frozen solid to the surface, and with crushing
The odd couple
They are unlike anything we’ve ever seen before. Stuart Clark meets the latest additions to the family of Earth-like worlds
40 | NewScientist | 16 December 2006
061216_F_SuperEarths.indd 40
gravity strong enough to level mountains, it should be a smooth sphere. Now that we know super-Earths exist, researchers are scratching their heads to understand how they form. At first sight there is nothing like them in our solar system. But Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says they may be right under our noses, hidden inside the gas giants. “The super-Earths are probably a class of planet similar to what Jupiter and Saturn looked like four-and-a-half billion years ago,” he says.
Supersize me At that time, according to the leading theory of planetary formation, the planets had recently formed from a disc of dust and gas surrounding the nascent sun, but the gas giants had yet to accumulate their colossal atmospheres. Out beyond Mars, far from the sun, low temperatures allowed volatile molecules to condense into solid material, boosting the size of the resultant planets to super-Earth proportions. This gave each planet enough gravity to attract a substantial atmosphere of hydrogen and helium. If that picture is correct, our solar system gave birth to four super-Earths that eventually became the gas giants we see today. “Neptune and Uranus are the closest thing we have to super-Earths in the solar system,” says Kenyon. “If you stripped them of their gaseous envelopes, you’d be left with a ball of rock and ice containing about five times the mass of the Earth.” So what about the super-Earths around other stars? “For me that’s the key question: what conditions make super-Earths in preference to Jupiters?” Kenyon adds. A major clue may be their location. The two known super-Earths orbit red dwarf stars, or M dwarfs as astronomers call them. These stars are cooler and smaller than the sun, typically just a few tenths of a solar mass, and there is a dearth of gas giants around such stars. “So far we find almost no Jupiter-mass planets among the M dwarf stars,” says Paul Butler of the Carnegie Institution in Washington DC, one of the discoverers of the first super-Earth. This may be because M dwarfs are unable to hold as much planet-forming material in orbit as stars the size of the sun, so even if a planet makes it to super-Earth size there will be little extra gas and it will stop growing. “Whether you get a Jupiter or a super-Earth must depend on the amount of mass available,” Beaulieu says. Alternatively, it could all be down to the speed with which a super-Earth forms. Researchers know from spectroscopic measurements of young stars that as planets form, radiation from the star purges the disc www.newscientist.com
11/12/06 9:07:34 am
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of its gas. “If it takes too long to get a superEarth, there might not be enough gas left for it to go on and become a gas giant,” Kenyon says. To pin down the answer, astronomers need to start trawling for more super-Earths. That’s where the Corot mission comes in. Set to blast off from Kazakhstan, the 30-centimetreaperture telescope will be the first satellite dedicated to the search for extrasolar planets. Sensitive enough to detect rocky worlds just a few times the mass of Earth, it will monitor more than 100,000 stars, watching for any tiny dip in brightness caused by a planet passing across them. The duration of the dip gives the orbital period of the planet, while the time from onset of eclipse to full dimming gives an estimate of the planet’s diameter. Follow-up observations from the ground will determine the mass of the planet by measuring the star’s gravitational wobble. The results will take years to analyse, but the result should be worth the wait. “In four years we will have a complete picture of the super-Earths,” says Beaulieu. If they appear around M dwarfs only, for instance, then location is likely to be the key to their development, whereas if they appear around other types of stars, then how fast they form is probably the decider. Might Corot even discover an Earth-sized planet? “In an ideal case,” cautions Malcolm Fridlund, ESA’s Corot project scientist.
“Super-Earths may be hiding right under our noses, inside gas giants like Jupiter”
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To be detected, such a planet would need to orbit its star very quickly, so researchers could watch it cross the star dozens of times to boost confidence in their observations. Whatever happens, more help is on the way: NASA’s Kepler mission, a kind of super-Corot with a 95-centimetre telescope, is scheduled for launch in late 2008. The most intriguing aspect of super-Earths is that they could in principle be home to extraterrestrial life. Of the two known supers, one is too hot and the other too cold to support life. The most spectacular result of the upcoming missions might be to find a “Goldilocks” planet: one that is just right ●
&$&'7K
www.newscientist.com
061216_F_SuperEarths.indd 41
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Stuart Clark is a science journalist based in the UK 16 December 2006 | NewScientist | 41
11/12/06 11:26:15 am
THEY are out there, and they are unlike anything previously discovered in the galaxy. Dubbed “super-Earths” because of their size, these planets are the first predominantly rocky worlds found outside our solar system. Big deal? You bet. The discovery of this new class of planet takes us a significant step closer to finding other Earth-like worlds that might be suitable for life. It also sheds new light on the physics of planetary formation. Indeed, the race to find Earth’s bigger cousins is hotting up: a French-led space telescope called Corot is scheduled to launch later this month, with the promise of detecting more of these mysterious worlds. What is most interesting is not that superEarths are big, but that they are relatively small. Since the discovery in 1995 of a Jupiter-sized planet around the star 51 Pegasi, astronomers have located more than 200 extrasolar planets. Until last year, all had been classified as gas giants like Jupiter, Saturn, Uranus and Neptune – most of them hundreds of times the Earth’s mass. (A few were originally mislabelled super-Earths but have since been judged Neptune-class.) Now, thanks to more sensitive instruments and better analytical techniques, researchers are detecting smaller planets which, crucially, should be more Earth-like. So far we know of just two super-Earths (see Diagram). They are less than half the mass of any previously known extrasolar planet, which is why researchers are confident they have crossed the threshold from gas giants to
rocky worlds. Although no telescope is yet powerful enough to reveal their surfaces, astronomers have plenty of ideas about what they might look like. The first was discovered in early 2005 by a team led by Geoff Marcy at the University of California, Berkeley. “Gliese 876 d” is roughly 7.5 times the Earth’s mass and orbits a small star 15 light years away from us. Researchers detected the planet and determined its mass by measuring the tiny wobble that its gravity induced on its star. It is so close to the star, completing an orbit in just two Earth days, that it is probably hotter than an oven, with a surface dominated by Promethean volcanic activity. Its atmosphere might be a thick blanket, producing a perpetual orange glow. A second super-Earth was found in August 2005 and announced in January this year by a group led by Jean-Philippe Beaulieu of the Institute for Astrophysics of Paris, France. “OGLE-05-BLG-390Lb” is smaller and much farther away, orbiting a star near the centre of our galaxy roughly 22,000 light years away. It was detected when it drifted in front of another more distant star; the planet’s gravitational field focused more of the star’s light towards Earth, thus boosting its brightness – a phenomenon known as microlensing, which gives an estimate of the planet’s mass. This super-Earth orbits its star at such a great distance that it is almost certainly a frozen ball of rock and ice, like a gigantic Pluto. Its atmosphere is probably frozen solid to the surface, and with crushing
The odd couple
They are unlike anything we’ve ever seen before. Stuart Clark meets the latest additions to the family of Earth-like worlds
40 | NewScientist | 16 December 2006
061216_F_SuperEarths.indd 40
gravity strong enough to level mountains, it should be a smooth sphere. Now that we know super-Earths exist, researchers are scratching their heads to understand how they form. At first sight there is nothing like them in our solar system. But Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says they may be right under our noses, hidden inside the gas giants. “The super-Earths are probably a class of planet similar to what Jupiter and Saturn looked like four-and-a-half billion years ago,” he says.
Supersize me At that time, according to the leading theory of planetary formation, the planets had recently formed from a disc of dust and gas surrounding the nascent sun, but the gas giants had yet to accumulate their colossal atmospheres. Out beyond Mars, far from the sun, low temperatures allowed volatile molecules to condense into solid material, boosting the size of the resultant planets to super-Earth proportions. This gave each planet enough gravity to attract a substantial atmosphere of hydrogen and helium. If that picture is correct, our solar system gave birth to four super-Earths that eventually became the gas giants we see today. “Neptune and Uranus are the closest thing we have to super-Earths in the solar system,” says Kenyon. “If you stripped them of their gaseous envelopes, you’d be left with a ball of rock and ice containing about five times the mass of the Earth.” So what about the super-Earths around other stars? “For me that’s the key question: what conditions make super-Earths in preference to Jupiters?” Kenyon adds. A major clue may be their location. The two known super-Earths orbit red dwarf stars, or M dwarfs as astronomers call them. These stars are cooler and smaller than the sun, typically just a few tenths of a solar mass, and there is a dearth of gas giants around such stars. “So far we find almost no Jupiter-mass planets among the M dwarf stars,” says Paul Butler of the Carnegie Institution in Washington DC, one of the discoverers of the first super-Earth. This may be because M dwarfs are unable to hold as much planet-forming material in orbit as stars the size of the sun, so even if a planet makes it to super-Earth size there will be little extra gas and it will stop growing. “Whether you get a Jupiter or a super-Earth must depend on the amount of mass available,” Beaulieu says. Alternatively, it could all be down to the speed with which a super-Earth forms. Researchers know from spectroscopic measurements of young stars that as planets form, radiation from the star purges the disc www.newscientist.com
11/12/06 9:07:34 am
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of its gas. “If it takes too long to get a superEarth, there might not be enough gas left for it to go on and become a gas giant,” Kenyon says. To pin down the answer, astronomers need to start trawling for more super-Earths. That’s where the Corot mission comes in. Set to blast off from Kazakhstan, the 30-centimetreaperture telescope will be the first satellite dedicated to the search for extrasolar planets. Sensitive enough to detect rocky worlds just a few times the mass of Earth, it will monitor more than 100,000 stars, watching for any tiny dip in brightness caused by a planet passing across them. The duration of the dip gives the orbital period of the planet, while the time from onset of eclipse to full dimming gives an estimate of the planet’s diameter. Follow-up observations from the ground will determine the mass of the planet by measuring the star’s gravitational wobble. The results will take years to analyse, but the result should be worth the wait. “In four years we will have a complete picture of the super-Earths,” says Beaulieu. If they appear around M dwarfs only, for instance, then location is likely to be the key to their development, whereas if they appear around other types of stars, then how fast they form is probably the decider. Might Corot even discover an Earth-sized planet? “In an ideal case,” cautions Malcolm Fridlund, ESA’s Corot project scientist.
“Super-Earths may be hiding right under our noses, inside gas giants like Jupiter”
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To be detected, such a planet would need to orbit its star very quickly, so researchers could watch it cross the star dozens of times to boost confidence in their observations. Whatever happens, more help is on the way: NASA’s Kepler mission, a kind of super-Corot with a 95-centimetre telescope, is scheduled for launch in late 2008. The most intriguing aspect of super-Earths is that they could in principle be home to extraterrestrial life. Of the two known supers, one is too hot and the other too cold to support life. The most spectacular result of the upcoming missions might be to find a “Goldilocks” planet: one that is just right ●
&$&'7K
www.newscientist.com
061216_F_SuperEarths.indd 41
&$'7K
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'&7K
Stuart Clark is a science journalist based in the UK 16 December 2006 | NewScientist | 41
11/12/06 11:26:15 am