Astronomers can detect an extrasolar planet by watching for the characteristic
dimming of a star as the planet's disk passes over, as shown in this graphic.
European astronomers say they have found a "super-Earth" that's less than twice as wide as our planet, but up to 11 times more massive and hellishly hot. It might turn out to be the smallest Earthlike planet yet discovered beyond our solar system - depending on how you define "smallest" and "Earthlike."
Discovery of the planet, known as COROT-Exo-7b, was announced today at a Paris symposium focusing on findings from Europe's Corot planet-hunting satellite, which was launched a little more than two years ago. The researchers are still working on a scientific paper that will lay out the details - but the planet's vital statistics already have set off an extrasolar buzz.
More than 300 planets have been detected beyond our solar system - but the most commonly used method for detecting them (known as the radial-velocity method) works best with big planets like Jupiter rather than small planets like Earth. Corot uses a different method, which looks for the telltale dimming of light as a planet's disk crosses its parent star. This transit method is how researchers detected COROT-Exo-7b, circling a sunlike star about 457 light-years from Earth. (Let's just call the planet Exo-7b from now on, OK?)
Even for transiting planets, Exo-7b is unusual: Just a couple of weeks ago, we were gushing over a transiting Super-Neptune that was 4.7 times as wide as Earth. Exo-7b is somewhere around 1.75 times as wide as Earth. It orbits its parent star once every 20 hours, at a distance that is about a sixtieth of our distance from the sun. That would put the temperature on Exo-7b around 1,800 to 2,700 degrees Fahrenheit (1,000 to 1,500 degrees Celsius).
Although the European Space Agency's news release says Exo-7b is the "smallest terrestrial planet ever detected outside the solar system," it's not yet clear how it really ranks among the full range of extrasolar worlds. The transit method used by Corot is great for measuring how wide a planet is, but other methods have to be used to figure out how massive it is.
We know for sure it's not the absolute smallest: That distinction belongs to the weird pulsar planets first found more than 15 years ago. And even if you're talking about Earth-scale worlds that circle normal stars, there are a few other contenders for the "smallest planet" title.
"It is the smallest-mass exoplanet found to date that transits, but other hot super-Earths have been found that do not transit but have lower masses (given the ambiguity in the orbital inclination)," the Carnegie Institution of Washington's Alan Boss, one of the world's foremost experts on planetary formation, told me in an e-mail.
For example, there's Gliese 876 d (minimum mass, 5.9 Earth masses; best guess, 7.5 Earths), another small world around HD 181433 (7.5 Earths), and still another around HD 69830 (11 Earths), Boss said. These planets were detected using the radial-velocity method, so their diameters haven't yet been nailed down precisely.
Responding to the questions raised about Exo-7b's mass, the researchers told me that today's estimate of 11 Earth masses, based on radial-velocity measurements, was merely an upper limit. "It may be as small as half that," Malcolm Fridlund, ESA's Corot project scientist, said in an e-mail. "We hope to know within a few weeks."
Daniel Rouan, a researcher at the Observatoire de Paris Lesia, said a smaller mass would be in line with the model for a planet with Exo-7b's dimensions.
Fire or ice?
Looking beyond the numbers, Exo-7b would be an exotic - if not exactly comfortable - place to visit. The researchers speculate that the planet might be rocky like Earth, and covered in liquid lava. Or it could be made up of water and rock in equal amounts, and possess a water-vapor atmosphere.
Wait a minute ... water? Wouldn't any water on Exo-7b boil off into space?
"It's even beyond the point of boiling: It's in the state called super-critical water, which is neither a gas nor a liquid," Rouan wrote. "However, vapor is its best approximation. Note that this possibility of having water on this planet is just a viable hypothesis, not the most probably at present. A more precise response in a few weeks or months probably."
Fridlund said the water might even exist as ice, if the conditions were just right. "If the rotation of the planet is bound (like our moon is), and it turns the same side toward its star at all times, the other hemisphere may be very cold, and water could exist as ice on the surface."
Boss said Exo-7b could have been an ice giant that migrated inward from the cold part of its solar system and melted into a water world, but he thought it was more likely to be a rocky planet. "Most likely these hot super-Earths formed inside their gas giants, in much the same way that Earth, Venus, Mars and Mercury did," he said.
Best is yet to come
No matter where you think Exo-7b fits in the planetary scale, everyone agrees that smaller finds are on the way. Planet hunters are particularly looking forward the launch of NASA's Kepler spacecraft, now set for March 5. Kepler and Corot use similar strategies for detecting planetary transits, but they look at different areas of the sky.
"Obviously, Kepler will have a better overall performance because of its larger surface (more flux), its larger field (more stars) and its longer survey of a given field," Rouan wrote. "True Earthlike candidates should be found. On the other hand, it will not be easy for Kepler to confirm its Earthlike planet candidates, because the ground-based instruments to do that do not really exist yet in the Northern Hemisphere, at least at the beginning of the mission. Confirmation by ground-based observations is absolutely mandatory with the transit technique."
Boss, who is part of Kepler's science team, said today's announcement was "hopefully a harbinger of more to come."
"There is a lot more for Kepler to do," he told me. "This by no means skims the cream away from Kepler's upcoming feast!"
Update for 5:40 p.m. ET: The last time we talked about extrasolar planets, I added a note calling attention to a New Scientist report claiming that scientists had downsized their estimate for the mass of the smallest known exoplanet orbiting a normal star. Since then, folks in the know have reported that New Scientist got it wrong, and that the planet known as MOA-2007-BLG-192Lb still has a mass estimated at 3.3 times that of Earth (with a large uncertainty factor). In any case, the planet has to be included as a contender for the title of smallest super-Earth.
The Corot team's findings are to appear in a paper titled "Transiting Exoplanets From the CoRoT Space Mission VII. COROT-Exo-7b: The First Super-Earth With Radius Characterized," by A. Leger, D. Rouan, J. Schneider, R. Alonso, B. Samuel, E. Guenther, M. Deleuil, H.J. Deeg, M. Fridlund et al., to be submitted to the journal Astronomy and Astrophysics.
To learn more about the search for extrasolar planets, just go ahead and search for "extrasolar" planets. You can also check out our interactive graphic on "Other Worlds," as well as this list of the 10 most intriguing extrasolar planets (which might have to be updated one of these days).