Back in February, astronomers announced the discovery of an alien planet dubbed CoRoT-7b. The discovery came from the French-built satellite CoRoT (short for Convection, Rotation, and planetary Transits), which detected this distant world repeatedly crossing in front of a 12th-magnitude star about 400 light-years away in the constellation Monoceros. The star, cataloged as CoRoT-7 or TYC 4799-1733-1, is a little cooler and dimmer than the Sun, at spectral type G9.
The planet orbits at an astoundingly close distance of 1½ million miles (2½ million km) from the host star, completing a "year" in just 20.4 hours. By measuring the minuscule amount of stellar dimming during transits, the discovery team determined that CoRoT-7b was 1.8 times Earth's diameter — and the ensuing press release trumpeted this find as the smallest exoplanet yet.
Well, I hate to be picky, but it wasn't and it isn't. According to the authoritative Extrasolar Planets Encyclopaedia, at least five (likely six) known planets beyond our solar system have lower masses than CoRoT-7b. Three of them circle the pulsar PSR 1257+12 and were announced so long ago (1992) that modern exoplanet hunters tend to forget they exist. But I digress . . .
There's new news this week about CoRoT-7b. While astronomers knew its diameter right away, from the depth of the transit dimmings accurately measured by CoRoT, they didn't know the planet's mass. Until now. A team led by Didier Queloz gathered 70 hours of observations using the HARPS spectrograph on the European Southern Observatory's 3.6-meter telescope in Chile. HARPS measured tiny wobbles of the star toward and away from Earth, the "reflex motion" induced by the gravitational pull of the little world whirling around it.
After crunching the numbers (and removing the effects of starspots rotating around the star itself every 23 days), Queloz deduced that CoRoT-7b must be a "super-Earth" with 4.8 plus or minus 0.8 Earth masses. Folding in its diameter, it has an overall density of 5.6 plus or minus 1.3 grams per cubic centimeter. That's about the same as Earth's dsnsity. For the first time, astronomers have found a world beyond the solar system that has to be rocky.
"We are coming tantalizingly close to reaching the ultimate goal of detecting a true Earthlike planet," notes CoRoT official Malcolm Fridlund in a press release issued today. (A press release from the European Southern Observatory is here.)
All that said, CoRoT-7b is anything but habitable. S&T's May 2009 cover story calls it the "Planet from Hell." It's so close to its star that one hemisphere must be tidally locked to permanently face the star. This eternal day side must be red hot and molten. The eternal night side could be unearthly cold — depending on whether an atmosphere exists to distribute heat globally.
So I guess astronomers can now fairly claim that CoRoT-7b is the smallest exoplanet whose diameter and mass have both been measured.
There's a bonus. All that scrutiny by HARPS turned up a second super-Earth around the star. CoRoT-7c is slightly farther out, orbits in 3.7 days, and weighs in at about 8 Earths. It does not transit the star's face.
The team's findings will appear in the October issue of Astronomy & Astrophysics. But you can get a peek now by downloading the online preprint here.