The most Earth-like exoplanet known (for now) is really, really unearthly, at least by human standards. CoRoT-7b is the so-called "Planet from Hell" of our May issue cover story, which described its discovery and preliminary characteristics. This morning the same team of astronomers has released their latest, improved measurements of the object. You wouldn't want to live there, but it would sure offer spectacular vacation sites — if you were wearing a very good spacesuit.
CoRoT-7b transits across the face of its star, a yellow, 11.7-magnitude G9 dwarf about 500 light-years away in Monoceros. That's how the CoRoT satellite discovered it. The tiny amount of the star's light that the planet blocks while in transit tells its diameter (based on reasonable assumptions about the diameter of the star, CoRoT-7). The planet works out to have 1.8 times the diameter of Earth, a slight refinement of the 1.7 Earth diameters we reported in May.
The real news is its mass. By measuring the star's radial-velocity wobbles for a total of 70 hours, a team including veteran planet hunters Didier Queloz and Michel Mayor (Geneva Observatory) determined that the planet throws around the gravitational heft of 4.8 ± 0.8 Earths. This makes it among the lowest-mass exoplanets measured.
The diameter and mass together yield the object's density: 5.6 ± 1.3 grams per cubic centimeter, similar to Earth's density. So it's the first clearly rocky world among the exoplanet menagerie.
Here's the European Southern Observatory's press release.
For the full information, here's the researchers' paper.