If you’ve been closely following the latest research on extrasolar planets on Astrobites, you might wonder why so much discussion is focused on the sizes or radii of planets. After all, what we’re really looking for in the search for Earth-like planets are exoplanets with compositions similar to Earth. Big is fine, as long as it’s rocky and not gaseous! New results published by Howard et al. point to what may be the most Earth-like extrasolar planet yet discovered, with respect to composition.
At issue is the Kepler spacecraft, which has provided astronomers with a bonanza of extrasolar planet discoveries. Kepler assesses planets’ radii at the same time it discovers them, by measuring the fraction of light blocked by the planet as it transits in front of its parent star. To measure mass, we instead need very high resolution spectroscopy of the star to watch as it wobbles in its orbit with the planet via the Doppler effect.
Howard et al. used this technique to measure the mass of planet Kepler-78b - what they found is a planet which is likely (see the gray probability distribution and red 1σ uncertainty ellipse) just a little bigger and more massive than Earth and Venus (green triangles). That makes it the lowest mass of all exoplanets (red circles) ever found to be consistent with a rocky density (lines trace theoretical models)! Future observations that fill in this plot with more exoplanets will allow us to explore the full diversity of planetary compositions in the universe.