Today's Kepler press conference found another small planet, Kepler 452b. It is 60% bigger than the Earth and orbits a yellow Sun. Kepler has delivered an exciting range of discoveries and will again deliver more in the future. Why would we need a Yellow Sun? Planets around Orange Suns are as exciting. 60% bigger than the Earth puts Kepler 452b on the limit of being rocky in the latest models (160% Earth-size seems to be the cutoff for rocky worlds, then they become Mini-Neptunes according to Leslie Rogers models at Chicago). We still use a radius below 2 Earth radii for rocky but that might change to 1.6 or below. If a planet is really hot - like Kepler 10c - then even planets with bigger radii can be rocky. They might even be exposed cores of evaporated Mini-Neptunes?
But the question remains - which of those worlds are more "Earth-like"? Any rocky planet in the Habitable Zone around another Sun is a fascinating world, still shrouded in mysteries. We have no way - yet - to say which one is better or worse. We'll need the planet's light for that. Planets that are thousands of lightyears away will keep their secrets for a long time. We won't be able to collect enough light even with the next generation of big telescopes.
To see in its light fingerprint if we can find the signatures for life or not. The work we do here at CSI. But the number of worlds we discover with potential is staggering. To collect their light, we'll need them to be closer than Kepler's targets. But NASA's new TESS mission is going to find exactly those worlds. Close enough to collect and read their light fingerprint for signature of life.
But today's Kepler discovery showed us another small worlds out there and lifts our veil a bit more, about what is around us in this beautiful universe.