Just two weeks after the confirmation of a planet that's within the habitable zone of a distant star, the Kepler team is back with the discovery of two Earth-sized planets orbiting in what is now a five-planet system (three other planets orbiting the star, Kepler-20, had been spotted earlier). Although these planets are much too hot to support liquid water, one of them (Kepler-20e) is the smallest exoplanet yet detected.

Kepler-20 was already a busy star system, with three small planets orbiting close in to the star: Kepler-20b is about twice the size of Earth and orbits once every 3.7 days; Kepler-20c is three times Earth's radius and orbits every 11 days; and Kepler-20d is 2.75 Earth radii with an orbit of 77.6 days. If that seems somewhat tightly packed, the new finds actually jam a couple more planets within the orbit of Kepler-20d. Kepler-20e has an orbit of six days, while Kepler-20f takes 19.6 days to orbit its host star.

Given that distance, it's possible to estimate the planets' radii based on the amount of light they block while transiting in front of their host star. And neither of these block very much at all. Kepler-20e results in a drop of only 82 parts-per-million in the light from its host star, which corresponds to a radius of 0.87R e . Kepler-20f is a bit larger, with a signal of 101ppm, placing its radius at roughly that of Earth's.

The fast orbits indicate that these planets are very close to their host star, which has a surface temperature of a toasty 5,500K, a few hundred K shy of our Sun's. That makes the planets correspondingly toasty. Kepler-20e is predicted to have a surface temperature of over 1,000K, and is close enough that any hydrogen atmosphere would have been heated off. Any water on its surface would have been boiled into vapor, broken down by UV exposure, and the resulting gasses also driven off.

Kepler-20f might have been able to hold onto its water if it had formed further out and then migrated inward to its current orbit. Its surface temperature is "only" 700K, and it's far enough from the host star that it could retain a water vapor atmosphere for several billion years.

Theoretical considerations based on what we know about planet formation suggest that both of these planets are rocky, and may even have a composition similar to Earth's. To confirm this, however, would require measuring the mass of the planets. Unfortunately, the best way to do this is to measure how much they pull their host star around as their orbits take them to opposite sides of Kepler-20. Their small size leads to correspondingly small Doppler shifts, however, and those are currently below our ability to detect. Improvements expected in our telescope hardware should allow us to do so within a few years, but we're stuck with theory for now.

The one caveat from all of this is that these signals haven't been confirmed with another piece of hardware, something the Kepler team normally requires before shifting something from the "candidate" to "confirmed" category. In this case, they have been able to call these candidates confirmed by showing that all the other possible sources of a signal are extremely unlikely. The host star shows no sign of having a companion brown dwarf or other dim star, based on the lack of any wobbles in its orbit. The chances that a second star in the same line of sight has a large planet that's producing these signals are very, very small.

The previous find of planets in this system also boost the odds that the new signal comes from a planet, since we already know that things are orbiting in the right plane to pass between Earth and Kepler-20. As a result, the authors conclude that these signals come from small planets with a greater than three sigma certainty (something that the Higgs hunters wouldn't find satisfying, but good enough for an astronomer).

NASA will be holding a press conference on the results shortly; we'll update this story if any additional information is provided during the event.

Nature, 2011. DOI: 10.1038/nature10780 (About DOIs).