Meet Kepler-36b and c, a Felix and Oscar among extrasolar planets. Kepler-36b is suspected to be a rocky super-Earth, packing 4.5 Earth masses into a planet about 1.5 times Earth's size. Orbiting a scant 1.2 million miles away is Kepler-36c, a Neptune-size gas planet with 8 times more mass than Earth. The planets are so close that, to an astronaut orbiting Kepler-36b, 36c would appear twice the size of a full moon. Kepler-36b makes one trip around its sunlike star every 13.8 days, compared with once every 16.2 days for Kepler-36c. The system is about 1,200 light-years from Earth.
The pair provide an interesting test of theories about how solar systems evolve. Our solar system indicates that gas giants form beyond the "snow line," where water, methane, and other compounds containing hydrogen appear as ice. Inside the line, rocky planets form.
The question: What could bring this pair together? Were they both gas giants that migrated inward, with the innermost planet losing its gas first? Or did they form on different sides of the snow line, to be brought together through gravitational interactions with the disk of dust and gas from which they grew? Scientists are working on it.