Data from NASA's Kepler spacecraft have revealed two extrasolar planets with similar orbits around the same star but radically different densities — challenging planetary formation theories.
Joshua Carter at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, Eric Agol at the University of Washington in Seattle and their team found the two planets, which have orbital periods of 14 and 16 days. The inner planet has a density consistent with a rocky planet like Earth, whereas the outer one has the density of a gaseous planet, such as Neptune. This is puzzling to theorists who had expected gaseous planets to form much further out, as in the Solar System. When viewed from the Earth-like planet, the Neptune-like planet would be 2.5 times the size of the Moon.
Science http://dx.doi.org/10.1126/science.1223269 (2012)