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The more planetary systems astronomers discovered, the more our own solar system looked like an oddball. Exoplanets – at least the ones big enough for us to see – tended to be bigger than Earth, with tight orbits that took them much closer to their host stars. In multi-planet systems, these orbits tended to be much closer together than they are in our solar system. For instance, the star known as Kepler-11 has six planets closer to it than Venus is to the sun. Why does our solar system look so different? Astrophysicists Konstantin Batygin of Caltech and Greg Laughlin...

Astronomers using NASA's Hubble Space Telescope have photographed one of the smallest objects ever seen around a normal star beyond our Sun. Weighing in at 12 times the mass of Jupiter, the object is small enough to be a planet. The conundrum is that it's also large enough to be a brown dwarf, a failed star.

Luhman and his colleagues used the Hubble Space Telescope to photograph an object, about 10 Jupiter masses, orbiting the star CHXR 73. The orbiting object's mass alone would typically identify it as a heavy planet spawned from a disk that once surrounded this young star. However, the newfound object lies about five times as far from CHXR 73 as Pluto's average distance from the sun. Theory suggests that a gas-and-dust disk isn't likely to contain enough material that far from a star to make a planet... Distinguishing between brown dwarfs and planets is important, says Luhman. A brown dwarf could...