Exoplanet hunting is a relatively new field in science, and astronomers are still working out the kinks. So, while the discovery of a planet that doesn't quite fit any of the existing molds is surprising, it's not so surprising as to be unbelievable. Space, after all, is a really strange place.
The newest planetary class, says Jonathan Amos for the BBC, is the “mega-Earth.” This class is based on the planet Kepler-10c, a rocky planet twice as wide as our planet and 17 times heavier.
Thanks to exoplanetary researchers, we know that there are a range of different types of planets in the universe. There are rocky planets, like Earth, and gas giants, like Uranus and Saturn. There are also more exotic planets—hot Jupiters, carbon-rich “diamond” planets and iron-heavy “cannonball” planets.
These various types of planets are thought to form under different sets of conditions. If a planet's orbit is relatively short distance from its star, that planet is more likely to be a hot planet than a ball of ice, for instance. Planets above a certain mass become gas giants, while tiny planets are more likely to be barren and to lack an atmosphere.
A planet as big as Kepler-10c isn't supposed to be a rocky planet, says Amos: “Theorists had always thought that any planet that large would pull so much hydrogen on to itself that it would look more like a Neptune or a Jupiter.”
But, given its huge mass and its spatial size, Kepler-10c is way too dense to be a gas giant. Astronomers think that the planet is a big, dense, dry world made of highly-compressed rock and only a minor amount of water. Its size would make gravity overwhelming—this isn't a place you'd want to visit.