Once we believed we lived in an absolutely typical planetary system. Around the Sun, small rocky planets huddle close in taking months or years to complete an orbit while huge and chilly gas giants slowly circle further out orbiting the Sun in decades or centuries. We expected that nature would use this template to build planetary systems across the Universe.
We were wrong.
Extra-solar planets, or exoplanets, vary enormously in size, mass, composition and every other parameter. Some take mere hours to orbit their stars, some take millennia. There are hot jupiters, mini-neptunes, chthonian planets, the list goes on. One well-established planetary type is the super earth, rocky planets heavier than Earth, perhaps as massive as ten Earth masses. Now we have a new addition to the bestiary of worlds, the “mega-earth”, a rocky planet with the mass of a gas giant.
The first mega-earth to be discovered is Kepler-10c which orbits a star lying about 560 light years from the Sun in the constellation Draco. The planet was found by a team led by Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA). This planet is estimated to be 17 times as heavy as Earth, making it about as massive as Neptune. Astronomers expected any planet as large as this should be a Jupiter-like gas giant world. Its strong gravity should have accumulated and held on to a thick hydrogen and helium atmosphere. Yet this planet appears to be an entirely solid world of rock and metal. It is so much bigger than any previously discovered super-earth planet that a new category had to be invented for it. As astronomers search the skies it is certain than many more mega-earth worlds like this will be added to the catalogues.
Kepler 10-c has been measured to be 2.3 times as wide as Earth, this is huge yet too small for it to be surrounded by a gas giant’s extended atmosphere. Existing planetary formation theories cannot fully explain how such a large, rocky world could form. What is more, this planet orbits an ancient star, believed to have formed about 10 billion years ago when the Universe was less than four billion years old. Heavy elements such as the iron and silicon needed to build rocky and metallic planets were rare then, hence finding so large a planet of this composition dating back to those distant days is even more surprising.
Kepler-10c orbits its star once every 45 days and is closer to the star than Mercury is to the Sun. It is safe to assume that the landscapes of Kepler-10c will be scorched wastelands. Surface gravity on Kepler-10c’s desolate plains will be a crushing three times that of Earth. Kepler 10-c has a sibling in the lava world Kepler-10b, a possible chthonian planet which whizzes around the parent star in a 20 hour orbit. The two planets’ star is very similar to our Sun, being a G-type but based on how close they are to the star, both planets are certainly barren of life as we know it.
Kepler-10c’s size was measured using the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands. The HARPS-N project is run by an international consortium which includes the Queens University of Belfast.
(Article by Colin Johnston, Science Education Director)