Since 1995 more than 500 planets have been discovered in orbit around distant stars. Most of these exoplanets are much larger than our Earth. However, smaller and smaller planets are now being found. Some of these are ‘super-Earths’, only a few times bigger than our world.
Giant planets that orbit close to their stars are much easier to find than small planets or worlds that orbit farther out. Not surprisingly, many of the known exoplanets are ‘hot Jupiters’ - gas giants with temperatures of more than 1,000 degrees Celsius. Most exoplanets are discovered by looking for wobbles in a star’s motion caused by the pull of a nearby planet. A few have shown up in images. One of these orbits the bright southern star Fomalhaut, and was imaged by the ESA-NASA Hubble Space Telescope.
A European spacecraft named COROT is searching for exoplanets that pass in front of stars. Such planets are normally impossible to see because they are hidden in the glare of their sun. COROT overcomes this problem by looking for a tiny dip in starlight caused by a planet slipping across the disk of its star.
Many of COROT’s discoveries have been hot Jupiters, but it has also detected some super-Earths, including the smallest planet yet found. Known as COROT-7b, the planet orbits very close to its star, zooming around it in less than one day. The alien world is less than twice as wide as Earth and less than 4 times its mass. This means it must be made of rock and metal, rather than gas.
COROT-7b was the first of many small, rocky planets yet to be discovered. It seems that such planets may form anywhere in the galaxy – and who knows how many support intelligent life?