The second theory is known as the gravitational collapse model, where the gas disc around the young star fragments and collapses directly into giant planets through its own gravity. This should form only large gas planets, and probably far out from the star, although you might be able to form rocky planets by this method if the high energy radiation from the star can evaporate the gas from the planet and leave behind the heavier rocky material.
“Most people favour the core accretion method for the formation of most planets,” says Dr Pete Wheatley at the University of Warwick, UK. However, many scientists now agree that the gravitational collapse model could be very important “especially when explaining multiple large planets a long way out from the star”.
Thanks to exoplanets the field is now paying more attention to other theories that have traditionally lacked widespread acceptance. One such idea is migration, in which Jupiter, Uranus and Neptune are possible candidate solar system immigrants. “They probably formed at longer distance then moved in,” says Adam Burrows at Princeton University, New Jersey. This would explain why hot Jupiters that are close to their parent star seem so odd to astronomers used to surveying our Solar System.
Other possibilities involve an early violent phase in a solar system’s formation, where planets start as unstable objects far from the parent star, and then settle in more stable orbits. Another theory to explain how rocky planets formed involves X-rays from a planet’s parent star heating a gas planet’s upper atmosphere so much that it evaporates, leaving behind the rocky core. “There are one or two planets very close to stars where we know this is likely to have happened,” says Wheatley, suggesting the event can be thought of like an extreme version of an aurora.
Certainly, the list of ideas considered plausible is only expanding. “Discovering so many new exoplanets hasn’t eliminated any theory of planet formation”, admits Burrows.
Perhaps a more profound aspect of exoplanet hunting is the possibility of finding a planet with environmental conditions similar to our own: an “Earth twin”. The hunt has thrown up plenty of potential candidates, although definitions can be misleading. There is no reason why life would be more likely to exist on a planet that is similar to our own than one with different characteristics. “Prejudices about our solar system slip into our terminology,” notes Bruce Macintosh, an astronomer at the Lawrence Livermore National Laboratory in northern California. Of all the planets discovered so far, a quarter are known as “super Earths”, defined as those with a mass substantially bigger than our own planet, but much smaller than the gas giants of the solar system. It’s unlikely that they bear much of a resemblance to our Earth.
Picking clues from the available data about what these planets are made of is no simple matter. What astronomers know depends on what they can observe. Nasa’s main planet-finding mission, Kepler, which launched in March 2009 is now defunct, leaving vast amounts of data still to be trawled. Direct imaging of exoplanets has only recently become possible.
Several projects that should reveal the densities and the atmospheric characteristics of hundreds of exoplanets are on the horizon. Nasa has The American Gemini Direct Imager, to be based in Chile, and the Transiting Exoplanets Survey Satellite, due to launch in 2017. The UK has a Next Generation Transit Survey in the works, with a focus on discovering objects of Neptune’s size and smaller around bright stars.