The gas giant exoplanets we have discovered so far seem very different from Jupiter, Saturn, Uranus, and Neptune. Now, however, a new model suggests a way for gas giant planets to form, and in the process indicates there’s a large population of exoplanets that we have yet to find.
The new approach, published in the Astrophysical Journal, is proposed by Alan Boss of the Carnegie Institute for Science and hopes to clarify where gas giants can form around a star.
These exogiants tend to either orbit very close to their stars and have incredibly high temperatures or they are found very far away from the star, at least dozens of times the distance between the Earth and the Sun – the so-called astronomical unit (AU).
The discovery of exoplanets are biased by the limited means we have to look for them, but the lack of similar gas giants in the Solar System has made astronomers question whether all Jupiter-like planets in all the galaxies form in the same way.
Two ideas have been put forward to explain gas giants. The first suggests they form like rocky planets by slowly acquiring material as they move around their star. The second one, called disk instability, suggests they form rapidly due to instabilities in the protoplanetary disk that surrounds newborn stars.
The first idea can’t explain gas giants that are too far from their star, and the second one can’t explain gas giants that are closer than 20 AU. Jupiter is located 5.4 AU from the Sun and Neptune is 30. Either proposed scenario is not ideal to explain our local gas giants.
“Given the existence of gas giant planets on such wide orbits, disk instability or something similar must be involved in the creation of at least some exoplanets,” Boss said in a statement. “However, whether or not this method could create closer-orbiting gas giant planets remains unanswered.”
By including different cooling mechanisms, Boss suggests that disk instabilities are actually possible between 6 AU and 16 AU under certain conditions. Clearly, the existence of Jupiter and the other gas planets tells us that there must be a way, even if we don’t know what it is yet.
Astronomers have also suggested that planets might migrate inwards over time. This could explain the current Solar System, although it requires a more complex scenario than just “gas giants can form at almost every distance”.
An upcoming NASA mission, the Wide Field Infrared Survey Telescope, will hopefully provide the necessary observations to test Boss' model – and maybe in the process find these missing gas giants.