In the rush to find planets orbiting other stars, a new method has been suggested. It won’t help us find Earth-like planets, but it may enable us to locate gas giants at an exceptionally early stage of their evolution, letting us see how they form.
The flood of data on new planets that has come from the Kepler Space Telescope, Doppler detection, and direct imaging has answered many of our questions about solar systems beyond our own, but raised plenty more. Most of the planets we have found are already fully formed, and tell us little about how their size and location came to be.
In the Astrophysical Journal Letters, Dr Ruobing Dong of Lawrence Berkeley National Laboratory reveals that the presence of spiral arms within a protoplanetary disk can tell us not only that a planet is present, but its size and location.
These disks are commonly seen around newly forming stars, and contain material that will become additional planets. Two such disks, around the stars SAO 206462 and MWC 758, contain arms that look a little like the spiral arms of galaxies like our own. Some other disks have fainter patterns that may be similar. Such spirals are expected in disks large enough to be gravitationally unstable, but Dong said these disks are far too small for that.
Dong wondered if these were created by the disturbance of planets on the move. The model confirmed his suspicion. When a planet orbits through a disk it clears gaps within it, a well-known phenomenon seen on a smaller scale in Saturn’s rings. Dong and his co-authors found it also creates a dense patch closer to the star, which gets smeared out through discrepancies in the orbit of the planet and dust, eventually taking on a spiral shape.
The finding offers us the opportunity to work backwards to locate planets hidden within disks. It should also help us understand why some giant planets form at distances similar to Saturn and Jupiter, but migrate in until they are closer to their star than Mercury is to the Sun – known as hot Jupiters.
Explaining this process could be important for the search for extraterrestrial life. One popular theory holds that advanced life requires not only an Earth-like planet, but a gas giant to protect against the rain of comets that would otherwise repeatedly press the restart button on evolution. If Jupiter-equivalents seldom survive in place, intelligent life may be rarer than we expect.
Moreover, another theory holds that friction from the disk would otherwise have caused Jupiter’s orbit to decay, like a satellite affected by the outer reaches of Earth’s atmosphere, had it not been counteracted by Saturn’s gravity. Seeing how planets and disks interact may enable us to see how well these ideas hold up in practice, and whether two giant guardians are required if a planet is to host intelligent life.