Computer simulations have revealed a plausible explanation for a phenomenon that has puzzled astronomers: Rather than occupying orbits at regular distances from a star, giant gas planets similar to Jupiter and Saturn appear to prefer to occupy certain regions in mature solar systems while staying clear of others.
“Our results show that the final distribution of planets does not vary smoothly with distance from the star, but instead has clear ‘deserts’ — deficits of planets — and ‘pileups’ of planets at particular locations,” said Ilaria Pascucci from the University of Arizona in Tucson.
“Our models offer a plausible explanation for the pile-ups of giant planets observed recently detected in exoplanet surveys,” said Richard Alexander from the University of Leicester in the United Kingdom.
Alexander and Pascucci identified high-energy radiation from baby Sun-like stars as the likely force that carves gaps in protoplanetary disks, the clouds of gas and dust that swirl around young stars and provide the raw materials for planets. The gaps then act as barricades, corralling planets into certain orbits.
The exact locations of those gaps depend on the planets’ masses, but they generally occur in an area between 1 and 2 astronomical units from the star. One astronomical unit (AU) marks the average distance from Earth to the Sun.
According to conventional wisdom, a solar system starts out from a cloud of gas and dust. At the center of the prospective solar system, material clumps together, forming a young star. As the baby star grows, its gravitational force grows as well, and it attracts dust and gas from the surrounding cloud.
Accelerated by the growing gravitation of its star, the cloud spins faster and faster, and eventually flattens into what is called a protoplanetary disk. Once the bulk of the star’s mass has formed, it is still fed material by its protoplanetary disk, but at a much lower rate.
“For a long time, it was assumed that the process of accreting material from the disk onto the star was enough to explain the thinning of the protoplanetary disk over time,” Pascucci said. “Our new results suggest that there is another process at work that takes material out of the disk.”
That process, called photoevaporation, works by high-energy photons streaming out of the star and heating the dust and gas on the surface of the protoplanetary disk.
“The disk material that is very close to the star is very hot, but it is held in place by the star’s strong gravity,” Alexander said. “Further out in the disk where gravity is much weaker, the heated gas evaporates into space.”
Even further out in the disk, the radiation emanating from the star is not intense enough to heat the gas sufficiently to cause much evaporation. But at a distance between 1 and 2 AU, the balancing effects of gravitation and heat clear a gap, the researchers found.
While studying protoplanetary disks, Pascucci found that gas on the surface of the disk was gravitationally unbound and leaving the disk system via photoevaporation, as Alexander had previously predicted. “These were the first observations proving that photoevaporation does occur in real systems,” she said.
Encouraged by those findings, Alexander and Pascucci then used the ALICE High Performance Computing Facility at the University of Leicester to simulate protoplanetary disks undergoing accretion of material to the central star that took the effects of photoevaporation into account.
“We don’t yet know exactly where and when planets form around young stars, so our models considered developing solar systems with various combinations of giant planets at different locations and different stages in time,” Alexander said.
The experiments revealed that just as observations of real solar systems have shown, giant planets migrate inward before they finally settle on a stable orbit around their star. This happens because as the star draws in material from the protoplanetary disk, the planets are dragged along, like a celebrity caught in a crowd of fans.
However, the researchers discovered that once a giant planet encounters a gap cleared by photoevaporation, it stays put.
“The planets either stop right before or behind the gap, creating a pileup,” Pascucci said. “The local concentration of planets leaves behind regions elsewhere in the disk that are devoid of any planets. This uneven distribution is exactly what we see in many newly discovered solar systems.”
Once surveys for discovering extrasolar planet systems such as the Kepler mission become more sensitive to outer giant planets, Alexander and Pascucci expect to find more and more evidence for the pileup of giant planets around 1 AU.
“As we discover more exoplanets, we will be able to test these predictions in detail and learn more about the conditions under which planets form,” said Pascucci