Study Sheds Light On Planetary Orbital Pile-Ups
March 20, 2012

Study Sheds Light On Planetary Orbital Pile-Ups

Researchers said they have found why some orbits seen in young solar systems seem to be more popular than others.

Astronomers have been puzzled as to why giant gas planets like Jupiter and Saturn appear to occupy certain regions in mature solar systems while staying clear of others.

Ilaria Pascucci, an assistant professor at the University of Arizona's Lunar and Planetary Laboratory, said the final distribution of planets does not vary smoothly with distance from the star, but has clear planet "deserts" and planet "pile-ups" in certain locations.

A solar system starts as a cloud of gas and dust, which clumps together to form a young star.  As the star grows, its gravitation force grows, attracting dust and gas from the surrounding cloud.

As the cloud spins faster from the growing gravitation of its star, it 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 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 in a press release. "Our new results suggest that there is another process at work that takes material out of the disk."

This process, known as photo-evaporation, 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," Richard Alexander of the University of Leicester in the United Kingdom said in a statement. "Further out in the disk where gravity is much weaker, the heated gas evaporates into space."

Further out in the disk, radiation emanating from the star is not intense enough to heat the gas sufficiently to cause much evaporation.  However, the researchers found that at a distance of between about 93 million miles and 186 million miles, the balancing efforts of gravitation and heat clear a gap.

Pascucci found that gas on the surface of the disk was gravitationally unbound and leaves the disk system through photoevaporation.

"These were the first observations proving that photoevaporation does occur in real systems," she said in a press release.

The researchers used ALICE High Performance Computing Facility at the University of Leicester to simulate protoplanetary discs undergoing accretion of material to the central star that took the effects of photo-evaporation 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.

They found that giant gas planets migrate inward before they finally settle on a stable orbit around their star.

They said this happens because as the star draws in material from the protoplanetary disk, the planets are dragged along.

The researchers discovered that once a giant planet encounters a gap cleared by photo-evaporation, it stays put.

"The planets either stop right before or behind the gap, creating a pile-up," Pascucci said in a press release. "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."

Pascucci presented the findings at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas on March 19. Results of the study are also to be published in the journal Monthly Notices of the Royal Astronomical Society.


Image Caption: Computer simulations suggest high-energy radiation from baby sun-like stars are likely to create gaps in young solar systems, leading to pile-ups of giant planets in certain orbits. (Illustration: NASA/JPL-Caltech)