Why Gas Giants Bunch Up Together (It's Not Because They're Lonely)

It seems like every week astronomers are finding more evidence of order underlying the general chaos of the universe. Take planetary orbits within a solar system, for example. It might seem like Mars, Earth, and the rest all plopped into their current orbits relatively randomly, or that they’d be aligned evenly like grooves on a record.

New research, published today in Monthly Notices of the Royal Astronomical Society, suggests otherwise, offering an astrophysical explanation for why some orbits are far more popular than others. That means that planets are neither even distributed as they radiate outward from a star, nor do they arrive in their orbits by purely random chance.

“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 ‘pile-ups’ of planets at particular locations,” study author Ilaria Pascucci, an assistant professor at the University of Arizona, said.

Pascucci and co-author Richard Alexander of the University of Leicester found that high energy radiation, produced by young Sun-like stars, likely sliced through the clouds of gas and dust that make up protoplanetary disks, the precursors to fully-formed planets. It’s this action of splitting the protoplanetary disks up that creates planets’ orbits.

The question that’s remained unexplained is why, as has been observed frequently in the widening search for exoplanets, giant planets like Jupiter and Saturn tend to bunch up close to each other. Pascucci and Alexander’s findings suggest that, as a growing young star sucks in material from a protoplanetary disk, there’s a sweet spot where material heated by the star floats off into space, a process they call photo-evaporation.

“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.”

All planets held in orbit by a star’s gravity tend to spiral in towards the star over time. However, this gap described by the researchers, which tends to be found between one and two astronomical units (one AU is the approximate distance from Earth to the Sun) from a young star, acts as a barrier for gas giants, which tend to clump up just outside the dead ring.

The pair used the ALICE High Performance Computing Facility at the University of Leicester to simulate the process of a protoplanetary disk getting split apart by the process of photo-evaporation.

“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.

Their models showed that the photo-evaporation process would indeed help cause the planetary bunching that has already been observed.

“The planets either stop right before or behind the gap, creating a pile-up,” 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.”

As the search for exoplanets continues, knowing how planets form and space themselves is crucial to pinpointing quality candidates for the next Earth. Plus, it’s truly fascinating to find out that planets are neither scattered throughout solar systems like paint flicked off a brush.

Follow Derek Mead on Twitter.

• Lonely Worlds
• The Odds Have Us Finding A New Earth By Next Spring
• To Make Planetary Classification Harder, Just Add Water