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Planets pile up
in popular orbits

Posted: 21 March 2012

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Computer simulations suggest that high-energy radiation from baby Sun-like stars carve out gaps in their proto-planetary discs that act as barricades, forcing giant planets to pile up in preferred orbits.

"In proto-planetary discs we see several competing processes: accretion, which slowly drains the disc material onto the star; photo-evaporation, which blows disc material off into space; and the formation of planets, which grow from the cold dust and gas in these discs," explains Richard Alexander of the University of Leicester. "Towards the end of the disc's lifetime photo-evaporation overcomes accretion, opening a gap at 1-2AU (astronomical unit, where 1AU is equal to the separation of the Earth and the Sun) for Sun-like stars."

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 planets in certain orbits. Image: NASA/JPL-Caltech. 

The location of the gap varies depending on the mass of the star; for lower mass stars photo-evaporation opens gaps closer to the star, while the gaps in discs around more massive stars open further out. The gaps act as barricades, forcing planets into certain orbits while leaving a deficit elsewhere in the disc.

"Our study focused mainly on stars like the Sun, but as we find more planets around stars of different masses we expect to see pile-ups and deserts at different locations," says Alexander. "We believe that photo-evaporation does occur in all proto-planetary discs, but it may well be more efficient in some systems than others because some stars emit more high-energy radiation than others. It's the competition between these different processes that leads to the diverse range of exoplanets we see."

Alexander and colleague Ilaria Pascucci of the Lunar and Planetary Laboratory, who presented the new results at the 43rd Lunar and Planetary Science Conference in Texas earlier this week, used the ALICE High Performance Computing Facility at the University of Leicester to simulate proto-planetary discs undergoing accretion of material to the central star along with the effects of photo-evaporation. They explored a variety of developing solar systems with various combinations of giant planets at different locations and different stages in time and found that as well as mimicking observations of planets migrating inwards before settling on a stable orbit, once a giant planet encounters a gap cleared by photo-evaporation, it stops dead.

"The planets either stop right before or behind the gap, creating a pile-up," says Pascucci, adding that planets that form much earlier than the gap carved by photo-evaporation move inward and likely fall onto the star. "The local concentration of planets leaves behind regions elsewhere in the disc that are devoid of any planets. This uneven distribution is exactly what we see in many newly discovered solar systems, and 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."