Early Planet Formation Triggers Planet Offspring

In a paper to be published in the December issue of Nature, Philip Armitage and Brad Hansen, formerly of University of Toronto's Canadian Institute of Theoretical Astrophysics, studied how early planet formation triggered the formation of other planets in developing solar systems.

"We're suggesting that it's the mass of the disk that influences the formation of planetary systems," says Armitage. "If the disk is lightweight, planet formation occurs fairly slowly - over 10 million years or so - and the result could look something like our own solar system. For a heavyweight disk, more violent processes can occur more quickly and lead to a very different-looking system of planets."

Using computer simulations, the researchers tested how a massive planet the size of Jupiter would interact with a massive disk, 10 times larger than the disk thought to have given rise to our own solar system. They found the extra gravitational force from the planet would cause parts of the disk to collapse and fragment into other planets. The resulting planets would also be gigantic, but would be mostly gaseous rather than solid like that of Earth.

According to Armitage and Hansen, their research indicates that there is an upper limit to the amount by which planets can grow. If the planets formed close together, the planetary system would become violently unstable - some planets would be ejected from the system and the remaining ones would be left with eccentric orbits.

"The paper provides a new way to understand how multiple planets could form in a relatively short space of time, roughly the first million years after the birth of the solar system," says Hansen. "The rapid creation of additional planets will result in competition during planet growth and so may explain why there appears to be a maximum mass for planets around other stars."

Whether habitable Earth-like planets can form and survive in such harsh environments and allow life to develop and grow remains unknown, say the researchers.

"This work, along with other theoretical explanations of planetary systems, suggests that planet formation can sometimes involve violent and chaotic processes that are different from those of our own early solar system," says Armitage. "We now know that the existence of planets themselves are common. However, conditions suitable for forming habitable planets - at least ones like the Earth - could still be rare."

Armitage is currently completing post-doctoral work at the Max-Planck-Institut for Astrophysik in Germany. Hansen is a Hubble post-doctoral fellow at Princeton University in the United States.

CONTACT:
Janet Wong
U of T Public Affairs
(416) 978-6974
jf.wong@utoronto.ca