Planet Formation Model Indicates Earthlike Planets Might Be Common

Astrobiologists disagree about whether advanced life is common or rare in
our universe. But new research suggests that one thing is pretty certain –
if an Earthlike world with significant water is needed for advanced life to
evolve, there could be many candidates.

In 44 computer simulations of planet formation near a sun, astronomers found
that each simulation produced one to four Earthlike planets, including 11
so-called “habitable” planets about the same distance from their stars as
Earth is from our sun.

“Our simulations show a tremendous variety of planets. You can have planets
that are half the size of Earth and are very dry, like Mars, or you can have
planets like Earth, or you can have planets three times bigger than Earth,
with perhaps 10 times more water,” said Sean Raymond, a University of
Washington doctoral student in astronomy.

Raymond is the lead author of a paper detailing the simulation results that
has been accepted for publication in Icarus, the journal of the American
Astronomical Society’s Division for Planetary Sciences. Co-authors are
Thomas R. Quinn, a UW associate astronomy professor, and Jonathan Lunine, a
professor of planetary science and physics at the University of Arizona.

The simulations show that the amount of water on terrestrial, or Earthlike,
planets could be greatly influenced by outer gas giant planets like Jupiter.

“The more eccentric giant planet orbits result in drier terrestrial
planets,” Raymond said. “Conversely, more circular giant planet orbits mean
wetter terrestrial planets.”

In the case of our solar system, Jupiter’s orbit is slightly elliptical,
which could explain why Earth is 80 percent covered by oceans rather than
being bone dry or completely covered in water miles deep.

The findings are significant because of the discovery in recent years of a
large number of giant planets such as Jupiter and Saturn orbiting other
suns. The presence, and orbits, of those planets can be inferred from their
gravitational interaction with their parent stars and their effect on light
from those stars as seen from Earth.

It currently is impossible to detect Earthlike planets around other stars.
However, if results from the models are correct, there could be planets such
as ours around a number of other suns relatively close to our solar system.
A significant number of those planets are likely to be in the “habitable
zone,” the distance from a star at which the planet’s temperature will
maintain liquid water on the surface. Liquid water is thought to be a
requirement for life, so planets in a star’s habitable zone are ideal
candidates for life. It is unclear, however, whether those planets could
harbor more than simple microbial life.

The researchers note that their models represent the extremes of what is
possible in forming Earthlike planets rather than what is typical of planets
observed in our galaxy. For now, they said, it is unclear which approach is
more realistic.

Their goal is to understand what a system’s terrestrial planets will look
like if the characteristics of a system’s giant planets are known, Raymond
said.

Quinn noted that all of the giant planets detected so far have orbits that
carry them very close to their parent stars, so their orbits are completed
in a relatively short time and it is easier to observe them. The giant
planets observed close to their parent stars likely formed farther away and
then, because of gravitational forces, migrated closer.

But Quinn expects that giant planets will begin to be discovered farther
away from their suns as astronomers have more time to watch and are able to
observe gravitational effects during their longer orbits. He doubts such
planets will be found before they have completed whatever migration they
make toward their suns, because their orbits would be too irregular to
observe with any confidence.

“These simulations occur after their migration is over, after the orbits of
the gas giants have stabilized,” he said.

The research is supported by the National Aeronautics and Space
Administration’s Astrobiology Institute, its Planetary Atmospheres program,
and Intel Corp.