Cambridge, MA — Of the first 100 stars found to harbor planets, more than
30 stars host a Jupiter-sized world in an orbit smaller than Mercury’s,
whizzing around its star in a matter of days (as opposed to our solar system
where Jupiter takes 12 years to orbit the Sun). Such close orbits result
from a race between a nascent gas giant and a newborn star. In the October
10, 2003, issue of The Astrophysical Journal Letters, astronomers Myron
Lecar and Dimitar Sasselov showed what influences this race. They found that
planet formation is a contest, where a growing planet must fight for
survival lest it be swallowed by the star that initially nurtured it.

“The endgame is a race between the star and its giant planet,” says
Sasselov. “In some systems, the planet wins and survives, but in other
systems, the planet loses the race and is eaten by the star.”

Although Jupiter-sized worlds have been found orbiting incredibly close to
their parent stars, such giant planets could not have formed in their
current locations. The oven-like heat of the nearby star and dearth of raw
materials would have prevented any large planet from coalescing. “It’s a
lousy neighborhood to form gas giants,” says Lecar. “But we find a lot of
Jupiter-sized planets in such neighborhoods. Explaining how they got there
is a challenge.”

Theorists calculate that so-called “hot Jupiters” must form farther out in
the disk of gas and dust surrounding the new star and then migrate inward. A
challenge is to halt the planet’s migration before it spirals into the star.

A Jupiter-like world’s migration is powered by the disk material outside the
planet’s orbit. The outer protoplanetary disk inexorably pushes the planet
inward, even as the planet grows by accreting that outer material. Lecar and
Sasselov showed that a planet can win its race to avoid destruction by
eating the outer disk before the star eats it.

Our solar system differs from the “hot Jupiter” systems in that the race
must have ended quite early. Jupiter migrated for only a short distance
before consuming the material between it and the infant Saturn, bringing the
King of Planets to a halt. If the protoplanetary disk that birthed our solar
system had contained more matter, Jupiter might have lost the race. Then it
and the inner planets, including Earth, would have spiraled into the Sun.

“If Jupiter goes, they all go,” says Lecar.

“It’s too early to say that our solar system is rare, because it’s easier to
find ‘hot Jupiter’ systems with current detection techniques,” says
Sasselov. “But we certainly can say we’re fortunate that Jupiter’s migration
stopped early. Otherwise, the Earth would have been destroyed, leaving a
barren solar system devoid of life.”

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists, organized
into six research divisions, study the origin, evolution and ultimate fate
of the universe.