Hot Jupiters may have formed through planetary billiards

were hot Jupiters , gas giants that orbit
their stars in days or even hours.
“The presence of hot Jupiters has been a
major surprise with planet-hunting, and
their existence has immediately challenged
planet-formation theory,” says Aaron
Boley of the University of British Columbia
in Vancouver, Canada.
Early theories suggested that these worlds
must have formed at least as far from
their stars as Earth is from our sun,
before moving inward.
But the influx of planets discovered with
NASA’s Kepler telescope challenged that
idea. Kepler has spotted large, rocky
planets called super-Earths and mini-
Neptunes orbiting near their stars. Such
planets should have gathered gas and dust
as they travelled inward, ultimately
becoming gas giants. The fact that they
remained rocky suggested they could
have formed closer to their stars.
Tightly packed
In addition, the telescope has turned up
systems where several rocky planets are
packed closely with their star, which
researchers call systems of tightly packed
inner planets (STIPs). Most of these
systems will eventually become unstable
and send their planets crashing into one
another .
If those collisions happened slowly
enough, the planets could stick together
and form the core of a new planet. And if
they happened before the material
around the star dissipated, which takes
about 10 million years, that core could
grab on to enough gas and dust to
become a hot Jupiter.
To test this idea, Boley and his colleagues
added instabilities to a computer model of
Kepler-11 , a system that contains six
rocky planets orbiting closer to their star
than Mercury does to the sun. Their
simulations produced several warm
Jupiters – gas giants just a bit further from
their stars than hot Jupiters. The team
attributed this difference to the particular
arrangement of Kepler-11’s planets, and
say different configurations should result
in the overheated gas giants.
This doesn’t mean that planets never
migrate in towards their stars, Boley adds
– but it might not be the dominant
method of building warm and hot
“It fits in really nicely with the idea of
STIPs becoming unstable,” says Kathryn
Volk of the University of Arizona in
Tucson. “This is a totally different way of
thinking about [hot Jupiter] formation.”