http://www.sciencenews.org | March 12, 2022 13L. CALÇADA/ESOATOM & COSMOSHot Jupiters may be kicked into place
Celestial neighbors can send giant exoplanets toward their sunsBY KEN CROSWELL
Strange giant planets known as hot
Jupiters, which orbit close to their suns,
got kicked onto their peculiar paths by
nearby planets and stars, a new study
finds.
After analyzing the orbits of dozens
of hot Jupiters, a team of astronomers
found a way to catch giant planets in the
process of getting uncomfortably close
to their stars. The analysis, published
in the Feb. 20 Astrophysical Journal
Letters, pins the blame for the weird
worlds on gravitational kicks from other
massive objects orbiting the same star,
many of which destroyed themselves in
the process.
“It’s a pretty dramatic way to create
your hot Jupiters,” says Malena Rice, an
astrophysicist at Yale University.
Hot Jupiters have long been mysteri-
ous. They orbit very close to their stars,
whirling around in a few days or less. In
contrast, all the giant planets in our solar
system lie at vast distances from the sun.
To explain the odd planets, astrono-
mers have proposed three main ideas
(SN: 4/28/18 & 5/12/18, p. 28). Perhaps
the hot Jupiters formed next to their
stars and stayed put, or maybe they
started off farther out and then slowlyCircling close to its star,
a type of planet known as
a hot Jupiter (illustrated)
can acquire its peculiar
orbit after another
planet or star
catapults it inward.spiraled inward. In either of those
cases, the planets should have circular
orbits aligned with their stars’ equators,
because the worlds acquired their paths
from the material in the protoplanetary
disks that gave birth to them.
The new study, though, favors a
third idea: Gravitational interactions
with another giant planet
or a companion star first
hurl a Jupiter-sized planet
onto a highly elliptical and
inclined orbit that brings
the planet close to its star.
In some cases, the planet
even revolves the wrong
way around its star, oppo-
site the way it spins.
In this scenario, every time the tossed
planet sweeps past its sun, the star’s
gravity robs the planet of orbital energy.
This shrinks the orbit, gradually making
it more circular and less inclined, until
the planet becomes a hot Jupiter on a
small, circular orbit, realigned to be in
the same plane as the star’s equator.
Stars usually circularize a planet’s
orbit before they realign it, and cool stars
realign an orbit faster than warm stars do.
So Rice and colleagues looked for rela-
tionships between the shapes and tilts ofthe orbits of several dozen hot Jupiters
that orbit stars of different temperatures.
Generally, the team found that the hot
Jupiters around cool stars tend to be on
well-aligned, circular orbits, whereas
the hot Jupiters around warm stars are
often on orbits that are elongated and
off- kilter. Put another way, many of the
orbits around warm stars haven’t yet
had time to settle down into their final
size and orientation. These orbits still
bear the marks of having been shaped
by gravitational run-ins with neighbor-
ing bodies in the system, the
team concludes.
It’s a “simple, elegant
argument,” says astrophysi-
cist David Martin of Ohio
State University in Colum-
bus. “They’re presenting
the evidence in a new way
that helps strengthen” the
idea that other massive objects in the
same solar system produce hot Jupiters.
Martin suspects this theory probably
explains the majority of these planets.
But it means that innumerable giant
worlds have suffered terrible fates. Some
of the planets that hurled their brethren
close to their stars ended up plunging
into those same stars themselves, Rice
says. And many other planets got ejected
from their solar systems altogether, so
today these wayward worlds wander the
deep freeze of interstellar space, far from
the light of any sun. sHot Jupiters
orbit very
close to their
stars, whirling
around in a few
days or less.hot-jupiter.indd 13hot-jupiter.indd 13 2/23/22 12:15 PM2/23/22 12:15 PM