Giant Exoplanet Hunters: Look for Debris Disks

There’s no map
showing all the billions of exoplanets hiding
in our galaxy — they’re so
distant and faint compared to
their stars, it’s hard to find them. Now,
astronomers hunting
for new worlds have established a possible signpost for
giant
exoplanets.

A new study finds
that giant exoplanets that orbit far from
their stars are more likely to be
found around young stars
that have a disk of dust and debris than those without
disks.
The study, published in The Astronomical Journal, focused on
planets more
than five times the mass of Jupiter. This study
is the largest to date of stars
with dusty debris disks, and
has found the best evidence yet that giant planets
are
responsible for keeping that material in check.

“Our
research is important for how future missions will plan
which stars to
observe,” said Tiffany Meshkat, lead author and
assistant research
scientist at IPAC/Caltech in Pasadena,
California. Meshkat worked on this study
as a postdoctoral
researcher at NASA’s Jet Propulsion Laboratory in Pasadena.
“Many
planets that have been found through direct imaging have
been in systems that had
debris disks, and now we know the
dust could be indicators of undiscovered
worlds.”

Astronomers
found the likelihood of finding long-period giant
planets is nine times greater
for stars with debris disks than
stars without disks. Caltech graduate student
Marta Bryan
performed the statistical analysis that determined this result.

Researchers
combined data from 130 single-star systems with
debris disks detected by NASA’s
Spitzer Space Telescope, and
compared them with 277 stars that do not appear to
host disks.
The two star groups were between a few million and 1 billion
years
old. Of the 130 stars, 100 were previously scanned for
exoplanets. As part of
this study, researchers followed up on
the other 30 using the W. M. Keck
Observatory in Hawaii and
the European Southern Observatory’s Very Large
Telescope in
Chile. They did not detect any new planets in those 30
systems,
but the additional data helped characterize the
abundance of planets in systems
with disks.

The research
does not directly resolve why the giant
exoplanets would cause debris disks to
form. Study authors
suggest the massive gravity of giant planets causes small
bodies called planetesimals to collide violently, rather than
form proper
planets, and remain in orbit as part of a disk.

“It’s
possible we don’t find small planets in these systems
because, early on, these
massive bodies destroyed the building
blocks of rocky planets, sending them
smashing into each other
at high speeds instead of gently combining,” said
co-author
Dimitri Mawet, a Caltech associate professor of astronomy and a
JPL
senior research scientist.

On the other
hand, giant exoplanets are easier to detect than
rocky planets, and it is
possible that there are some in these
systems that have not yet been found.

Our own solar
system is home to gas giants responsible for
making “debris belts” —
the asteroid belt between Mars and
Jupiter, shaped by Jupiter, and the Kuiper
Belt, shaped by
Neptune. Many of the systems Meshkat and Mawet studied also
have two belts, but they are also much younger than ours — up
to 1 billion
years old, compared to our system’s present age
of 4.5 billion years. The youth
of these systems partly
explains why they contain much more dust — resulting
from the
collisions of small bodies — than ours does.

One system
discussed in the study is Beta Pictoris, which has
been directly imaged from
ground-based telescopes. This system
has a debris disk, comets and one
confirmed exoplanet. In
fact, scientists predicted this planet’s existence well
before
it was confirmed, based on the presence and structure of the
prominent
disk.

In a different
scenario, the presence of two dust belts in a
single debris disk suggests there
are likely more planets in
the system whose gravity maintains these belts, as
is the case
in the HR8799 system of four giant planets. The gravitational
forces of giant planets nudge passing comets inward toward the
star, which
could mimic the period of our solar system’s
history about 4 billion years ago
known as the Late Heavy
Bombardment. Scientists think that during that period, the
migration of Jupiter, Saturn, Uranus and Neptune deflected dust
and small
bodies into the Kuiper and asteroid belts we see
today. When the Sun was young,
there would have been a lot
more dust in our solar system as well.

“By
showing astronomers where future missions such as NASA’s
James Webb Space
Telescope have their best chance to find
giant exoplanets, this research paves
the way to future
discoveries,” said Karl Stapelfeldt of JPL, chief
scientist of
NASA’s Exoplanet Exploration Program Office and study
co-author.

For more
information about exoplanets, visit:

https://exoplanets.nasa.gov

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, CA
818-354-6425
elizabeth.landau@jpl.nasa.gov

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