Giant exoplanet hunters: Look for debris disks

This artist’s rendering shows a large exoplanet causing
small bodies to collide in a disk of dust. Credit:
NASA/JPL-Caltech

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 are more likely to be found around young
stars that have a of dust and debris than those
without disks. The study, published in The Astronomical
Journal
, focused on 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 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 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 , 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 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 . 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 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.

Explore further:

Hubble images three debris disks around G-type stars

Journal reference: Astronomical
Journal

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