Report: Young Giant Planet Offers Clues to Formation of Exotic Worlds

Jupiter-size
planets orbiting close to their stars have upended ideas about how giant
planets form. Finding young members of this planet class could help answer key
questions.

For most of human history our understanding of how planets
form and evolve was based on the eight (or nine) planets in our solar system.
But over the last 25 years, the discovery of more than 4,000 exoplanets, or
planets outside our solar system, changed all that.

Among the most intriguing of
these distant worlds is a class of exoplanets called hot Jupiters. Similar in
size to Jupiter, these gas-dominated planets orbit extremely close to their
parent stars, circling them in as few as 18 hours. We have nothing like this in
our own solar system, where the closest planets to the Sun are rocky and
orbiting much farther away. The questions about hot Jupiters are as big as the
planets themselves: Do they form close to their stars or farther away before migrating
inward? And if these giants do
migrate, what would that reveal about the history of the planets in our own
solar system?

To answer those questions, scientists will need to observe many
of these hot giants very early in their formation. Now, a new study in the Astronomical
Journal reports on the detection of the exoplanet HIP 67522 b, which
appears to be the youngest hot Jupiter ever found. It orbits a well-studied
star that is about 17 million years old, meaning the hot Jupiter is likely only
a few million years younger, whereas most known hot Jupiters are more than a
billion years old. The planet takes about seven days to orbit its star, which has
a mass similar to the Sun’s. Located only about 490 light-years from Earth, HIP
67522 b is about 10 times the diameter of Earth, or close to that of Jupiter.
Its size strongly indicates that it is a gas-dominated planet.

HIP 67522 b was identified as a planet candidate by NASA’s
Transiting Exoplanet Survey Satellite (TESS), which detects planets
via the transit method: Scientists look for small dips in the brightness of a
star, indicating that an orbiting planet has passed between the observer and
the star. But young stars tend to have a lot of dark splotches on their
surfaces – starspots, also called sunspots when they appear on the Sun – that
can look similar to transiting planets. So scientists used data from NASA’s
recently retired infrared observatory, the Spitzer
Space Telescope, to confirm that the transit signal was from a
planet and not a starspot. (Other methods
of exoplanet detection have yielded hints at the presence of
even younger hot Jupiters, but none have been confirmed.)

The discovery offers hope for finding more young hot
Jupiters and learning more about how planets form throughout the universe –
even right here at home.

“We can learn a lot about our solar system and its
history by studying the planets and other things orbiting the Sun,” said
Aaron Rizzuto, an exoplanet scientist at the University of Texas at Austin who
led the study. “But we will never know how unique or how common our solar
system is unless we’re out there looking for exoplanets. Exoplanet scientists are
finding out how our solar system fits in the bigger picture of planet formation
in the universe.”

Migrating
Giants?

There are three main hypotheses for how hot Jupiters get so
close to their parent stars. One is that they simply form there and stay put.
But it’s hard to imagine planets forming in such an intense environment. Not
only would the scorching heat vaporize most materials, but young stars
frequently erupt with massive explosions and stellar winds, potentially
dispersing any newly emerging planets.

It seems more likely that gas giants develop farther from
their parent star, past a boundary called the snow line, where it’s cool enough
for ice and other solid materials to form. Jupiter-like planets are composed
almost entirely of gas, but they contain solid cores. It would be easier for
those cores to form past the snow line, where frozen materials could cling
together like a growing snowball.

The other two hypotheses assume this is the case, and that
hot Jupiters then wander closer to their stars. But what would be the
cause and timing of the migration?

One idea posits that hot Jupiters begin their journey early
in the planetary system’s history while the star is still surrounded by the disk
of gas and dust from which both it and the planet formed. In this
scenario, the gravity of the disk interacting with the mass of the planet could
interrupt the gas giant’s orbit and cause it to migrate inward.

The third hypothesis maintains that hot Jupiters get close
to their star later, when the gravity of other planets around the star can drive
the migration. The fact that HIP 67522 b is already so close to its star so
early after its formation indicates that this third hypothesis probably doesn’t
apply in this case. But one young hot Jupiter isn’t enough to settle the debate
on how they all form.

“Scientists would like to know if there is
a dominant mechanism that forms most hot Jupiters,” said Yasuhiro Hasegawa,
an astrophysicist specializing in planet formation at NASA’s Jet Propulsion
Laboratory who was not involved in the study. “In the community right now
there is no clear consensus about which formation hypothesis is most important
for reproducing the population we have observed. The discovery of this young
hot Jupiter is exciting, but it’s only a hint at the answer. To solve the
mystery, we will need more.”

TESS is a NASA Astrophysics Explorer mission led and operated by
MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight
Center. Additional partners include Northrop Grumman, based in Falls Church,
Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s
Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore.
More than a dozen universities, research institutes and observatories worldwide
are participants in the mission.

NASA’s Spitzer Space Telescope was
retired on Jan. 30, 2020. Science data continues
to be analyzed by the science community via the Spitzer data archive located at
the Infrared Science Archive housed at IPAC at Caltech in Pasadena, California.
JPL managed Spitzer mission operations for NASA’s Science Mission Directorate
in Washington. Science operations were conducted at the Spitzer Science Center
at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin Space
in Littleton, Colorado. Caltech manages JPL for NASA.

Rizzuto is a 51 Pegasi b Fellow
which is funded by the Heising-Simons Foundation.

For more information about TESS visit:

https://www.nasa.gov/tess-transiting-exoplanet-survey-satellite