Astronomers agree that planets are born in protoplanetary disks—rings of
dust and gas that surround young, newborn stars. While hundreds of these
disks have been spotted throughout the universe, observations of actual
planetary birth and formation have proved difficult within these
environments.
Now, astronomers at the Center for Astrophysics | Harvard & Smithsonian
have developed a new way to detect these elusive newborn planets—and with
it, "smoking gun" evidence of a small Neptune or Saturn-like planet lurking
in a disk. The results are described today in The Astrophysical Journal
Letters.
"Directly detecting young planets is very challenging and has so far only
been successful in one or two cases," says Feng Long, a postdoctoral fellow
at the Center for Astrophysics who led the new study. "The planets are
always too faint for us to see because they're embedded in thick layers of
gas and dust."
Scientists instead must hunt for clues to infer a planet is developing
beneath the dust.
"In the past few years, we've seen many structures pop up on disks that we
think are caused by a planet's presence, but it could be caused by something
else, too," Long says. "We need new techniques to look at and support that a
planet is there."
For her study, Long decided to re-examine a protoplanetary disk known as
LkCa 15. Located 518 light years away, the disk sits in the Taurus
constellation on the sky. Scientists previously reported evidence for planet
formation in the disk using observations with the ALMA Observatory.
Long dove into new high-resolution ALMA data on LkCa 15, obtained primarily
in 2019, and discovered two faint features that had not previously been
detected.
About 42 astronomical units out from the star—or 42 times the distance Earth
is from the Sun—Long discovered a dusty ring with two separate and bright
bunches of material orbiting within it. The material took the shape of a
small clump and a larger arc, and were separated by 120 degrees.
Long examined the scenario with computer models to figure out what was
causing the buildup of material and learned that their size and locations
matched the model for the presence of a planet.
"This arc and clump are separated by about 120 degrees," she says. "That
degree of separation doesn't just happen—it's important mathematically."
Long points to positions in space known as Lagrange points, where two bodies
in motion—such as a star and orbiting planet—produce enhanced regions of
attraction around them where matter may accumulate.
"We're seeing that this material is not just floating around freely, it's
stable and has a preference where it wants to be located based on physics
and the objects involved," Long explains.
In this case, the arc and clump of material Long detected are located at the
L4 and L5 Lagrange points. Hidden 60 degrees between them is a small planet
causing the accumulation of dust at points L4 and L5.
The results show the planet is roughly the size of Neptune or Saturn, and
around one to three million years old. (That's relatively young when it
comes to planets.)
Directly imaging the small, newborn planet may not be possible any time soon
due to technology constraints, but Long believes further ALMA observations
of LkCa 15 can provide additional evidence supporting her planetary
discovery.
She also hopes her new approach for detecting planets—with material
preferentially accumulating at Lagrange points—will be utilized in the
future by astronomers.
"I do hope this method can be widely adopted in the future," she says. "The
only caveat is that this requires very deep data as the signal is weak."
Reference:
Feng Long et al, ALMA Detection of Dust Trapping around Lagrangian Points in
the LkCa 15 Disk, The Astrophysical Journal Letters (2022).
DOI: 10.3847/2041-8213/ac8b10
Tags:
Space & Astrophysics