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EI2GYB > ASTRO 13.09.21 11:01z 113 Lines 6328 Bytes #999 (0) @ WW
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Subj: Astronomers Detect Potential Moon-forming Disk around an Ex
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Astronomers Detect Potential Moon-forming Disk around an Exoplanet
Astronomers have made the first clear detection of a dusty disk surrounding an
exoplanet, which could eventually go on to form moons.
For the first time, astronomers have clearly detected a dusty disk around a
young giant planet that may go on to form moons. The results appear in the July
20th Astrophysical Journal Letters.
Disks of gas and dust left over from stellar formation can create circumstellar
disks, shrouding a newborn star in planet-making potential. Planets that form
carve their way through the dust to trace out rings and other structures,
perhaps gathering their own personal dusty disk, called a circumplanetary disk,
in the process.
Astronomers had previously found a circumstellar disk around PDS 70, a young
star nearly 400 light-years away in the constellation Centaurus, the Centaur.
Last year, they also confirmed the presence of two planets - a Jupiter-Saturn
pair, dubbed PDS 70b and c. These baby planets dwell in a cavity between two
rings of dust, one close to the star, the other farther out. Observers thought
they saw hints of a smaller disk around just PDS 70c, but they couldn't
distinguish it from the brighter stellar disk ring nearby.
Now, a team led by Myriam Benisty (University of Chile and University of
Grenoble, France) presents high-resolution data from the Atacama Large
Millimeter/submillimeter Array (ALMA) that clearly shows PDS 70c has a disk of
its own, separate from the larger, encompassing circumstellar disk. This giant
world is the first exoplanet to have a directly detected circumplanetary disk,
making this system "the textbook example of planet formation," according to
Sebastiaan Haffert (University of Arizona), who wasn't involved with the study.
Such a discovery confirms astronomers' theories about how moons and planets
form.A Disk Around PDS 70c
Circumplanetary disks are vital to the formation of an exoplanet, since they
control how much material the growing planet accumulates. Planetary disks also
set the budget for satellite formation, determining how much material will be
left over for moons to coalesce from.
Both PDS 70b and c are still gathering mass, albeit slowly, but only PDS 70c
unambiguously hosts its own disk. This planet, a few Jupiter masses at most,
resides 34 times farther from its star than Earth does from the Sun (1
astronomical unit, or a.u.) - that's a little farther than Neptune's orbit.
Even though the circumplanetary disk is distinct from the stellar disk's outer
ring in the data, ALMA could not resolve its structure, so Benisty's team is
still unclear just how far it extends or how massive it is. At its largest, PDS
70c's disk would be 1.2 a.u. in diameter, holding roughly 3 lunar masses (about
3% Earth's mass) of material.
Characteristics like mass are hard to pin down in part because the size of dust
particles - which is hard to determine - influences many aspects of a
circumplanetary disk, like how moon formation could happen.
PDS 70c's circumplanetary disk could make moons in different ways. Small dust
particles can get trapped in the disk, creating conditions just right so that
these particles stick together, forming pebbles, then rocks, then moons. On the
other hand, if larger particles get trapped, they can spontaneously congregate
into clumps, causing them to collapse into the beginnings of a satellite.
Of course, these are only a couple of ways to build a moon. "We know so little
about satellite formation," says Jason Wang (California Institute of
Technology), who has independently studied the PDS 70 system. He adds that
while this discovery does not solve all moon-related mysteries, "it is
certainly a stepping stone in understanding when and how efficiently satellites
form around planets."
Why Doesn't PDS 70b Have a Disk?
The other planet, PDS 70b, shows signs of tenuous dust near its orbit, but
nothing actually encircling the planet. Maybe the dust is trapped at one of the
gravitationally stable points along 70b's orbit, or maybe it's part of a stream
of material, linking each planet to the inner disk surrounding the star. Either
way, PDS 70b doesn't have the circumplanetary disk that its sibling shows off.
"The largest surprise is the non-detection of a circumplanetary disk around PDS
70b," says Haffert, who has also studied this system. "This probably means that
it already has completed its potential satellite system." He adds that even
though the process has obviously halted for 70b, any moons that do form around
these planets will sadly not be detectable with current technology.
Benisty's team also has a few theories for why one planet is hogging all the
goods. Since PDS 70b is closer to the star than its sibling, perhaps its realm
of influence is much smaller, meaning that any dust that would orbit PDS 70b is
instead pulled towards the star. Another explanation is that PDS 70c starves b
of dust: Circumplanetary dust must come from the outer, cooler ring of the
stellar disk, and since PDS 70c is closer to that region, it may catch what it
can and allow only a trickle of tiny dust particles to drift towards 70b.
Whatever the reason, Haffert and Wang agree with Benisty's collaboration that
this study presents the most compelling evidence yet for a bona fide
circumplanetary disk. With this new real-life laboratory to study growing
planets and moons, astronomers can construct a more complete picture of planet
formation.
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