Astronomers detect signs of an atmosphere stripped from a planet in a
giant impact
Such planetary smashups are likely common in young solar systems, but
they haven't been directly observed

Date:
October 20, 2021
Source:
Massachusetts Institute of Technology
Summary:
A team has discovered evidence of a giant impact in the nearby HD
17255 star system, in which an Earth-sized terrestrial planet and
a smaller impactor likely collided at least 200,000 years ago,
stripping off part of one planet's atmosphere.



FULL STORY ========================================================================== Young planetary systems generally experience extreme growing pains, as
infant bodies collide and fuse to form progressively larger planets. In
our own solar system, the Earth and moon are thought to be products of
this type of giant impact. Astronomers surmise that such smashups should
be commonplace in early systems, but they have been difficult to observe
around other stars.


==========================================================================
Now astronomers at MIT, the National University of Ireland at Galway,
Cambridge University, and elsewhere have discovered evidence of a
giant impact that occurred in a nearby star system, just 95 light years
from Earth. The star, named HD 172555, is about 23 million years old,
and scientists have suspected that its dust bears traces of a recent
collision.

The MIT-led team has observed further evidence of a giant impact around
the star. They determined that the collision likely occurred between a
roughly Earth-sized terrestrial planet and a smaller impactor at least
200,000 years ago, at speeds of 10 kilometers per second, or more than
22,000 miles per hour.

Crucially, they detected gas indicating that such a high-speed impact
likely blew away part of the larger planet's atmosphere -- a dramatic
event that would explain the observed gas and dust around the star. The findings, appearing today in Nature, represent the first detection of
its kind.

"This is the first time we've detected this phenomenon, of a stripped protoplanetary atmosphere in a giant impact," says lead author Tajana Schneiderman, a graduate student in MIT's Department of Earth, Atmospheric
and Planetary Sciences. "Everyone is interested in observing a giant
impact because we expect them to be common, but we don't have evidence
in a lot of systems for it. Now we have additional insight into these dynamics." A clear signal The star HD 172555 has been an object of
intrigue among astronomers because of the unusual composition of its
dust. Observations in recent years have shown that the star's dust
contains large amounts of unusual minerals, in grains that are much
finer than astronomers would expect for a typical stellar debris disk.



========================================================================== "Because of these two factors, HD 172555 has been thought to be this
weird system," Schneiderman says.

She and her colleagues wondered what the gas might reveal about the
system's impact history. They looked to data taken by ALMA, the Atacama
Large Millimeter Array in Chile, which comprises 66 radio telescopes, the spacing of which can be adjusted to increase or decrease the resolution of their images. The team looked through data from the ALMA public archive, seeking signs of carbon monoxide around nearby stars.

"When people want to study gas in debris disks, carbon monoxide is
typically the brightest, and thus the easiest to find," Schneiderman
says. "So, we looked at the carbon monoxide data for HD 172555 again
because it was an interesting system." In the aftermath With a careful reanalysis, the team was able to detect carbon monoxide around the
star. When they measured its abundance, they found the gas amounted to
20 percent of the carbon monoxide found in Venus' atmosphere. They also observed that the gas was circling in large amounts, surprisingly close
to the star, at about 10 astronomical units, or 10 times the distance
between the Earth and the sun.



==========================================================================
"The presence of carbon monoxide this close requires some explanation," Schneiderman says.

That's because carbon monoxide is typically vulnerable to
photodissociation, a process in which a star's photons break down and
destroy the molecule. At close range, there would typically be very
little carbon monoxide so close to a star.

So, the group tested various scenarios to explain the gas' abundant,
close-in appearance.

They quickly ruled out a scenario in which the gas arose from the debris
of a newly formed star, as well as one in which the gas was produced
by a close-in belt of icy asteroids. They also considered a scenario in
which the gas was emitted by many icy comets streaking in from a far-out asteroid belt, similar to our own Kuiper belt. But the data didn't quite
fit this scenario either. The last scenario the team considered was that
the gas was a remnant of a giant impact.

"Of all the scenarios, it's the only one that can explain all the
features of the data," Schneiderman says. "In systems of this age,
we expect there to be giant impacts, and we expect giant impacts to
be really quite common. The timescales work out, the age works out,
and the morphological and compositional constraints work out. The only plausible process that could produce carbon monoxide in this system in
this context is a giant impact." The team estimates that the gas was
released from a giant impact that occurred at least 200,000 years ago
-- recent enough that the star would not have had time to completely
destroy the gas. Based on the gas' abundance, the impact was likely
massive, involving two proto-planets, likely comparable in size to
the Earth. The impact was so great that it likely blew off part of one
planet's atmosphere, in the form of the gas that the team observed today.

"Now there's a possibility for future work beyond this system,"
Schneiderman says. "We are showing that, if you find carbon monoxide in
a place and morphology consistent with a giant impact, it provides a
new avenue for looking for giant impacts and understanding how debris
behaves in the aftermath." This research was supported, in part, by
the ALMA Observatory and the Simons Foundation.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Schneiderman, T., Matra`, L., Jackson, A.P. et al. Carbon
monoxide gas
produced by a giant impact in the inner region of a young
system. Nature, 2021 DOI: 10.1038/s41586-021-03872-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211020135916.htm

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