An enormous comet—approximately 80 miles across, more than twice the width
of Rhode Island—is heading our way at 22,000 miles per hour from the edge of
the solar system. Fortunately, it will never get closer than 1 billion miles
from the sun, which is slightly farther from Earth than Saturn; that will be
in 2031.
Comets, among the oldest objects in the solar system, are icy bodies that
were unceremoniously tossed out of the solar system in a gravitational
pinball game among the massive outer planets, said David Jewitt. The UCLA
professor of planetary science and astronomy co-authored a new study of the
comet in the Astrophysical Journal Letters. The evicted comets took up
residence in the Oort cloud, a vast reservoir of far-flung comets encircling
the solar system out to many billions of miles into deep space, he said.
A typical comet's spectacular multimillion-mile-long tail, which makes it
look like a skyrocket, belies the fact that the source at the heart of the
fireworks is a solid nucleus of ice mixed with dust—essentially a dirty
snowball. This huge one, called Comet C/2014 UN271 and discovered by
astronomers Pedro Bernardinelli and Gary Bernstein, could be as large as 85
miles across.
"This comet is literally the tip of the iceberg for many thousands of comets
that are too faint to see in the more distant parts of the solar system,"
Jewitt said. "We've always suspected this comet had to be big because it is
so bright at such a large distance. Now we confirm it is."
This comet has the largest nucleus ever seen in a comet by astronomers.
Jewitt and his colleagues determined the size of its nucleus using NASA's
Hubble Space Telescope. Its nucleus is about 50 times larger than those of
most known comets. Its mass is estimated to be 500 trillion tons, a hundred
thousand times greater than the mass of a typical comet found much closer to
the sun.
"This is an amazing object, given how active it is when it's still so far
from the sun," said lead author Man-To Hui, who earned his doctorate from
UCLA in 2019 and is now with the Macau University of Science and Technology
in Taipa, Macau. "We guessed the comet might be pretty big, but we needed
the best data to confirm this."
So the researchers used Hubble to take five photos of the comet on Jan. 8,
2022, and incorporated radio observations of the comet into their analysis.
The comet is now less than 2 billion miles from the sun and in a few million
years will loop back to its nesting ground in the Oort cloud, Jewitt said.
Comet C/2014 UN271 was first serendipitously observed in 2010, when it was 3
billion miles from the sun. Since then, it has been intensively studied by
ground and space-based telescopes.
The challenge in measuring this comet was how to determine the solid nucleus
from the huge dusty coma—the cloud of dust and gas—enveloping it. The comet
is currently too far away for its nucleus to be visually resolved by Hubble.
Instead, the Hubble data show a bright spike of light at the nucleus'
location. Hui and his colleagues next made a computer model of the
surrounding coma and adjusted it to fit the Hubble images. Then, they
subtracted the glow of the coma, leaving behind the nucleus.
Hui and his team compared the brightness of the nucleus to earlier radio
observations from the Atacama Large Millimeter/submillimeter Array, or ALMA,
in Chile. The new Hubble measurements are close to the earlier size
estimates from ALMA, but convincingly suggest a darker nucleus surface than
previously thought.
"It's big and it's blacker than coal," Jewitt said.
The comet has been falling toward the sun for well over 1 million years. The
Oort cloud is thought to be the nesting ground for trillions of comets.
Jewitt thinks the Oort cloud extends from a few hundred times the distance
between the sun and the Earth to at least a quarter of the way out to the
distance of the nearest stars to our sun, in the Alpha Centauri system.
The Oort cloud's comets were tossed out of the solar system billions of
years ago by the gravitation of the massive outer planets, according to
Jewitt. The far-flung comets travel back toward the sun and planets only if
their orbits are disturbed by the gravitational tug of a passing star, the
professor said.
First hypothesized in 1950 by Dutch astronomer Jan Oort, the Oort cloud
still remains a theory because the comets that make it up are too faint and
distant to be directly observed. This means the solar system's largest
structure is all but invisible, Jewitt said.
Reference:
Man-To Hui et al, Hubble Space Telescope Detection of the Nucleus of Comet
C/2014 UN271 (Bernardinelli–Bernstein), The Astrophysical Journal Letters
(2022).
DOI: 10.3847/2041-8213/ac626a
Tags:
Space & Astrophysics