But we’re not in danger, says professor — ScienceDaily

A huge comet – about 80 miles in diameter, 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 come within a billion miles of the sun, which is slightly further from Earth than Saturn; it will be in 2031.

Comets, among the oldest objects in the solar system, are icy bodies that were thrown unceremoniously out of the solar system in a game of gravitational pinball among the massive outer planets, David Jewitt said. The UCLA planetary science and astronomy professor is co-author of a new comet study in the Astrophysical Journal Letters. The expelled comets have taken up residence in the Oort Cloud, a vast reservoir of distant comets circling the solar system billions of miles into deep space, he said.

The spectacular millions of miles long tail of a typical comet, which makes it look like a flare, belies the fact that the source at the heart of the firework is a solid core of ice mixed with dust – essentially a dirty snowball. This huge, called comet C/2014 UN271 and discovered by astronomers Pedro Bernardinelli and Gary Bernstein, could be up to 85 miles across.

“This comet is literally the tip of the iceberg for several thousand comets that are too faint to be seen in the most distant parts of the solar system,” Jewitt said. “We always suspected that this comet must be big because it’s so bright at such a great distance. Now we’re confirming that.”

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 at 500 trillion tons, one hundred thousand times greater than the mass of a typical comet found much closer to the sun.

“It’s 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 at UCLA in 2019 and works now at Macau University of Science and Technology in Taipa, Macau. “We guessed the comet might be quite large, but we needed the best data to confirm that.”

The researchers therefore used Hubble to take five photos of the comet on January 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 it will return to its nesting place in the Oort Cloud, Jewitt said.

Comet C/2014 UN271 was first observed by chance in 2010, when it was 3 billion kilometers from the sun. Since then, it has been intensively studied by ground and space telescopes.

The challenge in measuring this comet was how to determine the solid core of the huge dusty coma – the cloud of dust and gas – that envelops it. The comet is currently too far away for its nucleus to be resolved visually by Hubble. Instead, the Hubble data shows a bright peak of light at the location of the nucleus. Hui and his colleagues then created a computer model of the surrounding coma and adjusted it to fit the Hubble images. Then they subtracted the coma glow, leaving behind the core.

Hui and his team compared the brightness of the core to earlier radio observations from the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. The new Hubble measurements are close to previous ALMA size estimates, but convincingly suggest a darker core surface than previously thought.

“It’s big and it’s blacker than coal,” Jewitt said.

The comet has been falling toward the sun for over a million years. The Oort Cloud is believed to be the nesting ground for billions of comets. Jewitt thinks the Oort cloud extends from a few hundred times the distance between the sun and Earth to at least a quarter of the distance of the stars closest to our sun, in the Alpha Centauri system.

The Oort Cloud comets were ejected from the solar system billions of years ago by the gravitational pull of the massive outer planets, according to Jewitt. Distant comets only return to the sun and planets if their orbits are disturbed by the gravitational tug of a passing star, the professor said.

Hypothesized for the first time in 1950 by the Dutch astronomer Jan Oort, the Oort cloud is still a theory because the comets which compose it are too faint and too distant to be directly observed. That means the largest structure in the solar system is virtually invisible, Jewitt said.

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