Unusual centaur emitting gas and dust

Astronomers and planetary scientists studying a little-known group of icy bodies called centaurs have found one that is emitting a cloud of gas and dust.

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This image of centaur 2014 OG392 and its extensive coma combines many digital images into a single 7700-second exposure. The dashed lines are star trails caused by the long exposure. Credit: Northern Arizona University

That’s unusual, says Colin Chandler, of Northern Arizona University, US, because only 17 other centaurs have ever before been observed to do so.

Studying it, he told last week’s virtual meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS), will provide important clues to the make-up of these unusual bodies, and thus to the history of the early Solar System.

Centaurs are small objects whose orbits lie in the three-billion-kilometre range between Jupiter and Neptune. They are thought to have originated even farther out, in the Kuiper Belt beyond Neptune, then migrated inward millions of years ago.

They are called centaurs because they are, in many ways, hybrids.

“Centaurs have a dual nature,” says Jordan Steckloff, of the Planetary Science Institute, US, who was not part of the study team. “Just as the centaurs in lore are half-horse, half-human, the centaurs in the sky are sometimes comet-like, and sometimes asteroid-like.”

Once in the centaur belt, these objects tend to be short-lived, at least by astronomical standards. Eventually they pass too close to Jupiter, Saturn, Uranus or Neptune and either get flung into interstellar space, get sent diving toward the Sun as comets, or crash into one of the planets.

Chandler’s centaur, named 2014 OG392, is, in fact, almost certainly slated to smash into Saturn, and according to his team’s calculations could only have been in its present orbit for at most a couple of million years (and possibly only a few thousand) without already having done so.

That makes it a relatively recent arrival to the centaur belt, where even the chilly temperatures in that zone of the Solar System (2014 OG392 is estimated to have a surface temperature of minus 210 degrees Celsius) are still enough to vaporize materials that presumably were long ago lost by most other centaurs.

Figuring out what that material is, Chandler says, will provide important clues to the composition of the icy bodies farther out from the Sun. At the distance of 2014 OG392, for example, it is still far too cold for it to emitting water vapor. That should still be safely frozen, as ice.

Conversely, other ices, such as frozen carbon monoxide and methane should have warmed up and steamed away long before it got as close as it presently is. More likely, he says, it’s jetting off a mix of carbon dioxide and ammonia.

All of which may sound arcane but has huge implications for how the infant Earth received its volatiles – the chemically lightweight materials that are now the stuff of life.

One way such materials may have reached the Earth is via impacts from objects such as 2014 OG392. Thus, the more we can learn from today’s centaurs, Chandler says, the better we may be able to figure out exactly how this all-important process happened.

Meanwhile, a team led by Laura Woodney, of California State University has found another active centaur on the verge of becoming a true comet.

This object, known as P/2019 LD2, has been determined to be in a “gateway orbit” from which it is poised to be launched into the inner Solar System by gravitational interactions with Jupiter in 2063, she told the DPS meeting,

“This is the first time we’ve discovered an object that we know is going to become a comet within [at least] the lifetime of our graduate students, so we can watch this transition process,” she says.

That’s important, she adds, because up until now her team is “reasonably certain” that P/2019 LD2 has never before passed close enough to the Sun for solar heat to vaporize its water and produce the type of dramatic tail most of us identify with comets.

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