Giant Oort Cloud Comet Lights Up in the Outer Solar System

Updates:

The Minor Planet Center has now officially designated this object a comet: Comet Bernardinelli-Bernstein (C/2014 UN₂₇₁).

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Astronomers have spotted the largest comet ever recorded coming from the Oort Cloud. And at 20 times the distance between Earth and the Sun (20 astronomical units), beyond Uranus’s orbit, it’s already venting gas.

New observations, taken on June 22nd with the 0.51-meter SkyGems Remote Telescope in Namibia, reveal “clear cometary activity, with a 15-arcsecond coma,” Luca Buzzi reported this morning on the Minor Planet Mailing List.

A Big Comet

Astronomers discovered the comet, dubbed 2014 UN₂₇₁, in data collected by the Dark Energy Survey, which recently released data on hundreds of millions of galaxies spread over one-eighth of the sky. Survey computers spent millions of hours automatically searching the entire data set for transient objects moving across the sky. And from 2014 to 2018 one of them was the comet, yielding the orbit reported June 19th in a Minor Planet Electronic Circular.

The orbit immediately drew attention because it showed the comet coming from deep in the Oort Cloud, a group of planetesimals surrounding the Sun at icy distances of about 1,000 to 100,000 a.u. The comet will continue inward almost to the orbit of Saturn before heading back out again. The new observations put it at about magnitude 20, which enables a rough estimate of its diameter of 160 kilometers (100 miles). That puts it at the large end of objects seen from the Oort Cloud, seen as long-period comets by the time they’re visible from Earth. But it’s a lightweight relative to the rest of the solar system, so it could be tossed around by a close encounter with a planet. 

That size isn't enough to make it a dwarf planet, but it is the biggest object from the Oort Cloud that we’ve seen so far. It’s ahead of the modern record-holder, Comet Hale-Bopp (C/1995 O₁), which was only about 60 kilometers across. The Comet of 1729 (C/1729 P1), may have spanned on the order of 100 km; however, it never came much inside Jupiter's orbit, so observers at the time were limited in what they could see.

The Orbit

“The orbit we've got [for 2014 UN₂₇₁] is actually quite good,” says Bill Gray of Project Pluto. It's based on 32 observations taken over four years with a 4-meter telescope and the Dark Energy Survey’s DECam high-performance camera. From that data, calculations predict that the comet will reach perihelion on January 23, 2031, at 10.95 a.u., just outside the most distant point of Saturn's orbit. Tony Dunn's Orbit Simulator site shows it approaching at an inclination of 95°.

Aphelion, or the farthest point from the Sun, is much more uncertain because the orbit is extremely elongated and the swing around Saturn will alter its orbit, but Gray says the comet will clearly turn around in the Oort Cloud, and it’s definitely not an interstellar object. When the MPEC orbit was updated with Buzzi’s new observations, the comet's  most recent aphelion was calculated to be 40,000 a.u. and the prediction for the next one became 55,000 a.u. That means the comet took 1.39 million years to reach its present position from the last aphelion, and will take about 2.2 million years going back to the outer edge of its modified orbit, says amateur astronomer Sam Deen.

Gray sees little likelihood of pre-discovery observations because the comet was a little fainter than magnitude 22 when first spotted, and earlier it would have been even fainter. On the other hand, he says, it’s possible the comet will show up in observations taken since 2018.

New, targeted observations will help constrain the comet’s orbit as well as other key characteristics, including its rotation period, satellites, and composition. The behavior of comets is notoriously difficult to predict, but this one is getting off to a very early start, and many more telescopes will be turned toward it in the coming months and years.