Universe Today
A team of astronomers has recently completed a long-range observation of a comet far from the Sun. This analysis proves that there’s lots going on, even in the icy depths of the solar system.
The targeted comet is C/2014 UN271 Bernardinelli-Bernstein. Comet UN271 is one of the largest Oort Cloud comets ever observed, measuring 140 kilometers across. It's currently at a distance of 16.5 Astronomical Units (AU) from the Sun, which makes it tough to observe with all but the largest telescopes. Astronomers have used the powerful Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe the comet, watching as jets of carbon monoxide gas are erupting from its nucleus. This is a surprising level of activity for a comet that's so far from the Sun.
ALMA telescopes on the Chajnantor Plateau. Credit: ALMA/NSF/ESO
Comet UN271 was discovered in 2014 by astronomers Gary Bernstein and Pedro Bernardinelli. They noticed the faint fuzzball in archival images from the Dark Energy Survey, moving slowly as a +22nd magnitude smudge through the constellation Sculptor.
A Far Ranging Orbit
Almost immediately, astronomers knew Comet UN271 was something special, as its slow motion through the sky suggested it was far out in the solar system, and therefore quite large. After following the comet for a bit, astronomers realized it was 29 AU from the Sun at the time of discovery—about ¾ of the way to Pluto from the Sun—and had a nucleus about 120 kilometers (75 miles) across. For context, Halley’s Comet has a nucleus just 15 kilometers across. This puts Comet UN271 up there in terms of size, as the largest Oort Cloud comet seen to date.
The discovery image for Comet UN271 (annotated). Credit: NOIRLab.
We’re fortunate to see Comet UN271 on its perihelion approach. In January 2031, the comet will pass 10.9 AU from the Sun, just outside the orbit of Saturn. On a 2.8-million-year orbit inbound, the comet will then head out of the solar system on a 4.6-million-year orbit outbound, for a far-off aphelion 55,000 AU from the Sun. That’s 87% of a light-year away, about a fifth of the way to Proxima Centauri, the nearest star.
The orbit of Comet UN271 through the solar system. Credit: NASA/JPL Horizons.
The difference in orbit inbound versus outbound for the comet is due to its interaction with solar system planets while it's near the Sun. Any new comet entering the inner solar system stands about a 40% chance of having its orbit altered. Likely, Comet UN271 was knocked off its Oort Cloud perch by a stellar pass near the solar system that sent sunward, long ago.
Looking at a cold distant object like Comet UN271 really pushed the sensitivity and resolution of ALMA to its limits. The thermal signal received by ALMA will not only help to refine the comet’s size, but also help to chronicle the dust production rate seen.
"The study gives us a look at the nature of the outgassing activity in this distant active comet," Nathan Roth (American University) told Universe Today. "It suggests that carbon monoxide was subliming from ice into gas directly from the nucleus surface, as opposed to being produced by chemistry in the coma."
False color images of Comet UN271, showing activity. Credit: ALMA/NSF/ESO.
Not only was this a record-setting detection in terms of distance, but it also demonstrates that comets can still display activity, far from the Sun.
"The comet was very far from the Sun during our observations (roughly halfway to Neptune), and since it is so cold at these distances, very few other molecules should have been present as gases in the coma," says Roth. "Our results mean it is likely that carbon monoxide is a key part of the engine that drives activity in this comet. Gaining insights into what makes comets tick at such large distances from the Sun was a key goal of our study."
ALMA's observations can even characterize the activity on the comet, even at the enormous distances involved. "Despite the large size of the comet's nucleus, carbon monoxide was being released from discrete jets on the nucleus surface in areas of high illumination from the Sun, rather than subliming uniformly from the entire nucleus," says Roth. "Furthermore, we observed the comet on two dates roughly a week apart, and we noticed that the orientation of these jets had changed from one observation to the next. This may be consistent with the comet's history of repeated outburst activity measured at optical wavelengths, but we need more observations to draw firm conclusions."
No missions are planned to give us a view of Comet UN271 up close. The European Space Agency does have a mission named Comet Interceptor in the works that would loiter in the inner solar system, ready to chase down the next big comet. Comet Interceptor is planned for launch in 2029.
The Hubble Space Telescope imaged Comet UN271 in January 2022. We should get good views of the comet from the James Webb Space Telescope leading up to perihelion if it's ever tasked to observe it. The Vera Rubin Observatory, which is set to reveal its very first images next week could provide amazing views of the comet as well in the years to come.
"We're looking forward to learning more about the comet as it continues to approach the Sun and warms up," says Roth. As the Sun continues to heat it, we may see new molecules begin to sublime from the nucleus, which will help us to measure its chemical composition in greater detail and may give us clues to how it formed."
Hubble's view of Comet UN271 in 2022. Credit: NASA/HST/STScI
And yes, a comet the size of UN271 would spell a bad day for Earth, though in this case, it isn’t coming anywhere near the inner solar system. UN271 is 12 times the size of the Chicxulub impactor (which was only about 10 kilometers across) that struck the Earth 66 million years ago, so it would definitely be an extinction-level event if something the size of UN271 ever came our way.
A size comparison for other known comets, versus UN271. Credit: NASA/ESA/Zena Levy/STScI
Too bad Comet UN271 won’t visit the inner solar system... (just not too close!) It’s bigger than Hale-Bopp, and would provide an amazing show. For now, we’ll have to wait for the next Oort Cloud interloper to turn up. Still, the appearance of Comet UN271 shows us just what might be lurking out there, in the remote realms of the outer solar system.
