Last year, news broke that scientists had discovered a huge inbound comet, named Bernardinelli-Bernstein. It was the most distant comet we’d ever discovered. And it was huge, so big scientists originally thought it was a minor planet. But it was so distant that all our pictures were still blurry. So, we waited. Now, NASA confirms that the Hubble Space Telescope (HST) picked up the object — and it’s absolutely smashing all our predictions. Comet Bernardinelli-Bernstein, or “BB” for short, is the biggest comet ever recorded.
The arXiv preprint from the comet’s discovery gave a low-end estimate of two trillion tons. For scale, that’s 10 times the mass of Hale-Bopp, and Hale-Bopp became known as the Great Comet of 1997. But Comet Bernardinelli-Bernstein is an absolute unit. The thing has smashed our earlier predictions. NASA estimates Comet Bernardinelli-Bernstein’s mass to be 500 trillion tons.
And that’s not the only place we got it wrong. When we reported on Comet BB last year, we said that it was twice the size of Phobos and Deimos, laid out longwise end-to-end. But BB made a liar of us all on that one too. It’s bigger. The previous all-time biggest comet ever recorded was 80 kilometers wide. Comet BB is eighty miles wide.
‘Dirtmosphere’
Despite its distance from Earth, Comet BB has already started to put on a show. When we picked it up last year, it was shedding enough debris that we called it a “dirtmosphere.” Now it’s close enough to the sun that the comet is starting to form a coma. But that haze of dirt and back-scattered light makes astronomers’ jobs harder.
“This is an amazing object, given how active it is when it’s still so far from the Sun,” said Man-To Hui, lead author of a report detailing these more recent observations. “We guessed the comet might be pretty big, but we needed the best data to confirm this.” So, Hui and colleagues waited until the comet was close enough for Hubble to capture its photo.
The comet is still too far away for Hubble to visually resolve its nucleus. Instead, Hubble’s CCD just shows us a “bright spike of light” where the nucleus ought to be. So, Hui and his team created a computer model of the surrounding coma, and then adjusted it to fit the Hubble images. Then, they subtracted out the glow of the coma, leaving behind only its “starlike” nucleus.
“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,” said coauthor David Jewitt, professor of planetary science and astronomy at UCLA.”We’ve always suspected this comet had to be big,” said Jewitt, “because it is so bright at such a large distance. Now we confirm it is.”
Initial Brightness Measurements Confirmed
Measurements of the comet’s composition turned out to be right. Prof. Gary Bernstein — one of Comet BB’s namesakes — said, “The measurement that Hui et al reported confirms our earlier measurements from the ground of the brightness of this body.”
“We measured its brightness before it had a coma around it,” Bernstein told ExtremeTech. “The new result takes an image with HST after the coma has formed, and tries to subtract away the coma light to isolate the comet’s nucleus’s light.”
Comet BB is a long-period comet, with an orbit that takes about three million years. It’s likely from the Oort Cloud, which is cool all on its own, because we don’t know much about the Oort Cloud. But this comet is also the largest Oort Cloud object ever identified, by a long shot. That makes this a unique chance to check our assumptions about what’s going on in the silent outer reaches of the solar system. Sadly, not even Voyager 1 or 2 will be able to reach the Oort Cloud before their fuel runs out.
Far From Home
The heliopause is the place where the solar wind loses its outbound momentum. In 2012 Voyager 1 flew through the heliopause and Voyager 2 passed through the same boundary back in 2018. Far beyond the heliopause, the Oort Cloud is a spherical shell of ice, rock and dust encircling the whole solar system. It has no proper edge, because that’s where the sun’s gravitational influence ends. Instead, they both fade off, like the glow of headlights in deep fog, into the big empty of the interstellar medium. Their porous outer boundary marks where the sun’s gravity gives way to the galactic tide.
We think other stars may well have their own faraway clouds of comets. Alpha Centauri AB and Proxima Centauri will make their closest approach to us in about thirty thousand years. When they do, their Oort Cloud will be separated from ours by a gap only as wide as our Oort Cloud itself.
The comets in the Oort Cloud probably didn’t form all the way out there. Instead, NASA explains, “they were tossed out of the solar system billions of years ago by a gravitational ‘pinball game’ among the massive outer planets, when the orbits of Jupiter and Saturn were still evolving.”
But the planets aren’t the only things capable of disturbing that distant cloud of comets. When Gliese 710 buzzes our solar system in about 1.3 million years, it’s expected to come within 20,000 AU of the sun. That’s close enough to go puddle-stomping through the Oort Cloud. It won’t disturb Pluto, nor planets sunward, but it may send us many more long-period comets.
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