We've Never Seen a Comet Do This

Just past Jupiter, a comet has been randomly exploding for years, and no one knows why. These aren't cute little bursts. This is believed to be the most active comet in the solar system. We've seen it get nearly 300 times brighter in just a couple of hours, explode 4 times in 2 days without warning, and spew over a million tonnes of debris out faster than the speed of sound. And what makes it even stranger is there's no obvious trigger for the explosions. No dramatic swing into the sun, just a peculiar icy body acting unlike anything we've ever studied. I'm Alex McColgan, and you're watching Astrum. Join me today on a tour of everything we know about Centaur 29P, and why we think it acts out so violently, and the recent discoveries of the James Webb Telescope that flip these assumptions on their head. In 1927, two astronomers at Hamburg Observatory, Arnold Schwarzmann and Arno Arthur Wachmann, discovered Centaur 29P while comparing a series of photographs of the night sky. Initially classified as a short-period comet, 29P was later reclassified as a Centaur, the name given to a group of small solar system bodies orbiting the Sun between Jupiter and Neptune. They exhibit characteristics of both comets and asteroids, much like mythical Centaurs were half-human, half-horse. It's thought that Centaurs originated in the Kuiper Belt, but eventually moved inward due to the subtle gravitational influences of the gas giants over the last few million years. Centaurs generally have elliptical and unstable orbits, most will eventually be flung into the inner solar system as comets join the Jupiter family comets, or be catapulted out of the solar system entirely. Because of this, Centaurs represent a short-lived transitional state in a small body's journey from the Kuiper Belt to the Jupiter family comets. We think Centaur 29P could be launched into the inner solar system as early as 2038, when a conjunction with Jupiter will change the path of its orbit. But Centaur 29P is different from most Centaurs in two big ways that will be relevant later. Firstly, its orbit isn't elliptical, it traces an almost perfect circle around the Sun. And secondly, it is big. Measuring 60km in diameter, it is wider than 96% of all Centaurs, and is one of the largest known cometary bodies. Oh, and it explodes. A lot. It's the second most active body in the solar system after Io, exploding on average about 30 times per year, and 40% of these strong outbursts don't happen in any kind of predictable fashion. These explosions are erratic, sudden, and very intense. Following an outburst, the comet reaches peak brightness in about 2-3 hours. In 2024, it made headlines when it erupted four times in a 48-hour period, appearing almost 300 times brighter than normal. Two years earlier, it shot more than 1 million tonnes of debris into space faster than the speed of sound, like popping the cork out of the world's most pressurised champagne bottle. The main outburst then triggered two smaller ones 5 and 7 days later, which seems to be a common occurrence for the comet. And even though those sound incredibly violent and powerful, things get even bigger. In September 2021, Centaur 29P underwent the largest outburst in 40 years, when it blew its top 5 times in a row. Despite the massive blast, it wasn't scientists who initially noticed, it was amateur astronomers. This is one of the curiosities of Centaur 29P's history. Most of its eruptions are first observed by hobbyist stargazers. It's easier to pull out your backyard telescope and get approval to point a multi-million dollar public-funded instrument on your special interest, randomly exploding Centaur. When scientists heard what had happened, they managed to secure precious Hubble time to take a closer look, but the universe had other plans. As bad luck would have it, the day before the scheduled observation of Centaur 29P, Hubble experienced a technical glitch and couldn't be used. So, what causes these outbursts? How do we know about Centaur 29P's properties that could possibly explain them? If you suspect volcanoes, you're kind of right, but not the hot, fiery kind. They are ice volcanoes. Centaur 29P is what we call a cryovolcanic comet. These are generally covered in an icy shell with ice, dust and gas inside, mainly carbon dioxide, carbon monoxide and nitrogen gases in a frozen, icy state. Over time, radiation causes the crust to weaken from solid to gas. This forces internal pressure to rise. Eventually, it gets too much and the outer shell cracks, unleashing a visceral eruption of cryomagma from the inside of the comet, a mix of frozen water, ammonia, salts and volatile gases like methane and nitrogen gas. See? Ice volcano. Centaur 29P's nucleus has a high escape velocity, and most debris isn't ejected fast enough to break free from its gravity. The result is most falls back onto the Centaur's surface to reform its crust. This may then trigger the knock-on outbursts 5 or so days later, as we saw in the 2021 outburst. Around the comet nucleus, the ejected dust and ice particles form a hazy, reflective cloud, lingering for days up to a couple of weeks, making the comet look brighter, sometimes by several orders of magnitude. Some cryovolcanic comets like 12P, also known as the Devil Comet for the horn-like plumes that form during its outbursts, are on highly elliptical orbits around the Sun. As you might expect, the closer to the Sun they get, the more active and volatile their behaviour becomes. But Centaur 29P is on a circular, not an elliptical orbit. It's always roughly the same distance from the Sun, so you'd expect a pretty even absorption of solar radiation and regular predictable activity. However, as we've seen, that's not the case at all. So why are these eruptions so erratic when they shouldn't be? What's going on here? Scientists have been trying to work out if there is any method to the madness of the Centaur's eruptions for years. Some have speculated that the eruptions may be related to its slow rotation period of 57 days. More specifically, they observed that major outbursts were more likely to happen every 52 to 60 days. But smaller outburst events also occurred every 30 and every 90 days, with enough regularity to make scientists think that there might be some kind of seasonal cycle governing the comet. While Centaur 29P's eruptions seem to happen quasi-periodically, they are utterly unpredictable in terms of intensity or precise timing. That didn't stop astronomers from trying to anticipate when our feisty friend would blow. In April 2023, scientists noticed the light surrounding the comet's nucleus had become fainter than ever, suggesting a slowed rate of outgassing. They predicted pressure was building up inside the nucleus at a faster and faster rate, making an eruption highly likely. And just like that, Centaur 29P gave off a mini-outburst later that very same day. It was the first successful prediction of an eruption on this weird and volatile little world. For the first time, we weren't just reacting to randomness we got on the inside track, it felt like a breakthrough. And thanks to James Webb, we were about to have another. Until recently, we had never detected carbon dioxide on Centaur 29P. Carbon monoxide was always seen as the driver of these eruptions, mainly because it is consistently found in high amounts around the comet. But in 2024, data from the James Webb Space Telescope showed us that was only half the answer. Data from the previous radio wavelength observations of Centaur 29P showed a carbon monoxide gas jet pointed towards Earth. But Webb's near infrared spectrograph brought the jet's composition into greater focus, revealing not just one, but two jets of carbon monoxide, one pointing to Earth and one pointing towards the North. And that wasn't all. The James Webb Space Telescope also revealed two jets of carbon dioxide emanating from the north and south directions, the first definitive detection of the gas on the comet. The difference in abundance of carbon monoxide and dioxide suggests Centaur 29P might not be one object, but several objects that have become stuck together. The cause of the outgassing itself? Well, scientists still can't say for sure, but one interesting theory has been put forth to explain the unusually high carbon monoxide levels associated with Centaur 29P. Carbon monoxide is extremely volatile, and pure carbon monoxide ice can sublimate at temperatures as low as 25 Kelvin. If it was stored near the Centaur's surface, it would have all escaped a long time ago, so it must instead be trapped or preserved deep below the surface somehow. One possible solution is amorphous water ice. Almost all the ice we know of on Earth is in crystalline form. Crystalline ice is characterised by a structured, ordered arrangement of molecules. Amorphous ice, on the other hand, is not tightly packed. This makes it less dense than crystalline ice, and it also means it can hold gases within its structure. At temperatures above 77 Kelvin, amorphous ice turns spontaneously into crystalline ice. The higher the temperature, the faster this conversion occurs. In this process, trapped gases are released. So in theory, if Centaur 29P still has amorphous ice in its core along with trapped carbon monoxide, and the ice is undergoing conversion into crystalline form, then that could be where the higher levels of carbon monoxide are coming from. How plausible is this theory? Let's break it down. Centaur 29P started in the Kuiper belt, with temperatures ranging from 30 to 40 Kelvin. At this temperature, all ice would be amorphous, since it only starts crystallising above 77 Kelvin. As it travelled inward over 10 million years to its current location at 6 astronomical units from the Sun, temperatures rose to 115 Kelvin, and its amorphous ice started converting to crystalline ice. But how long will it take for all of its amorphous ice to turn crystalline? I mentioned earlier that Centaur 29P's size was going to play a role, remember? This is that part. First, researchers took four known small Centaurs. They all measured between 1 and 6km across, and were in a similar orbital position to Centaur 29P. They calculated that it would take those Centaurs between 0.1 to 6 million years to convert their amorphous ice into crystalline ice. In other words, by the time they arrive in this position, they are already fully crystalline inside. All trapped gases released, no aces up their sleeves. Predictable behaviour. However, a Centaur the size of 29P would take between 60 and 100 million years to convert all its amorphous ice. If these calculations are correct, it is possible the Centaur has only converted 50 to 65% of its amorphous ice, and this has huge implications for its behaviour. Converting lower density amorphous ice into higher density crystalline ice would shrink the overall volume of the nucleus. This could induce structural failures in the Centaur's surface and interior. Some believe the subsequent cave-ins, landslides and sinkhole creation events could be the driving forces behind the Centaur's erratic outbursts. But regardless of the elegant explanation, another problem exists. Centaur 29P doesn't slowly leak gas. It erupts explosively, sometimes multiple times per year. How that pressure is able to build up fast enough to elicit several explosions back to back in quick succession is still not fully understood. Why Centaur 29P undergoes such bursts of brightness, and how its outgassing actually works remain two of the biggest areas of investigation relating to this highly active cosmic body. It challenges our assumptions of just how fiery icy bodies can be in the outer solar system, and we still don't know as much about it as we'd like. But we've made progress. We're slowly starting to unravel its mysteries, and thanks to incredible tools like the James Webb Space Telescope, we're only going to keep learning. Of course, always keep learning is a great mantra for life in general, not just when learning about cryovolcanoes on distant comets. Are there questions you have about why things work the way they do, or are there skills you'd like to master, for work or pleasure? There's a joy that comes from learning, you just maybe need the right guide, one just like Brilliant, the sponsor of today's video. If you haven't noticed already, I'm a big fan of Brilliant, which is why I keep having them back as a sponsor on the channel. They're an online learning platform with really easy, accessible courses on a massive range of subjects, digital circuits, how AI works, as well as the classics of physics, maths and science. Their lessons are conveniently bite-sized, making them easy to do whenever you have a few minutes, and it's a great way to develop a lifelong habit of learning. If there are subjects you'd like to know more about, or if you want to master new skills in your spare time, head on over to brilliant.org forward slash astrum, scan the QR code on screen, or click on the link in the description below. Brilliant's also given our viewers 20% off an annual premium subscription, so why not give it a try? Thanks for watching, and thanks to our crew of astrum lords over at Patreon, who help us make science knowledge freely available to everyone. Chasing the algorithm can be hit and miss sometimes, so your contributions help us keep making the content we love. And if you want to join the Patreon, there's never been a better time to get in on the party. Just sign up with the link in the description. When you join, you'll be able to watch the whole video ad-free, see your name in the credits, and submit questions to our team. Meanwhile, click the link to this playlist for more Astrum content. I'll see you next time.