discovered Centaur 29P while comparing a series of photographs of the night sky.
Initially classified as a short-period comet, 29P was later re-classed 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 60 km 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 two to three 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 tons 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 five
and seven 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 five 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 star-gazers.
It’s easier to pull out your backyard telescope than 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 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? What 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… 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, solar radiation causes the
crust to weaken and the icy interiors to sublimate 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 five 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 which lingers 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 explosions 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 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 previous radio-wavelength observations of Centaur 29P showed a
carbon monoxide gas jet pointed toward Earth. But
Webb’s Near-Infrared Spectrograph brought the jet’s composition into greater focus,
revealing not one but two jets of carbon monoxide – one pointing to Earth and one pointing toward
the North. And that wasn’t all. The JWST also revealed two jets of carbon dioxide emanating
in 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 were 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 K, 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 that ice is undergoing conversion into crystalline form,
then that could be where the high levels of carbon monoxide are coming from.
How plausible is this as a 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 crystallizing above 77 Kelvin.
As it travelled inward over 10 million years to its current location at 6 AU 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 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 one
and six kilometres 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 persists:
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,
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