Perhaps you’re a bit fuzzy on the differences between comets and asteroids. You’d like someone to make it easy, memorable – like how “stalagmites” has a “g” in it, so you know those rise up from the ground, while “stalactites,” with a “c,” come down from the ceiling. (You’re welcome.)
You’d like me to review the basics.
Asteroids circle the Sun as inert chunks of rock, mostly in orbits between Mars and Jupiter. Comets, on the other hand, can be found farther out, in the Kuiper Belt or the Oort Cloud. Comets travel along more elliptical paths that sometimes take them to the inner solar system, where they puff off clouds of gas and dust into comas and tails.
The last thing you want is for someone to take that simple dichotomy and confuse things with wacky fringe cases. So if you’re reading this with a growing sense of dread, and you’re sneaking peeks at that adorably WTF animal pictured below – well, buckle in. We are going to talk about exceptions to the rules.
We’re going to discuss the platypuses of the solar system.
A platypus in Geelong, Australia. Credit: Wikimedia Commons User TwoWings
Some background seems in order. In 1798, the first platypus pelt arrived in England from Australia. The animal was just so darn goofy, so impossible, that naturalist George Shaw thought it was a hoax, to the point of checking the bill to see if it was stitched on. Yet fortunately, comically, the platypus turned out to be real. With its duck-like bill, its venomous spur, and – oh yeah – the fact that it lays eggs, the platypus challenges simple notions of what it means to be a mammal.
The field of astronomy isn’t immune to its own quirky, confusing classifications. A paradigm-shaking, platypus-like discovery occurred in 1996, when astronomers noticed that asteroid 133P had started sporting a dust tail. That’s an awfully comet-like thing for an asteroid to do, and it brings us to our first planetary-science platypus: main-belt comets.
Asteroids that look like comets
Main-belt comets get their name because they orbit the Sun in the “main belt” of asteroids, between Mars and Jupiter. Yet they also display comet-like tails and comas. They aren’t comets, really – just asteroids masquerading like them.
Planetary scientist Dave Jewitt, among others, has argued that “active asteroids” would be a better name. The fact that these asteroids are active at all, however, is surprising.
This 2010 picture of asteroid (596) shows an extended, comet-like tail. Credit: Steve Larson and A. Gibbs, University of Arizona/Catalina Sky Survey
Comets like ISON are “active” – that is, they produce gaseous comas and dusty tails – because they’ve spent little or no time in the inner solar system. They have carbon and water ices on their surfaces that sublimate into gas when heated, expelling dust grains into space. But these ices just sit there, quiescent, during the long periods comets spend far away from the Sun. When comets finally do fall inward, these pristine ices are fuel for outbursts.
Unlike comets, the orbits of many asteroids stay permanently inside the snow line – the zone in our solar system where there’s too much sunlight for water to freeze. That means that any ice on their surfaces has been vaporized away for eons. There’s nothing to form a tail from.
Notice, though, that I said “on their surfaces.” Pockets of ice may still exist inside these asteroids, buried away from the Sun. When two asteroids collide, that fresh material can be suddenly exposed.
Another, more exotic way for this underground ice to be liberated is when main-belt comets spin so fast that they literally centrifuge themselves apart. The force of sunlight on an asteroid is just a feathery touch, but after 4 billion years of subtle pressure, it can add up. Some asteroids have been gradually spun up by the Sun to the point where the centripetal force they feel is stronger than their ability to hold together, which can expose icy or dusty innards.
In a press release today, astronomers announced the discovery of a main-belt "comet" with six separate tails. They hypothesize that dust grains are being swept back into trails by solar radiation as the asteroid rotates. Credit: NASA, ESA (STSCI/AURA)
Only about 10 active asteroids have been discovered, but astronomers are hungry to find more – and to understand what's causing these abrupt outburts.
Comets that look like asteroids
With active asteroids, we had the orbital characteristics of asteroids but the signature tails of comets. Here, we’ve got the opposite: extinct comets travel in comet-like orbits but behave like asteroids.
It’s actually quite simple. When a comet runs out of juice (surface volatiles) it still keeps following the same path as before. But observationally, although we can solve for their orbits, these objects no longer show the bright, expansive clouds that would signal their comet-hood.
A possible extinct comet, asteroid (3200) Phaethon. Credit: Wikimedia Commons User Marcoaliaslama
Although they aren't as exciting as active asteroids, inactive comet nuclei make up a much larger fraction of our local environment. It’s estimated that about 6% percent of near-Earth objects are in this category.
"…dogs and cats living together, mass hysteria…"
- Dr. Peter Venkman, Ghostbusters
The existence of active asteroids and extinct comets makes it clear: whether in biology or astronomy, taxonomy is hard. The platypus is a mammal, but of a special, weird kind. Ceres, the first asteroid ever discovered, was at first sight thought to be a comet, and then hailed as the “missing planet” between Mars and Jupiter. After a few years, it was called an asteroid.
Then, in 2006, Ceres got promoted to “dwarf planet” – where it joined Pluto, subject to its own infamous demotion from planet status.
Apparently, nature doesn’t crave organizing everything into neat categories the way humans do. The order in which discoveries happen, often a product of chance, biases us toward thinking we know what to expect next. Then, when something unexpected shows up, we’re forced to question our prior classifications.
That’s why it’s important to recall that the “comets vs. asteroids” question doesn’t have just a binary answer. Everything gets a little more complicated when you throw in main-belt comets and extinct comet nuclei.
And sometimes, when scientists are really lucky, they find something so weird that their first thought is that it can’t be real.