After all, that's the whole point of studying comets. They're old and cold, having been formed in or flung out into the far reaches of the solar system. That combination of age and chilliness means that they're full of materials from our solar system's early days and that those materials are pretty much unchanged — at least deep down in the comet's frozen core.
According to observations published last week in Science Advances and led by Karen Meech of the University of Hawaii's Institute for Astronomy, the first ever so-called Manx comet — named for a tailless breed of cat — was likely booted into the Oort Cloud at the solar system's outer limits billions of years ago, when the young planets were jostling for space near the sun and causing all sorts of gravitational mayhem. It was detected while it passed through the inner solar system in 2014, but it's already headed back out into the Oort Cloud.
Generally speaking, asteroids — made mostly of metal and rock — don't have tails, because they don't melt. And comets, which are usually made up of dirt, rock and ice, always have them, at least until they run out of ice and "die." But Manx comets defy categorization.
In studying the light reflected by C/2014 S3, scientists found that it is actually made up of the kinds of rocky materials you'd expect in an asteroid. It has something like 1 million times less sublimation activity than a normal comet. It could even be a fragment left over from Earth's formation. But it remains "uncooked," like a comet, unlike the objects in the asteroid belt between Mars and Jupiter.
“If you’d shown me the spectrum, I would have just said this is another stupid asteroid,” study author Olivier Hainaut of the European Southern Observatory told Gizmodo. “If you showed me the orbit, I’d say yeah, it’s a standard long-period comet. But you don’t at all expect to find a rocky asteroid on an Oort cloud orbit. That’s wrong.”
"We already knew of many asteroids, but they have all been baked by billions of years near the Sun," lead author Meech said in a statement. "This one is the first uncooked asteroid we could observe: it has been preserved in the best freezer there is."
This tricky, blurring line between comet and asteroid shows just how little we know about even the bodies within our own solar system. It's reminiscent of our confusion over the gap between planets and stars: Gas giants like Jupiter come uncomfortably close to stars called brown dwarfs — so much so that the two types of objects are both sometimes called "failed stars". And then there are dwarf planets, the tiny planetoids that pepper our solar system and forced scientists to reevaluate just what qualities make a true planet, leading to Pluto's infamous demotion. Even inside the fences of our own cosmic backyard, our semantics leave a lot to be desired.
Since this particular strange body is headed very far away, we're not likely to grab a chunk of it for investigation. But now scientists will be on the lookout for similar "comets" as they visit the inner solar system.
And even if we never study the ancient planetary building blocks chilling inside comets like these, they could still help us understand the early solar system. If scientists can find more of these Manx comets, it could help them figure out what ratio of Oort Cloud objects are rocky versus icy. One study estimated that 8 billion asteroids orbit in the Oort Cloud's icy clutches. If icy objects formed far out at the edge of the solar system and rocky ones formed close to the sun, which many scientists believe to be the case, then the number of rocky objects with comet-like orbits could tell them something about the gravitational climate of our solar system's early days — and by extension, we could finally figure out just how much the planets in our system moved around before settling into the orbits they have today.
"We've found the first rocky comet, and we are looking for others," Hainaut said in a statement. "Depending on how many we find, we will know whether the giant planets danced across the Solar System when they were young, or if they grew up quietly without moving much."