Twist in the tail: ‘Oumuamua was a comet, not an asteroid

‘Oumuamua, humanity’s first known interstellar visitor, may have been a comet, not an asteroid, scientists say.  

That might sound like a minor difference, but it’s not, says Humberto Campins, a planetary scientist from the University of Central Florida in the US, who was not a part of the research team. 

“This is the very first interstellar object to visit the solar system,” he says, “so the more we know about it, the better.”

About the size and shape of a large skyscraper, ‘Oumuamua whizzed through the inner solar system in 2017, slingshotted around the sun, and headed back to interstellar space. 

Unlike a similar object in Arthur C. Clarke’s 1973 science fiction novel Rendezvous with Rama, it is not an alien spacecraft … although there were suggestions that in Clarke’s honor it be named Rama. Instead, it was named ‘Oumuamua — a Hawaiian word for “scout”.{%recommended 6332%}

Until recently, it was classified as an asteroid because even though it passed closer to the sun than Mercury and was still not much farther out than the Earth when it was first spotted, it produced no visible tail. 

But now, a team led by Marco Micheli, an astrophysicist at the European Space Agency’s Near-Earth Object Coordination Centre in Frascati, Italy, has looked more carefully at ‘Oumuamua’s movements and concluded that it was being accelerated by some force other than the combined gravity of the sun, planets, and large asteroids. 

That doesn’t mean it was an alien rocket ship, like Clarke’s Rama. There are a number of natural forces that can do this, the most obvious of which are jets of gas too small to produce a visible tail – meaning that it was a comet, but not a very active one.

Figuring this out, however, wasn’t an easy task. 

“Non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational accelerations, [so] the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc,” the scientists write in the journal Nature.

Furthermore, gas jets aren’t the only things that can alter a small object’s motion. Pressure from solar radiation and interactions with the sun’s magnetic field can also produce tiny changes, the scientists say. All of these had to be ruled out by careful analysis of ‘Oumuamua’s orbit using a combination of space and ground-based instruments. 

Campins is impressed. “The analysis is sound and the results are credible,” he says.

And while it’s hard to generalise from a single extrasolar object, he says, the fact it’s now shown to be a comet is important because the theory of planetary system formation says that icy objects — the ones capable of becoming comets — are more easily kicked out of their own solar systems, freeing them to pass through ours.

Asteroids form close in to their stars, inside the “snow line”, where it’s too hot for vapor to condense into ice. Comets not only form further out, but in a region where it’s also cold enough for the formation of giant planets like Jupiter, Saturn, Uranus, or Neptune. These are the type of monster worlds with gravities that can most easily eject other bodies from their systems. 

So, Campins says, the difference between ‘Oumuamua being a comet or an asteroid “is more important than you would think”.