Astronomers around the world were positively tickled last year when we detected our first interstellar object inside the solar system. The object was named ‘Oumuamua, a term of Hawaiian origin meaning “a messenger from afar arriving first.” Telescopes around the world turned to watch as our visitor rocketed off into oblivion, and scientists eventually decided it was an asteroid. However, some new data calls that assessment into question. ‘Oumuamua may have been a comet after all.
The scientific community has long expected an interstellar visitor to show up, and ‘Oumuamua was undeniably from another star. It shot past us with such haste there was no way it could have started in our solar system — even a gravity boost from Jupiter wouldn’t have given a local object matching speed and trajectory.
So, ‘Oumuamua’s appearance wasn’t a shock, but the 800-meter cigar-shaped object still wasn’t exactly what we expected. Scientists believed the first interstellar object in our solar system would be a comet. If other solar systems are anything like ours, there should be a cloud of comets at the very edge. It wouldn’t take much for these objects to break away and sail through interstellar space. However, ‘Oumuamua looked more like an asteroid than a comet.
The hallmark of a comet is the way the icy body heats up as it passes close to the sun, leading to the formation of a “coma.” That’s why comets have tails and asteroids don’t. ‘Oumuamua didn’t have a visible tail. Researchers from the University of Hawaii Institute for Astronomy (where ‘Oumuamua was discovered) have kept watch on the object using the Hubble Space Telescope, and the latest data suggests something other than gravity has affected its path.
Astronomers have attempted to determine if the solar wind or radiation could have tweaked ‘Oumuamua’s trajectory using computer models, but nothing fit with the observations. The best explanation was outgassing. The process that produced a comet’s coma as it heats up. The release of gas can actually nudge the orbit. If that’s what caused the change to ‘Oumuamua’s orbit, it would have to be a comet.
There are still a lot of unanswered questions. We didn’t see a coma, so maybe a journey through interstellar space alters comets in some way. Or perhaps ‘Oumuamua is an asteroid, and there’s some mechanism at work that we’ve never seen before. We might never know — ‘Oumuamua is leaving, and it isn’t coming back.
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