'Oumuamua Is Not Aliens

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Matt O'Dowd (1:03)

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To repeat the Space Time, Maxim, it's never aliens.

Until it is. So let's talk about Oumuamua.

Last year, a visitor came to our inner solar system. To us, it looked like a faint spot of light, moving quickly relative to the fixed stars. We'd seen similar things countless times before. Lonely rocks, remnants of the formation of our solar system, drifting on their endless orbits around the sun, and faintly glimmering with its reflected light. Nothing unusual about such asteroids and comets. But what made this object special was its trajectory. It was not in orbit around the sun at all. Rather, it appeared to have fallen in from interstellar space, and was on its way back out again. The object is 1I-2017-U1, better known as Oumuamua.

It's the first chunk of interstellar space debris we've ever spotted passing through our solar system. It caused a justifiable stir at the time. Hell, even we did an episode on it. But Oumuamua has been making the news again. This time it's for a new paper by Shmuel Bialy and Avi Loeb that investigates the possibility that Oumuamua is aliens.

Or more accurately, an artificial light sail presumably attached to an alien probe. Today on Space Time Journal Club I want to do two things. First, assess the plausibility of this proposal. And second, talk about the media hype around it. First up, here's what we know about Oumuamua. Its changing brightness suggests it's a highly elongated object tumbling through space. It appears to be a few hundred meters long, but only a few tens of meters across. It may or may not also be flat. It's a space baguette, perhaps after going through a space sandwich press. Initially, its reddish color looked comet-like, but then it didn't produce a cometary tail, so it was assumed it must be more rocky, more asteroid-like, at least on the surface. A couple of strange things right there. The geometry is very unusual for an asteroid or comet. It's also a little surprising that we should have seen an interstellar object at all. We certainly expect there to be a bunch of space junk floating between the stars, probably ejected in the violent early stages of formation of planetary systems.

But in order for our encounter with Oumuamua to have been likely, that population needs to be huge. Much more interstellar debris than our theoretical models predict. Those things are odd, but not odd enough to start assuming aliens. But now that Oumuamua is on its way out of the solar system, it's doing something even weirder. It appears to be accelerating. Or at least it's not slowing down as much as you'd expect due to the sun's gravitational pull. This non-gravitational acceleration was reported in a nature paper in June. The extra acceleration is tiny, but it's clear. Something is pushing the body outwards. By itself this wouldn't be anything especially weird. Comets show the same sort of acceleration on their way out from the sun.

In their case, it's a result of outgassing. As the sun heats the rear surface of the comet, water ice and frozen gases, so-called volatiles, evaporate. The ejected gases act like jets to propel the comet forward. In regular comets, the outgassing is typically visible as sunlight reflects off dust carried along with the jets. And we see it as the comet's coma and tail.

But no coma or tail was observed in Oumuamua. Also, outgassing should probably change the way the object is tumbling, but Oumuamua's tumbling appears consistent.

SPEAKER_1 (5:00)

College football is back. So Hilton called on me, the superstition concierge, to make your fan rituals a reality. Need a room to match your lucky number? We got you. Want to make sure our team doesn't wash your lucky jersey? Oh, that smells lucky.