What do we know about ‘Oumuamua?


A strange reflector

How else is ‘Oumuamua strange? Astronomers used the Spitzer Space Telescope for over 30 hours of observations of ‘Oumuamua in the infrared — or rather, Meech says, they tried. “They detected nothing,” she says, when looking in the thermal infrared, which detects heat. “So it was just an upper limit.” But sometimes even non-detections can provide information. “From this, they said, ‘Okay, if it has thermal properties typical of comets, then it’s going to have to be brighter than everyone thought.’”

Reflectivity, or how much light an object reflects, can tell astronomers a lot about its shape, size, and composition. “We just always assumed [the reflectivity] was cometlike and very low,” Meech says. But “instead of the normal four percent reflectance, which is typical of comets, they were getting more like 10 percent reflectance. So that again isn’t quite matching a cometary body. The only way they could get it to agree with a four percent reflectance is if you assume it has very different thermal properties on its surface than normal comets. So again, a little bit peculiar,” she says.

Those same infrared observations also failed to see significant carbon monoxide or carbon dioxide outgassing from ‘Oumuamua. That’s important because Meech’s team had also looked at the possibility that it was spitting off those two gases, but in much higher quantities than is indicated by the limits set by the Spitzer observations.

“It’s always a little bit tricky to interpret too much out of not seeing something,” Meech cautions. But “yet again, it’s not quite hanging together as a typical comet.”

Where’s home?

‘Oumuamua was born in another solar system, so perhaps all these oddities are normal for something that formed somewhere else. The next question then becomes, “Where did it form?”

“The whole point of getting HST time was to get the longest possible arc so that we could try to trace back where it came from,” says Meech. Coryn Bailer-Jones of the Max Planck Institute for Astronomy in Heidelberg, Germany, calculated ‘Oumuamua’s track back to before it first entered the solar system, and compared that path to the past positions of 7 million stars measured by the Gaia satellite. Because stars are moving in the galaxy over time, he had to rewind their orbits, in addition to ‘Oumuamua’s, to look for places where the two might reasonably overlap.

He found four possible candidates that passed close enough to the rewound track of ‘Oumuamua’s motion that they might be its home. (By “close enough,” Meech says, Bailer-Jones was looking at stars ‘Oumuamua’s path had come within one to two Oort Cloud radii — 100,000 to 200,000 AU.)

But even here, all was not straightforward. “The only problem with these four candidates is that the encounter velocities were relatively high, between about 10 and 25 kilometers per second,” she says. “‘Oumuamua is probably an ejected planetesimal or building block from the birth process of planets, and it’s pretty hard to get ejection velocities this high during that birth process, just from a planetesimal getting too close to giant planets. We would have expected only a few kilometers per second. So these are high. It’s not impossible but it is pretty unlikely.”

It would be more likely, she says, if ‘Oumuamua came from a binary star system. But, of course, none of the four candidates found are known to be binary stars. The next Gaia data releases, she says, may help, allowing astronomers to trace back the motions of stars nearby even further in time, and possibly find a better candidate for ‘Oumuamua’s home. “It would be great to revisit this in a couple of years,” she says, when more data from Gaia is available.

An ongoing mystery

‘Oumuamua remains the first and (thus far) only interstellar visitor we’ve caught skipping through our solar system. The data available on it are limited, but that doesn’t stop researchers around the world from wondering what they can find within its depths. “What’s especially fascinating is that people are still writing papers on it,” says Meech, “even though we effectively got a couple weeks of data with everyone trying, and then the HST and the Spitzer data went a little longer, but that’s it. And people are still writing papers.”

What’s more, A recent TED talk Meech gave on ‘Oumuamua currently has more than 2 million views and is continuing to gain more, she says. “Clearly people are interested.”

It’s certainly an interesting object. Its strange properties have even prompted some to suggest it’s of truly alien — as in, alien-built — origin. But those arguments don’t really hold up against the data we do have, Meech says, limited though it may be.

So while it’s not an alien spaceship or an artificial light sail, ‘Oumuamua is still a mysterious, transient visitor that has captured the attention of researchers and the public alike.

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