What have we learned about this mysterious visitor?
Astronomers were able to record data on ‘Oumuamua (which means “scout” or “messenger” in Hawaiian) for only a brief period of time, although it’s been in our solar system for over a century. “It came into the solar system, inside 1,000 AU, around 1837 or so,” says Karen Meech of the Institute for Astronomy at the University of Hawaii. (1 AU, or astronomical unit, is equivalent to the average Earth-Sun distance of 93 million miles [150 million kilometers]). But ‘Oumuamua wasn’t spotted until last year because it was too far from the Sun to reflect enough light for astronomers to pick it up, even with today’s best telescopes. And once it got close enough to the Sun, it was moving fast, remaining for very little time in the relatively small region where it is visible to modern-day telescopes.
After it sped around the Sun, ‘Oumuamua was again on its way out, growing ever fainter. “Our very last observations from Hubble were January 2nd of 2018 at fainter than magnitude 27,” says Meech. “By May 3rd, it was again outside of Jupiter’s orbit. And … once it gets beyond about 1.2 AU or so, it’s 25th magnitude, so beyond Jupiter’s orbit, forget it. It’s gone.”
Gone from sight, but not yet from the solar system. According to Meech, ‘Oumuamua is expected to reach the Kuiper belt in about 2024, and pass the edge of the Kuiper belt in late 2025. It will pass the most distant location the Voyagers have reached, she says, in about 2038. By 2196, it will again be 1,000 AU from the Sun, although our Oort Cloud is projected to extend beyond 100,000 AU, so when ‘Oumuamua truly passes the “edge” of the solar system depends on where you define that edge.
Regardless, since January, ‘Oumuamua has been beyond astronomers’ reach. Now, they are working with what data they were able to gather over the course of just a few months to learn all they can about this strange interstellar interloper.
At first, observers assumed ‘Oumuamua, which is about 2,600 feet (800 meters) long and about 260 feet (80 m) wide, was an asteroid. But in a June Nature paper led by Marco Micheli, astronomers (including Meech) reported that ‘Oumuamua was not moving as it should. Instead, it showed a “really strong non-gravitational acceleration,” Meech explains, which means its motion indicated that gravity was not the only thing dictating its path.
“There were a number of things that can cause acceleration in that direction,” she says. Her team investigated at least six of those things that could cause the acceleration, throwing out several along the way because they either required the object to be so very strange it wasn’t plausible (e.g., less dense than aerogel), or because the acceleration they produced didn’t match what was observed. The team ultimately concluded “the most likely one [was] comet outgassing, because we see that pretty often in comets.”
Outgassing is simply the process of gas and dust escaping from a comet as it nears the Sun and heats up, sublimating ices on the surface — turning them directly from a solid to a gas. As gas escapes, it gives the comet an extra push in the opposite direction, affecting its speed and rotation, giving it an extra push that deviates from gravitational effects. ‘Oumuamua’s motion “was the same level of strength as you would typically see comets,” says Meech. “So that didn’t seem unusual.”
What was unusual, she says, is that no dust was seen coming from the object. When gas escapes from a comet, it generally carries surface dust with it, and the stronger the outgassing, the larger the grains of dust it can liberate. That didn’t kill the comet theory, however — Meech says that even if the comet was devoid of small dust, large dust could have been coming off the surface and affecting its motion. And large dust is harder to see at visible wavelengths, where astronomers were looking.
So — no dust was seen escaping ‘Oumuamua, but what about gas? “We didn’t see any evidence of gas” either, says Meech. That was strange, because astronomers expected “a fair bit” of gas to be escaping to affect ‘Oumuamua’s motion in the way that was observed. The caveat here is that Meech’s team looked for CN gas, which, she says, is bright and easy to spot. “CN is usually dragged off the comets in our solar system when water comes out,” she says, so it can be used as a proxy for water, which is harder to observe. But down to their detection limit, “there was no CN at all seen. And that upper limit was strong enough that if there was that much water coming off, it says this comet’s also unusual, chemically.” After all, comets in our solar system can be pretty abundant in water.
“But again,” she adds, “it comes from another solar system, so maybe that’s not so surprising.”
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.”
‘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.