On October 19, 2017, astronomers first saw an object from another solar system traveling through our own. Zipping into our solar system from above, the interloper, now known as 1I/2017 U1 (âOumuamua), swung around the Sun and shot away again, never to return once it leaves our neck of the woods for interstellar space once more.
What have we learned about this mysterious visitor?
âOumuamuaâs Path
Astronomers were able to record data on âOumuamua (which means âscoutâ or âmessengerâ in Hawaiian) for only a brief period of time, although itâs been in our solar system for over a century. âIt came into the solar system, inside 1,000 AU, around 1837 or so,â 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 âOumuamua wasnât spotted until last year because it was too far from the Sun to reflect enough light for astronomers to pick it up, even with todayâs 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, âOumuamua was again on its way out, growing ever fainter. âOur very last observations from Hubble were January 2nd of 2018 at fainter than magnitude 27,â says Meech. âBy May 3rd, it was again outside of Jupiterâs orbit. And ⊠once it gets beyond about 1.2 AU or so, itâs 25th magnitude, so beyond Jupiterâs orbit, forget it. Itâs gone.â
And while it may be gone from sight, itâs not yet gone from the solar system. According to Meech, âOumuamua 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 âOumuamua truly passes the âedgeâ of the solar system depends on where you define that edge.
Regardless, since January, âOumuamua has been beyond astronomersâ 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.
Speeding Up
At first, observers assumed âOumuamua, 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 âOumuamua was not moving as it should. Instead, it showed a âreally strong non-gravitational acceleration,â Meech explains, which means its motion indicated that gravity was not the only thing dictating its path.
âThere were a number of things that can cause acceleration in that direction,â 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 wasnât plausible (e.g., less dense than aerogel), or because the acceleration they produced didnât match what was observed. The team ultimately concluded âthe most likely one [was] comet outgassing, because we see that pretty often in comets.â
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 â 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. âOumuamuaâs motion âwas the same level of strength as you would typically see comets,â says Meech. âSo that didnât seem unusual.â
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 didnât kill the comet theory, however â 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 â no dust was seen escaping âOumuamua, but what about gas? âWe didnât see any evidence of gasâ either, says Meech. That was strange, because astronomers expected âa fair bitâ of gas to be escaping to affect âOumuamuaâs motion in the way that was observed. The caveat here is that Meechâs team looked for CN gas, which, she says, is bright and easy to spot. âCN is usually dragged off the comets in our solar system when water comes out,â she says, so it can be used as a proxy for water, which is harder to observe. But down to their detection limit, âthere 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 cometâs also unusual, chemically.â After all, comets in our solar system can be pretty abundant in water.
âBut again,â she adds, âit comes from another solar system, so maybe thatâs not so surprising.â
âOumuamuaâs Strange Reflections
How else is âOumuamua strange? Astronomers used the Spitzer Space Telescope for over 30 hours of observations of âOumuamua in the infrared â or rather, Meech says, they tried. âThey detected nothing,â she says, when looking in the thermal infrared, which detects heat. âSo it was just an upper limit.â But sometimes even non-detections can provide information. âFrom this, they said, âOkay, if it has thermal properties typical of comets, then itâs going to have to be brighter than everyone thought.ââ
Reflectivity, or how much light an object reflects, can tell astronomers a lot about its shape, size, and composition. âWe just always assumed [the reflectivity] was cometlike and very low,â Meech says. But âinstead of the normal four percent reflectance, which is typical of comets, they were getting more like 10 percent reflectance. So that again isnât 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,â she says.
Those same infrared observations also failed to see significant carbon monoxide or carbon dioxide outgassing from âOumuamua. Thatâs important because Meechâs 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.
âItâs always a little bit tricky to interpret too much out of not seeing something,â Meech cautions. But âyet again, itâs not quite hanging together as a typical comet.â
Whereâs Home?
âOumuamua was born in another solar system, so perhaps all these oddities are normal for something that formed somewhere else. The next question then becomes, âWhere did it form?â
âThe 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,â says Meech. Coryn Bailer-Jones of the Max Planck Institute for Astronomy in Heidelberg, Germany, calculated âOumuamuaâs 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 âOumuamuaâs, to look for places where the two might reasonably overlap.
He found four possible candidates that passed close enough to the rewound track of âOumuamuaâs motion that they might be its home. (By âclose enough,â Meech says, Bailer-Jones was looking at stars âOumuamuaâs path had come within one to two Oort Cloud radii â 100,000 to 200,000 AU.)
But even here, all was not straightforward. âThe only problem with these four candidates is that the encounter velocities were relatively high, between about 10 and 25 kilometers per second,â she says. ââOumuamua is probably an ejected planetesimal or building block from the birth process of planets, and itâs 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. Itâs not impossible but it is pretty unlikely.â
It would be more likely, she says, if âOumuamua 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 âOumuamuaâs home. âIt would be great to revisit this in a couple of years,â she says, when more data from Gaia is available.
An Ongoing Mystery
âOumuamua remains the first and (thus far) only interstellar visitor weâve caught skipping through our solar system. The data available on it are limited, but that doesnât stop researchers around the world from wondering what they can find within its depths. âWhatâs especially fascinating is that people are still writing papers on it,â says Meech, âeven 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 thatâs it. And people are still writing papers.â
Whatâs more, A recent TED talk Meech gave on âOumuamua currently has more than 2 million views and is continuing to gain more, she says. âClearly people are interested.â
Itâs certainly an interesting object. Its strange properties have even prompted some to suggest itâs of truly alien â as in, alien-built â origin. But those arguments donât really hold up against the data we do have, Meech says, limited though it may be.
So while itâs not an alien spaceship or an artificial light sail, âOumuamua is still a mysterious, transient visitor that has captured the attention of researchers and the public alike.