Two new studies leave most options open for the origin and composition of 'Oumuamua, the solar system's first interstellar visitor

It’s been a year since the detection of 'Oumuamua (1I/2017 U1), the first known object to enter the solar system from interstellar space. In the aftermath of the discovery, astronomers have been reviewing their observations and sustaining a lively debate about the possible origin and nature of the visitor.

Now, two papers stake new ground in this debate. One claims to have narrowed in on the object’s home star, the other argues that it is not a comet, contrary to earlier reports.

First caught by the robotic telescope Pan-STARRS in Hawaii, ground- and space-based telescopes monitored 'Oumuamua as it quickly left the solar system, disappearing from view only a few months after it was discovered. At first, astronomers thought 'Oumuamua was a dry chunk of rock and metal, devoid of any gas or ice. They couldn’t see the typical signs of comets such as a tail or coma, which would have revealed a release of gas triggered by proximity to the Sun.

But in June 2018, a team of astronomers led by Marco Micheli (ESA SSA-NEO Coordination Center, Italy) announced that some force was slightly changing 'Oumuamua’s trajectory. The researchers argued that the most likely source of this thrust was ice or gas erupting into space that was otherwise invisible. Maybe the dust grains in the surface were too thick, or the composition of the gas made it invisible to telescopes. In any case, they claimed that 'Oumuamua was a comet after all.

But a new study by Roman Rafikov (University of Cambridge, UK), published September 17th on the astronomy preprint arXiv, makes a simple yet compelling argument against the cometary nature of 'Oumuamua. Rafikov argues that if 'Oumuamua released enough gas to change its own trajectory, it should also spin increasingly faster, something that the observations didn’t show. Instead observations show a steady, though tumbling, rotation roughly once every eight and a half hours. The only exception, according to Rafikov, would have been if the force propelling 'Oumuamua was directed very close to its center of mass. “For an object with a very elongated shape and dimensions of hundreds of meters, this sounds close to impossible,” he says.

Although Rafikov doesn’t directly question the results obtained by Micheli and colleagues, he suggests that another explanation for the trajectory is needed and encourages “further analyses of the astrometry data to better characterize its anomalous acceleration.”

Micheli declined to comment for this article.

Determining the nature of 'Oumuamua and other future visitors could reveal clues about the makeup of other planetary systems in the Galaxy. Current theories argue that during planet formation, lots of material is thrown into space. Most of this material is scattered out of the system by giant planets like Jupiter. But how close to the sun those planets reside determines what kind of material gets tossed out. If the giant planets are in the outskirts of the systems, such as in our solar system, the material would be cold and icy. If the planets are really close to the star, like in the case of hot Jupiters, the material is expected to be hot and dry. 'Oumuamua and its ilk could tell astronomers what planetary arrangements are most common.

Tracking the interstellar traveller to its origins

Just a few days after Rafikov posted his paper, a different group of researchers led by Coryn Bailer-Jones (Max Planck Institute for Astronomy, Germany) reported their attempt to find 'Oumuamua’s home star, or at least narrow the search to a few most likely candidates. Their study appeared on the astronomy preprint arXiv on September 24th.

The team used Micheli’s estimated trajectory to carry out the search—indeed Micheli is a co-author of the paper. This trajectory presumes that 'Oumuamua is accelerating on its own. Tracking the trajectory backward and comparing it to the observed positions, motions, and distances for 1.3 billion stars contained in the second data release of the Gaia mission, they identified four dwarf stars as the most likely candidates. These stars were chosen based on similarities between their speeds and that of 'Oumuamua, because astronomers think that ejected objects don’t typically leave their home systems at large speeds.

The best candidate is HIP 3757, a red dwarf star. 'Oumuamua’s estimated trajectory took it about 2 light-years from this star about 1 million years ago. Another of the finalists is HD 292249, a star more similar to the Sun that might have had a close encounter with 'Oumuamua about 3.8 million years ago but with a lower relative speed. The other candidates have combinations of relative speeds and distances that are more unlikely.

Not much is known about any of these stars. None of them have been probed for planets yet—so it’s unknown whether they harbor giant worlds that could slingshot 'Oumuamua—but now that astronomers have a mystery to unravel, they might take a closer look in the near future. Also, Gaia’s third data release is expected in 2021, and it will include ten times as many stars, providing more candidates whose speed and trajectory might be a better match.

Maybe by then we will have figured out if 'Oumuamua is a comet or not.