In Search Of ‘Oumuamua’s Home

‘Oumuamua (OH-mu-uh-mu-uh) races toward the outskirts of our solar system in this artist’s concept. The object, possible a mildly-active comet is estimated at 800 long by 100 feet (230 x 35 meters) wide. The last observations of ‘Oumuamua were in January by the Hubble Space Telescope before it became too faint to see. NASA/ESA/STScI

Ever since astronomers realized that ‘Oumuamua wasn’t a member of our solar system but instead a far-flung visiting asteroid or comet, they’ve been trying to figure out from whence it came. We knew the object wasn’t from here because it was moving so fast it would soon escape the solar system. Now, using data from the European Space Agency’s Gaia stellar surveyor, project scientists have identified four stars that are possible places of origin of ‘Oumuamua, an interstellar object spotted during a brief visit to our solar system in 2017.

Animation of the ‘Oumuamua passing through the solar system. It’s moving along a trajectory unbound to the sun.

It’s not gone just yet. Currently 1.9 billion miles away, a little further than the planet Uranus, ‘Oumuamua’s on its way to stellar encounters in the far distant future. At first, astronomers thought the object was an asteroid, but later observations revealed it wasn’t slowing down as fast as should have under the influence of the sun and planets’ gravitational attraction. The best explanation for the change in speed is to reconsider the object as a comet. Comets are made of dusty ices that turn directly from solid to gas (vaporize) when warmed by the sun. Gas released this way can push a comet enough to change its course.

But where in our galaxy did it come from? One imagines that it must have first formed from a cloud of gas and dust around a star much like our planets, asteroids and comets did around the sun. It’s possible that during the formation of ‘Oumuamua’s solar system, it was ejected when it passed too close to a giant planet there and has been wandering the cold spaces between the stars since.

‘Oumuamua outgassing and rotating. 

To look for its home, astronomers had to trace back in time not only the trajectory of the interstellar comet, but also of a selection of stars that might have crossed paths with this object in the past few million years while still in the vicinity of the sun. That’s where Gaia comes in. It’s surveyed the positions and motions of more than a billion star in our galaxy. Among these there’s a subset of seven million stars for which we also know how fast they’re moving toward or away from us, which enabled the team to reconstruct their trajectories from the past into the future.

These animations show the relative trajectories of ‘Oumuamua (shown in blue), the sun (yellow) and one of the four stars that could be ‘Oumuamua’s home (shown in red) over the past seven million years. The left panel shows the view on the plane of our Galaxy, while the right panel shows the view perpendicular to the plane. ESA

All four are dwarf stars – with masses similar to or smaller than our sun’s – and had their close encounter with the comet between one and seven million years ago. Unfortunately, none of them is known to have planets or to be part of a binary star system; a giant planet or companion star would be the preferred way for ‘Oumumua to get kicked out of the system.

In the next couple years, Gaia will deliver data on many more stars which will allow astronomers to replay the trajectories of these stars to see if they can find a better match. In case you’d like to know here are the particulars for two of the four candidate stars I was able to pinpoint on a typical star atlas:

  • HIP 3757 — a 12th magnitude red dwarf in Cetus
  • HD 292249 — a faint, sun-like star in Monoceros

Here’s the scientific paper on the topic for all the details.

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