Was Interstellar Object `Oumuamua a Chunk of Exo-Pluto?

Humans have observed two interstellar objects to date, and have struggled to understand their origins and nature. At the Exoplanets in Our Backyard 2 workshop held in Albuquerque, New Mexico, Steve Desch (Arizona State University) presented a plausible scenario that accounts for all aspects of the first known interstellar object, 1I/`Oumuamua.

To recap, `Oumuamua was first spotted by the Pan-STARRS telescope in Hawai`i on October 19, 2017, already past perihelion and on its way back out of the solar system. Its strong brightness variations hinted at a non-spherical shape. It also slowed more than expected — 10 times more than it would if it were a typical comet — as it exited the solar system, a phenomenon called non-gravitational acceleration. There was also no visible coma, nor a tail of either dust or gas.

`Oumuamua is long beyond the reach of telescopes now, but studies continue. In 2019, Sergey Mashchenko performed a careful analysis of its light curve and found that the best-fit shape for Oumuamua is not a cigar, as it’s often depicted, but a pancake about 6 times wider than it is thick (it measures 113 by 111 by 19 meters).

Such an extremely flattened shape is almost unheard of in the solar system. What could account for `Oumuamua’s strange shape, extra-cometary acceleration, lack of visible cometary activity, and extrasolar origin?

All observed comets have very low albedo, reflecting only a few percent of the light that strikes them. But what if, Desch and team member Alan Jackson (also Arizona State) surmised, `Oumuamua was made of something brighter, some kind of ice?

Desch and Jackson investigated the sublimation behavior of various ices common in the outer solar system, such as carbon dioxide, ammonia, oxygen, nitrogen, carbon monoxide, neon, and methane. They found that if `Oumuamua were made of nitrogen ice, it would have the right albedo and the right mass to produce the exact amount of non-gravitational acceleration observed by astronomers as it retreated from the Sun. And if it were nearly pure nitrogen ice, it would exhibit this cometary behavior without any of the hallmarks of comets, neither reflecting sunlight from dust nor lighting up with emission from water or other gases.

Hypothesizing pure nitrogen ice for Oumuamua’s composition solves some other puzzles, too. The body passed within 0.2 astronomical units (a.u.) of the Sun (20% of the distance from the Sun to Earth), and yet it survived to exit the solar system. But only barely, according to Desch and Jackson’s model. A nitrogen-ice `Oumuamua would have lost 95% of its mass by the time it exited the inner solar system; evaporative cooling would have insulated the remaining morsel through the harrowing passage.

That much mass loss also explains the extreme shape. If you add 20 times the present mass in concentric layers around the present pancake, reversing its evaporation by the Sun, the original body would have had a much more normal 2:1 aspect ratio.

Where would such a big chunk of nitrogen ice have come from? Within our solar system, there are a few worlds in the Kuiper belt, such as Pluto and 225088 Gonggong, that might have lost chunks of dust-free, nearly pure nitrogen ice crust to impacts. In a 2021 paper, Desch and Jackson worked out that during the formation of the solar system, about 100 trillion ice fragments would’ve been ejected from our solar system, two-thirds of them nitrogen ice. For perspective, that’s more ice fragments ejected from our solar system than there are comets in the Oort Cloud!

But even though 100 trillion is a big number, statistically speaking it just isn’t enough to make enough pieces that one of them is likely ever to have come through our solar system when we were capable of looking.

Here is where we get to the new (and, as yet, unpublished) work. Our Sun isn’t the most common type of star; cooler M-type stars are much more common. M stars are more favorable environments for the creation of worlds covered in nitrogen ice. In our solar system, you have to be nearly at the orbit of Neptune, at 15 a.u., for nitrogen ice to be stable on the surface. However, stars at the lower end of the mass range (technically classified as M8) can host worlds with nitrogen ice at only 1 a.u.

Taking into account the huge population of M stars and their more favorable environments for hosting nitrogen ice, Desch and Jackson found that M stars will have ejected 40 times more nitrogen ice fragments than stars like our Sun.

That contrast is enough for `Oumuamua’s appearance in our backyard to be a likely accident. Its trajectory, which lies in the galactic plane and has relatively low speed for an interstellar interloper, indicates that it hasn’t been roaming the galaxy on its own for very long; it probably exited its parent solar system up to a few hundred million years ago.

Desch and Jackson point out that if the object were about 500 million years old, it would likely have come from a young star in the Perseus arm of the Milky Way. If so, our observations of `Oumuamua were of a fragment of the surface of a young exoplanet — and we’re likely to be visited by more such interplanetary travelers in the future.

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