New research concludes that the interstellar object that entered our solar system and then rocketed out was a small comet and not a spacecraft, as some had speculated. (JPL/NASA)

In 2017, an  unusual small object flew into our solar system from afar,  approached the inner solar system and the Sun and then sped back out to interstellar space.  In all, it was detected and followed for 11 days.

The object was puzzling because such interstellar visitors had not been observed before, and most mysterious because it accelerated in a most unusual way out of the solar system.  This was not the normal behavior of any object in the solar system.

The object, a few hundred meters in length, was first identified as an asteroid because it had not of the sparkle of a comet, and a “dark comet” was proposed, and  then  something perhaps sent by aliens to explore the solar system.  After all, the shape of the object known as ‘Oumuamua — Hawaiian for “Scout”– was described as reddish and sometimes shaped like a pancake and sometimes like a cigar.

‘Oumuamua became an object of great fascination among space scientists and even became the subject a popular book by a Harvard astronomer who argued that it was clearly an alien lightsail. That is, a probe that is propelled by the propulsive radiation of starlight itself.

Now, a paper offers a very different, and apparently quite compelling, explanation.,

In Nature, University of California, Berkeley astrochemist Jennifer Bregner and Cornell University astronomer Darryl propose that the comet’s mysterious deviations from a typical object’s path around the Sun can be explained by a simple physical mechanism likely common among many icy comets: outgassing of hydrogen as the comet warmed up in the sunlight.

What made ‘Oumuamua different from every other well-studied comet in our solar system was its size. It was so small that the gravitational nudge it received around the Sun was slightly altered by the tiny push created when hydrogen gas spurted out of the ice.

And that’s what caused the acceleration, the scientists say.

An artist rendering of ‘Oumuamua, the first known alien object to enter our solar system. It was identified by the Pan-STARRS 1 telescope in Hawaii. It was later followed by observatories around the world and astronomers generally concluded that it had been traveling for millions of years before its chance encounter with our solar system. (ESO/E. Kornmesser)

Before we go further, first some background about comets:

Comets are icy rocks (or “dirty snowballs”) left over from the formation of the solar system 4.5 billion years ago, and they periodically approach the Sun from the outer reaches of our solar system.  (In 2019, a comet from outside the solar system — comet Borisov — became the second interstellar object detected inside our system.)

Because they are so ancient, comets can tell astronomers about the conditions that existed when our solar system formed. Interstellar comets can also give hints to the conditions around other stars surrounded by planet-forming disks.

When warmed by sunlight, a comet ejects water and other molecules, producing a bright halo or coma around it and often tails of gas and dust. The ejected gases act like the thrusters on a spacecraft to give the comet a tiny kick that alters its trajectory slightly from the elliptical orbits typical of other solar system objects, such as asteroids and planets.

When ‘Oumuamua was discovered, it had no coma or tail and was too small and too far from the Sun to capture enough energy to eject much water.  Thus the broad speculation about what it might be.

It was not really surprising that the field came up with so many possible answers to the question of ‘Oumuamua.  After all, comets observed in our solar system are a half-mile to hundreds of miles across.  ‘Oumuamua was not only tiny and thin, but didn’t have many features of a comet either.

Jennifer Bergner, the UC Berkeley astrochemist, explained the solution proposed in a release:  “A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result.”

“Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?” Bergner said. “Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?”

The answer that she and Cornell postdoctoral fellow Darryl Seligman  came to was that indeed it could.

Halley’s comet as it traveled through the sky in 1986. The coma (or head) of the comet is seven-miles wide. It is one of the few comets that approach Earth at a distance that allow it to be seen with the naked eye. But it only comes every 75 to 79 years. (Wikipedia)

When she began studying ‘Oumaumua, Bergner found found that experimental research published in the 1970s, ’80s and ’90s demonstrated that when ice is hit by high-energy particles akin to cosmic rays, molecular hydrogen (H2) is abundantly produced and trapped within the ice. In fact, cosmic rays can penetrate tens of meters into ice, converting a quarter or more of the water to hydrogen gas.

“For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both in terms of composition and any acceleration, you wouldn’t necessarily expect that to be a detectable effect,” she said. “But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.”

And that could explain the otherwise mysterious speed-up as it headed past the Sun and back into interstellar space.

Seligman was taken with the simplicity of the solution.

“What’s beautiful about Jenny’s idea is that it’s exactly what should happen to interstellar comets.” he said. ” We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation.”

Jennifer Bergner in her Ph.D. lab at Harvard University. In the background is instrumentation for studying ice chemistry at the frigid temperatures characteristic of interstellar space. (Luke Kelley)

Seligman and Bergner began their work on ‘Oumuamua when they were both postdoctoral fellows at the University of Chicago.

Their paper was well received by scientists in the field, though it is unlikely to be the last word on the nature of the strange object that visited us briefly in 2017.  Other objects like it have been detected since by Seligman and colleagues — “dark” and small comets with no observable comas or tails — now that astronomers know what to look for.

Compared with some of the other explanations for the origins and nature of  ‘Oumuamua  including the ones featuring alien technology — this explanation has the advantage of being firmly grounded in  is science.  It may not have the dazzle of some impossible-to-prove alien visitation,  but it is based on the well-understood astrochemistry of comets.