Here’s a thought that puts a different spin on exoplanet studies. The speaker is Darryl Seligman (Cornell University):
“The comets and asteroids in the solar system have arguably taught us more about planet formation than what we’ve learned from the actual planets in the solar system. I think that the interstellar comets could arguably tell us more about extrasolar planets than the extrasolar planets we are trying to get measurements of today.”
Seligman’s comment plays into the growing interest in interstellar objects that drift into our Solar System like 1/I ‘Oumuamua and 2/I Borisov. These may be the initial members of what is actually a large class of debris from other stars that we are only now learning how to detect. Among the many things we have yet to refine in our understanding of ‘Oumuamua is its actual size. Projections of 115 by 111 by 19 meters are deduced from its brightness and the changes produced by its apparently tumbling motion. The interstellar interloper is too far from Earth and too small to resolve.
Image: This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. The large range of brightness — about a factor of ten (2.5 magnitudes) — is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object’s shape or surface albedo. Credit: ESO/K. Meech et al.
I mention this just to underline how difficult it is to make sense of ‘Oumuamua at present. Absent a fast mission to catch up with the object (and there are ideas out there, as we’ve discussed in these pages before), its dimensions will remain ambiguous. And what of the anomalous non-gravitational acceleration that astronomers noted in 2018? Seligman, who along with Gregory Laughlin has written about fast missions to ‘Oumuamua in a paper from that year, is also behind the conjecture that the object could be composed of molecular hydrogen ice. It’s no wonder, then, that his interest was piqued by Jennifer Bergner at UC-Berkeley, whose work involves chemical reactions on objects in space. And it has led to an alternative explanation for the anomalous acceleration of 1/I ‘Oumuamua, one he likes better than hydrogen ice.
Explanations for the acceleration have proliferated, many of them aimed at discrediting the concept that the object might be technological, a thought too radical for many scientists. But are ideas like hydrogen icebergs and shards of nitrogen credible in their own right? Bergner wasn’t satisfied with either pole of the controversy, and her initial work revealed that there was an explanation well documented in earlier research. Experiments beginning in the 1970s and continuing later showed what happens when ice is impacted by high-energy particles like cosmic rays. The process produces molecular hydrogen in quantity that remains trapped within the ice.
Warming up these pockets of molecular hydrogen would cause the outgassing of the H2, even if the gas were trapped tens of meters inside cometary ice. Says Bergner:
“For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn’t necessarily expect that to be a detectable effect. But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.”
That would be a helpful explanation if it can be made to fit. With the brightness of ‘Oumuamua changing periodically by a factor of 12 and varying asymmetrically, the object’s shape appeared to be elongated and its tumble was apparent. A ‘local’ comet leaving the Sun after perihelion is known to eject water and gas from its surface, producing the familiar gaseous coma and releasing dust. A dusty cometary tail forms, while a secondary tail of vapor, dust and possibly organic materials pushed by the momentum of solar photons can appear, this one pointing away from the Sun.
It was the absence of these processes that flagged ‘Oumuamua as unusual. There was no coma here, nor were outgassed molecules or dust observed, and as the authors note in their paper in Nature, solar energy hitting the object in its trajectory would not cause enough water or organic compounds to sublimate to account for the acceleration. Without a dusty coma, explaining ‘Oumuamua’s acceleration was a problem, which is why hydrogen icebergs and nitrogen shards emerged as hypotheses, for they could conceivably produce enough kick to supply the needed force.
Bergner’s search of the literature found earlier experiments where ice chilled to the temperatures of deep space is converted into molecular hydrogen by high-energy radiation. The release of that hydrogen by a heat source is sufficient, her work with Seligman confirmed, to produce a gas discharge that would affect ‘Oumuamua’s orbit. No dusty coma, then, is needed, and indeed, none is discharged by this process. Cometary ice is not sublimating but rearranging itself as H2 is released.
Seligman seems confident in the analysis:
“Jenny’s definitely right about the entrapped hydrogen. Nobody had thought of that before. Between discovering other dark comets in the solar system and Jenny’s awesome idea, I think it’s got to be correct. Water is the most abundant component of comets in the solar system and likely in extrasolar systems, as well. And if you put a water rich comet in the Oort cloud or eject it into the interstellar medium, you should get amorphous ice with pockets of H2.”
And in fact, working with Bergner, Seligman and colleagues have identified six other comets that lack an observable coma but demonstrate small non-gravitational accelerations. Usefully, one of these is comet 1998 KY26, which is targeted by Japan’s Hayabusa2 mission, the same mission that collected samples of the asteroid Ryugu. It was only in December of 2022 that 1998 KY26 was identified as a comet rather than an asteroid. Is hydrogen outgassing like this in fact common to deep space comets? The Rubin Observatory Legacy Survey of Space and Time (LSST), which will go into operation in 2025, may give us a notion of the distribution of such objects.
The paper is Bergner and Seligman, “Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice,” Nature 22 March 2023 (abstract).