![]() It was the absence of these processes that flagged ‘Oumuamua as unusual. 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. 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. 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. That would be a helpful explanation if it can be made to fit. But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.” “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. Warming up these pockets of molecular hydrogen would cause the outgassing of the H 2, even if the gas were trapped tens of meters inside cometary ice. The process produces molecular hydrogen in quantity that remains trapped within the ice. Experiments beginning in the 1970s and continuing later showed what happens when ice is impacted by high-energy particles like cosmic rays. 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. And it has led to an alternative explanation for the anomalous acceleration of 1/I ‘Oumuamua, one he likes better than hydrogen ice.Įxplanations 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. 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 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. 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. I mention this just to underline how difficult it is to make sense of ‘Oumuamua at present. 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. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. 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. ![]() Image: This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. ![]() The interstellar interloper is too far from Earth and too small to resolve. Projections of 115 by 111 by 19 meters are deduced from its brightness and the changes produced by its apparently tumbling motion. Among the many things we have yet to refine in our understanding of ‘Oumuamua is its actual size. 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. 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. 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.” “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. ![]() The speaker is Darryl Seligman (Cornell University): Here’s a thought that puts a different spin on exoplanet studies.
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