Scientists have found more evidence that may help shed light on what ‘Oumuamua, a mysterious interstellar object seen flying by Earth four years ago, really is—or what it’s not.
‘Oumuamua, officially named 1I/2017 U1, was detected in October 2017 by the Pan-STARRS1 telescope in Hawaii. (‘Oumuamua, pronounced “oh-mooah-mooah,” roughly means “scout” in Hawaiian.) It has intrigued the science community since then, because astronomers couldn’t make out what it was.
‘Oumuamua’s trajectory and speed suggested that it was likely a comet from outside the solar system. But unlike any comet observed in the past, it didn’t have a visible gas tail, leading some scientists, notably Harvard University’s Astronomy Department chair Avi Loeb, to wonder if it could be some sort of an artifact of an alien civilization.
The hypothesis was called into question by a study published in March by two Arizona State University astrophysicists, which suggested that ‘Oumuamua was more likely a nitrogen iceberg chipped off from an “exo-Pluto,” or a Pluto-like planet in another star system.
However, a more recent study conducted by Loeb’s team ruled out that possibility. A research paper accepted for publication in the academic journal New Astronomy last month argues that for ‘Oumuamua to be a nitrogen iceberg ejected by an ex-Pluto, its parent planet would have an unreasonably large mass density.
“We show that the mass budget in exo-Pluto planets necessary to explain the detection of `Oumuamua as a nitrogen iceberg chipped off from a planetary surface requires a mass of heavy elements exceeding the total quantity locked in stars with 95% confidence, making the scenario untenable because only a small fraction of the mass in stars ends in exo-Plutos,” the researchers wrote in a pre-print of the study. The paper is authored by Loeb and his colleague Amir Siraj, a theoretical astrophysicist at Harvard.
Loeb and Siraj calculated the required mass density of ‘Oumuamua’s parent planet using its observed dimensions, inferred initial mass, the estimated time it had traveled in space before passing by Earth, and the estimated abundance of ‘Oumuamua-like objects based on the fact that no similar object has been detected since 2017.
“The nitrogen fragment hypothesis is strongly disfavored, since no known physical process could accommodate such a mass budget” of an exo-Pluto planet, the authors concluded, adding that the mass budget would be even larger if erosion of nitrogen icebergs by cosmic rays is taken into account.
More research will be needed to determine what ‘Oumuamua really is. Before the nitrogen iceberg hypothesis, one prevailing theory went that it might be a hydrogen iceberg, in which case we would not see a gas tail even it had one because hydrogen is transparent. But an earlier study led by Loeb proved that a hydrogen iceberg from interstellar space would have long evaporated before it reached our solar system.