It has been 2½ years since astronomers in Hawaii discovered a strange, cigar-shaped object speeding through the solar system on a trajectory from far away and toward even farther away. Today Oumuamua, the Hawaiian term for “scout,” as the object was named, is now somewhere between the orbits of Saturn and Neptune and on its way to the Great Out There, but astronomers are still wondering and debating what it was.
The cosmic interloper was first thought to be an interstellar asteroid, a chunk of rock shed by another star system. Then astronomers decided it must be a comet, flung loose from some faraway star and planetary system. Briefly they speculated that it could be an alien artifact, a derelict probe or a fragment from a planetesimal that was ripped apart.
Now a pair of Yale astronomers have suggested that Oumuamua was neither an asteroid nor a comet. Rather, it was a cosmic iceberg: a chunk of frozen hydrogen.
Moreover, it was a primordial leftover, originating from a place and time where stars and planets didn’t exist yet: the deep, dark core of an interstellar cloud, one of the assemblages of gas and dust that shadow the starry lanes of the Milky Way, and where stars are sometimes born.
That might not sound as exciting as an alien spaceship, but if the assessment is accurate, it would provide astronomers with direct insight into stellar nurseries, a part of the universe that human technology cannot access.
These clouds, composed mostly of molecular hydrogen left over from the Big Bang, can contain the mass of tens of thousands of suns and span hundreds of light-years.
At their center, where no sun yet shines, protected from radiation, the temperature can plunge to a few degrees above absolute zero, cold enough for hydrogen itself, the lightest, most volatile and most common element in the universe, to freeze. In turn, these frozen particles stick to small grains of interstellar dust, growing in a few thousand years into an ice cube 1,000 feet wide.
Darryl Seligman, postdoctoral researcher at the University of Chicago, said that if he and his colleagues are right, more cosmic icebergs will surely come to be detected by facilities like the new Vera Rubin Observatory in Chile.
Astronomers will be able to watch them light up and evolve as the erosive power of sunlight gets to work.