Last year, the Solar System was treated to its first known tourist. ‘Oumuamua, an odd, cigar-shaped body, shot through our neighborhood at high speed, following an orbit that indicates it arrived from somewhere else. Although bodies ejected from other solar systems are expected to make regular visits, this was the first one that we’d imaged sufficiently to determine that its origins were elsewhere.
The imaging, however, didn’t resolve a somewhat different debate: what, exactly, is ‘Oumuamua? Its odd orbit had initially had it categorized as a comet, as these tend to have more extreme orbits. But imaging didn’t show any indication of gas and dust being released, as is typical when a comet approaches the Sun. That imaging also revealed that it had an elongated, cigar-like shape. Combined with its relatively rapid rotation, this would indicate that ‘Oumuamua had to be fairly robust, leading to the conclusion that it was probably an asteroid.
But now, a large international team of researchers is weighing in with another vote for comet. The argument, says the team, is based on the odd behavior of ‘Oumuamua, which appears to have been accelerating away from the Sun.