The first-known visitor from outside the Solar System, an object dubbed ‘Oumuamua, is an icy comet rather than a rocky asteroid. New measurements help to confirm early guesses as to the composition of the interstellar interloper, and could also aid researchers in their hunt for similar objects in our Solar System.
Careful observations of ‘Oumuamua’s orbit showed that as the object flew through space, something continually nudged it a tiny bit farther from the Sun than expected. That something was probably ice that warmed up and sprayed gas into space. This process is characteristic of a comet, rather than an asteroid, even though ‘Oumuamua never displayed the glorious tail of gas and dust that accompanies most comets.
“It’s an unusual comet, and that’s pretty exciting,” says Karen Meech, an astronomer at the University of Hawaii in Honolulu. She and her colleagues, led by astronomer Marco Micheli of the European Space Agency in Frascati, Italy, report the discovery on 27 June in Nature.
