When a rocky interstellar object now known as 'Oumuamua was first detected in our solar system in October, it was thought to be an asteroid. Now, astronomers say it could be a comet whose core has withstood intense heat from the Sun thanks to a thick crust that insulated it, Science News reports.

Why it matters: Most objects that move between solar systems are thought to be comets so there was excitement that the presumed asteroid 'Oumuamua could help researchers to better understand how other solar systems formed. "Everybody's been assuming that this is just a lump of rock," study author Alan Fitzsimmons of Queen's University Belfast in Northern Ireland told Science News. "This may not be the case."

Comets v. asteroids: Unlike typically rocky asteroids, comets have an icy core that warms as they pass close to the Sun and releases gas and dust seen as a tail. 'Oumuamua traveled close to the Sun but had no tail which led researchers to first believe it was an asteroid.

The evidence: The spectrum of light reflected by 'Oumuamua's surface "similar to organically rich surfaces found in the outer Solar System," the authors wrote. "An internal icy composition cannot therefore be ruled out..." despite it passing close to the Sun and not producing a tail.