What is ’Oumuamua?


’Oumuamua means ‘messenger from faraway’ or ‘a scout that arrived first’, in Hawaiian. It’s the first object ever discovered in the Solar System that comes from outer space. We can tell that because it’s moving too fast to be bound to the Sun gravitationally.

What’s so unusual about it?

It appears to have an extreme shape that’s five to 10 times longer than it is wide, based on the reflected sunlight. If it’s a perfect reflector, it’s at least 20 metres in size; if not, then it’s hundreds of metres in length. So it doesn’t look like any asteroid we’ve ever seen.

It also deviates from the orbit that we’d expect based on the Sun’s gravity, and that’s due to some extra force acting on it. For comets, such a force would be provided by the evaporation of ice on the surface. But there’s no cometary tail around the object and, moreover, you would expect a significant fraction of its mass to be evaporated to give it the boost that’s needed. Plus, it rotates every eight hours – its rotation period would have changed if it was propelled by the rocket effect from cometary evaporation.

: Planets form in a star’s protoplanetary disc, but such a mechanism is unlikely to have formed a cigar-shaped object such as ’Oumuamua © NASA
Planets form in a star’s protoplanetary disc, but such a mechanism is unlikely to have formed a cigar-shaped object such as ’Oumuamua © NASA

How did the object reach us?

In our paper, we basically explain the extra push that it has based on sunlight pushing it – the radiation pressure. Just as a sail boat moves forward due to the reflection of air from the wind, so light exerts a force when it’s reflected off a surface. So we’re suggesting maybe it’s a light sail, because I couldn’t imagine making a thin structure just naturally in the interstellar medium or a protoplanetary disc [dust and gas that goes on to form planets]. If anyone else has a better explanation, then they should put it forward.

Could it really be an alien probe?

Yeah, that’s one possibility, that it may be of artificial origin. It could be some defunct technological equipment, it could be on a reconnaissance mission. We [humans] are now developing light sail technology and it’s possible that another civilisation out there has already mastered this technology.

More like this

People are contemplating a mission to chase down this object and take a photograph by flyby, but it moves faster than chemical rockets, so we need to develop technology that’s more capable. A simpler approach would be to look for other objects of the same type.

Will ’Oumuamua change astronomy?

I think it opens a new frontier for research. The traditional way to search for extraterrestrial intelligence is by looking for signals. This is just another way, looking for space debris, searching the sky for objects that may be artefacts of other civilisations. The mere discovery of this object is a surprise, because you need 1015 or a thousand trillion objects ejected by every star in the Milky Way galaxy in order for us to have seen one at random.

A decade ago, I predicted there shouldn’t be any observed with existing surveys. Within a few years, there will be a new survey by the Large Synoptic Survey Telescope that will be much more sensitive.

Earth’s water may not all have come from asteroids © NASA


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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.