Image copyright Science Photo Library Image caption Artist's impression: 'Oumuamua is unusually elongate

The interstellar object which passed through our solar system in late 2017 is still surprising scientists.

When researchers first spotted 'Oumuamua, it did not appear to have the characteristic tail or coma (cloud of ice and dust) that define comets, and was thought to be an asteroid.

But that didn't fully explain the object's movements.

Now scientists think it may be a comet after all, with valuable information about distant planetary systems.

When studied by a team led by Dr Marco Micheli, part of 'Oumuamua's observed acceleration was best accounted for by the effect of the Sun's heat on its icy surface.

Image copyright Science Photo Library Image caption The comet Hale-Bopp, with a tail of ice and dust

Comet vs asteroid

Comets consist of both rock and ice, and form beyond the 'ice line', where it is cold enough for water to remain frozen.

In our Solar System, this lies almost as far from the Sun as Jupiter.

Asteroids are rocky objects which mainly orbit in a belt between Mars and Jupiter.

However 'Oumuamua isn't the only case in which the distinction hasn't been clear.

"There's becoming an increasingly blurry line between the two as we are finding comet-like objects in the main asteroid belt," says Prof Sara Russell of London's Natural History Museum.

Image copyright Science Photo Library

Why is the difference important?

As our first visitor from another solar system, the comet can tell us more about how planets form.

"Comets likely formed towards outer regions of other planetary systems, so perhaps they can escape the gravity of their parent star and go into interstellar space more easily than an asteroid," Prof Russell told BBC News.

"'Oumuamua and other interstellar travellers that may visit our solar system can potentially give us some brilliant clues about the nature and composition of other planetary systems. Ultimately these objects may show us whether our solar system is unique, or one of many habitable systems in our galaxy."

Will we learn more about 'Oumuamua?

The object was visible to powerful ground-based telescopes for about 2.5 months after its discovery.

According to Dr Micheli, it was last seen by the Earth-orbiting Hubble Space Telescope in early 2018.

Researchers can continue to work on the information gathered from its one way trip through the Solar System, but we won't see 'Oumuamua again.

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