Interstellar nomad Oumuamua found to be a comet

In a handout image released by the European Southern Observatory, an artist’s impression of the Oumuamua asteroid, thought to be 800 yards long and 80 yards wide. The discovery of Oumuamua on October 19 set off a worldwide scramble for telescope time to observe it zooming through the solar system at 40,000 miles per hour. (M. Kornmesser/European Southern Observatory )

'Oumuamua' - the first interstellar object found within the solar system - has been reclassified as a comet, after scientists undertook intense scrutiny since its discovery last year.

By combining data from the the Very Large Telescope and other observatories, astronomers has found that the object is moving faster than predicted. The measured gain in speed is tiny and ‘Oumuamua is still slowing down because of the pull of the Sun - just not as fast as predicted by celestial mechanics.

The team, led by Marco Micheli from European Space Agency, explored several scenarios to explain the faster-than-predicted speed of this peculiar interstellar visitor.

The most likely explanation is that 'Oumuamua' is venting material from its surface due to solar heating - a behaviour known as outgassing. The thrust from this ejected material is thought to provide the small but steady push that is sending Oumuamua hurtling out of the Solar System faster than expected - as of June 1, 2018 it is travelling at roughly 114,000 kilometres per hour.

Such outgassing is a behaviour typical for comets and contradicts the previous classification of Oumuamua as an interstellar asteroid.

"We think this is a tiny, weird comet. We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets," said Micheli.

Usually, when comets are warmed by the Sun they eject dust and gas, which form a cloud of material - called a coma (cometary) - around them, as well as the characteristic tail . However, the research team could not detect any visual evidence of outgassing.

"We did not see any dust, coma, or tail, which is unusual. We think that Oumuamua may vent unusually large, coarse dust grains," said Karen Meech of the University of Hawaii in the US.

The team speculated that perhaps the small dust grains adorning the surface of most comets eroded during Oumuamua's journey through interstellar space, with only larger dust grains remaining.

Though a cloud of these larger particles would not be bright enough to be detected, it would explain the unexpected change to Oumuamua's speed.

Not only is Oumuamua's hypothesised outgassing an unsolved mystery, but also its interstellar origin. The team originally performed the new observations on Oumuamua to exactly determine its path which would have probably allowed it to trace the object back to its parent star system. The new results means it will be more challenging to obtain this information. 

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