New results suggest that 'Oumuamua is a comet
New results suggest that 'Oumuamua is a comet
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Artistic illustration of 'Oumuamua. Asteroid or comet? Credit: ESA / Hubble, NASA, ESO, M. Kornmesser.
'Oumuamua - the first interstellar object discovered in the Solar System - has been the object of study since its discovery in October 2017. Now, an international team of astronomers has discovered that the object moves faster than expected. 'Oumuamua is still slowing down due to the attraction of the Sun, but not as fast as the celestial mechanics predicts.
The team, led by Marco Micheli (European Space Agency) explored several scenarios to explain why the speed of this peculiar interstellar visitor is faster than predicted. The most likely explanation is that 'Oumuamua is releasing material from its surface due to the warming caused by the Sun, a phenomenon known as degassing. It is believed that the momentum generated by this ejected material provides the small, but constant push that is making 'Oumuamua leave the Solar System faster than expected.
Such emission of gases is a behavior typical of comets and contradicts the previous classification of 'Oumuamua as an interstellar asteroid. "We think it's a small, strange comet," says Marco Micheli. "We can see in the data that its momentum is getting smaller as it moves away from the Sun, which is typical of comets."
Generally, when comets are heated by the Sun, they eject dust and gas that form a cloud of material around them called a coma, as well as the characteristic tail. However, the research team has not detected any visual evidence of the emission of gases.
"We have not seen dust, coma or tail, which is unusual," explains one co-author, Karen Meech, of the University of Hawaii (USA). Meech led the team that discovered and characterized 'Oumuamua in 2017. "We believe that' Oumuamua can release unusually large and thick dust grains."
Predicted position of 'Oumuamua in front of its observed position. Credit: ESA.
Most comets have small dust grains on their surfaces, but the team speculates that perhaps those from 'Oumuamua have eroded during the trip through interstellar space, leaving only large grains of dust. Although a cloud of these larger particles would not be bright enough to be detected, it would explain the unexpected change in speed of 'Oumuamua.
Oumuamua's degassing hypothesis is not his only unsolved mystery, so is its interstellar origin. At first, the team made new observations of 'Oumuamua to determine exactly its trajectory, which could have allowed to trace the path of the object to its star system of origin. The new results show that obtaining this information will be more difficult than previously thought.
"The true nature of this enigmatic interstellar nomad can remain a mystery," said team member Olivier Hainaut, an astronomer at ESO. "The speed increase recently detected in 'Oumuamua makes it more difficult to trace the route he took from his home, his extrasolar star system."
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