In a study published in March, astronomers suggested that "Oumuamua could be an asteroid that flies past our solar system after being ejected from a binary star system, although they rely on models to show that this is a possibility Lead author Alan Jackson remarked, "It's really strange that the first object we would see from outside our system would be an asteroid, because a comet would be much easier to see, and the solar system throws many more comets than asteroids out. "
Now, by combining telescope data from the Hubble Space Telescope and other ground-based observatories, an international team of astronomers has shown that 'Oumuamua is indeed a speed-famous comet rather than an asteroid as astronomers previously thought.
In the new study published in the journal Nature on June 27, researchers found that "Oumuamua accelerates slowly and steadily away from the sun, meaning it moves faster as predicted by celestial mechanics. "a very well-understood branch of astronomy that deals with the motions of cosmic objects.
" Our high-precision measurements of Oumuamua's position revealed that there was something other than the gravitational forces of the sun and planets "said team principal Marco Micheli of the European Space Agency in a NASA press release.
Recent observations by & # 39; Oumuamua show that the interstellar object does not follow the well-understood laws of celestial mechanics, suggesting that another force is changing its motion through space.
NASA / JPL-Caltech
Researchers investigated a number of possible scenarios to explain the faster than expected speed at which Oumuamua slips out of the solar system. But after considering all the possibilities (such as solar radiation pressure, friction-like forces, and magnetic interactions with the solar wind), the team concluded that the Sun causes Oumuamua to release gas and dust from within it in a process called outgassing occurs almost exclusively in icy comets, not in rocky asteroids.
This ejection of material, caused by the evaporation or sublimation of the sun caught just below the surface of Oumuamua, produces a tiny amount of thrust. And although this outgass-induced thrust is small, it is enough, according to the researchers, that Oumuamua's speed surge was observed.
"We think this is a tiny, strange comet," said lead author Marco Micheli of the European Space Agency (ESO) in a press release. "We can see in the data that his boom gets smaller the farther he is from the sun, which is typical of comets." However, the researchers point out that Oumuamua does not seem to be the most outgassing other comets found within the solar system.
In particular, when comets from our solar system are heated by the sun, outgassing typically results in the ejection of material that forms a turbid cloud around it, called a coma, plus a characteristic tail. For Oumuamua, however, the researchers found no visual evidence for these features.
"We have not seen dust, coma or tail, which is unusual," said co-author Karen Meech of the University of Hawaii. "We think that & # 39; Oumuamua can vent unusually large, coarse dust grains."
These large dust grains – which are surprisingly harder to detect than smaller grains – may be weak, but they are also massive enough that they emit produce enough thrust to account for the unexpected increase in speed of & # 39; oumuamua , The researchers also suggest that "Oumuamua could eject large grains because all of the smaller dust grains that adorn most of the comet's surfaces have already been eroded during their long journey through interstellar space."
Although the suspected outgassing tells us something about the composition of & # 39; Oumuamua, unfortunately, makes the determination of the origin of this enigmatic object a lot more challenging.
"The true nature of this enigmatic interstellar nomad can remain a mystery," co-author Olivier Hainaut of ESO said "Oumuamua's recent speed gain makes it harder to trace the route his extra-solar home star has traveled."
Using data from ESO's Very Large Telescope, NASA's Hubble Space Telescope, and ESA, astronomers have determined that "Oumuamua is likely a comet experiencing outgassing."