(Written by Promit Chakroborty)
The mystery of the small, dark red cigar-shaped object that shot across our cosmic neighbourhood late last year, has been solved — it is a comet, after all. And in identifying this first interstellar visitor ever seen in the Solar System, astronomers have discovered an entirely new way to detect comets, those icy chunks of frozen gases, space rock, and dust that have fascinated humankind for over 2,000 years.
‘Oumuamua — Hawaiian for “a messenger from afar arriving first” — was first spotted on October 19, 2017 by the Pan-STARRS 1 telescope on Haleakala, Maui, Hawaii. It was dark and faint, with no visible ‘coma’ (atmosphere of dust and gas around a comet’s core) or ‘tail’ (elongated cloud that points away from the Sun) — signature identifiers of comets as they approach the inner Solar System.
“Typically, comets are identified by seeing the tail, or the coma,” Professor Marco Micheli of the European Space Agency’s SSA-NEO Coordination Centre in Frascati, Italy, said in an email interview. The absence of a visible tail in the case of ‘Oumuamua had initially led to its being classified as an asteroid. But an analysis by Dr Micheli, who was also part of the team that had discovered the object last year, has shown that comets do not necessarily vaporise and light up as they get close to the Sun. This absence of brightening also means that only powerful telescopes with a wide field of view like Pan-STARRS can detect them.
When it was discovered, the oddly-shaped, about 800-m-long ‘Oumuamua was racing through space. “It is now moving away from us, and it is already farther away than planet Jupiter. It was last seen in early January 2018, and it’s now too faint to see even with the most powerful telescopes,” Dr Micheli said.
There isn’t a lot of information about ‘Oumuamua, but the discovery provides exciting insights into the chemistry of objects born in other solar systems. “We don’t know a lot of the composition, but its spectrum (i.e. the colour of the light it reflects) is very similar to our own Solar System comets. This supports our identification of it as a comet,” Dr Micheli told The Indian Express.
‘Oumuamua’s nucleus, he said, “is probably similar to comets of our Solar System, although some aspects have to be different. For example, the dust grains have to be larger than typical for our comets. On the other hand, the complex rotation of ‘Oumuamua is not very unusual, there are other objects in the Solar System displaying a similar rotational state.” Astronomers noticed the rotation from a dramatic brightening and dimming of ‘Oumuamua every 7.3 hours, which suggested rotation about a short axis.
The larger grain sizes could be a reason for the absence of the characteristic cometary tail. “The tail we see in comets is typically made of fine dust lifted off the nucleus and dragged away by the emitted gas. In the case of ‘Oumuamua, we think that the dust is not visible because it’s made of larger grains, that are too heavy to be lifted off by the gas. The gas itself is actually very difficult to detect, because the specific molecules composing it (water, CO and CO2) are difficult to see in optical images… [‘Oumuamua] probably does [have a tail], we have not seen it, but in the [research] paper we show that the amount of gas we need to explain the effect would not be visible, nor will the dust it lifts off when being emitted (assuming the dust is made of grains larger than for normal comets),” Dr Micheli said. The research was published last week in the journal Nature.
So, does the example of ‘Oumuamua show that it is more difficult to identify comets than was first thought? “The lack of visible tail and activity makes objects harder to see, because they appear less bright. If an object is active, the dust around it contributes to the overall brightness, making the object also easier to discover,” Dr Micheli said.
But once it has been discovered, this new ‘indirect’ method should prevent faulty classifications. “It has happened a few times that an object that looked asteroidal at the time of discovery was then noticed to have a coma or a tail, and therefore reclassified as a comet. The ‘indirect’ way we used on ‘Oumuamua, by seeing a change in the orbit and modeling it as a cometary effect, is to my knowledge done here for the first time, and it was made possible by the very good data we were able to collect on this object.”
Clearly, the little comet heralds a big step in our understanding of interstellar cosmic objects. Perhaps it’s just as well that ‘Oumuamua is shaped like a cigar — as Pink Floyd sang in 1975, Come in here, dear boy, have a cigar, / You’re gonna go far, / You’re gonna fly high, / You’re never gonna die, / You’re gonna make it if you try, / They’re gonna love you.