Guest from outer space

Astronomers had expected a comet but got an asteroid instead. Aswin Sekhar talks of its importance

An artist’s impression of Oumuamua released 
by Nasa

In recent weeks, the global media has been buzzing with stories about the entry of the first confirmed interstellar body into our solar system. Although we have plenty of homegrown asteroids —which are observed frequently — it is extremely unusual for an asteroid from a different stellar system to visit the inner solar system. This object was initially named A/2017 U1 but within a few weeks the International Astronomical Union (IAU) invented a new system of giving formal names to interstellar objects and called it I1/Oumuamua, where I1 stands for first interstellar object and Oumuamua means scout or messenger in Hawaiian. “From an observational point of view, we now know these objects exist, confirming various theoretical works which suggest their existence,” says Robert Weryk, the ace observer who first discovered this object using the Pan-STARSS telescope at the University of Hawaii in the US. 

This is the first time in recorded history that an interstellar object has been observed by telescope. Astronomers had long theorised that interstellar debris might invade the solar system from time to time but they thought these wanderers would act like comets, lighting up and vapourising when they got close to our sun. Oumuamua did no such thing. It stayed dark and faint and was only detected because Pan-STARRS is a very powerful telescope with a wide field of view. It was first noticed on October 19, moving out of a constellation about 32 million kilometres from Earth. It is about 40 metres long and was travelling at 93,000 kmph during the time of discovery. It will pass the planet Jupiter next May, on its way out of the solar system. 

How are we so sure that Oumuamua is from outer space? First, it was observed before it entered the solar system. Second, its orbit.  “The other 7,50,000 known objects [in space] travel in ellipses, orbiting the sun. This orbit is the fi rst one that is clearly hyperbolic (not bound to the sun). This object cannot have originated in our solar system: it must have formed around some other star and travelled here through interstellar space. That’s why we call it an interstellar object,” explains Paul Chodas, chief manager of the Center for Near Earth Object Studies at JPL-NASA, US.

The Oumuamua resembles the asteroids we already know in colour (reddish) but it dims and brightens every 7.3 hours, leading scientists to think that it actually revolves around an axis. Its shape is rather singular, it is about 10 times as long as it is wide. “This first positive detection of an asteroid from another solar system passing through ours will firm up constraints of how often we should be seeing them and, ultimately, how many such objects are cruising unseen in interstellar space,” says Apostolos Christou, a top solar system dynamicist based at Armagh Observatory and Planetarium, Northern Ireland. Thousands of such objects may lurk undetected in our solar system, surmise scientists.
The visit of this guest from outer space can also trigger studies in planet formation around other star systems. “Planet formation is understood as a by-product of star formation. By carefully determining how frequently the material ejected from other solar systems is passing through our own, we can understand the processes and frequency of planet formation in our neighbourhood,” says Matthew Holman, Director of International Astronomical Union-Minor Planet Center (based at Harvard-Smithsonian Centre for Astrophysics, US). 

It also raises interesting questions about the formation and evolution of our own solar system. “This body appears to be an ‘unused brick’ in the building of a planet, that has taken place somewhere in our galaxy. Interstellar space hosts an enormous number of such bodies, that are much more numerous than stars in the galaxy. There is even a very small chance that this asteroid was formed in, and expelled from, our own planetary system, and has just paid a visit back home,” says Giovanni Valsecchi, a celestial mechanics expert based at INAF, Rome, Italy.

But the biggest question is why was it not a comet. “I think everybody was expecting to see an interstellar object doing a fly-by with the solar system, sooner or later. But everybody was expecting a comet. Instead, this object has no activity whatsoever. As a general rule, the comet reservoirs are more massive and, being further out, they are easier to eject in interstellar space. So, why is the first hyperbolic object we see an asteroid and not a comet?” asks Alessandro Morbidelli, a noted solar system formation expert based at the Observatory of Nice in France. Many astronomers are trying to answer this intriguing question with logical arguments bringing diverse fields of astrophysics together. Follow-up astronomical observations are being done on this object by the best telescopes and observers in the astrophysics community.

“The recent discovery is a landmark event for astronomy. It implies, as people have anticipated for years, that interstellar space must contain not just many dust particles of interplanetary dust size but also occasional rare ‘orphan’ planets ejected from star systems and objects of all sizes and types in between. The link between the galaxy and the solar system has just got a whole lot more interesting!” remarks Mark Bailey, a renowned comet expert and organising committee member of IAU Commission on the Study of Comets and Minor Planets.

This latest discovery can easily keep galactic astronomers, solar system astronomers and exoplanetary astronomers busy for the next few decades. Many telescopes, theoreticians and observers are going to sacrifice a lot of sleep to learn more about this intriguing object. When it comes to the cosmos, there are always more questions than answers.

The writer is an Indian astrophysicist based at CEED, University of Oslo, Norway


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