A fast moving, cigar-shaped object of extrasolar origin was discovered on October 19, 2017 by the Pan-STARRS 1 telescope in Hawai’i. Named 1I/2017 U1 (‘Oumuamua), the object was first spotted three days after its closest approach to Earth, well after it had passed closest to the Sun on September 9, 2017. Its discovery provoked intense and continuing interest from the scientific community and the general public. Reports of its unusual characteristics even led to speculation that ‘Oumuamua might be an interstellar spacecraft. Now, a review of all the available evidence strongly suggests that the object has a purely natural origin.
This artist’s impression shows the first interstellar asteroid — 1I/2017 U1 (’Oumuamua). Image credit: M. Kornmesser / ESO.
“While ‘Oumuamua’s interstellar origin makes it unique, many of its other properties are perfectly consistent with objects in our own Solar System,” said Dr. Robert Jedicke, an astronomer at the University of Hawaii’s Institute for Astronomy and a member of an international group of asteroid and comet experts called the ‘Oumuamua ISSI Team.
“We have never seen anything like ‘Oumuamua in our Solar System. It’s really a mystery still,” said team member Dr. Matthew Knight, a researcher at the University of Maryland.
“But our preference is to stick with analogs we know, unless or until we find something unique. The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it.”
According to the authors, ‘Oumuamua is red in color, similar to many small objects observed in our Solar System. But that’s where the familiarity ends.
The interstellar object likely has an elongated, cigarlike shape and an odd spin pattern — much like a soda bottle laying on the ground, spinning on its side.
“Its motion through the Solar System is particularly puzzling. While it appeared to accelerate along its trajectory — a typical feature of comets — we could find no evidence of the gaseous emissions that typically create this acceleration,” Dr. Knight said.
“The motion of ‘Oumuamua didn’t simply follow gravity along a parabolic orbit as we would expect from an asteroid. But visually, it hasn’t ever displayed any of the cometlike characteristics we’d expect. There is no discernable coma — the cloud of ice, dust and gas that surrounds active comets — nor a dust tail or gas jets.”
Montage of potential formation scenarios of ‘Oumuamua as a natural planetesimal. Image credit: NASA / Hubble Heritage Team / STScI / AURA / C.R. O’Dell, Vanderbilt University / ESA / A. Feild, STScI / JAXA / Susanne Pfalzner / ESO / L. Calçada / JPL / D. Seal / CfA / Mark A. Garlick.
The team considered a number of mechanisms by which ‘Oumuamua could have escaped from its home system.
For example, the object could have been ejected by a gas giant planet orbiting another star.
According to theory, Jupiter may have created the Oort cloud — a massive shell of small objects at the outer edge of our Solar System — in this way. Some of those objects may have slipped past the influence of the sun’s gravity to become interstellar travelers themselves.
“We tend to assume that the physical processes we observe here, close to home, are universal,” Dr. Knight said.
“And we haven’t yet seen anything like ‘Oumuamua in our Solar System. This thing is weird and admittedly hard to explain, but that doesn’t exclude other natural phenomena that could explain it.”
The team’s paper was published online in the journal Nature Astronomy.
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Michele T. Bannister et al (The ‘Oumuamua ISSI Team). The natural history of ‘Oumuamua. Nature Astronomy, published online July 1, 2019; doi: 10.1038/s41550-019-0816-x
