An interstellar visitor both familiar and alien

Visit the galaxy before the galaxy visits you. This fall, the galaxy came calling in the form of a small reddish cigar-shaped object named Oumuamua by astronomers in Hawaii, USA. They discovered it in October, careening through the solar system at 40,000 mph, an interstellar emissary from points unknown. Oumuamua, Hawaiian for ‘scout’ or ‘messenger’, was not here long. It was first noticed zooming out of the constellation Lyra on October 19, about 20 million miles from Earth.

Mysterious shape

The discovery set off a worldwide scramble for telescope time to observe the object. Astronomers from the Search for Extraterrestrial Intelligence Institute even got into the act, swinging into action to look for alien radio signals. For now, however, those are just science fiction thrills.

“Our observations are entirely consistent with it being a natural object,” said Karen Meech of the University of Hawaii’s Institute for Astronomy, USA and leader of the international collaboration that discovered Oumuamua with the Pan-STARRS 1 telescope on Haleakala, Maui.

Karen’s team has now published the first report of their observations in Nature. The paper describes the interstellar visitor as both reassuringly familiar and utterly alien. “We don’t see anything like that in our solar system,” Karen said. In its colour and other imputed properties, Oumuamua resembles the asteroids we already know and fear will one day smash the Earth to smithereens. But asteroid’s shape is weird. It is extremely elongated, at least 10 times as long as it is wide, perhaps 800 yards by 80 yards.

Though the mysterious object is nearly gone, thousands like it probably lurk undetected in our solar system, according to the scientists. The Pan-STARRS telescope was built to patrol the sky for dangerous asteroids in our own system, not interlopers from beyond. But astronomers got a surprise. Karen learned that her colleagues had found one whose path seemed to originate beyond the solar system altogether.

Astronomers had long surmised that interstellar debris might invade the solar system from time to time, in the form of icy chunks spit from the rocky disks forming faraway planets. Such wanderers would manifest themselves as comets when they got close to our sun, vaporising and lighting up; however, they have not been seen.

Oumuamua showed no such cometary brightening. It is so dark and faint that it could only have been detected by a powerful telescope with a wide field of view, like Pan-STARRS. Oumuamua brightens and dims dramatically every 7.3 hours, which suggests that it is rotating about its short axis.

Spectral measurements have revealed that Oumuamua is dark red. Iron can also contribute that colour, Karen said. How Oumuamua got its shape is a mystery for now. Perhaps, Karen said, it was shot away from its home star by a supernova explosion. Or perhaps it was formed by a pair of objects that collided and stuck together.

A different picture

But where did it come from? Karen said the astronomers were initially excited when the orbit appeared to point to the brightest star in Lyra, Vega, which is known to have a debris disk. It would have taken the object about 6,00,000 years to get here from there, astronomers estimated. But further refinements in the trajectory have made it less likely that Vega actually was the source.

The fact that Oumuamua was travelling at about the same speed relative to the sun suggests that this is the asteroid’s first encounter with a new star system. Still, the authors write in Nature, “The possibility that Oumuamua has been orbiting the galaxy for billions of years cannot be ruled out.” Where it’s going is equally in the dark.

The adventures of this asteroid and its ilk paint a very different picture of the galaxy than you might imagine while gazing up at a sky in which the stars seem separate and sovereign, beaming away in solitude.

We now know that meteorites sprung by asteroid impacts on Mars land on Earth all the time. Otherwise respectable astronomers speculate that one of them might have seeded Earth with life that started on Mars when it was warm and wet long ago. But we can look even further out and backward in time for our connection to the cosmos. Consider the hundreds of thousands of years that Oumuamua might have taken to get here. While that might sound like a long time, it is a blink of cosmic time.

The Milky Way galaxy is 10 billion years old. Which means that over the course of our galaxy’s lifetime so far, Oumuamua might have cruised through some 20,000 star systems. Oumuamua would have trailed behind bits of dust and debris, and so the stars and the worlds of the galaxy mix it up. It may be that the universe is a small place after all.