Commentary Magazine

Greetings, Earthlings

Wikipedia Commons/ESO/M. Kornmesser

The solar system received an extraordinary visitor this fall: an object from another solar system, the first such object ever recorded.

It was at first classified as a comet, but its orbit was unlike that of any solar system object. It came into the neighborhood at an angle of 60 degrees from the ecliptic, the plane defined by the earth’s orbit around the sun. Most orbits in the solar system, and all major ones, lie fairly close to the ecliptic. Pluto’s orbit, quite eccentric, is inclined by only 17 degrees.

More, the object’s speed, about 59,000 miles an hour, is well above the sun’s escape velocity. Having shot around the sun (reaching a speed of 196,000 miles an hour at its closest approach, about 26 million miles from the sun), it is now rapidly leaving the solar system, never to return. It whizzed by earth only 19 million miles away. It has been named ‘Oumuamua, the Hawaiian word for “scout.”

Where could it have come from? Almost certainly another stellar system somewhere in the Milky Way. The early days of stellar systems are chaotic (and even later days, just ask the dinosaurs). Many smaller objects get ejected in the process by gravitational interaction with larger objects (much as space probes pick up speed by passing close to a planet such as Jupiter). ‘Oumuamua has in all probability been wandering interstellar space for billions of years, and it will wander for billions more before approaching another star. If it were heading for the nearest star, Proxima Centauri, which it is not, it would take about 50,000 years to reach it. Space is very big and very empty.

Perhaps the most interesting thing about ‘Oumuamua is its shape. It is about 400 meters long but only about 40 meters wide. In other words, it’s cigar-shaped. Astronomers know of no solar system object that looks like this. It rotates in about eight hours, spinning like a majorette’s baton around its center of gravity.

As this is the first time something like this has ever been detected, astronomers have no idea how frequently an interstellar object pays a visit to the solar system. It might be a few times a century or a few times in a billion years. But with our rapidly increasing observational powers, perhaps we will have an idea soon.

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