Nov 1, 2017
Expert Voices

An interstellar interloper in the solar system

The path of A/2017 U1 as it passed through our inner solar system in September and October 2017. Credit: NASA / JPL

On October 19 astronomers with the Pan-STARRS facility in Hawaii spotted an otherwise unremarkable small object tumbling through our solar system, but initial and follow-up observations revealed several intriguing characteristics:

  • It was moving faster than 50,000 mph before it arrived, more than enough to ensure it wouldn't be captured by the sun.
  • It entered the inner solar system from an angle far steeper than other distant visitors like comets.
  • It appears to be more like an asteroid than a comet — if it were a comet, it would have produced a tail as it neared the sun.

Why it matters: All the evidence together suggests that A/2017 U1, as it's currently named, is not from around here. It likely formed in another stellar system altogether, was ejected who knows how long ago, and has been traveling interstellar space before a chance encounter with our sun. It's already on its way out — past the orbit of Mars — traveling back into the void.

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