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SYFY WIRE Bad Astronomy

An interstellar rock gets a name

By Phil Plait
When it was still called A/2017 U1, the object was spotted by the William Herschel Telescope on the island of La Palma. The telescope guided on the motion of the object, so stars appear to trail.

Recently, I told you about some very exciting news: A newly discovered comet from the frozen depths of the solar system turned out to be neither: It was actually an asteroid, and it came from another star!

The orbit of this object wasn’t known well at first, but as more observations came in it became clear that it was not from our solar system at all. It literally came from another star, an alien rock traveling through interstellar space for who knows how long before whipping around the Sun and heading back out.

Even then, what to call this object was a problem. The first designation for it was C/2017 U1 (Pan-STARRS). The name of a comet tells you a lot about it. The C indicates it was a comet with an open orbit; that is, not bound to the Sun (P is used for periodic comets, D for ones that disintegrate, and X if the orbit isn’t well established).  The number is obviously the year of discovery. The next letter designates when in the year it was discovered (each letter represents half a month (skipping I, which can be confused with 1), so U is the second half of October), and then numbered in order for that time period. Finally, the name of the discoverer or observatory (in this case, Pan-STARRS) is in parentheses.

But when it was found that this object didn’t behave like a comet — it exhibited no fuzziness, which a comet would do that close to the Sun as ice turned into a gas and blew away from it — it was realized it was an asteroid. The C was changed to an A.

But this is still a bit confusing. Since it came from another star (!!!!), and we’re likely to find more as technology gets better (these alien rocks are probably whizzing past us all the time, but are very faint), perhaps a new nomenclature is needed.

So astronomers from the International Astronomical Union (the gatekeepers of naming celestial objects) and the Minor Planet Center (who maintain the list of smaller objects in the solar system) got together and came up with a new way to designate these interstellar visitors: the letter I.

So it now has the official designation of I/2017 U1. I have to admit, that’s pretty cool. The first of its class!

When it was still called A/2017 U1, the object was spotted by the William Herschel Telescope on the island of La Palma. The telescope guided on the motion of the object, so stars appear to trail.

But there’s more. Due to its unique nature, there was also pressure to give it an actual name. In general, minor bodies don’t get real names until they have been observed for a long time, and the process is somewhat complicated. But again, this object is so special they decided to ease that restriction this one time. The Pan-STARRS team chose an excellent name: ʻOumuamua*. This is a Hawaiian name, meaning, roughly, “very first scout.” How great is that?

There are several ways to refer to it, then. As the announcement notice states,

Correct forms for referring to this object are therefore: 1I; 1I/2017 U1; 1I/ʻOumuamua; and 1I/2017 U1 (ʻOumuamua).

After this one, any discovered will just get a designation and likely not an actual name. Why not? Because the rules state that an object has to be seen at multiple oppositions, when it is opposite the Sun in the sky. For distant objects the time between oppositions is a little more than one Earth year; if it’s far enough away it isn’t moving much relative to background stars, and the Earth’s motion around the Sun is far faster.

The problem is that these objects will likely be small and moving very fast (they fell from freaking interstellar space!), so over the course of a year they’ll get so faint as they pull away they won’t be visible for more than a single opposition. Perhaps that rule will be eased eventually as well, but for now ʻOumuamua stands alone.

And when will we find more objects like it? It’s hard to say, of course. But with Pan-STARRS’ big eye, and LSST coming pretty soon, I doubt we’ll have too long for our next alien visitor.

* Hawaiian words are pronounced just as they are spelled with every letter pronounced, but in this case the name has an ʻokina in front, which is like a glottal stop (I saw one description that it’s like the stop when you say, “uh-oh”). So this is pronounced Oh-oo-moo-ah-moo-ah, with the glottal stop in front.

Personally, I was hoping for Rama (which, amazingly, hasn’t been used yet), but come on, 'Oumuamua is a really cool name.