From the archives (May 9, 2005)…

Sedna’s Moon

Mysteries of the solar system’s most distant member

I would like astronomer Mike Brown to know that he should not feel any shame or embarrassment. He made what we can in retrospect consider a minor, honest mistake—and though it was, unfortunately, advertised in a widespread, public manner, it was, in all fairness, of a much smaller magnitude than many mistakes I have made. This is something human beings do, and it’s OK. We’ve all been there, Mike, and we don’t hold it against you. In fact, it’s rather nice to be reminded that scientists are just ordinary people after all. You will put this flub behind you and, I’m certain, have a long and illustrious career. After all, you did discover a rather amazing planetoid. Not many of us can say that.

Far Out
On November 14, 2003, a trio of astronomers—Mike Brown from the California Institute of Technology, Chad Trujillo from the Gemini Observatory, and David Rabinowitz from Yale—discovered a large, previously unknown object while working at Palomar Observatory near San Diego. The object is a planetoid (or “minor planet”) orbiting the Sun at a distance of about 8 billion miles (13 billion km)—making it three times as distant as Pluto, and the most distant object known to exist in our solar system. And because its orbit is highly elliptical—reaching distances from the Sun as far as 84 billion miles—a single trip around the Sun (or a “year” from the perspective of this planetoid) takes a staggering 10,500 Earth years. The astronomers announced their discovery on March 15, 2004, giving the planetoid the provisional name “Sedna,” after the Inuit goddess of the sea. (The name was later made official by the International Astronomical Union.)

Apart from its sheer distance from the Sun and unusually elongated orbit, Sedna had one other particularly surprising characteristic: an incredibly slow rate of rotation. Brown and his colleagues noticed that the planetoid dimmed and brightened slightly in cycles of about 20 Earth days. They reasoned that this was probably due to bright and dark spots on the surface, which come into and out of view as the planetoid rotates. Considering that Earth rotates in (about) 24 hours, and that some other planets rotate even faster, a 20-day rotation was quite unexpected. The astronomers thought the most plausible explanation for this long period of rotation was the existence of a moon exerting a significant gravitational pull on Sedna. In fact, they were so certain of this that the obligatory “artist’s rendition” of Sedna that accompanied the announcement included a moon. So the team’s next step was to find the moon.

No Moon
About a month later, the Hubble Space Telescope took a series of high-resolution pictures of Sedna, but much to everyone’s surprise, the photos showed no moon. And so began a long and highly publicized debate: What happened to Sedna’s moon? Astronomers around the world (including, naturally, Brown and his co-discoverers) posited numerous theories. Maybe the moon just happened to be hiding behind the planetoid when the pictures were taken. Maybe it was so small and dim that it didn’t show up in the photos. Maybe it once existed, but was destroyed or knocked out of orbit long ago. Or maybe the moon was in fact some exotic object such as an extinct comet. And so on. In a great many articles and news reports, Brown was quoted as being “completely baffled” at why there was no moon, which was seemingly the only way of explaining Sedna’s extraordinarily slow rotation. He admitted that, among other possibilities, the planetoid might in fact be rotating much more quickly than he’d thought, but after reanalyzing his data, he remained convinced that Sedna’s period of rotation was somewhere between 20 and 50 days.

In early 2005, the mystery was finally solved, rather anticlimactically. Astronomers Scott Gaudi and Krzysztof Stanek at the Harvard-Smithsonian Center for Astrophysics took new measurements using a much larger telescope than Brown’s team had used. Their observations showed conclusively that Sedna’s rotation is in fact quite normal: no faster than once every 5 hours, but definitely no slower than once every 10 days (with a probable speed of one rotation every 10 hours). This makes Sedna about the same in terms of rotational speed as other bodies its size, and completely eliminates any need to posit a moon.

The latest series of stories about Sedna, naturally, quote Gaudi, rather than Brown. And they remind us that although one particular mystery has been solved, there are still plenty of things about Sedna that we don’t know, such as why it has such an unusual orbit. Mike Brown may or may not be the one to figure out Sedna’s other secrets, but long after the “missing moon” story is forgotten, he’ll have an honored place in history as the person who discovered this amazing planetoid. —Joe Kissell

More Information about Sedna’s Moon…

Mike Brown has an extensive Sedna page, which, curiously, has not (yet?) been updated to reflect the solution to the missing moon problem.

To get a feel for the timeline of discovery surrounding Sedna and its absent moon, see: