NEWSWEEK: Astronomers can see galaxies billions of light years away. What took you so long to find Sedna?

Brown: Astronomers now have these fabulous telescopes for looking at ever-fainter objects, but to find things close by, in our own solar system, you have to look for things that move. If you take a picture of the sky, and if you don’t come back an hour later and take another picture, you don’t realize that one thing you think is a star, isn’t.

Clyde Tombaugh found Pluto in 1930 by using big photographic plates and staring at each image. We’ve finally been able to catch up with Tombaugh by using computers and big digital cameras. It’s completely robotic. We tell the telescope every morning where to point the following night, and the next morning we get the data that comes back down.

When did you make this discovery?

November 14. I sat down and looked through the data from the night before. I saw this thing move. It was the most slowly moving object I’d ever seen. And I just didn’t believe it. I had to run to teach a class. I gave the lecture I’d prepared, saying that the Kuiper Belt ends but there’s nothing out farther than that. At the end of it, I had to say, well, I may just have found something out there, so this may all be wrong.

What did your students think of that?

They sort of sat there in shock. I always say outrageous things to them just for fun, so I’m not sure they even believed me.

What do you make of Sedna itself?

It’s red, which is not initially a surprise. If I take a snowball, sprinkle it with dirt and put it in space, it turns red. But it turns really dark, and Sedna is quite bright, which is a big surprise to us. We’re trying to figure it out.

Does it have a moon?

We have indirect evidence of a moon. We know Sedna rotates once every 40 days, which is extremely slow. Most objects rotate in a matter of hours. Our guess is that Sedna is locked in a 40-day rotation with a satellite. There may be other explanations, but we can’t think of them right now.

Will the Hubble telescope take a look?

It already has. I have the data sitting here on my computer, and as soon as I can I’m going to look at it. If there’s a moon, that’ll be quite obvious.

If it has a moon, why isn’t it a planet?

The first asteroid was called a planet, until lots of others were discovered. The same thing happened with Pluto. Now we know of 800 objects out there in the Kuiper Belt, and Pluto is just the largest. So calling it a planet just doesn’t make sense. Sedna is the only object we know of in that [more distant] region of space, so you could argue we should call it a planet. But I think over the next five or 10 years, we’ll be finding a lot more.

How big is this for you personally?

That’s a hard one to answer. It’s been such a crazy week. It’s huge. We set out looking for things like this two and a half years ago. We still want to find something bigger. It’s been a goal for a long time to find something larger than Pluto. I’m convinced there are things out there, and I just would like to be able to find them. This has been fun, but it’s definitely not the end of the story. The nice thing is, people really like astronomy and planets. So many people just want to know about it, which makes it really gratifying.

Has this changed our view of the solar system?

I think it has to. Until now, there was a big empty space between the Kuiper Belt and the Oort Cloud [a so-far undetected cloud of asteroids and planetoids thought to exist hundreds of billions of miles from the sun]. If Sedna is part of the Oort Cloud, then what we used to think of as being inconceivably far away is actually on our doorstep. Though 8 billion miles is quite a doorstep.

Should we worry about the Oort Cloud?

The probability is high that there’s an Earth-sized object in the Oort Cloud. Eventually, one of them will come screaming into the inner solar system, and it’ll be the most amazing sight that anyone has ever seen in the sky.

Eventually?

Sometime in the next 10 billion years.