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Spitzer Finds Hints of Planet Birth Around Dead Star
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Narrator: I'm Jane Platt, and you're listening to a podcast from JPL, NASA's Jet Propulsion Laboratory in Pasadena, Calif. During its more than two-and-a-half years in orbit, NASA's Spitzer Space Telescope has gathered a lot of information about how planets form in dusty disks around stars. Today, we're going to learn about one more piece of the puzzle. Joining us is Dr. Deepto Chakrabarty of MIT - Massachusetts Institute of Technology in Cambridge. He's the principal investigator for this new Spitzer research, which appears in the April 6th issue of Nature. Welcome.

Chakrabarty: Thanks very much, Jane.

Narrator: What can you tell us about your new discovery, just to sum it up?

Chakrabarty: To sum it up, we found evidence for another debris disk, but now around an old dead star, a pulsar, or neutron star, which is quite a surprise. It's very different than the ordinary young stars around which debris disks had been previously found.

Narrator: What's so special about a pulsar?

Chakrabarty: A pulsar, which is a type of neutron star, is formed at the end of the life of a massive star, a star say 10 to 20 times the mass of our sun. So it's sort of a stellar corpse. And it's formed when a massive star ends its nuclear burning and collapses and explodes in a brilliant explosion called a supernova explosion. So pulsars are essentially the opposite of the ordinary young stars around which most debris disks have been previously seen. We usually associate debris disks and planet formation with young stars, the birth of new stellar systems, whereas pulsars and neutron stars are representing the other end of the stellar life, the end of their lives.

Narrator: So it is sort of like Phoenix rising from the ashes, where you have these disks where you think conceivably planets could be forming, correct?

Chakrabarty: Exactly.

Narrator: And you say it was a surprise, but there had been theories to that effect, right?

Chakrabarty: That's true. So the disk that we're seeing we believe is leftover debris from the explosion that formed, from the supernova explosion that formed the neutron star. Essentially, the explosion rips away the outer part, the outer 90-percent of the massive star that dies, and we believe this debris disk that we found represents a small fraction of that material that didn't quite achieve enough velocity to escape and fell back toward the neutron star or pulsar.

Narrator: It really is the remnants?

Chakrabarty: That's right. So it was predicted, there was a theory that predicted that fallback disks should exist, and indeed that's why we were looking for them. But the surprise for us is how much this disk that we found resembles the debris disks that are seen around young stars. And astronomers know that these debris disks around young stars are the sites where planet formation can happen. And so, it suggests that planet formation may also be able to happen around pulsars and that it's a very robust process that can happen in many different environments in the universe, and not just around young stars. Chakrabarty

Narrator: But the environment around a pulsar is not very friendly.

Chakrabarty: That's exactly right, the environment is quite hostile. The environment is bathed in very intense x-ray radiation and possibly in very strong particle radiation of the sort that we see in particle accelerators on the Earth. And that's part of the reason for the surprise. I mean, one of the things that our discovery indicates is that a debris disk can survive even in this harsh environment.

I should stress we haven't discovered a planet around this particular pulsar, but what we've found is the conditions under which we think planets could form.

Narrator: And pulsars actually, you had already known, that pulsars can have planets. Tell me about that.

Chakrabarty: Yes, remarkably, the first planets that were ever discovered outside of our own solar system were discovered around a pulsar about 15 years ago by Alex Wolczcan of Penn State University. And when those planets were found, that was really a surprise because at that point no one had ever contemplated the idea that planets could form around very old dead stars. And so when that discovery occurred, it was a real puzzle as to how these planets might have got there, how they might have formed. And further work on those planets in the past 15 years has strongly indicated that they almost certainly form from some kind of a disk. And so we had the idea that maybe there would be the possibility of a debris disk around a pulsar, but many people have searched for a debris disk around pulsars and not found any evidence of them. And so we've never seen such a disk until now. So this discovery really helps close the circle and answers the question that was first raised 15 years ago when planets were first, when the first planets outside our solar system were found.

Narrator: What's the name of this pulsar and where's it located?

Chakrabarty: So the pulsar around which we've found the debris disk with Spitzer is called 4U0142+61. It's located in the northern constellation of Cassiopeia at a distance of about 13-thousand light years.

Narrator: OK, before we wrap up, there's one factoid that I'd like you to share with our listeners, and that is, it's actually one of my favorite factoids about astronomy that I've heard, about pulsars and neutron stars and how dense they are.

Chakrabarty: Pulsars are about one-and-a-half times the mass of our sun, but they're squeezed into a volume, a sphere that's only about 10 miles across. So a teaspoon of a pulsar would weigh nearly two billion tons. It's the densest form of matter known in the universe.

Narrator: That's absolutely mind-boggling. Alright, well, thank you very much for your time, I appreciate it.

Chakrabarty: Thanks very much.

Narrator: We've been talking today with Dr. Deepto Chakrabarty of MIT. More information on the Spitzer Space Telescope is online at http://www.spitzer.caltech.edu . For more NASA podcasts, go to www.nasa.gov/podcast . Thanks for joining us for this podcast from NASA's Jet Propulsion Laboratory.