Planets Birth Around Dead Stars: the First Proofs!

David Kaplan thank you for answering Science at Stake’s questions.

Can you describe us what you’ve seen around this dead stars?

What we have seen is a small disk of material that’s orbiting the dead star. This disk has been expected for number of decades. When stars explode, as this one did a hundred thousand years ago, most of the material fly away at very high speed and wives the stars entirely in form as a big growing cloud around it but some small parts of that material will probably not fat enough to escape the gravitational pull of the star, like if you throw a ball on earth if you don’t’ throw it fast enough it will fall back down. So some small part of that material will move way from the star but will eventually fall back towards it and because it was spinning to begin with when it will fall back to the star it will form a disk around it. The reason why we are very excited is first of all people have suspected that a disk like this existed for, as I said, decades but there where of real proof of any of them and our observations are the first proofs of that. The second reason is that the disk that we see is very much like the disk that people have seen for a number of years around stars, around normal stars, and these are the disks out of which people believes planets form. So by analogy if you look a the disks where planets form and look at the disk around our dead stars one could suppose that in the future planets will form around our dead star or pulsar.

Does it mean there should be many a lot of planets orbiting around dead stars?

Right now there probably aren’t that many planets as it’s very young. It’s only a hundred thousand years old which on a scale of astronomical objects is very young. Most likely if planets do form, they will form over much longer time, millions of years. But I do want to emphasize that you do not really know what will happen: it could be that planets will form, it could be entirely different. It’s a very unique environment. The type of dead star, pulsar, is a very very strong X-Ray source so instead of just having his normal light in the sky all the material is constantly bombarded with X-Rays. It make the conditions very different from the conditions that most people consider so we don’t really know how things will progress in the future. But there is another dead star which is considerably older, perhaps a hundred millions years old, where people do see planets around it. That is an indication that in least some of these scenarios planets will form eventually. In that one, which was discovered ten years ago, I believe there are already three planets known and they are very small planets unlike the extreme large planets that people are finding around most stars these days. The planets discovered on this pulsar are each of the size of the earth instead of being larger than the Jupiter.

Do you think that these planets are hospitable for life?

For life as we know it, this is pure speculation, it would be very difficult , just because as I said, it’s bombarded constantly by X-Ray and X-Ray are not very good the type of life that we know: it would break apart DNA or stuff like that. But I’m not saying there couldn’t be an entire different sort of life, something on that planet that would stand the X-Ray. Our concept of life is very depending on what we see here on earth and it’s hard to extrapolate to other systems.

Can you tell us a little bit on how you did the observation?

We used NASA Spitzer Space telescope which is designed to work in the Infra Red. It’ a telescope that is orbiting the earth. Using the combining of data from Spitzer and data that other people had already taken from other telescopes from the ground we where able to see that there is a large excess of emission that looks like coming from glowing dust. It looks like there is dust orbiting the star which is, compared to the star, quite cold but compared to the stuff on earth it’s very warm, maybe two thousand degrees. It’s warm enough to glow and therefore we can see it with the Spitzer Space telescope.

What was your strategy? Why did you select this pulsar?

The main strategy was that it’s quite bright. The brighter the pulsar is the more the dust is heated the more it glows. Now we are looking at the others that are quite as bright but not that bright.

What are the next steps now?

The next steps are to study this object in more detail. We don’t know yet very much of the disk itself. We think it’s only about a million kilometers from the pulsar itself and goes out from one million to ten million kilometers. The total mass of the material of the disk is very roughly around ten times the mass of the Earth but those are all very rough numbers and we are doing more observations of the object with the Spitzer Space telescope to get more details on what is actually there. We are also trying to go looking at other objects, we are trying to see if this phenomenon is common to all similar objects or if this is a special case.

David Kaplan, thank you.
By Gilles Prigent and Francesca Gilibert