Dr Maggie Turnbull, Space Telescope Science Institute, Washington DC
Chris - Now Maggie you've come up with a short list of planets which are in the habitable zones of stars. In other words they are at just the right distance from the star so that they are not too hot and not too cold, so they are what's called Goldilocks planets, but how do you look for that kind of thing?
Maggie - Well it depends on what you are looking for. If you want to actually try to detect the planets themselves, then you have to build a space telescope that is able to somehow black out the light of the star, which is extremely bright, so that you can see the very faint light coming from the tiny planet.
Chris - People have come up with various ways to do that haven't they? There is this petal-shaped star shade so that you can use to blot out the sun and leave the planets on the outside. Other than that, are there any other ways to do it?
Maggie - Well the easier way to do it is if that planet had some sort of intelligent civilisation that was broadcasting its presence to the universe we could just listen for those signals. Which is of course what Search for Extra- Terrestrial Intelligence (SETI) scientists are trying to do. That puts the burden of technology on them, so that they have to outshine their stars at certain frequencies so that we can see them.
Chris - This seems like a good opportunity to talk to Tony, who is in Westcliffe, because he wants to ask something about that, Hi Tony.
Tony - Good evening Sir. We've been sending out radio waves for about 80 years, and given that they travel at the speed of light, the nearest star that could hear them would be about 80 light years away, wouldn't it? So I wondered how many planets fall within that range?
Maggie - You can imagine this sphere around the Earth, going out into space, that is filled with radio noise that we have emitted because of our technological activities. If you want to count the very first broadcasts, then the radius of that sphere around us is something like 80 or as much as 100 light years. I don't think in terms of light years, I think in terms of parsecs and 1 parsec is about 3 light years. If we say that this sphere is about 30 parsecs in radius, I know that there are about 3 to 5 thousand stars inside that sphere around us, and of those, about 2 thousand are sun-like stars.
Chris - So like our own star?
Maggie - Not exactly like our own, but similar enough to our star that they could live long enough and have planets that could have life on them that are somewhat similar to our own.
Chris - That's quite a lot, isn't it?
Maggie - Yes, its a lot but you also have to consider that not every one of those stars is going to have planets and out of those that do, they are not all going to have Earth sized planets that are in the habitable zone. So I would say that a reasonable number would be about 10% of those, and anywhere between 1 to as many as 5 hundred habitable planets are within that sphere of noise that we've created. You have to keep in mind that most of the transmissions that are leaking out into space are doing just that, they are leaking, they are not beamed intentionally at any star system, so they are very weak by the time they get there. Also, all the signals that we leak out into space are not going equally in all directions, sometimes you have to be at a certain spot to be able to pick them up.
Chris - Great question Tony, thank you very much. So carrying on with that theme how do we try to pick up alien signals? Assuming that those people are technologically ahead of us and they are sending things our way, how do we try to find them?
Maggie - Well, we have to listen! Getting time on a large enough telescope to be able to detect transmissions of the type that we are actively leaking out into space, is actually a challenge. But one of the exciting things that is happening soon with regard to SETI is the construction of the Allen Telescope Array in northern California. When that's completed that will really be our first opportunity to begin monitoring the skies 24-7 for intelligent signals coming from other star systems.
Chris - You have narrowed down the number of planets which you think, fairly locally, might play home to foreign life. How have you done that and how many of them are there?
Maggie - I have attempted to do this exercise by picking out stars that are similar to the Sun with regard to a lot of different factors. We donít want to spend our telescope time looking at stars that are young as those systems are probably still filled with the debris of planetary formation; youíve got huge collisions going on with comets and asteroids. Those planets are not very nice places to live right now and no way do they have radio telescopes!
So we donít want to look at the really young ones, but on the other hand really old stars tend to not have a lot of the heavy metals like iron and nickel which are needed in order to form terrestrial planets. So really old stars or stars that just donít have enough of these metals in them are not good places to look either because theyíre not likely to have planets like the Earth.
Chris - So what does that leave you with?
Maggie - Well, we end up with a pretty substantial chunk of stars. I have a list in the order of 18 thousand stars that are really our top targets for SETI.
Chris - Are they anywhere near here?
Maggie - Some of them are very nearby. I think most people will have heard of Alpha Centauri. These are just examples though, Iím not saying these are the best places to look!
Chris - "This is where youíll find UFOs!?"
Maggie - Do not put me on the spot like that! [Laughs] I am not going to tell you whatís the best one to look at but, for example, Alpha Centauri is the closest Sun-like star to us, itís only 1 parsec away, about 4 light years. It is actually in a triple star system, but maybe it could still be habitable to Earth. The second star actually gets a little closer than Iíd like it to, so itís not ideal, but itís interesting because itís so nearby and itís so much like the Sun.
Epsilon Eridani is another bright star, which is actually a little bit less luminous than the sun. Itís kind of an orange star; itís a little cooler so planets that are habitable would be a little closer to that star than they are in our solar system. Thatís one of my very favourite stars that I feel like we absolutely must look at.