Focus on Stars in Search for Earth-Like Worlds Researchers release a "short list" of stars to target in the search for life elsewhere in the universe. An astronomer discusses how to narrow down the possible universe of star targets to find stars likely to have worlds similar to our own, or even to have life.
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Focus on Stars in Search for Earth-Like Worlds

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Focus on Stars in Search for Earth-Like Worlds

Focus on Stars in Search for Earth-Like Worlds

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Alien life? You know, we've been looking for it, for signs of it for centuries. You had Percival Lowell and his canals on Mars, and now you have SETI and its radio ears listening for signals from any civilization on planets orbiting distant stars. But, with so many billions of stars in the galaxy, how are we supposed to know where to look? How do you know where to start?

Well, one astronomer has sifted through a catalog of thousands of stars and come up with a short list of top ten, the top ten stars that have the conditions needed to support life, and are close enough for us to take a look or a listen. And what's so special about these stars that they made the cut and what exactly are scientists looking for when they turn their telescopes on them? Well, my next guest has some of the answers to these and other questions.

Maggie Turnbull is a post-doctoral research associate with the Department of Terrestrial Magnetism at the Carnegie Institute of Washington and you can talk to us at 1-800-989-8255, 1-800-989-TALK. Welcome to Science Friday, Dr. Turnbull.

Dr. MARGARET TURNBULL (Research Associate, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C.): Hello, thank you.

FLATOW: Why do you even have this list?

Dr. TURNBULL: Well, this list, motivated really by the search for extra-terrestrial intelligence, which is programmed to listen for technological signals from any other civilizations in the neighborhood of the sun. Now, the SETI Institute is working now on building a telescope array called the Allen Telescope Array, after Paul Allen, and what they need most urgently is a list, a target list of good SETI stars.

FLATOW: Ah-hah. So, how with all these thousands and thousands, I mean, billions of stars, potentially, how do you know which ones to listen in on?

Dr. TURNBULL: Right, well, the first thing is that close stars are better, so that we can have a chance of actually detecting any signals. But, the primary thing that I was focusing on is finding stars that behave in a way that is not hostile to any life forms that may be on planets orbiting those stars.

FLATOW: Ah-hah. So is there a criteria, a list of things that a star has to pass or it sort of gets eliminated?

Dr. TURNBULL: Right, so there are several criteria. I spent some time when I was given the charge of making the target list for SETI of just sitting down and thinking about, well, what is it about the sun that makes it such a good parent for life forms on earth? Obviously, the earth has been a habitable place for life for the last four and a half billion years and we've had plenty of time during that time for biology to evolve to the point where it can build radio telescopes and communicate in between the stars. So, one of my criteria was stability. I didn't want stars that are changing a lot in their brightness level over periods of less than a few billion years.

FLATOW: That would mean that would allow life to evolve there?

Dr. TURNBULL: Exactly.


Dr. TURNBULL: Right. So, it turns out that that requirement all by itself actually rules out quite a few stars and, in particular, massive stars. The most luminous stars in the galaxy are immediately ruled out because they simply do not live long enough.

FLATOW: Yeah. And so you've got it down to a top ten list? Is that what you're saying? Are you looking for just single cell life or crude life or does this list also include intelligent life that could be sending things to us?

Dr. TURNBULL: Well, it's kind of a trick question, actually, when you say, well...

FLATOW: Well, I love that, too...

Dr. TURNBULL: ...which star should we look at? I mean, that is a trick question because it depends on how you're planning on going about looking for those life forms. If you're talking about a project like SETI where you're listening for intelligent, you know, signs of intelligent civilizations, technology, then you need to be fairly rigorous about your criteria, because we are looking for a technology like ours, we want stars that are just like us.

And so, I started with a list, a catalog, of about 120,000 stars, and from those I was able to whittle it down, given everything I know about those stars and everything I know about life and technology on planet Earth, to come with a short list of 19,000 or so. Now, from that, obviously 19,000 is still quite a lot of stars and it will take time to go through all those, so we wanted to be able to prioritize those according to, well, I mean, just how nearby are they and how much do we know about the stars and how likely do we think they are to be able to support life exactly like what we have on earth.

So, from that I've come up with a very short list for the purposes of communicating with the public and for the purposes of prioritizing our observations, of five stars.

FLATOW: So, give us your top five then.

Dr. TURNBULL: Okay, well, number one, I have in, and, you know, you should keep in mind always that these top stars, really the top 50 or 100, these are all excellent targets and so it's hard for me to say, okay, this is the best star. That's just not realistic.

FLATOW: I think stars get insulted if they're not included in this list...

Dr. TURNBULL: Exactly, right, exactly, so the top 100, they're all tied for first place but I'm just gonna give you five of them. Beta CVn, kind of a strange name, but that's the name of this very sun-like star. It's just a tiny little bit brighter than the sun. You can see this one with the naked eye. It has a metal content in its atmosphere a little bit less than the sun, about 30% less than the sun, of iron and heavy elements in its atmosphere, and metal content in the star is an indicator of the presence or the potential presence of planets around that star.

The more metals are in the star, the more likely it is to have planets, at least planets that we've been able to detect so far, giant planets. Of course, we know nothing about terrestrial planets at this point. That star is pretty nearby in the grand scheme of things, 25 light years away.

FLATOW: It's already heard I Love Lucy by now.

Dr. TURNBULL: Absolutely...

FLATOW: And they may be sending us a reaction from out there...

Dr. TURNBULL: Sure, we just have to wait around for a few more years. Right, so my number two on my list here, and, of course, these are all switched around depending on which article you read and who you talk to, but on my little list right in front of me I've got 18 Sco in the constellation Scorpios next. That one's a little fainter, maybe if you have really good eyes and you're out in the country, you might be able to see that one. That's a star very much like the sun. In fact, we call that one a solar twin. It's got about the same metal content. It's a single star. It's about the same age as the sun, same luminosity.

FLATOW: Have you seen any planets around any of these stars yet?

Dr. TURNBULL: Yes, in fact, the next one on my list, 51 Peg, is known to have a giant planet orbiting. In fact, 51 Peg was the first star to be announced as having a giant planet through the radial velocity searches. It's in a very tight orbit around the star, interior to what we call the habitable zone. Now, the habitable zone is that region around the star where an earth-like planet would be at the right temperature to have liquid water on its surface. So, if 51 Peg's planet were in our system, that would be interior to our orbit, interior even to Mercury's orbit for that matter.



FLATOW: Pretty hot.

Dr. TURNBULL: Yeah. In fact the presence of that planet, while it does not interfere with the habitable zone today, does raise some questions about the history of that system because we are pretty sure that planets of that mass cannot form so close to the star. It's probably a gas giant planet at that mass, and it probably formed much further out, say at about Jupiter's distance. But as it was accreting material, it gradually spiraled inward and ended up where it is today, passing through the habitable zone on the way, sweeping up any terrestrial planets, making life miserable for any young life forms getting started there.

FLATOW: But why would you look there if life is miserable for any life-forms to get started? Why even look at that place?

Dr. TURNBULL: Well, you know, we haven't ruled out the possibility that after the planet swept through, terrestrial planets were able to form. It's all about time-scales. If that happened very early on in the planet forming history of that system, it's possible that terrestrial planets were still able to form in the habitable zone afterwards.

FLATOW: Ah-hah. There is enough time then and that must be an element that you use in your criteria.

Dr. TURNBULL: It is.

FLATOW: To allow enough time for evolution to happen.

Dr. TURNBULL: Yes, it absolutely is, and all of these stars that I'm talking about are, at least the five that I'm giving you right now, they are all old. Like this one, old as in around five billion years, at least three billion years old, any younger than that and I refuse to allow it on to my SETI target list.

FLATOW: How is this been paid for, this search for these planets?

Dr. TURNBULL: Well, in the case of searching for technological signals, the SETI Institute and the Berkeley Radio Astronomy Laboratory are partnering up with Paul Allen, co-founder of Microsoft, to build the Allen Telescope Array in Northern California.

FLATOW: Mm-hmm.

Dr. TURNBULL: And that is going to be an array, when it's finished, of about 350 radio dishes, small radio dishes, and we'll be able to do SETI 24/7 from that telescope.

FLATOW: Is this program affected by any of the cuts in the government, or NASA funding, that we keep hearing about?

Dr. TURNBULL: SETI not so much. However, you know, remember I was telling you that's kind of a trick question when you ask what's the best place to look for life in the solar neighborhood. SETI is only, SETI is one way of doing this.

Another way of doing this is to try to directly image, just take pictures of planets like Earth orbiting in the habitable zones of nearby stars. We want to see these planets with out own eyes. We want to hone in on those planets and see if we can see bio-signatures, signs of water, oxygen, methane, carbon dioxide, and so on. And there is, in fact, a mission in the planning stages to do just exactly that. It's NASA's terrestrial planet finder mission, and this mission represents the absolute state of the art in imaging technology.

FLATOW: And is it funded, is it going to happen?

Dr. TURNBULL: It has been funded up until this year at which point its funding was cut by about a factor of ten.


Dr. TURNBULL: So, even just to get through the remainder of 2006, it's not entirely clear where the money is going to come from. And, in fact, in the president's budget for 2007, it has been completely zeroed out.

FLATOW: Oh, well...

Dr. TURNBULL: However, that's still a proposal. That is still a proposal.

FLATOW: Right.

Dr. TURNBULL: And if Congress is behind this mission and taxpayers are behind this mission, the funding can be restored.

FLATOW: Well, let's, we'll hope for you in keeping your fingers crossed, Maggie Turnbull.

Dr. TURNBULL: Thank you.

FLATOW: Thank you for taking time to be with us.

Dr. TURNBULL: Thanks a lot.

FLATOW: Dr. Turnbull is a post-doctoral research associate at the Department of Terrestrial Magnetism at Carnegie Institution of Washington. We're going to take a break and come back, switch gears and talk about nuclear power. Stay with us, we'll be right back.


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