Science Interviews


Sun, 18th Nov 2007

An Alien Solar System

Professor Geoff Marcy, who's from the University of Berkley,

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55 CancriKat - Now in the news last week was the story around what, essentially, is a very boring star. Now the star is called 55-Cancri; it's 41 light years away; found in the constellation Cancer; and it seems to be very similar to our Sun.  To get the full story, earlier this week Ben Valsler met up with Professor Geoff Marcy, who's from the University of Berkley, to find out.

Prof Marcy - Way back in 1989 my team began taking Doppler shift measurements of this rather obscure sunlight star 55-Cancri. Every year we would take a few more Doppler measurements and we didn't see anything for quite a few years. Then around 1996 we noticed that the star was wobbling.

We quickly realized that it was wobbling because a planet was pulling on the star with an orbital period of about 14 days. We were delighted but then we got worried because over the next few years the star didn't behave as if it just had this planet around it. It was wobbling for other reasons that we didn't understand. Soon we realized that there was a second planet.

By the late 90s we actually discovered yet a third planet. Now in 2007 we can see the effects of yet a fourth and even a fifth planet. What's remarkable in hindsight is to notice that over 18 years ago we began observing a star with no sense, no knowledge that there were any planets at all. It's turned out to be a goldmine, this star.

Ben - So how do you go about spotting a planet that's so far away you can't actually see any light reflected off the planet?

Prof Marcy -   The sad truth is that even with the Hubble Space Telescope you can point it at a nearby star and any planet that might be orbiting that star would be lost in the glare of the host star. So we use a trick. The idea is to watch the star, not the planet and we look to see if the star is stationary, which as Newton said, it would stay unless it's acted upon by an outside force. And in fact, any star orbited by a planet would feel the gravitational tug by the planet, acting on the star, causing the star to wobble. We watch star to see if they're stationary or if they move. We do that by measuring the Doppler effect that all of us are familiar with. When you hear a train whistle go by the pitch changes and so it is with star light. The frequency of the light waves change as a star wobbles in space. We measure that and thereby detect the planets.

Ben - So you're looking for a signature of a planet rather than the planet itself? How to you tease out which wobbles are caused by which planet?

Prof Marcy - A star that's orbited by just one planet in the simplest case will execute a small circular motion as the star is yanked on by the planet. If there are two planets there, you'll see the star execute a curlicue pattern, sort of circular motion superimposed on a larger circular motion. And so we have to pull the planets apart one by one from the motion of the star and eventually we're able to detect all five planets. Not unlike your ear hearing five notes on the piano. Eventually, of course, you can tell there's not just one or two notes but a full five notes.

Ben - What is special about the star 55-Cancri?

Prof Marcy - What's lovely about 55-Cancri is that it's a Sun-like star. It has nearly the same mass as the Sun - even the same age of about 5 billion years and nearly the same chemical composition. In fact 55-Cancri's a little richer in the heavier elements like oxygen, silicon and iron. Soit's a Sun-like star. Of course, what's wonderful about our solar system is that there are eight major planets orbiting it and now we have 55-Cancri with its five planets that we've so far been able to detect. It may well be that there are smaller planets orbiting 55-Cancri, Earth-like planets that our technology can't at present detect at all.

Ben - Four of the five planets are what we would think of as being as gas giants. So why do you think that there's a likelihood that we'll find rocky or Earth-like planets?

Prof Marcy - It's amazing that this 55-Cancri system has four gas giants.The innermost planet, the fifth one, being smaller (about 13 Earth masses).There's a gap between planets four and five, a large gap, within which we detect nothing at all. Without the ability to detect Earths it could well be that there are rocky planets in there: Mars-, Venus-like or Earth-like planets. They may in fact be warmed up to nearly lukewarm temperature with a little greenhouse effect added in. So that gap offers an opportunity for us to go with next generation equipment and try to detect any Earth-like planets that may be there.

Ben - So what is the next step for you?

Prof Marcy - Well, there are several exciting next steps. The Europeans and Americans are trying to work together on a spectacular new telescope called Darwin/TPF (the Terrestrial Planet Finder). The idea of Darwin and TPF isto take the first pictures of an Earth-like planet. I think it would bemarvellous, some day, to open up the newspaper and see above the fold ayellow dot, the host star, with a small pale blue dot orbiting that yellow star. For the first time we would know that there are other Earths out there in the universe.


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