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He is one of the world’s leading authorities on planets outside our Solar System. But although Geoffrey Marcy has been officially credited with finding 38 of the 53 suspected worlds that have been identified so far, he was beaten into the history books by a European team which discovered the first extrasolar planet. Marcy, who is professor of astronomy at the University of California at Berkeley, is now in open competition with this team, and others, in the race to find and announce new worlds. Stuart Clark explores Marcy’s universe.]

Planet hunting seems intensely competitive, a bit like Formula 1 racing. Is that how you see it?

Mostly yes, but it is a good-natured competition. All of us in the extrasolar planet business sense that we are in the middle of a remarkable 10-year era of discovery. It reminds me of the transoceanic voyages of the Europeans during the 1500s, when they realised that Earth had so much that was ripe for discovery.

Which are the top three teams?

There are well over a hundred people working on extrasolar planets, with varying degrees of effort and success. As far as searching for planets is concerned, two teams are far ahead of everyone else: Michel Mayor’s team at the University of Geneva and our team. Together we have found 95 per cent of the planets. We are close friends who often get together over a good glass of wine. A third team is led by Tim Brown at the High Altitude Observatory in Boulder, Colorado. He has done some marvellous work with ground-based telescopes and the Hubble Space Telescope, studying the reflected light from planets and the transits that cause the stars to dim as planets cross in front.

With this degree of pressure to succeed, what would you do if you made a mistake?

We never want to make a mistake and we haven’t announced a planet which then turned out to be wrong. Before 1995, there’s quite a sordid history of claims that were later found to be bogus. So Paul Butler and I vowed not to add any new headstones to the already large graveyard of defunct planets. If we were going to find planets, we would not only have to find them, but we would make sure our claims were of the highest integrity. So when we first see the star wobbling, we don’t believe it. When we are 99 per cent sure, we predict what the star should do over the next three months and collect more data. If the new data meets the prediction, we announce and publish it. This is a little slow and gives other people a chance to sneak in. In fact, we have had one or two teams beat us to the announcement, but we would rather be right.

Why did you bother getting involved in such a tarnished field of study?

I remember it like it was yesterday. In 1982, I received my PhD from the University of California at Santa Cruz for research measuring the magnetic fields of stars. It had not gone well, my answers were not very accurate and there was one astronomer who doubted I was measuring magnetic fields at all. As a young person I felt insecure and almost ashamed that my research was so uncertain. I was just beginning a two-year Carnegie postdoctoral fellowship in Pasadena and I was so depressed I decided I had to change direction. I had nothing to lose, so I could try something outlandish. Back then, detecting planets was equivalent to trying to understand UFOs or the power of the pyramids.

Did you also think it might be a direct route to fame as an astronomer?

No, it was about survival. I was so desperate for some inkling of happiness in my life that I even went to a psychotherapist. I wanted to address a scientific question that appealed to me on an emotional level, not just a scientific one. Finding out whether planets were common appealed to me. At that stage fame did not matter, nor what other people thought. Not even failure mattered because I thought I was a failure.

Before the first planets were discovered, your ideas received some of the lowest ratings from funding bodies. What kept you going?

I was often turned down or given very little funding. Now it’s very different, but I’m not bitter-I understood it was risky. Government agencies often act in conservative ways, but Paul and I thought we were in a win-win situation. If we found planets people would be excited-if we didn’t, it would be one of the most frightening non-detections in the history of science because it would render our own Solar System truly anomalous.

Despite your efforts, Mayor’s team beat you to discover the first extrasolar planet. How did you feel?

First, I was truly mesmerised and stunned by the discovery itself. Here was a question I wanted to know the answer to and, within my lifetime, I had got the answer. The Swiss may have come up with it first but it was just wonderful to know there are planets out there. I did feel a bit sad that we had been scooped-it’s always nice to be first, but it wasn’t a serious downward spiral because immediately Paul and I found two planets of our own. We had been monitoring 120 stars, we had excellent data and we had excellent software. So we knew that since 51 Pegasi was correct, we would find many planets of our own.

But buried in your unanalysed data were the signatures of other planets around other stars. If you had analysed it earlier, you could have made that first discovery…

In retrospect, we feel like fools. The really laughable thing is that we believed the theoreticians. They agreed that Jupiter and Saturn-like planets form far from their host stars with long orbital periods of 10 or 20 years. So we thought we could spend 10 years working on our optics, hardware and software. Of course, 51 Pegasi proved that Jupiter-like planets could exist close to their parent star, in orbits of just a few days. With that realisation, we knew that sitting in our data, literally on hard disc, would be planets in shorter periods than the theoreticians had predicted.

How do you choose which stars to monitor?

The stars must be close to Earth, since these are the ones we will be able to investigate with new techniques in the coming decades. Most importantly, the close stars are the brightest ones, making them easiest to measure. We also select stars that are at least middle-aged or older. We avoid stars younger than 2 billion years old because they still have strong magnetic fields, they are rotating rapidly and have turbulence on their surface-all of which could mask the wobble or make it difficult to interpret.

How can you tell that what you’ve detected is really a planet?

We make a graph of the star’s Doppler shift over the course of time. These graphs look a bit like an ECG of a patient’s heartbeat. If the star is wobbling, the graph will go up and down. If this is caused by a planet, the graph has to be a shape that is consistent with Newton’s laws. If it is, that confirms there is a companion and allows us to calculate its mass.

Some astronomers claim that more than half your planets are not planets but failed stars known as brown dwarfs…

It is difficult to comment because it is wrong. I have written pages and pages on my website that describe why all of this is wrong. It can be shown mathematically that it is not right. There is no way that half the planets or even a small fraction are actually brown dwarfs or stars. It is not possible.

How long will it be before we have an instrument that can take a picture of an extrasolar planet?

I think it’s going to be a long time. There is a lot of optimism and media hype, but we are at least twenty years away from launch. This space-borne telescope would consist of a fleet of four or five craft like the Hubble Space Telescope, flying in formation, accurate to within a thousandth of the wavelength of light. It’s the grandest astronomical instrument humans have ever conceived and it is beyond our abilities at the moment.

What will a picture tell us about these new worlds?

Detecting a pale blue dot orbiting a yellow star would be the prettiest picture I could ever imagine. But seriously, astronomers want a picture so they can gather that light into a spectrometer, analyse the planet’s chemical composition, and discover if it shows signs of life. We would like to know whether the planet has oxygen or nitrogen or carbon dioxide in its atmosphere. If you find both methane and oxygen in an atmosphere then you have a strong indication of something on the surface, possibly life, keeping these gases abundant in the atmosphere. What really excites us is the same thing that excites a six-year-old child-the prospect of learning something about our position in the Galaxy. Is life on the Earth some remarkable fluke of biochemistry or is life a dime a dozen in the Milky Way?

Could any of the planets discovered so far could sustain life?

Those planets are just big balls of hydrogen and helium gas like Jupiter and Saturn so none of them has properties that we would say are amenable to life as we know it. However, one of the planets we announced last month, Epsilon Reticulum, is in the most Earth-like orbit ever discovered. It is in a nearly circular orbit, about 10 per cent farther from its star than the Earth is from the Sun. The planet’s mass is a little bit more than Jupiter, but it must have a temperature similar to Earth’s.

If it is a planet like Jupiter, might it have moons like Europa?

Absolutely. And if this planet has moons like Europa, the water, instead of being locked in ice on the crust, would be liquid. It would be hard to argue that there are no such moons around that planet, because the giant planets in our own system all have them.

So you believe there are many inhabited worlds in the Galaxy?

Let me answer that three ways. Out of 200 billion stars in our Galaxy, I would bet there are billions of Earth-like planets with rocky surfaces at the right temperature for water to be in liquid form on their surfaces. There is just no question about that. The next issue is whether biochemistry on those planets would lead to replicating molecules and life. Biochemists agree that if you have liquid water, carbon, nitrogen, oxygen, hydrogen, phosphorus-and most planets would because these are normal atoms in the Universe-then simple microbial life is almost certainly going to occur. The third part is the most difficult. Are human attributes, such as intelligence, dexterity and speech, common outcomes of Darwinian evolution? It could be that humans represent some tiny quirk of a twist on the Darwinian tree of life and that very few species will use these traits for their survival. Our Galaxy could have very few intelligent species.

If you could visit an extrasolar planetary system, what would you expect to see?

As you approached, you’d see the largest planets. Then, as you got closer, you’d see the smaller planets, maybe so small that they had never been detected from Earth. Closer still, you would begin to see the moons around those planets, some of which might have geophysics, meteorology and, most interestingly, oceanography that we had never even conceived of before. To see how our Solar System fits into the grand scheme of planetary systems, I think it would bring tears to my eyes.