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Astronomer Geoff Marcy is the Usain Bolt of planet hunters, but now he is turning his gaze towards potential alien civilisations. The good news, he tells Anil Ananthaswamy, is that ET may already be calling

Why have you decided to join the search for extraterrestrial intelligence ()?
I鈥檓 at a funny crossroads, personally. I really want to turn my attention away from planet-hunting towards SETI. I鈥檓 in this lucky position that my career has been more successful than I could have ever imagined. It鈥檚 time for me to roll the dice, try something that鈥檚 a long shot. Younger scientists can鈥檛 put their eggs in that basket, because if you spend your time on SETI, your chances of success are low. But I have the luxury. There are some experiments we can do to hunt for the great galactic internet.

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Are you serious? What鈥檚 your plan to find aliens?
[Laughs] If Gene Rodenberry is right and the Klingons and the Romulans are really out there, they have to communicate with each other. They aren鈥檛 going to do this by stringing fibre optic cables between the stars, they are going to do it with lasers. Lasers are a logical way to go, because you can maintain a level of privacy by confining your laser to a beam narrow enough that it just hits a spacecraft or the civilisation that鈥檚 around another star 3 light years away. Not to mention, you save energy. Why spread energy everywhere like a radio transmitter does?

If our galaxy is teeming with advanced technological life, it has lasers criss-crossing it 鈥� tens of thousands, millions of them 鈥� and we should be able to pick up some spillover. Also, some aliens are going to try to communicate with us. Maybe they are literally pointing their lasers at us and we just aren鈥檛 looking.

鈥淚f our galaxy鈥檚 teeming with advanced life, it has lasers criss-crossing it鈥�

You think aliens may have identified Earth as a habitable planet?
In the next century or two, we humans will have planet-finder telescopes that span our solar system with mirrors strewn from here to Jupiter, giving us enormous angular resolution so we can do the kind of science that a self-respecting advanced civilisation ought to be doing. We should someday be imaging the continents on other planets. We can鈥檛 do that yet, but aliens can do that already, so they know we are here.

What makes you sure aliens can do this already?
Oh, because our galaxy is 10 billion years old. The Earth is only 4.5 billion years old. We are a firefly flicker in the great astrobiology of the galaxy. They presumably have had their light bulbs on for much longer.

And what makes you think aliens are training their lasers at us?
Here鈥檚 a funny thing. Our ground-based telescopes nowadays have lasers attached to them. It鈥檚 called adaptive optics. The lasers are used to create artificial stars in the sky, which the telescopes use to correct for atmospheric aberrations. When you point your telescope, with its associated laser, at galaxies, stars or planets, those photons are going there. We have sent a signal betraying our own existence specifically to other objects in the universe.

This is not a lively repartee. The stars we鈥檙e studying are 1000 light years away, so our beam won鈥檛 get there for 1000 years. And if they want to answer us, that鈥檚 another 1000 years. But we are communicating whether we know it or not with pretty powerful and well-pointed lasers. The flip side is that maybe they are studying us with their own lasers, for whatever reason, and we should be looking for that. And that鈥檚 what I plan to do.

We have yet to detect any signs of alien life. Is that telling you something?
There is the distinct possibility that Earth is a relatively rare kind of planet. Another explanation is that the typical lifetime of an advanced civilisation could be 1000 to 5000 years. What possibility do we have for surviving another 1000 or 10,000 years?

Maybe the paucity of such advanced civilisations in the galaxy is a sign of the challenge of surviving. Ironically, that is the most poignant message the aliens could have ever sent us.

These aliens would need somewhere to live, which leads me to your work hunting for exoplanets. What鈥檚 the state of the field?
It鈥檚 changing by the week, thanks to NASA鈥檚 Kepler space telescope. We just submitted a paper on 1090 new planets, adding to the 1235 that we announced last year, on top of the 700 or so that had been found before Kepler. The sheer number of planets is astonishing for me personally, because it was only 16 years ago that I was wondering if humanity would ever find any exoplanets at all.

What are you learning about planet categories?
That there are three main groups: Jupiter-like planets that are predominantly hydrogen and helium; water-dominated planets like Uranus and Neptune; and rocky planets like Earth, Venus and Mars.

How common are Earth-like planets in the habitable zone 鈥� lukewarm with liquid water?
No one knows. A major puzzle for which nobody has an answer is this: is there some size at which the planets change their nature from water-rich planets like Neptune, to rocky planets like the Earth? We have found two planets that are the size of the Earth in radius, but they are very close to their host star, so water on the surface would evaporate away.

How do you make planets like the Earth in a universe that has far more water than iron and nickel? It鈥檚 possible that Earth, with its mostly rocky composition, is rare in the universe.

What kind of telescopes do we need to find habitable Earth-like planets?
What we really want is an enormous space-borne telescope the size of a football field that can image Earth-like planets around nearby stars, taking a light spectrum of these planets directly. Does the spectrum show water, methane, carbon dioxide, maybe even ozone? If you find oxygen in another planet鈥檚 atmosphere, that鈥檚 a sign that there is some photosynthesis going on. NASA was planning such a telescope, called the Terrestrial Planet Finder, and the European Space Agency was planning its Darwin telescope.

Where do these two efforts stand?
Dead in the water. Completely, totally dead. There is no money.

Could the exciting Kepler findings change that?
That鈥檚 the hope.