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Sunday, March 15, 2009

Planets like Earth appear to be out there


LONDON — The real wonder of our age is this. You can go on the Web, type in PlanetQuest New Worlds Atlas, or Extrasolar Planets Encyclopedia, or NASA Star and Exoplanet Database, and directly access the data on 340 new planets that have been discovered in the past five years.

That number is set to grow very fast now, for on March 6 The U.S. National Aeronautics and Space Administration (NASA) successfully launched the Kepler telescope, which will find many more planets including potentially Earth-like ones. It will stare unblinkingly at an area of space containing about 100,000 relatively near stars, watching for the tiny dimming of a star that happens when one of its planets passes between the star and us.

I enjoyed writing that last sentence. I couldn't have written it 10 years ago, because we still didn't know then whether it was normal for a star to have planets. Maybe planets were very rare, and life a thousand times rarer, and we were the only intelligent life in the galaxy. That always seemed pretty unlikely, but you couldn't prove otherwise.

Well, now we know that planets are as common as dirt. New techniques that can see past the blinding glare of the parent star to pick out only the faint light reflected from a planet's surface have found them around more than a hundred nearby stars. It's like spotting a candle burning next to a lighthouse from a thousand kilometers away, but it works.

The Kepler telescope mechanizes the process. If any of those 100,000 stars have planets that orbit in a plane that causes them to pass between the star and us, Kepler will spot them by the dimming they cause as they pass in front of the star. Probably thousands of the stars have planets orbiting in that plane, so the tally of "exoplanets" (planets orbiting other suns) is going to go up very fast.

Even in that tiny section of sky, Kepler will probably miss tens of thousands of other planets whose orbits don't bring them between their star and us. Moreover, the great majority of the planets it does find will be gas giants like our own Jupiter and Saturn, just because gas giants orbiting close to their stars are the easiest planets to spot. But those are planets that cannot support our kind of life; the real triumph is finding planets like our own Earth.

The closest astronomers have come so far is a planet called "Gliese 581 c." It's the middle planet of three orbiting Gliese 581, a star about 20 light-years from here — it may have other planets, but we can't see them — and it's only 1 1/2 times the diameter of Earth. It is a rocky planet like our own, not a gas giant, and it is in the "Goldilocks Zone" around its star where the temperature is neither too hot nor too cold to permit liquid water on the surface.

Gliese 581 c is not another Earth. The gravity is much higher, it is very close to its sun (which is smaller, dimmer and cooler than our own), and it whips around it every 13 days compared to our 365 days. But it could potentially support our kind of life — which makes it, just for the moment, the second most interesting object in the universe after our own planet.

We still cannot see if it has an atmosphere, and if so whether it contains the telltale gases that indicate the presence of life, but a new generation of orbiting observatories that are planned for the next decade — NASA's Terrestrial Planet Finder and the European Space Agency's Darwin project — could give us the answers. Darwin, for example, is going to survey 1,000 of the closest stars, looking for small, rocky planets and seeking signs of life on them.

Two big consequences are going to come out of this. One is a long and tempting list of Earth-like planets in our own stellar neighborhood. It's quite likely that there will be evidence of life on many of them.

Unless we discover some loophole in the laws of physics, we may never reach them, for the distances involved are immense, but from now on they will always be there, beckoning us to visit, even to come and settle them. The knowledge that there is a destination worth going to can be a powerful spur to innovation.

The other consequence is a huge question about intelligent life in the universe. If planets capable of supporting life are so commonplace — last month Dr. Alan Boss of the Carnegie Institution of Science told the annual meeting of the American Association for the Advancement of Science in Chicago that there could be a hundred billion of them in this galaxy alone. If so, then where is everybody?

Is intelligence a rare accident in the evolutionary process, or such a self-destructive attribute that intelligent species don't usually survive more than a couple of centuries after they industrialize? Are they all observing radio silence because there is something dreadful out there? Or have we just not figured out yet how mature galactic civilizations communicate?

Gwynne Dyer is an independent journalist whose articles are published in 45 countries.


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