There's another Earth out there – and we'll find it

The first Earth-like planet orbiting a distant star could be discovered within four years, astronomers believe. None of the 300 "extra-solar" planets so far identified beyond our own system is thought to be suitable for life, so the discovery of an Earth-like planet made of rock rather than hot gas or frozen ice would significantly increase the chances of finding the second habitable world, scientists said.

A leading astronomer confidently predicted yesterday that the discovery of an earth-like planet – possibly in the water-friendly "habitable zone" around a nearby star – would soon be announced, after two satellite studies by the European Space Agency (ESA) and the US National Aeronautics and Space Administration (Nasa).

"Within about three or four years, there will be a press conference at Nasa headquarters and they will tell us just how frequently Earth-like planets occur and once we know that we will know how to take the next steps in the search for habitable planets," said Alan Boss of the Carnegie Institution for Science in Washington.

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"I think it's inevitable that there are Earth-like planets out there. I suspect that every star we look at in the night sky has an Earth-like planet around it. We already know that most stars have planets," Dr Boss told the American Association for the Advancement of Science in Chicago.

"We've been able to find planets a few times the mass of the Earth around lower-mass stars but while they are not quite earth analogues, they are getting closer and closer to Earth. The fact that we can find them already implies we are seeing just the tip of the iceberg."

Scientists believe up to a third of the sun-like stars in the Milky Way galaxy have planets that are several times larger than Earth. Dr Boss said the figure could be even higher for smaller planets the size of Earth. "I think we are talking about a number that is very close to one, one Earth-sized planet around each sun-like star," he said. This would mean there could be 100 billion earth-like planets in our galaxy alone, and at least 100 billion galaxies in the Universe.

"What we're going to find is that the number of Earths is quite large and that's going to tell us how to build the next telescopes to go out and examine these finds," Dr Boss said.

At present, ground-based telescopes able to detect only planets that are substantially larger than Earth, which means the "gas giant" planets such as Jupiter or the ice-giants such Neptune which are both unsuitable for life.

But smaller, rocky planets could be identified for the first time with two space-based telescopes, ESA's Corot satellite, which began collecting data last year, and Nasa's Kepler mission, which is scheduled for launch next month. "I think we'll be absolutely astonished if Kepler and Corot did not find any Earth-like planets because basically we're finding them already, so I don't think it's possible that we won't find some," Dr Boss said.

Suggestions that there could be as many as 100 billion Earth-like planets in the galaxy, with a sizeable fraction of them orbiting with the habitable zone where liquid water exists, means that extraterrestial life is inevitable, Dr Boss told the meeting.

"If you have a habitable world that is sitting around for four, five or ten billion years around a star, how are you going to stop it from forming life? It's like taking a refrigerator, unplugging it, shutting the door and then coming back a couple of months later; you'd be amazed to find what's growing there," he said. "That's what life's like. The fridge analogy may not be the same as the origins of life, but life is so tenacious, it's hard to stop. If you had a planet sitting there at the right temperature with water for a million years, something's going to come out of it."

But finding a planet with life is not the same as finding intelligent life. "It is quite a bit harder to estimate what the likelihood is of finding intelligent life, probably because intelligent life, we believe, probably exists for only a fraction of the time period of when a planet may be inhabitable," Dr Boss said.

"Do you really think that our civilisation is going to last for a billion years? That's the timescale we talk about in astronomy. I don't think we are going to be around in a billion years. On the other hand, there are just real practical reasons why we may not have detected intelligent life. Maybe we haven't found them yet because we just haven't look far enough and long enough in the galaxy," he added.

... and there may be another form of life right here

We know that life evolved at least once in the Universe but could it have originated twice on the same planet – just not as we know it? That is the question posed by a scientist who believes that a second form of "weird life" which has yet to be discovered may have originated on Earth.

Paul Davies, a British-born physicist at Arizona State University in Tempe, said that the conventional view that all life on Earth originated once from a common ancestor may be wrong and that another form of microbial life may exist in parallel to life as we know it.

"There has been no systematic search for life as we don't know it on Earth. We're not talking about some kind of life we can't see for mystical reasons," Professor Davies said at the American Association for the Advancement of Science in Chicago. "We're talking about microbial life, and in this realm some little organisms may have an alternative biochemistry derived from a second or subsequent genesis event."

Scientists believe that the first lifeform originated between 3.5 and 4 billion years ago when the Earth was a very different place. It was being bombarded by comets and meteoroids and bathed in ultraviolet radiation to create a kind of "primordial soup" of pre-biotic chemicals that began to replicate and evolve. Professor Davies believes that genesis event may have occurred more than once and that it is possible that two or more forms of life came about which used quite different sets of biological molecules.

"If we imagine the origin of life on Earth some three and half to four billion years ago, we can imagine a series of stop-go experiments where life was formed and annihilated, formed and annihilated," Professor Davies said.

"As a result of that sequence it is entirely possible that another form of life was left, and then the issue is whether it would have survived today and formed a shadow biosphere," he said.

"It could be right under our noses, or even in our noses. It could be that weird life and regular life are intermingled."