Just 15 years ago, a planet outside the solar system was the topic of speculation. Many astronomers suspected the presence of planets around some stars, but no one knew for sure. Astronomers did not spot anything till 1992, when the first one was discovered by Polish and Canadian astronomers. It was orbiting a pulsar (fast-rotating neutron star) 1,000 light years from the earth.

Hunting for extrasolar planets has now become one of the hottest areas of astronomy. So far, astronomers have discovered 209 planets outside the solar system. And there’s a new problem: of finding worlds similar to the earth. Few planets discovered so far are like the earth; most of them are gaseous giants orbiting stars that are also very different from the sun. Does it mean we will find it difficult to find planets that can sustain life? Probably not, say astronomers.

At a recent American Astronomical Society meeting in Seattle, researchers were confident of finding planets similar to the earth, or at least with conditions that could support life. They are also revising the notion that a planet needs to have earth-like conditions to harbour life, as long as they fall within the “habitable zone” of a planetary system.

“Things look good for finding lots of new planets,” says David Morrison, senior scientist, Nasa Ames Research Center, “but finding life is much harder and unlikely in the next 10 years.”

The fact that we have found so many gas giants in the planetary systems could be a bias of observation: the bigger a planet, the more likely it is to be found from a distance. However, astronomers also think that big gaseous planets reach close to their stars after migrating from the edges of the planetary system. A giant migrating planet is not good news for the smaller, rocky planets near the stars in the habitable zone. In fact, many had thought that the inner terrestrial planets may not form at all if the gas giants migrate towards the star. Remember the asteroid belt: Jupiter’s gravity kept the rocks from coming together. So, in the last 10 years, the discovery of so many gas giants near their stars had reduced hopes of finding rocky planets there.

But now there is evidence that rocky planets can form even when the giants migrate. Researchers at Pennsylvania State University have shown using simulation that many rocky planets can form and survive even if their paths cross that of the migrating gas giants. Terrestrial planets will be the target of the European space mission Corot (Convection Rotation and Planetary Transits) launched last month.

Similarly, new studies have shown that red dwarfs can also have planets with potential for life formation. Compared to the sun they are small, not very hot and burn for a long time. Red dwarfs can have planets, but the catch so far has been this: a planet has to be too close to a red dwarf to be warm enough for liquid water.

This distance may seem too dangerous for life, but studies show that planets close to red dwarfs can sustain life under certain conditions. They must have a thick atmosphere to circulate the heat and a strong magnetic field to ward off harmful radiation. Geoff Marcy, professor of astronomy at the University of Berkely, California, and planetary hunter says, “Low-mass stars emit most of their light at visible and infrared wave lengths, and very little of the harmful ultraviolet light that can damage fragile organic molecules. So they may be even more suitable for organic life than sun-like stars.” Marcy’s team has embarked on a major search for rocky, earth-like planets around such low-mass stars, using both the world’s largest telescope (the Keck, in Hawaii) and also a specialised telescope his team is building in northern California.

As the number of exoplanets increases, so does the possibility of finding planets that can sustain life. It does not matter that stars like the sun are not abundant, or that too many stars have gas giants orbiting them. The Nasa Kepler mission, to be launched in 2009, is designed to search for earth-like planets in the habitable zone. But to detect life, we may have to wait for the launch of Nasa’s Terrestrial Planet Finder, after the year 2020.