Is there life on worlds other than our own? In the legendary science-fiction TV series, Star Trek, when Captain Kirk and the crew of the starship Enterprise set out “to explore strange new worlds,” they carried a nifty gizmo that could pick out life forms from afar. Present-day scientists no doubt wish they had access to such a device. Instead, they must make do with current technology. Spacecraft can be sent to planets and their moons in the Solar System, seeking clues to whether they ever harboured life. Mars looks quite promising and life might even be lurking in the icy waters of Jupiter's moon, Europa. What about the billions of stars in the universe? If life arose here on Earth, it may well have done so on planets orbiting other stars. Telescopes on the ground and in space offer the obvious way to make a start in assessing if life could have sprung up on such faraway worlds. That, of course, raises the question of what to look for. Based on the only case we know of life having arisen, the idea is to seek Earth-like planets that are just the right distance from their sun (known as the ‘habitable zone') for liquid water to exist on the surface.
In March 2009, the U.S. space agency, NASA, launched the Kepler space telescope. Its mission is to help assess how many Earth-size planets there could be, orbiting in the habitable zone in our region of the Milky Way galaxy. To do so, it stares unwaveringly at a large area of the sky, keeping watch over more than 100,000 stars and picking up signs of the faint dimming of light that occurs when a planet passes in front of its star. Recently, the mission scientists confirmed the existence of a planet, Kepler-22b, which, in the words of a press release, “is the smallest yet found to orbit in the middle of the habitable zone of a star similar to our sun.” This planet is larger than Earth, with a radius 2.4 times that of the latter. The scientists do not know yet whether it is predominantly rocky, gaseous, or liquid. In addition, the Kepler telescope has identified 2,300 planet candidates and they await further evaluation. Ten of them appear to be Earth-size and in the habitable zone. But finding a planet similar in size to Earth orbiting in the ‘Goldilocks zone' where it is getting just the right amount of energy from its sun is only a beginning. Such a planet probably also needs a ‘Goldilocks atmosphere' to be conducive for life. If, for instance, Kepler-22b has an atmosphere like that of Earth, its surface temperature would be a comfortable 22º Celsius. Such assessments of the atmosphere are tasks that must be carried out with future space telescopes like the James Webb telescope that NASA expects to have ready by 2018. Thus the search goes on.