One of most stunning finds

An Earth-like planet orbiting the star closest to our Sun, revealed yesterday, is the most recent — and among the most stunning — in a string of exoplanet discoveries going back 20 years. What do we know about the search for life beyond our solar system? What is an exoplanet? Simple: any planet outside our solar system. The first exoplanet was detected in 1995, but the number has exploded in the last few years. A recent statistical study estimated that there are a trillion in our galaxy alone. Today, according to a tally by Nasa, there are 3374 known exoplanets. Of those, 1248 are so-called ice giants, 990 are gas giants, and 775 are “super Earths” with masses many times higher than the rock we call home. There are only 347 smaller terrestrial planets with Earthlike mass, and of those only a handful in a “temperate” zone that would allow for the presence of liquid water — a key ingredient for life (as we know it). None of these were within reach until today, with the discovery of Proxima b revealed. How are exoplanets detected? There are several ways to find planets that cannot be directly observed, says Nasa. Wobble watching — This involves looking for changes in the colour spectrum emitted by a star due to the gravitational pull of one or more invisible planets. If these patterns are regular and cyclical, corresponding to a tiny wobble in the star, chances are they are caused by a planet. Also called radial velocity, this is how Proxima b was discovered. Exoplanets found this way: 17.6 per cent. Shadow searching — When a planet passes directly between its star and an observer it dims the star’s light by a tiny but measurable amount. This transit method has been the most successful — Nasa’s Kepler spacecraft used it to find thousands of candidate planets from 2009 to 2013. Obviously, if a planet is not on the same plane as the star and the observer, it doesn’t work. Exoplanets found this way: 79 per cent. Picture producing — Snapping a picture of an exoplanet in front of its star is like trying to photograph a microscopic speck of dust on a glowing lightbulb. But by removing the star’s blinding glare, astronomers can capture an image, a method called direct imaging. Only a tiny fraction have been detected this way: 1.2 per cent. Beam bending — Light from a distant star is bent and focused by gravity as an orbiting planet passes between the star an Earth. Called gravitational microlensing, the gravity of the planet and star focus light rays of the distant planet on an observer in the same way a magnifying glass focuses the Sun’s light onto a tiny, bright spot. Only a handful of exoplanets have been found using this method. What conditions support life? That depends on what the meaning of “life” is! For life as we know it, liquid water is essential. Of the exoplanets found to date, however, only a handful are in a “temperate” zone in relation to their star: not so hot water can evaporate, not so cold it freezes rock solid. Life on Earth is also unimaginable without an atmosphere, containing in our case, the oxygen we need to survive. An atmosphere also protects against damaging highenergy radiation from the Sun — ultraviolet and X-rays. But it is possible creatures elsewhere in the universe could thrive in conditions lethal for our species. — AFP