Editorial: Searching for Earth’s twin

Published in The Hindu on January 14, 2013

The recent announcement at the American Astronomical Society meeting in California dramatically increases the possibility of finding Earth-sized extra-solar planets, or exoplanets, and in particular, those that are in a habitable ‘Goldilocks’ zone — neither too close nor too far from a star, so liquid water might be present on the surface. One study looked at the number of habitable Earth-sized exoplanets (those one-half to twice the size of the Earth) orbiting the smaller and cooler M dwarf stars. By studying about 3,600 of the 5,000 M dwarf stars present in Kepler’s field of view, researchers calculated that each of these stars had an 87 per cent possibility of having Earth-sized exoplanets, and nearly six per cent of the M dwarfs studied could be in the habitable zone. M dwarfs account for about 75 per cent of the stars in the Milky Way. “The chance of discovering a habitable-zone super Earth transiting [passing] a low-mass star in the immediate future is tremendously high,” notes a 2010 paper. Another preliminary study estimated that one in six stars in our galaxy has an Earth-sized exoplanet, increasing the abundance to an astronomical 17 billion. But not all are in the habitable zone. With the addition of 461 new candidate exoplanets identified by Kepler during the last 22 months, the total number of candidates has shot up to 2,740. Of these, 105 have been confirmed as planets.

Advanced and sophisticated methods have helped in not only discovering exoplanets orbiting sun-like stars and M dwarfs but also in studying their atmospheric composition, accurately calculating their size and mass, and also the temperature at different altitudes (gradient). The launch in 2018 of the James Webb Space Telescope increases the possibility of studying the atmospheres of a few M dwarf stars. Having succeeded in studying the atmospheres of exoplanets, scientists have moved on to get a deeper understanding of their atmospheric biosignatures to ascertain the possibility of life. But the search for alien life has traditionally been based on our knowledge of living forms on Earth, which are carbon based. But can silicon-based life be completely ruled out? Similarly, should oxygen be present for life to exist? After all, the earliest life forms on Earth existed well before oxygen became dominant. Extremophiles on Earth have been found in hostile environments — extremely hot, cold, acidic, alkaline, dry and desiccating conditions, deriving energy from hydrothermal vent chemicals and certain elements found in rocks. Hence there is a possibility of alien life on exoplanets. The only limitation in identifying them, if they exist, is our technological capability. And sense of imagination.