Widening the search for alien life Widening the search for alien life Megan Whewell explains how other planets are classified,Since the first planet was confirmed orbiting a star other than our own Sun in 1995, this field of exoplanet research has grown hugely. We are now aware of over 700 other planets, in over 500 different systems. Like any new branch of science, its focus is now beginning to shift from discovery to classification and understanding, and this is where a newly proposed scheme of two ‘habitability indices’ comes in. The scheme has been proposed by scientists from a variety of organisations, including Nasa and the SETI Institute, and involves two ways of classifying exoplanets by the likelihood of life existing there. The first uses a traditional method of comparing the new planet to Earth and giving it a rating accordingly. This is called the Earth Similarity Index (ESI) and ranges from 0.0 (not similar at all) to 1.0 (the highest rating, and unsurprisingly the rating ...