THE HUNT FOR EARTH’S SISTER

Earth is, as far as we know, unique in the universe for bearing life. What’s not unique about it is that it’s a rocky ball orbiting at a distance from its star that allows liquid water to exist on its surface. The number of these planets we know of is quite large, and is getting bigger all the time.

Since the discovery of the first confirmed exoplanet in 1992, we’ve found over 4,000 planets orbiting more than 3,000 stars. Earth isn’t alone in the universe, and humanity is launching more planet-hunting missions to discover just how large its family is. One of these is CHEOPS, the CHaracterising ExOPlanet Satellite from the European Space Agency (ESA), which will blast off on a Soyuz ST-B rocket later this year. It is the first S-class, or ‘small’ mission in the ESA’s Cosmic Vision programme. CHEOPS actually has a rather large task: rather than discovering new exoplanets, it will reanalyse those already known and attempt to accurately measure their size. Its targets will all be in the range of one to six Earth radii, from