Where Are All The Habitable Planets?

The number of known planets is increasing all the time, but how soon can we expect to find life? Stuart Clark takes a closer look.

- Stuart Clark

Back in March, a team at the Niels Bohr Institute in Copenhagen used a 250-year-old equation called the Titius-Bode Law to predict the number of habitable planets. The researchers stated that billions of stars will have one to three planets in their ‘Goldilocks Zone’, also known as a habitable zone (see p56). While the law gives a simple means of predicting the orbits of planets around a star, it isn’t particularly accurate – even when applied to the Solar System.

Still, a lot of researchers believe that there are a significant number of Earth-like planets out there somewhere, many even within the Milky Way.Astronomers call these Earth-like planets ‘Earth analogues’. At the time of writing, there are 1,211 known planetary systems, with 482 of those sporting more than one planet. The current total of known planets sits at 1,918. These numbers increase all the time as new discoveries are made by various space programmes. Some of the planets so far observed are Earth-sized, some are in similar orbits to Earth, and some are around Sun-like stars. But not a single one ticks all three of these criteria. Astonishingly, of these hundreds of planets, not one is Earth’s twin. Does this mean that Earth analogues are rare? With several missions planned for the coming years by NASA and others, will we soon find our planet’s sibling?

Prof Geoffrey Marcy, from the University of California at Berkeley, was one of the first people to find planets around other stars. Back in 1995, he began reporting a string of planetary discoveries that continues to this day. In 2013, he and two colleagues began to wonder how close the nearest Earth-twin might be.