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An introduction to exoplanets
An introduction to exoplanets

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1  Welcome to the neighbourhood

You learned in Week 6 that large exoplanet surveys such as Kepler have allowed statistical estimates to be made of how many stars have planets. Astronomers have recently discovered it’s likely that stars have an average of one or more planets each. That means that there must be several planets around the stars nearest to the Sun.

Many of the exoplanets discussed in Week 7 are relatively close to Earth, residing in a region of space around 300 light years across called the ‘Local Bubble’. This includes GJ 1214 b, HD 189733 b, HD 209458 b and, of course, the recently discovered TRAPPIST-1 planets. But a number of exoplanets have now been found even closer by. By the end the first decade of this century the closest confirmed detections of exoplanets were around stars 15 light years away. In 2015 the discovery of the first ‘close’ potentially rocky habitable planet was announced, orbiting the M dwarf star Wolf 1061, around 14 light years away. This ‘super-Earth’ was discovered using the radial velocity method and does not transit its star, so unfortunately we know relatively little about it.

In Week 6 you briefly reviewed our solar neighbourhood, out to a distance of about 15 light years. One current estimate is that there might be around 30 stars with Earth-sized planets in this region of space, as well as larger planets. The statistics suggest that eight or more of these Earth-sized planets will reside in the habitable zone of their star. Earth-sized planets are difficult to find though, so it’s possible there are actually more – the estimates of numbers of small planets might be increased as astronomers identify new small planets on which to base the statistics.

While it’s thrilling to know that there are planets around other stars neighbouring the Solar System – even possibly supporting life – the distances between stars is huge, so we are still a long way from being able to visit them. While people are beginning to explore ideas for technology that would allow us to visit these systems, it certainly doesn’t exist yet. But if we were going to visit another planetary system, we’d pick the closest one possible. Before sending people to another star, we will undoubtedly send machines, just as we did with the exploration of the Moon. The Soviet Union’s Luna 2 probe reached the Moon a decade before the NASA astronaut Neil Armstrong walked on the Moon’s surface. Already, the Breakthrough Starshot project plans to send tiny laser-propelled ‘nanocraft’ to the nearest star.