A PASSION FOR
The Sky at Night presenter looks back on the menagerie of exoplanets out in the cosmos
Over the past 20 years or so exoplanets have been transforming our understanding of what a planet and even a star system can be. The very first exoplanet was found in 1991, but since then many thousands have been detected; there are around 1,900 confirmed exoplanets out there to date. In the past we considered our Solar System fairly unique, with multiple planets going around a star, but now we are very much aware that this seems to be the norm and most of the stars we look at seem to have planets in orbit around them. Current estimates suggest that one in five Sun-like stars have an Earth-sized planet in orbit.
The types of planets that we have discovered have been bewildering. There are super-Jupiters, rocky giants, miniNeptunes and gas dwarfs, and that’s before getting to the Earth-like, Earth-sized and the super-Earths out there.
But detection and confirmation of the presence of an exoplanet is challenging. The first planets detected were found using the radial velocity method. This involves measuring the wobble of a star that is caused by the movement of a planet in orbit about it, a movement that is miniscule and very hard to detect. As a result, the first exoplanets detected were Jupiter-sized or bigger, since they exerted the greatest movement on the star being observed.
Transit triumphs
There are now other detection methods, one of the most popular being the transit technique, which registers a dip in a star’s brightness as a planet travels in front of it. The transit method gives us information about a planet’s orbit and size. The radial velocity method gives us information about the planet’s mass, so by combining the methodologies much information can be interpolated about the planets and this gives us the large list of planet types above.
However, these methods aren’t foolproof and even NASA’s Kepler Space Telescope, which has detected around 1,000 of the confirmed exoplanets found to date, has found many thousands more candidates, 11 per cent of which will be false positives. A case that highlights the point is the detection of Alpha Centauri Bb. Using the radial velocity method, a tiny wobble in this star’s motion was attributed to the presence of a rocky, Earth-sized world sitting at an amazingly close distance to the star (around one-tenth of the orbital distance of Mercury). But other observations have found the signature movement to be transitory. Models suggest it could be due to sunspot activity or the gravitational pull of another star. The jury is still out on this one but it shows how difficult the detections can be.
The future looks bright for exoplanet detection. Kepler continues to do sterling work and has been joined by ESA’s Gaia mission, which is also designed to find larger exoplanets. In the future we will have the James Webb Space Telescope joining the hunt and TESS, which will be looking for smaller, more Earth-Like planets but could detect planets that we cannot even envisage today.