It has been only about 20 years since the first planets were found outside our solar system. Today, almost a thousand exoplanets have been confirmed, and the rate of discovery is going up all the time. A big reason for this explosion of discoveries is NASA’s Kepler space telescope, which, since 2009, has been gazing at a small patch of sky in the constellation Cygnus that contains about 100,000 stars. It’s looking for tiny variations in the light coming from each of these stars, which might be due to planets passing in front of them.
The first planets to be found around other stars were oddballs. They were discovered in 1992 circling around pulsars—dead, dense stars that send out radiation in narrow beams, much like a lighthouse does. Four years later, the first planet to be detected around a Sun-like star was announced by astronomers at the University of Geneva. Known as 51 Pegasi b, it had about half the mass of Jupiter and moved around its parent star in a tiny orbit, much closer than Mercury is to the Sun. For a gas giant to be so close to its central star was totally unexpected, and it wasn’t long before many more such “hot Jupiters” turned up. The explanation seemed to be that, like the gas giants in our solar system, they had formed much further out but had then been deflected inward in a process called orbital migration.
Astronomers seek out exoplanets in a number of different ways. The one that brought most success to begin with and is still used in the majority of search programs carried out by telescopes on the ground is called the “radial velocity” method. It involves looking for wobbles in the motion of a star caused by the to-and-fro tugging of an orbiting planet. If a planet is on “our” side of the star, it will pull it slightly towards us, causing a blueshift in the star’s spectral lines; when on the side away from us, its gravity pull will cause a redshift. The more massive the planet and the closer it is to the star, the bigger these effects will be and the easier they will be to spot. That’s why a disproportionate number of the first exoplanets found were giant worlds in tiny orbits.
As time has gone on, the instruments used in exoplanet searches have become more sensitive, and, in some cases, data has been collected over many years, making it possible to find smaller and smaller planets in wider orbits. Today, along with gas giants the size of Jupiter and bigger, astronomers can detect rocky exoplanets, as small or even smaller than the Earth.
A second highly successful technique for tracking down exoplanets is known as the transit method. This is how Kepler works. It involves monitoring the light output from a star and looking for tiny regular dips in brightness due to planets “transiting,” or passing in front of, the star as seen from our vantage point. To date, Kepler has discovered 114 new planets and pointed to another 2,740 candidates—about 90 percent of which, it’s estimated, will eventually be confirmed.