A CENTURY ago astronomers knew of only eight planets in the entire universe: those around the Sun, the most distant of which, Neptune, had been discovered in 1846. Today the tally of known planets stands at nearly 900, thanks to the development in the past 20 years of techniques to detect planets around other stars, known as extrasolar planets, or exoplanets. In addition there are thousands of “planetary candidates”—suspected exoplanets whose existence has yet to be confirmed by follow-up observations. How do you find a planet around another star? Unlike stars, planets do not emit light of their own, but are merely illuminated by the stars they orbit. Given the great distances involved, looking for a planet around another star is like trying to spot a moth flying around a floodlight from several miles away. But it is not necessary to observe a planet directly to tell that it is there. The two most widely used planet-hunting techniques rely on more subtle clues, as the motion of orbiting planets affects the light from the stars they orbit, as seen from Earth.

The first technique, the “radial velocity” method, exploits the fact that an orbiting planet causes its star to wobble slightly, and the heavier the planet and the tighter its orbit, the bigger the wobble. Such wobbling can be detected from Earth by measuring regular variations in the star’s speed along the line of sight to Earth—its radial velocity. (As the star moves back and forth, the Doppler shift causes a slight change in its apparent colour which can be detected using spectroscopy.) This method was used to discover the first planet around a sun-like star, 51 Pegasi b, in 1995, and has since been used to discover dozens more planets and multi-planetary systems. The drawback, however, is that it is best at finding large planets in tight orbits, known as “hot Jupiters”. The second technique, the transit method, is more egalitarian. It involves continuously monitoring the brightness of a star to look for periodic dimming caused by a planet passing between the star and the observer. The size and frequency of these dips allows the size and orbital period of the planet to be determined. This approach only works for stars whose planets are orbiting “edge on” as seen from Earth. Even so, monitoring tens of thousands of stars at once, as the Kepler probe did between 2009 and 2013, produced evidence of thousands of planets of varying sizes and periods.

The data from Kepler are still being analysed, even though the probe itself has stopped working. Meanwhile, new methods are being developed to detect exoplanets. A new technique called relativistic beaming, which has just notched up its first success, works by looking for a characteristic brightening of a star as it is tugged towards the observer by an orbiting planet. Another emerging technique exploits the fact that planets go through phases, as the Moon does (as seen from Earth), causing tell-tale fluctuations in the apparent brightness of a star. Like the radial-velocity method, both of these techniques are best for spotting hot Jupiters. In a few cases planets can even be seen directly, using clever tricks to blot out the light of the stars they orbit, and researchers are starting to analyse the light from planets to determine the characteristics of their atmospheres. The range and sophistication of planet-hunting techniques, along with the number of known exoplanets, will only increase in the coming years.