There is no longer any doubt that planets are common around other stars. Until comparatively recently, this was a matter for speculation,but by 2010 the number of stars proven to have at least one planet orbiting them had exceeded 400. Allowing for how diffi cult it is to make these detections, it is clear that the majority of Sun-like stars must be accompanied by planets. To avoid confusion, professionals usually refer to them as ‘extrasolar planets’ or ‘exoplanets’. The exoplanet tally excludes exotic dim objects exceeding 13 Jupitermasses,which is the threshold above which nuclear fusion of deuterium (heavy hydrogen) can occur. Those are called ‘brown dwarfs’ and are regarded as more star-like than planet-like.
Detection methods
Evidence that most young Sun-like stars have a surrounding ring of dust began to accumulate in the late 1970s. Initial clues came from the infl uence of dust on a candidate star’s infrared spectrum,then images of dust discs began to be obtained in the 1980s.Irrespective of whether these discs are like the solar nebula before planets formed or are remnant dust surviving in the equivalent of a star’s Kuiper belt, their mere existence showed that there ought to be plenty of planets out there too. The fi rst defi nite exoplanet discovery was made in 1995, after which discoveries gathered pace year by year.
Radial velocity
The fi rst discovery, and the majority ever since (more than 300 by2010), was made by detecting slight changes in a star’s radial velocity. This is the speed at which a star is travelling towards or away from the Earth, irrespective of any movement across the line of sight. Radial velocity changes can be determined to a remarkable precision of one metre per second by measuring shifts in the exact wavelength at which characteristic absorption lines appear in a star’s spectrum. These shift to shorter wavelengths (‘blue shift’) if the star is moving towards us and to longer wavelengths (‘red shift’)if the star is travelling away, in a phenomenon called the Doppler effect. Variations in radial velocity had long been used to measure orbital speeds (and hence to infer masses) of double stars, but the tiny infl uence of a much less massive exoplanet on a relatively much more massive star requires very sensitive modern instrumentation.Radial velocity changes caused by the Earth’s own orbital motion have to be accounted for before the more subtle changes attributable to the tug of the exoplanet on its star become apparent.