ESA Science & Technology - Exoplanet detection methods

Wobbling stars hint at exoplanet presence

The first planets found orbiting Sun-like stars were detected by the radial velocity technique. A single star devoid of a planetary system will have its centre of gravity located in the centre of the star. However, when a planet orbits a star, the centre of gravity of the star-planet system becomes offset from the centre of the star, causing the star to "wobble" back and forth, from an observer's perspective, as both the star and the planet orbit their common centre of mass. Subtle changes in the radial (line-of-sight) velocity of the star can – in principle – be measured to reveal the presence of otherwise invisible planets.

51 Pegasi b – the exoplanet discovered by Mayor and Queloz – is a Jupiter-mass planet that orbits close to its star, which results in a relatively large radial velocity signal. This type of planet, a gas giant orbiting very close to its parent star and known as a hot Jupiter, came as a complete surprise when discovered since theories of planet formation stated that such a planet could not form so close to a star due to the lack of material for it to sweep up. This "impossible planet" was explained in 1996 by Douglas Lin at Lick Observatory, California, and colleagues. They suggested that the gas giant had indeed formed further out, but then migrated towards the star as a result of interactions with the circumstellar disc from which the planet was born.

Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. This technique can also be used to identify planets around a star by measuring tiny changes in the star's position as it wobbles around the center of mass of the planetary system. However, the precision required to detect a planet orbiting a star using this technique is extremely difficult to achieve and for this reason only one planet has been discovered by this method, although astrometry has been used to make follow-up observations for planets detected via other methods. ESA's Gaia mission is expected to detect some tens of thousands of exoplanets out to 500 parsec (around 1600 light-years) from the Sun, using the astrometric technique.

Transiting planet causes dip in stellar light

Radial velocity was the primary method for detecting exoplanets until the start of this century when the periodic dip in stellar light arising from the transit of a planet across the face of its host star was made by David Charbonneau (from the Harvard-Smithsonian Center for Astrophysics) and colleagues. The planet that he detected, known as HD 209458b, was already known from the radial velocity method, so the first planet actually discovered through the transit method was OGLE-TR-56b, detected in 2003 by Maciej Konacki (from the California Institute of Technology) and colleagues.

In 2006, the first space mission dedicated to exoplanetary research was launched. Named CoRoT, for Convection, Rotation and planetary Transits, the mission was led by the French space agency CNES, with contributions from ESA, Austria, Belgium, Germany, Spain and Brazil. Within a few months of launch CoRoT had discovered its first planet, a hot Jupiter orbiting a Sun-like star. In the next few years, CoRoT placed exoplanetary research from space on a firm footing with the steady detection of unusual planets.

When NASA's Kepler mission was launched in 2009, the number of known planets started to soar. Along with the transit technique, a variation – known as transit timing variation (TTV) – was also used to find additional planets in a system. Tiny variations in the transit time of a planet can be used to reveal other companions in the system and a team led by Sarah Ballard at the Harvard-Smithsonian Center for Astrophysics detected the first TTV planet, Kepler-19c, in 2011.

2011 was also the year that Canada's MOST (Microvariability and Oscillation of STars) space telescope detected the exoplanet 55 Cancri e as it transited its host star.

Gravitational microlensing – chance alignment reveals planet

While both the radial velocity and transit methods rely on detecting variations in light from the star, a completely different method uses the effect of gravity on light. Gravitational microlensing was predicted by Albert Einstein in his general theory of relativity. It relies on the fact that objects with a large mass can bend light around them. If the right alignment occurs, light travelling towards an observer from a distant star can be bent around an intervening star, which acts as a lens. The light from the background star becomes amplified and if there is a planet in orbit around the star acting as a lens, a bump appears in what would otherwise be a smooth light curve.

A team led by Ian Bond at the University of Edinburgh revealed the first microlensing planet in 2004. Microlensing is extremely sensitive and can detect very small planets. The main disadvantage is that microlensing is a chance occurrence that will not be repeated.

Picture this – direct imaging of exoplanets

The brown dwarf 2M1207 and its planetary companion. Credit: ESO

It is extremely difficult to directly image exoplanets, as the light from the star overwhelms the planet – by more than a factor of a million.

Even when the light of the star is blocked, most planets are too faint or too close to the star to be seen. However, it is possible to image young, massive planets at a great distance from their host star. The first directly imaged planet – young, with a mass a few times that of Jupiter's – was discovered in 2004 by a team led by Gaël Chauvin at the European Southern Observatory in Chile.