Extrasolar Planet - Detection Methods

Detection Methods

Planets are extremely faint compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare, while leaving the light from the planet detectable; doing so is a major technical challenge.

All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. Most of them are also very hot, so that they emit intense infrared radiation; the images have then been made at infrared where the planet is brighter than it is at visible wavelengths.

Though direct imaging may become more important in the future, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:

• Radial velocity or Doppler method

As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity — that is, the speed with which it moves towards or away from Earth — can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less. This has been by far the most productive method of discovering exoplanets. It has the advantage of being applicable to stars with a wide range of characteristics. One of its disadvantages is that it cannot determine a planet's true mass, but can only set a lower limit on that mass. However if the radial-velocity of the planet itself can be distinguished from the radial-velocity of the star then the true mass can be determined.

• Transit method

If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary. The transit method reveals the radius of a planet, and it has the benefit that it sometimes allows a planet's atmosphere to be investigated through spectroscopy.

• Transit Timing Variation (TTV)

When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet WASP-3b suggest the existence of a second planet in the system, the non-transiting WASP-3c. If multiple transiting planets exist in one system, then this method can be used to confirm their existence. In another form of the method, timing the eclipses in an eclipsing binary star can reveal an outer planet that orbits both stars; as of November 2011, five planets have been found in that way.

• Gravitational microlensing

Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. This method has resulted in only 13 detections as of June 2011, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.

• Astrometry

Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method has not yet been very productive. It has produced only a few disputed detections, though it has been successfully used to investigate the properties of planets found in other ways.

• Pulsar timing

A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.

• Circumstellar disks

Disks of space dust surround many stars, believed to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared radiation. Features in the disks may suggest the presence of planets, though this is not considered a definitive detection method.

Most confirmed extrasolar planets have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT (launched December 2006) and Kepler (launched March 2009) are the two currently active space missions dedicated to searching for extrasolar planets. Hubble Space Telescope and MOST have also found or confirmed a few planets. The Gaia mission, to be launched in March 2013, will use astrometry to determine the true masses of 1000 nearby exoplanets.