How to Find an Exoplanet

By Tim Trott, 11th November 2008 in Astrobiology

How to find an exoplanet, a very faint and very small planet compared with stars, and very difficult to see. New techniques are being used which mean we do not have to see the planet in order to detect it.

Currently it is only possible to directly image exoplanets when they are especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

There are other indirect methods we can use, and these have all been used to discover and confirm the existance of exoplanets.

Astrometry and Radial velocity

As a sufficiently large planet orbits its star, it will exert a tiny gravitational "tug" on the star giving it the appearance of wobbling. Depending on the angle that the planet orbits, with regards to Earth, the star will appear to move in a tiny circular (or elliptical orbit) about their common centre of mass, or if we see the orbit "end on" we can use radial velocity (Doppler shift) to record changes in the stars velocity.

These two animations show how a planet orbiting will tug on the star producing the wobble. These animations are not to scale, and are greatly exaggerated. Jupiter causes the Sun to change velocity by about 13 m s-1 over a period of 12 years. Long-term observations by instruments with a very high resolution are required in order to detect exoplanets by this method.

An exoplanet orbiting a larger star could produce changes in position and velocity of the star as they orbit their common center of mass.

Seen from above, the orbit of a planet will cause the star to wobble as they both orbit around the common centre of mass.
Seen from above, the orbit of a planet will cause the star to wobble as they both orbit around the common centre of mass.
Viewed end on we can see the star moving towards us, then away.
Viewed end on we can see the star moving towards us, then away.

A series of observations can be made of the spectrum of light emitted by a star and periodic variations in the stars spectrum may be detected. The wavelength of characteristic spectral lines in the spectrum will appear to increase and decrease regularly over a period of time and are indicative of changes in the radial velocity of the star.

If an extrasolar planet is detected, its mass can be determined from the changes in the stars radial velocity.

Transit method

If a planet crosses (or transits) in front of its parent stars disk, then the observed energy output of the star will decrease by a small amount. The amount by which the star dims depends on the size of the star and on the size of the planet.

Transit method for observing exoplanets
Transit method for observing exoplanets

Pulsar timing

A pulsar (the small, ultra dense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. Slight anomalies in the timing of its observed radio pulses can be used to track changes in the pulsar's motion caused by the presence of planets.

Gravitational microlensing

Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Possible planets orbiting the foreground star can cause detectable anomalies in the lensing event light curve.

Circumstellar disks

Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in dust disks may suggest the presence of planets.

Eclipsing binary

In an eclipsing double star system, the planet can be detected by finding variability in minima as it goes back and forth. It is the most reliable method for detecting planets in binary star systems.

Polarimetry

Stellar light becomes polarized when it interacts with atmospheric molecules, which could be detected with a polarimeter. So far one planet has been studied by this method.

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