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What is an Exoplanet?

By on in Astrobiology

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Exoplanet is a relatively new word used to describe a planetary body orbiting a star other than our own Sun. The word is a combination of Extra-Solar Planet.

The first exoplanet was detected in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang. Their radial-velocity observations suggested that a planet orbited the star Gamma Cephei. They remained cautious about claiming a true planetary detection, and it wasn't until 1996 that the discovery was confirmed.

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around pulsar PSR 1257+12. This discovery was quickly confirmed, and is generally considered to be the first definitive detection of an exoplanet.

Artist's impression of the extrasolar planet HD 189733 b
Artist's impression of the extrasolar planet HD 189733 b

Since then Exoplanets have been discovered at an ever increasing rate, and as of November 2008, 322 exoplanets have been detected and confirmed.

HD189733b is the third planet of the red dwarf star Gliese 581 and is roughly the size of Jupiter. It is now known to contain methane and water in its atmosphere, the first time organic molecules have been detected in an extrasolar planet. Astronomers used the Hubble Space Telescope with the help of the Spitzer Space Telescope and studied how light from the host star filters through the planet's atmosphere using a process called spectroscopy.

In April 2005 astronomers using the NACO adaptive optics facility at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory photographed the first image of an exoplanet orbiting a star.

The planet is near the southern constellation of Hydra and approximately 200 light years from Earth.

The Brown Dwarf 2M1207 and its Planetary Companion
The Brown Dwarf 2M1207 and its Planetary Companion

Exoplanet Hunting

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 existence 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 star's 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 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.
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 star's 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 star's 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, ultradense 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 polarised when it interacts with atmospheric molecules, which could be detected with a polarimeter. So far one planet has been studied by this method.

Last updated on: Wednesday 24th January 2018

 

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