What is an Exoplanet? How Can We Detect Exoplanets?Exoplanet is a word used to describe a planetary body orbiting a star other than our own Sun. It is a combination of Extra-Solar Planet.
This article is part of a series of articles. Please use the links below to navigate between the articles.
- Constellation Guide and Associated Mythology
- What are Asteroids, Meteors and Comets?
- What Are Binary Stars and Double Stars
- What are Variable Stars and How to Observe Them
- What are Supernova and Supernovae?
- What Nebula and Nebulae, What are the Types of Nebula?
- What Are Black Holes? Black Holes Explained - From Birth to Death
- What Are Quasars (QUAsi-Stellar Radio Source)?
- Pulsars - Natures Lighthouses Key to Astronomy
- What is a Neutron Star and What Are They Made Of?
- What Are Gamma Ray Bursts and Where Do They Come From?
- What is the Kuiper Belt and Kuiper Belt Objects?
- What is an Exoplanet? How Can We Detect Exoplanets?
- What is a galaxy? What Types of Galaxy Are There? Where Do They Come From?
- The Messier Catalogue of Objects To Observe
- The Caldwell Catalogue
- 25 Stunning Sights Every Astronomer Should See
The first exoplanet was detected in 1988 by the Canadian astronomer's 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.
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.
Exoplanet Hunting
Currently, it is only possible to directly image exoplanets when they are especially large (considerably larger than Jupiter), widely separated from their 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 a 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 12 years. Long-term observations by instruments with a very high resolution are required to detect exoplanets by this method.
An exoplanet orbiting a larger star could produce changes in the position and velocity of the star as they orbit its common centre of mass.
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 some 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 of Exoplanet Detection
If a planet crosses (or transits) in front of its parent star's 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 the size of the planet.
Pulsar timing Exoplanet Detection
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.