Discovering planets
Author: Róża Chojnacka
"Earth is the cradle of humanity, but
one cannot live in a cradle forever."
Konstantin Tsiolkovsky
Extrasolar planets
An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of May 2007, the count of known exoplanets stands at 233. The vast majority have been detected through various indirect methods rather than actual imaging. Most of them are giant planets likely to resemble Jupiter more than Earth.
The beginning
A Polish astronomer, Aleksander Wolszczan carried out astronomical observations from the Arecibo Observatory which led him to the discovery of the pulsar PSR B1257+12 in 1990. The data analysis gathered thanks to the discovery showed that the pulsar is orbited by two planets with mass 4.3 and 2.8 times that of Earth's mass. Their orbits are 0.36 and 0.47 AU respectively. This planetary system was the first extra-solar system discovered in the Universe whose existence was proved.
Prof. Bohdan Paczyński (1940-2007) said „Undoubtly the biggest achievement of Polish astronomy not only last decade but the centenary, is a discovery of first extrasolar planets by prof. Aleksander Wolszczan”.
Detection methods
Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. In addition to the intrinsic difficulty of detecting such a faint light source, the parent star causes a glare that washes it out.
For those reasons, current telescopes can only directly image exoplanets under exceptional circumstances. Specifically, it may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and young (so that it is hot and emits intense infrared radiation).
The vast majority of known extrasolar planets have been discovered through indirect methods. At the present time, six indirect methods have yielded success:
Astrometry
Astrometry consists of precisely measuring a star's position in the sky and observing the ways in which that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit about their common center of mass.
Radial velocity
This is also known as the Doppler method. Variations in the speed with which the star moves towards or away from Earth — that is, variations in the radial velocity of the star with respect to Earth — can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. This has been by far the most productive technique used by planet hunters.
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.
Transit method
If a planet crosses (or transits) in front of its parent star's disk, then the observed visual 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.
Gravitational microlensing
Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect.
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 sometimes suggest the presence of full-sized planets.
In the future, several space missions are planned that will employ already proven planet-detection methods. Astronomical measurements done from space can be more sensitive than measurements done from the ground, since the distorting effect of the Earth's atmosphere is removed, and the instruments can view in infrared wavelengths that do not penetrate the atmosphere. Some of these space probes should be capable of detecting planets similar to our own Earth. Huge proposed ground telescopes may also be able to directly image extrasolar planets.
References
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