The Search for Extrasolar Planets
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Man's curiosity is boundless and insatiable. We seek to understand ourselves and the world in which we live, but what is more fascinating to many is not just the planet we inhabit but also the other planets we share the universe with. We search the heavens for other worlds, but after the discovery of Pluto in 1930, no other planets were found in our solar system, or any other, until 1994. In 1994, a pulsar-orbiting planet was found by Alexander Wolszczan around the pulsar PSR B1257+12 (in the constellation Virgo), and the first planet orbiting a star like our sun, was found the next year around 51 Pegasi (in the constellation Pegasus) by Michel Mayor. Since that time, more than 100 extrasolar planets have been found, though none of them have ever been observed in any direct manner; the sole exception being the planet HD 209458 b, orbiting the star HD 209458. Its presence was confirmed by dips in its host star's luminance as it passed in front of it. Normally, the only way a planet's presence can be detected, is by its effects on the orbit of its host star.
One of the difficulties in searching for extrasolar planets is that no one can agree on exactly what a planet is, and that some objects in space fulfill common definitions of what stars and planets are, but aren't stars or planets themselves. Simply saying that a planet is an object orbiting a star, which is not a star itself, is inadequate. It doesn't allow for comets and asteroids, which also orbit stars, and it also doesn't allow for objects like brown dwarfs, which are too massive to be considered planets, but which failed to fully evolve as stars. The astronomers Geoff Marcy and Paul Butler have defined a planet as follows: A "planet" is an object that has a mass between that of Pluto and the deuterium-burning threshold and that forms in orbit around an object that can generate energy by nuclear reactions - specifically, an object that is less than 13 Jupiter masses (13 MJUP), because such objects never burn deuterium nor generate significant energy from any nuclear reactions, and which has an upper mass limit of about 10 MJUP.
A host star's luminosity is so great that it can render orbiting objects invisible to us. We can only detect the presence of these orbiting objects by effects on the host star itself.
Planets don't emit their own light. Only stars with their tremendous gravitational pressure have the power to ignite the thermonuclear reactions that give off energy as light. In principle, planets could be seen by reflected starlight. However, even a high-albedo, Jupiter-like planet is down by a factor of more than a million in brightness relative to its host star. With our largest telescopes and best detectors, planets are absolutely invisible next to the bright spotlights of the stars they orbit. Invisible, but not undetectable. (Debra Fischer)
One of the ways in which we do this detection is by observing the Doppler effect. When an object moves towards an observer on Earth, the light is emitted in shorter wavelengths relative to the observer, and appears to shift towards the blue end of the spectrum. When an object moves away from the observer, the light is emitted in longer wavelengths, and shifts to the red end of the spectrum. These emissions are studied via a spectrometer (spectrograph), a device that separates light into various wavelengths - into a spectrum - so that it can be studied. The spectrum of a star can tell you such things as its composition, and its radial velocity - its velocity as directed towards, or away from, Earth. It is by viewing the results of the spectrometer that we can detect a star's wobble as it moves towards or away from us.
The motion of our own sun, the planets around it, and their effect on the motion of the sun, led scientists to surmise that planets orbiting other stars would produce similar effects on their own host stars, that circular orbits would be produced. However, as stated by Geoff Marcy and Paul Butler, the expectation of circular orbits was thwarted when astronomers found that most extrasolar planets have eccentric orbits, some "twice as elongated as the orbits of Earth, Jupiter, or Saturn." They theorise this is so due to evidence of the orbit of comets, which started out with circular orbits but then attained eliptical ones when their orbits brought them close to planets, those planets essentially, gravitationally, "flinging" the comets out of their circular orbits. A planet in a disturbed orbit will likely travel close enough to other planets to disturb their orbits also, perhaps, if they are large enough, even forcing smaller mass planets out of a system altogether.
These massive, Jupiter-like planets are the easiest ones to confirm the presence of, as they cause the most wobble in their host stars' orbits. They exert a gravitational force on their host stars, which pulls the star around in a circular or eliptical path. The velocity of the wobble of the host star depends on the mass of the planet doing the tugging, and its distance from the star. Jupiter (which is 7.8 billion km from the sun, about 5.2 AU), for example, causes the sun to wobble with a velocity of 12.5 meters per second over a period of 12 years, and because it is one thousandth the mass of the sun, the sun oscillates in a circle that is one thousandth the size of Jupiter's orbit every 11.8 years - Jupiter's orbital period.
Interest in the search for extrasolar planets is growing every day, and groups such as the Space Interferometry Mission (developed by the Jet Propulsion Laboratory under contract with NASA), Kepler (in partnership with Ball Aerospace and NASA), The Geneva Extrasolar Planet Search (in partnership with the University of Geneva), the California and Carnegie Planet Search (supported by NASA, the NSF, and Sun Microsystems), as well as others, are all engaged in that search. While California and Carnegie Planet Search and Geneva Planet Search have been searching for extrasolar planets for many years, Kepler and SIM aren't yet launched; they are slated to launch within the next five or six years. Perhaps, sooner than we think, we might find an Earth-like planet orbiting some distant star.