Methods for Finding Extrasolar Planets

1. Direct Observations

Direct observations can take several forms. One is the astrometry discussed previously, where an attempt is made to precisely measure a star's position at various times, so that any motion induced by a companion can be seen. This has not proved successful in locating planets. Another direct observation technique is monitoring the light output of a star, in hopes of catching a transit by a planet. This may work if we are in the planet's orbital plane, if the star's output is sufficiently constant and there are no large starspots. This technique does appear to have worked once so far, and is expected to be used more in the future. Eventually, giant adaptive mirrors in space should be capable of imaging exoplanets directly.

Recently astronomers using the Hubble Space Telescope captured this image of a planet orbiting Gliese 229A (see photo below). It is about the size of Jupiter and lies 44 AU from the star. (By comparison, Pluto is 40 AU from our Sun.)

Doppler shift of starlight caused by planetary companion. As the planet moves away from the observer, it pulls the star with it and the star's light is reddened. When the planet moves toward the observer, the star's light is shifted toward the blue end of the spectrum. (Courtesy: University of California Planet Search Project, California & Carnegie Planet Search )
2. Spectroscopy

As mentioned previously, spectroscopy has been an extremely effective tool in astrophysics whereby we can deduce temperature and pressure conditions, magnetic fields and other parameters of interest in stars and galaxies far, far away. This technique has been used, via Doppler shifting of spectral lines, to detect almost all of the exoplanets which have been found to date. But the signals are small and deeply buried in the noise; it may be difficult to improve this technique enough to detect terrestrial planets like our own Earth.

The Very Large Array (VLA) located in Socorro, New Mexico. The array is made up of 27 antennas arranged in a y-shaped pattern that spans 22 miles. The individual antennas are on railroad tracks so that they can be moved. (Courtesy: Emma Ryan)
3. Interferometry

Interferometry is a technique that has been used for decades in radio astronomy, where it is used to generate radio maps sometimes even more detailed than optical ones. However, optical maps should be far more detailed than radio ones, because of the very small wavelengths of optical radiation compared to radio waves. The advantage of the interferometry technique is that it allows two or more telescopes to act as one large one, that has a size the same as the distance between the telescopes. Much like the Very Large Array (VLA) pictured below, it is also possible to use telescopes on opposite sides of the earth and later combine the signals in a computer. This produces the effect of a radio telescope as big as the Earth!

The Very Large Array (VLA ) viewed from above (Courtesy: NASA)
Interferometry is beginning to be used at optical wavelengths, although the separations for now are much more modest, typically tens of meters. The two Keck telescopes atop Mona Kea in Hawaii were planned for use in interferometric mode (see photo below). The electromagnetic waves are combined over pathlengths that may differ by less than a wavelength (cm to meters in radio, 10,000,000 times less in the optical). The waves may cancel each other out in "troughs" or add together in "peaks" of a "fringe patterns". In this way, the interferometer might be arranged so that a bright star would be in a trough, while a planet, a million times less bright, might be in a peak and thus detectable, where it would be drowned out otherwise. Interferometry will become increasingly important in astronomy and may be the only way of directly detecting and eventually imaging small terrestrial planets like our own

Twin Keck telescopes on the top of Mauna Kea in Hawaii. Each telescope contains a 10meter(33 foot) mirror. (Courtesy: University of Hawaii)