Extrasolar Planets

The recent rush of discoveries has been made possible by new methods of search. Direct visual observation of extrasolar planets remains difficult; all the recent discoveries have been made, therefore, by indirect means, that is, by observing their effects on either the motions or brightness of the stars they orbit.

Apart from the been detected by analyzing the perturbations (disturbances) they cause in their star's motions. A planet does not simply orbit around its star; rather, a star and its planet both orbit around their common center of gravity. Because a star weighs more than a planet, it follows a tighter orbit, but if a planet (or other companion) is massive enough, the orbital motion of a star—its "wobble"—may be detectable from Earth. Several techniques have been and are being developed to detect the orbital wobbles caused by planet-size bodies.

All extrasolar planets so far detected (apart from the three orbiting PSR1257.12) have been detected by the radial-velocity technique. This uses spectroscopy (analysis of the electromagnetic spectra emitted by stars) to detect perturbations of stars orbited by planets. The mutual orbital motions of a star and a planet around each other manifest in the star's light via the Doppler effect, in which spectroscopic lines from a light source such as a star are shifted to longer wavelengths in the case of a source moving away from the observer (red shift), or to shorter wavelengths in the case of a source approaching an observer (blue shift). These shifts are measured relative to the wavelengths of spectral lines for a source at rest. Small changes in the wavelengths of spectroscopic lines in the star's spectrum indicate changes in its line-of-sight (radial) velocity relative to the observer; periodic (regularly repeated) spectral shifts probably indicate the presence of a planet (or planets) perturbing the star's motion by swinging it toward us and then away from us. (This is true only if the planetary orbits happen to be oriented flat-on to us in space.)

Another new techniques for detecting extrasolar planets involves searching for transits. A transit is the passage of a planet directly between its star and the Earth, and can occur only when the planet's orbit happens to be oriented edge-on to us. When transit does occur, the star's apparent brightness dims for several minutes—perhaps by only a percent or so. The amount of dimming and the speed with which dimming occurs as the planet begins to move across the star's disk reveal the planet's diameter, which the "wobble" method cannot do. Furthermore, since a planet with an atmosphere does not block all the light from the star behind it but allows some of that light to filter through its atmosphere, precise measurements of changes in the star's spectrum during transit can supply information about the chemical composition of the transiting planet's atmosphere. In 2001, the Hubble Space Telescope detected sodium in the atmosphere of a transiting extrasolar planet approximately 150 light years away. This was the first information ever obtained about the composition of an extrasolar planet. This study was limited to the wavelengths in which sodium absorbs light, and were not expected to detect other chemicals; using different wavelengths, astronomers intend to search for potassium, water vapor, methane, and other substances in the atmosphere of this and other transiting extrasolar planets.