Extrasolar planets, or exoplanets, are planets that orbit stars other than the sun. The search for them is one of the most exciting phases of astronomy today.Planets have no light source of their own, and most extrasolar planets are too far away to be seen by the light they reflect from their own suns; however, over 400 have been detected with indirect methods, such as Doppler spectroscopy (determining the "wobble" of the parent star due to the orbiting planet's gravitational tug) or transit photometry (measuring the ever-so-slight dimming of the parent star when the planet passes through the line of sight, obscuring part of the star). Actually photographing the planet's faint speck of light against the host star's overwhelming glare is much more challenging, yet ultimately more rewarding because it provides critical information about the planet's orbit and the temperature and composition of its atmosphere.
Methods of detection
A star and planet orbit about a spot that is the center of gravity of the system known as the barycenter. For the Sun and Jupiter, the barycenter is 1.068 solar radii. With larger planets orbiting closer to the star, it is possible to detect the movement of the star against the distant background. This method was used to detect a planet orbiting Gliese 876.
Instead of mapping the position of the star against the background, it is also possible to detect the minute Doppler shift in the star's spectrum as it is tugged by the planet. This method is responsible for the majority of extrasolar planet detection.
When a planet passes in front of its star, it blocks some of the light of the star. By measuring this periodic momentary dimming of the light, it is possible to identify a planet that is properly aligned. This method does not work if the planet itself does not eclipse the star from our perspective.
In the situation where a star is not extremely bright, it is possible to detect a planet orbiting it. Such was the case with the brown dwarf 2M1207 which had a detectable companion that was named 2M1207b.
On November 13, 2008 a planet orbiting the star Fomalhaut at a distance of approximately 115 AU became the first exoplanet observed directly in visible light. The planet, whose existence was theoretically predicted, was photographed by the Hubble Space Telescope. It is believed to be a few times the mass of Jupiter and to posess an extrensive ring system.
As of December 2009, the number of extrasolar planets detected stands at more than 400. Since the detection methods devised to date favor large, gaseous planets, most detected planets are quite dissimilar to Earth, and they are not thought to be good candidates for finding life. However, the planet Gliese 581c orbiting the red dwarf star Gliese 581 (approximately 20 light-years from earth) appears to be a terrestrial (that is, rocky) planet that orbits in the "habitable zone" of space surrounding its star. The term "habitable" in this context simply means that the stellar light flux it receives is similar to that of Earth, and it therefore might have an Earthlike temperature. Whether it is in fact habitable depends on many other factors, and is pure speculation at present.
- Thalmann, Christian, et al. "Discovery of the Coldest Imaged Companion of a Sun-Like Star," submitted for publication, Nov. 2009, on the Subaru discovery