Detecting Extrasolar Planets
There are two basic methods of detecting a planet circling a star outside of our own solar system; indirect methods and direct methods. At present, most of the extrasolar planet discoveries have been made via indirect means. As a light source, planets are extraordinarily faint in relation to their parent stars. With the current state of technology, this makes it difficult to directly image a planet unless all circumstances at the time of viewing are absolutely perfect.
In order for there to be any chance whatsoever of directly imaging an extrasolar planet, several conditions must be met. The planet must be exceptionally huge, on the order of many times the size of our own gas giant, Jupiter. The giant planet must be a considerable distance from its parent star, so that the planet is well beyond the range of glare created by the star's own magnitude of brightness. Additionally, the planet itself must be so hot that it gives off a substantial amount of infrared radiation of its own.
The first extrasolar planet to be directly imaged via infrared—2M1207 b in 2004—was also the first to be found orbiting a brown dwarf star. The image was collected by a group of astronomers at the European Southern Observatory's Very Large Telescope (VLT) array in Chile. The status of 2M1207 b as a planet was confirmed in December of 2005.
For now, indirect methods—which are most effect when the extrasolar planet has its orbit in roughly the same plane as Earth's—are the most commonly used method of detection. The transit method, whereby the amount of light given off by a star is dimmed by a planet passing in front of it from Earth's perspective, has also proven to be reliable. Transit and gravitational microlensing are currently the most common methods of extrasolar planetary detection. The so-called wobble method—where a planet's existence can be discerned by the impact it has on its resident star's behavior—was in the past the most commonly used detection method. Today, the technique is used primarily to support data already collected by more modern means.
One of the most exciting extrasolar planet discoveries was announced on April 24, 2007 regarding the most Earth-like extrasolar planet discovered to date, Gliese 581 c. The excitement was generated due to this planet's positioning in its parent star's so-called habitable zone, which would allow water to exist in a liquid state: a necessity (as far as we know) for supporting life. It was later determined that another planet in the same system, Gliese 581 d, was a more viable candidate for supporting life as we know it. Subsequent studies have determined that it's more reasonable that Gliese 581 c has conditions similar to our solar system's planet Venus.