What is an Exoplanet?
An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. Over 3,000 exoplanets have been discovered since 1988. HARPS (since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand candidate planets of which about 11% may be false positives. On 10 May 2016, NASA verified 1,284 new exoplanets found by Kepler; the largest single finding of planets to date. On average, there is at least one planet per star, but most star systems have multiple planets. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.
What is the Goldilocks Zone?
The habitable zone is nicknamed the “Goldilocks Zone”. It is the zone around a star where the temperature is just right to allow liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.
How do we detect exoplanets?
Planets are extremely faint compared to their parent stars. They usually have less than a millionth of their host star's brightness. It is difficult to detect such a faint light source, and the glare of the parent star tends to wash a planet out. It is necessary to block the light from the parent star in order to reduce the glare while leaving the light from the planet detectable. Some large planets (gas giants) have been detected using direct imaging. All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. Most of them are also very hot, so that they emit intense infrared radiation; the images have then been made at infrared where the planet is brighter than it is at visible wavelengths.
The most common indirect method is the transit method. If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This method suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary. The transit method reveals the radius of a planet, and it has the benefit that it sometimes allows a planet's atmosphere to be investigated through spectroscopy. The Kepler telescope uses this method.