Artist's impression of the nonexistent Fomalhaut b, an exoplanet directly observed by the Hubble telescope

Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)

Discovery image of the Gliese 758 system, taken with Subaru telescope in the near infrared. It is unclear whether the companion should be regarded as a planet or a brown dwarf.

2MASS J044144 is a brown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.

Exoplanet discoveries by year

An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system. A related concept is an exomoon, a natural satellite orbiting an exoplanet.

In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion to at least 144 billion. The smaller estimate studied planet candidates gathered by the Kepler space observatory. Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".