A hot Neptune is an extrasolar planet

Extrasolar planet

An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 in an orbit close to its star (normally less than one astronomical unit

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 away), with a mass

Mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 similar to that of Uranus

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 or Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. Recent observations have revealed a larger potential population of hot Neptunes than previously thought. The first hot Neptune discovered was Mu Arae c

Mu Arae c

Mu Arae e is one of the four extrasolar planets orbiting the star Mu Arae. Its discovery was announced on June 13, 2002. Mu Arae e is a gas giant at least 1.8 times as massive as Jupiter. The planet orbits at Jupiter-like distance at 5.235 AU.-References:...

 (or HD 160691 c).

General characteristics

Hot Neptunes have some common characteristics:

• They have a much greater chance of transiting
  Astronomical transit
  The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
  
   their star as seen from a farther outlying point than planets of the same mass in larger orbits. The most notable of these are Gliese 436 b
  Gliese 436 b
  Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...
  
  , the first transiting hot Neptune found, and HAT-P-11b
  HAT-P-11b
  HAT-P-11b is an extrasolar planet orbiting the star HAT-P-11. This planet was discovered by the transit method and submitted for publication on January 2, 2009....
  
  , which was recently observed by the Kepler mission
  Kepler Mission
  The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
  
  .