A hot Neptune is a type of

giant planet A giant planet, sometimes referred to as a jovian planet (''Jove'' being another name for the Roman god Jupiter (mythology), Jupiter), is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling ...

with a

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

similar to that of

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

orbiting close to its star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was

Gliese 436 b Gliese 436 b (sometimes called GJ 436 b, formally named Awohali) is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty (in 2007) and was among the smallest-known transiting plane ...

(Awohali) in 2007, an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

about 33 light years away. Recent observations have revealed a larger potential population of hot Neptunes in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

than was previously thought. Hot Neptunes may have formed either ''

in situ is a Latin phrase meaning 'in place' or 'on site', derived from ' ('in') and ' ( ablative of ''situs'', ). The term typically refers to the examination or occurrence of a process within its original context, without relocation. The term is use ...

'' or ''ex situ''.

General characteristics

Because of their close proximity to their parent

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s, hot Neptunes have a much greater rate and chance of transiting their star as seen from a farther outlying point, than

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

s of the same mass in larger orbits. This increases the chances of discovering them by transit-based observation methods. Transiting hot Neptunes include

(Awohali) and

HAT-P-11b HAT-P-11b (or Kepler-3b) is an extrasolar planet orbiting the star HAT-P-11. It was discovered by the HATNet Project team in 2009 using the transit method, and submitted for publication on 2 January 2009. This planet is located approximatel ...

. Gliese 436 b was the first hot Neptune to be discovered with certainty in 2007. The exoplanet

Mu Arae c Mu Arae c, also known as HD 160691 c, formally named Dulcinea (pronounced or ), is an extrasolar planet orbiting the star Mu Arae of the constellation Ara. It was the first ' hot Neptune' to be discovered. None of the four planets orbiting Mu A ...

(Dulcinea) discovered in 2004 might also be a hot Neptune, but it has not been determined definitively. Another may be Kepler-56b, which has a mass somewhat larger than Neptune's and orbits its star at 0.1 AU, closer than Mercury orbits the Sun. The first theoretical study of how hot Neptunes could form was carried out in 2004. If these planets formed ex situ, i.e., by migrating to their current locations while growing, they may contain large quantities of frozen

volatiles Volatility or volatile may refer to: Chemistry * Volatility (chemistry), a measuring tendency of a substance or liquid to vaporize easily ** Volatile organic compounds, organic or carbon compounds that can evaporate at normal temperature and pre ...

and

amorphous ice Variations in pressure and temperature give rise to different phases of ice, which have varying properties and molecular geometries. Currently, twenty-one phases, including both crystalline and amorphous ices have been observed. In modern histor ...

s. Otherwise, if they formed in situ, their inventory of heavy elements should be made entirely of

refractory materials In materials science, a refractory (or refractory material) is a material that is resistant to Thermal decomposition, decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are Inorganic c ...

. Yet, regardless of the mode of formation, hot Neptunes should contain large fractions (by mass) of gases, primarily hydrogen and helium, which also account for most of their volume.

Ultra-hot Neptune

LTT 9779 b (Cuancoá) is the first ultra-hot Neptune discovered with an orbital period of 19 hours and an atmospheric temperature of over 1700 degrees Celsius. Being so close to its star and with a mass around twice that of Neptune, its atmosphere should have evaporated into space so its existence requires an unusual explanation. A candidate planet around

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, and ...

slightly more massive than Neptune was detected in 2021. It orbits Vega, an A-class star, every 2.43 days, and with a temperature of about 2500 degrees Celsius would be the second-hottest planet on record if confirmed.

References

Sources

* * A discussion of the "Detection of transits of the nearby hot Neptune GJ 436 b" paper. {{Portal bar, Astronomy, Stars, Outer space Types of planet *

General characteristics

Ultra-hot Neptune

See also

References

Sources