A hot Neptune or Hoptune is a type of

giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. The ...

with a

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

similar to that of

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

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

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) 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 planets in mass and radius, ...

in 2007, an exoplanet about 33 light years away. Recent observations have revealed a larger potential population of hot Neptunes in the Milky Way than was previously thought. Hot Neptunes may have formed either

in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...

or ex situ.

General characteristics

Because of their close proximity to their parent stars, hot Neptunes have a much greater rate and chance of transiting their star as seen from a farther outlying point, than planets of the same mass in larger orbits. This increases the chances of discovering them by transit-based observation methods. Transiting hot Neptunes include

and HAT-P-11b. Gliese 436 b (also known as GJ 436b) was the first hot Neptune to be discovered with certainty in 2007. The exoplanet

Dulcinea Dulcinea del Toboso is a fictional character who is unseen in Miguel de Cervantes' novel ''Don Quijote''. Don Quijote believes he must have a lady, under the mistaken view that chivalry requires it. As he does not have one, he invents her, m ...

(or HD 160691 c) 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

'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 Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ...

and

amorphous ice Amorphous ice (non-crystalline or "vitreous" ice) is an amorphous solid form of water. Common ice is a crystalline material wherein the molecules are regularly arranged in a hexagonal lattice, whereas amorphous ice has a lack of long-range ord ...

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

refractory materials In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

. 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 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.An Ultra-Hot Neptune in the Neptune desert
James S. Jenkins et al, 27 Sep 2020 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, a ...

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, Spaceflight, Outer space, Solar System Types of planet *

General characteristics

Ultra-hot Neptune

See also

References

Sources