Neptunian planet

 Gaseous worlds similar to Neptune that are about Neptune's size                                  

A Neptunian planet is what?

Neptunian exoplanets resemble our solar system's Neptune or Uranus in size. (Neptune has a radius almost four times that of Earth and a mass nearly 17 times that of Earth.) The interiors of Neptunian exoplanets could vary, but they are all likely to be rocky with heavy metals in their cores. The atmospheres of Neptunian planets are often dominated by hydrogen and helium. Mini-Neptunes, planets that are larger than Earth but smaller than Neptune, are also being found. In our solar system, there are no planets like these.

Discover some planets that resemble Neptune.

HAT-P-26_b                                     GJ_436_b

While hydrogen and helium make up the majority of Uranus and Neptune's compositions, both planets also contain water, ammonia, and methane. Uranus and Neptune are frequently referred to as "ice giants" since these three substances are typically found frozen as ices in the chilly outer solar system (though their interiors are warm enough that the "ices" inside them are not frozen). 2014 saw the discovery of an ice giant exoplanet located 25,000 light-years distant. It orbits its star similarly to how Uranus orbits our Sun, albeit we don't know much about its composition, what it's made of, or what elements exist in its atmosphere.

The compositions of far-off planets can be difficult to infer. Space observatories like NASA's Hubble (and old Spitzer) use starlight analysis to get data about planet atmospheres. A planet's atmosphere scatters starlight at specific wavelengths, which atoms and molecules absorb and prevent the telescope from seeing. A planet looks to be larger the more light it blocks. Researchers can identify the substances that make up the atmosphere by examining the amount of light filtered by the planet at various wavelengths using a technique called spectroscopy. The goal of NASA's Transiting Exoplanet Survey Satellite (TESS) is to find planets that are smaller than Neptune and transit stars that are bright enough to support subsequent spectroscopic investigations that can reveal the atmospheric compositions of the planets.

Thick clouds on Neptunian exoplanets frequently prevent any light from passing through, masking the signature of the molecules in the atmosphere. After its 2021 launch, the James Webb Space Telescope will be able to observe exoplanet atmospheres more closely.

In 2017, astronomers were thrilled to discover clear skies on HAT-P-11b, a planet of the size of Neptune. They were able to distinguish water vapour molecules in the exoplanet's atmosphere since there were no clouds to obscure their view. HAT-P-11b resembles our own Neptune in that it is gaseous with a rocky core. Although there may be clouds in its atmosphere farther up, Hubble, Spitzer, and Kepler's combined observations revealed that there are none in the upper region. Due to the clear visibility, researchers were able to identify molecules of water vapour in the planet's atmosphere.

A desert on Neptune

Astronomers have discovered sizzling super-Earths and scorching, Jupiter-sized planets in close proximity to their sun. However, it has been considerably more difficult to detect so-called "hot Neptunes," whose atmospheres are heated to temperatures greater than 1,700 degrees Fahrenheit (more than 900 degrees Celsius). Only a small number of hot Neptunes have actually been discovered thus far.

Because they orbit further from their stars than those in the area where scientists would expect to find hot Neptunes, the majority of the known Neptune-sized exoplanets, like HAT-P-11b above, are only "warm." The enigmatic hot-Neptune desert argues that either these extraterrestrial worlds are uncommon or that they were numerous once but have since undergone transformations.

One of the warmest Neptunes known as GJ 436b is shedding its atmosphere, according to research conducted a few years ago using Hubble. It's unlikely the planet would disappear, but hotter Neptunes could not have been so fortunate. The tremendous radiation from a star can heat an atmosphere to the point where it defies the gravitational pull of the planet, much like an untethered hot air balloon. Around the globe, a massive cloud of escaping gas builds before dissipating into space.

This could apply to the planet GJ 3470b, a "very warm Neptune" that is losing its atmosphere 100 times more quickly than GJ 436b. The distance between the two planets and their stars is roughly 3.7 million miles (5.5 million kilometres). The Sun and Mercury, the innermost planet in our solar system, are separated by only one-tenth of that distance. Due to its lower density and hence less ability to gravitationally hold onto the hot atmosphere, GJ 3470b may be evaporating more quickly than GJ 436b.

In contrast to GJ 3470b, which orbits a star that is between 4 and 8 billion years old, GJ 3470b orbits a star that is just 2 billion years old. In comparison to GJ 436b, the younger star is more active and so bombards the planet with more intense radiation.

The discovery of two heated, evaporating Neptunes supports the theory that these hotter versions of these typically far-off planets may be a type of planets in transition. It's possible that hot and very warm Neptunes will ultimately become mini-Neptunes, which are planets with dense atmospheres made primarily of hydrogen and larger than Earth but smaller than Neptune. Or they could shrink even further to become super-Earths, larger, rocky replicas of Earth.

Past the "Snow Line"

Neptune-mass worlds may be the most typical sort of planet to develop in the frigid, outer regions of planetary systems, according to a 2016 study. The discovery of planets using gravitational microlensing was the main focus of the study. The study gave the first clue as to the kinds of planets that may be lurking at the distance from a host star, where planets are thought to form most effectively.

"We've located the seeming sweet spot for cold planet sizes. We find that the frequency of Neptune-mass planets in these outer orbits is roughly ten times greater than that of Jupiter-mass planets in Jupiter-like orbits "the study's principal investigator, Daisuke Suzuki, who was a post-doctoral researcher at the University of Maryland Baltimore County and NASA's Goddard Space Flight Center in Greenbelt, Maryland, at the time, said.

They discovered that cold, Neptune-mass worlds are probably the most frequent types of planets beyond the so-called snow line, which is the distance from a star beyond which water remains frozen during planetary formation. They made this discovery by comparing planet frequency to the mass ratios of planets and stars and the distances between them. The snow line is considered to have been in the middle of the main asteroid belt today, at a distance from the Sun that is approximately 2.7 times that of Earth.