The term Neptunian Desert refers to an orbiting area around a star where Neptune-like planets are very rare. Therefore, the term Neptunian Desert Planet refers to an exotic type of planet.
Occurrence[]
The Neptunian Desert is an area located very close to a star. At such distance, planets would experience very strong solar winds and temperatures over 1500 degrees K. Planets have been confirmed to exist at such torch orbits, but they are larger then Jupiter or tiny rocky bodies. It is not clearly known why Neptune-like or Super Earths are so rare in the Neptunian Desert. The fact is, such a planet would have its atmosphere blown into space relatively fast on an astronomical timescale. A planet at least the size of Jupiter, especially with a strong magnetic field, would protect its atmosphere for much longer.
A Neptunian Desert Planet is NGTS-4b.
It is not known if NGTS-4b was formed where it currently orbits or if it was pushed on its current orbit by something else. For future settlers, it would not be so important how the planet came there, but what economical value it can have.
Properties[]
A Neptunian desert planet is unable to keep its atmosphere locked and is losing mass over time. While this mass loss is not significant for a human civilization letime, it still creates several problems. One major concern is that gasses pushed out of the atmosphere enrich the solar wind and can create problems to passing ships or human colonies on a further planet. This could be a particular problem for NGTS-4b, which orbits an M - type star very close. Habitable zones for M-Type Stars are sometimes less then 0.1 AU.
Given the small distance to the host star, Neptunian desert planets have small Hill spheres and are not thought to have moons. And even if they have, in time, they will lose the moons as they lose mass.
The temperature of such a planet is expected to be above 1500 K. Adding a greenhouse effect, which certainly exists, we can presume that beneath the surface clouds temperatures exceed 2000 K. At such values, many substances that we would find in solid phase on Earth, should be in gaseous phase. The upper atmosphere would contain sodium, potassium or even magnesium. If that is true, the planet would appear dark in color. It is possible that, beneath the upper sodium clouds we could find a second layer of silicon or even iron clouds, as these elements are gasses at high temperatures. With such a complex chemical composition, solar winds might be able to blow away not only light atoms like hydrogen (which will be the first removed), but even heavier elements, creating a chemically complex radiation belt and solar wind around the planet and among its orbit around the star.
Water, methane and ammonia molecules will tend to disintegrate, producing organic molecules and free gasses (like hydrogen, nitrogen and oxygen). Organic compounds will not get complex, as extreme heat disintegrates them. The process will ensure a supply of hydrogen to be released into space, even if the initial molecular hydrogen was depleted.
Aged Planet[]
We can think what would happen to a Neptunian Desert Planet after most of its atmosphere was eroded. With the reduction of mass, it would become an exotic Super-Earth, with very little atmosphere. If it maintains its current orbit, it will be a Vulcanoid Type Planet, impossible to terraform, but suitable for Industrial Colonization. Its exposed core will be stripped of volatiles and light elements, but rich in metals.
Super - Earths are usually hard to terraform because they have dense atmospheres. However, an aged Neptunian Desert Planet, moved out of its initial orbit to the Habitable Zone, can offer good opportunities for colonization. Of course, this must be a rare case, since Neptunian Desert Planets are already very rare and moving a planet from a very close orbit requires a lot of energy. Such a planet can accumulate some water and gasses from cometary bombardment and be suitable for terraforming. Its gravity would be between 1.3 and 1.7 that of Earth. Having a much larger surface, it can host more people then Earth currently does.