"Forbidden" Planet Discovered In The So-Called Neptunian Desert

Artist impression of NGTS-4b. University of Warwick

There is a huge variety of planets in our galaxy. The last decade or so has shown some truly peculiar worlds exist out there, from magma planets and water worlds, to puffy hot Jupiters that are so close to their stars they have an atmosphere rich in iron.

A particular class of planet that has been missing is the so-called hot Neptunes. These are planets bigger than Earth but smaller than Jupiter, that orbit their star very closely. Astronomers call this apparent scarcity the "Neptunian desert" and evidence suggests that this absence is due to most planets evaporating away. This dramatic evaporation has been witnessed in exceptional objects such as GJ 436b and G 3470b.

However, a new object has been found in the Neptunian desert, and its properties are so peculiar that the international team has nicknamed it “forbidden”. NGTS-4b, its technical name, has 20 times the mass of Earth and a radius 20 percent smaller than Neptune. It orbits its star in just 1.3 days and has a temperature of 1,000°C (1,832°F).

According to the study published in the Monthly Notices of the Royal Astronomical Society, the planet is being bombarded by X-rays and extreme ultraviolet photons, and researchers estimate that it might be losing about 10,000 tons of its atmosphere every second.

This high number, about 10 times higher than GJ 436b, combined with the fact the planet has apparently survived the eons when the star was more active is truly impressive. The team suggests that either the planet migrated inward recently or that it must have a large dense core whose gravity has fought well against the powerful light of its star.

“This planet must be tough – it is right in the zone where we expected Neptune-sized planets could not survive,” lead author Dr Richard West from the University of Warwick said in a statement. “We are now scouring out data to see if we can see any more planets in the Neptune Desert – perhaps the desert is greener than was once thought.”

The observation was possible due to the transit technique, where the light of the star is dimmed slightly by the planet passing in front of it. Ground observatories can usually detect transiting planets that dim their star more than 1 percent, so ones that are relatively close and big. But the Next-Generation Transit Survey (NGTS) used in this discovery was able to detect a dimming far below that.

“It is truly remarkable that we found a transiting planet via a star dimming by less than 0.2% – this has never been done before by telescopes on the ground, and it was great to find after working on this project for a year,” Dr West said.  

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