A previously undiscovered exoplanet that is about three times the size of Earth is being dubbed the “forbidden” planet by researchers who are puzzled by its very existence.

NGTS-4b is a gaseous medium-sized planet that is located 920 light-years away from Earth in an area often referred to as the “Neptunian Desert,” which is broadly defined as an area close to stars where no Neptune-sized planets are found.

Researchers previously believed this region was inhospitable to Neptune-sized planets because it receives strong heat and radiation from the star, which prevents the planets from retaining their gaseous atmospheres. Instead, it’s thought the atmospheres would just evaporate, leaving only a rocky core.

That is, until the team of astronomers discovered NGTS-4b, a planet with a radius only 20 per cent smaller than Neptune, orbiting a distant star with its very own atmosphere intact.

Daniel Bayliss, an assistant professor in the department of physics at the University of Warwick who worked on the research, said the newly discovered exoplanet is “very, very close” to its star.

To get a sense of how close NGTS-4b is to its star, Bayliss said the planet orbits its star every 1.3 days.

“When you're that close to a star, then you get a lot of radiation from the star and it's enough to strip off the layers of an atmosphere on a planet that's about the size of Neptune,” he told The Associated Press on Wednesday. “We think it's probably about 1,000 degrees Celsius on this planet."

Despite the intense heat, researchers said NGTS-4b’s atmosphere has not evaporated.

“For this ‘Forbidden Planet’ to exist at all, there must be something else going on that we haven’t quite worked out,” Bayliss said. “It’s possible that the system is very young, and it just hasn’t had time to evaporate off the planet. Or perhaps the planet has just moved in there quite recently.”

Bayliss also suggested the star the planet is orbiting might not be emitting the amount of radiation they expected and “somehow, it hasn’t been enough to evaporate the planet.”

The astronomers were able to detect NGTS-4b using the Next-Generation Transit Survey (NGTS), which is comprised of a dozen 20-centimetre telescopes housed in the Paranal Observatory in Chile’s Atacama Desert.

In their search for new planets, the research team looked for a dip in the light of a star that signals an exoplanet has passed between it and Earth during its orbit.

“What we do is we monitor the star's brightness over many, many months and we look for very minute dips in the flux of the star that tells us that the planet's passing in front of us,” Bayliss said.

Until now, astronomers have only been able to detect larger exoplanets approximately the size of Jupiter that dim a star’s light by 1 per cent when they pass.

In the case of NGTS-4b, however, the astronomers’ telescopes were able to detect the planet even though it only dimmed the star’s light by less than 0.2 per cent.

“This is the first time from the ground we've been able to discover such a tiny signal and that's why we were able to discover a planet that's much, much smaller than Jupiter,” Bayliss said.

Richard West, a professor in the department of physics at the University of Warwick who worked on the research, called the discovery “truly remarkable” in a press release.

“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,” he said.

The researchers’ findings were published Monday in the journal Monthly Notices of the Royal Astronomical Society.

With files from The Associated Press