Scientists Discover Rogue 'Forbidden Planet' Three Times the Size of Earth in a Place Where It Shouldn't Exist

forbidden planet
Exoplanet NGTS-4b, also known as 'The Forbidden Planet.' University of Warwick/Mark Garlick

Astronomers have discovered a "forbidden" planet three times the size of Earth in a region of a distant star system where theory suggests it should not exist.

The planet—which has a radius about 20 percent smaller than Neptune and a mass of 20 Earths—is very close to its star, completing an orbit once every 1.3 days. Being so close to the star means it is heated to a scorching 1,000 degrees Celsius (1,830 degrees Fahrenheit,) but despite this, it still retains its own atmosphere.

According to a study published in the Monthly Notices of the Royal Astronomical, the planet—officially called NGTS-4b—is the first exoplanet of its kind to be found in what scientists call the "Neptunian Desert."

This is the region immediately surrounding stars where no planets similar in size to Neptune are found. Usually, the radiation in this zone is so strong that any gas in the atmosphere of a planet evaporates leaving behind only a rocky core. But unusually, NGTS-4b bucks this trend.

"This planet must be tough—it is right in the zone where we expected Neptune-sized planets could not survive," Richard West, author of the study from the Astronomy and Astrophysics Group at the University of Warwick in the U.K., said in a statement.

The researchers propose two possible explanations for the planet's location: firstly, the planet may only have moved into its current position recently—perhaps in the last million years or so—or the atmosphere could have been very big and it is still evaporating.

The planet was identified by an international team of astronomers using the Next-Generation Transit Survey telescopes (NGTS) at the European Southern Observatory's Paranal facility in the Atacama Desert, Chile. Like many other planets, it was discovered using the "transit method."

This involves looking for "dips" in the light coming from a star, indicating that an orbiting planet has moved in front of it and blocked the light. Normally, ground-based telescopes can only detect dips in light of 1 percent of more. But the NGTS observatory can pick up dips as small as 0.2 percent.

"It is truly remarkable that we found a transiting planet via a star dimming by less than 0.2 percent—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," West said.

"We are now scouring our data to see if we can see any more planets in the Neptune Desert—perhaps the desert is greener than was once thought."