Science Reporting

Planet found in the “neptunian desert” of a star

An international group of astronomers has identified a rogue planet orbiting its star in the so-called “Neptunian desert.” The Neptunian desert is a region close to the stars where large planets with their Neptune-like atmospheres are not expected to survive, as the strong radiation from the star would cause the evaporation of any gaseous atmosphere, leaving behind only a rocky core.

However, NGTS-4b, nicknamed the “Forbidden Planet,” still has its atmosphere intact and is the first exoplanet of its kind to be found in the Neptunian desert. The results are reported in Monthly Notices of the Royal Astronomical Society.

NGTS-4b is smaller than Neptune and three times larger than Earth. It is dense and warm, with a mass equal to 20 times that of the Earth and an average surface temperature of 1000 degrees Celsius. The planet orbits its star closely, completing a complete orbit in just 1.3 days.

The planet was identified using the Next-Generation Transit Survey (NGTS) observation facility at the Paranal Observatory of the Southern European Observatory in the Atacama Desert in Chile. NGTS is a collaboration between the universities of Warwick, Leicester, Cambridge and Queen’s University of Belfast, together with the Observatoire de Genève, the DLR Berlin and the Universidad de Chile.

When looking for new planets, astronomers use structures such as NGTS to look for a flexion of the light of a star, which occurs when an orbiting planet passes in front of it, blocking part of the light. Usually, these decreases are 1% and more can be detected by ground-based research, but NGTS telescopes can record a drop of just 0.2%.

This sensitivity means that astronomers can now detect a wider range of exoplanets: those with a diameter between two and eight times that of the Earth, between the smallest rocky planets and gas giants.

“This is a very rare planet, and it is the first time that a small planet has been detected by a large field Earth telescope,” said co-author Ed Gillen of the Cavendish Laboratory in Cambridge, who conducted the data analysis to determine the mass, radius and orbit of NGTS-4b.

Researchers believe that the planet may have moved into the Neptunian desert recently, in the last million years, or that it has been very large and the atmosphere is still evaporating.

“This planet must be resilient – it’s precisely in the area where we expected that the Neptune planets would not survive,” reports Chief Richard West of Warwick University. “It’s really amazing that we found a planet passing through a star that oscillates less than 0.2% – this has never been done before by telescopes on the ground, and it was great to find it after working on this project for a year.”

“We’re now looking for our data for other similar planets to help us figure out how dry this Neptunian desert is or whether it’s greener than we once thought,” said Gillen.

The research was supported in part by the Science and Technology Facilities Council of the United Kingdom.