Astronomers discover ‘forbidden planet’ in barren zone

An exoplanet with its own atmosphere has been discovered in the Neptunian Desert — an area previously believed to be uninhabitable for planets with atmospheres.

The rogue planet, named NGTS-4b — also dubbed ‘The Forbidden Planet’ by researchers — is smaller than Neptune but three times the size of Earth.

It has a mass of 20 Earth masses, and a radius 20% smaller than Neptune, and is 1000 degrees Celsius —making it hotter than Mercury in our solar system. NGTS-4b has an extremely short orbital period, taking only 1.3 Earth days to circle its host star.

It is the first exoplanet of its kind to have been found in the Neptunian Desert — a region close to stars where Neptune-sized planets are not normally found.

This area receives strong irradiation from the star, meaning the planets in this zone do not retain their gaseous atmosphere as they evaporate leaving just a rocky core. However, NGTS-4b bucks this finding, as it still has an atmosphere of gas.

The new research that spotted this rogue planet was led by Dr Richard West and a team including Professor Peter Wheatley, Dr Daniel Bayliss and Dr James McCormac from the Astronomy and Astrophysics Group at the University of Warwick.

They made the discovery using the state-of-the-art Next-Generation Transit Survey (NGTS) observing facility. NGTS is designed to search for transiting planets on bright stars. NGTS-4b is so small other ground surveys wouldn’t have spotted it.

This nighttime long-exposure view shows the NGTS telescopes during testing. The very brilliant Moon appears in the centre of the picture and the VISTA (right) and VLT (left) domes can also be seen on the horizon. (ESO/ G.Lambert)

NGTS is situated at the European Southern Observatory’s Paranal Observatory in the heart of the Atacama Desert, Chile — a collaboration between UK Universities Warwick, Leicester, Cambridge, and Queen’s University Belfast, together with Observatoire de Genève, DLR Berlin and Universidad de Chile.

When looking for new planets astronomers look for a dip in the light of a star — this the planet orbiting it and blocking the light. Usually, only dips of 1% and more are picked up by ground-based searches, but the NGTS telescopes can pick up a dip of just 0.2%

Dr Richard West, from the Department of Physics at the University of Warwick, says: “This planet must be tough — it is right in the zone where we expected Neptune-sized planets could not survive.

“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.

West concludes: “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.”

Original research: https://academic.oup.com/mnras/article/486/4/5094/5475662