An international team of astronomers has discovered a Neptune-sized exoplanet orbiting NGTS-4, a K-dwarf star located 922 light-years from Earth. Named NGTS-4b, the planet orbits around its host star in only 1.3 days and therefore is very hot (1,832 degrees Fahrenheit, or 1,000 degrees Celsius). It has a mass of 20.6 Earth masses and a radius 20% smaller than Neptune, which places it well within the so-called ‘Neptunian desert.’
An artist’s impression of NGTS-4b and its star. Image credit: Mark Garlick / University of Warwick.
The Neptunian desert is broadly defined as the lack of short-period Neptune-sized exoplanets — those with masses around 0.1 Jupiter’s mass and periods less than 2-4 days.
Short-period planets receive strong irradiation from their star and do not retain their gaseous atmosphere. However NGTS-4b still has its atmosphere of gas.
“This planet may have moved into the Neptunian desert recently, in the last one million years, or it was very big and the atmosphere is still evaporating,” said Dr. Richard West from the University of Warwick and colleagues.
NGTS-4b was discovered using the Next-Generation Transit Survey (NGTS) facility, designed to search for transiting planets around bright stars. The facility is situated at ESO’s Paranal Observatory in the heart of the Atacama Desert, Chile.
“When looking for new exoplanets we look for a dip in the light of a star — this the planet orbiting it and blocking the light,” the astronomers said.
“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%.”
“NGTS-4b must be tough — it is right in the zone where we expected Neptune-sized planets could not survive,” Dr. West added.
“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.”
“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 team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
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Richard G. West et al. 2019. NGTS-4b: A sub-Neptune transiting in the desert. MNRAS 486 (4): 5094-5103; doi: 10.1093/mnras/stz1084
