An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the discovery of an exoplanet belonging to the mini-Neptune class in an orbit close to its star, where a planet of that type should not exist, so much that it’s called Neptunian Desert. A team of astronomers coordinated by the University of Warwick used the Next-Generation Transit Survey (NGTS) array in Chile to locate the exoplanet, which was therefore cataloged as NGTS-4b. It may have got closer to its star or was originally larger but part of its atmosphere was consumed by stellar radiation.
A little more than 920 light years away from Earth, the star NGTS-4 is a little smaller and less massive than the Sun. Object of observations through the NGTS array, an exceptional instrument to identify exoplanets with the transit method that detects the a slight brightness dimming of a star when a planet passes between it and the Earth, it revealed so far the presence of a single planet around it but it’s really special since it was found very close to its star, where a planet of that type should not exist.
The exoplanet cataloged as NGTS-4b is very close to its star, to the point that its year lasts only 1.3 Earth days. According to current knowledge, a planet with its characteristics – a mass about 20 times the Earth’s for a radius about 20% smaller than Neptune’s – shouldn’t be so close to its star. A rocky planet is compact enough to survive at such a short distance, a hot Jupiter is quite massive for its gas atmosphere to resists stellar radiation but a mini-Neptune such as NGTS-4b shouldn’t be able to hold its own atmosphere. For this reason that area was nicknamed the Neptunian Desert.
The exoplanet NGTS-4b was nicknamed the forbidden planet due to its presence in the Neptunian Desert and astronomers wonder how it can survive. One would expect that a mini-Neptune so close would lose its gas atmosphere a long time ago leaving a rocky planet with a very hot surface since the one on the current gas planet was estimated at around 1,000° Celsius.
The researchers have two possible explanations for the presence of NGTS-4b in the Neptunian Desert: that exoplanet got closer to its star in recent times, where the term is meant from the astronomical point of view and therefore in the last million years, or originally it was more massive and has long been losing its atmosphere. The answer may depend on some characteristics of the exoplanet NGTS-4b and in particular on its core because if it’s massive and dense enough its gravity could help retain its atmosphere.
Dr. Richard West of the University of Warwick, lead author of the article, stated that this is a remarkable discovery also because the star dimming at the transit of the exoplanet NGTS-4b was less than 0.2%, an unprecedented result for ground-based telescopes. His team is examining the data collected by the NGTS array to look for more planets in the Neptunian Desert to see whether that’s an isolated case or the desert is greener than expected. After all, the known exoplanets are only a few thousand in the cosmic neighborhood so other anomalous situations are likely to be discovered and will help improve our models of star systems’ formation and evolution.