A strange couple formed by a red dwarf star and a gas giant planet

Artist's impression of the planet NGTS-1b and its star (Image University of Warwick/Mark Garlick)
Artist’s impression of the planet NGTS-1b and its star (Image University of Warwick/Mark Garlick)

An article published in the magazine “Monthly Notices of the Royal Astronomical Society” describes the discovery of a hot Jupiter-type gas giant planet orbiting the star NGTS-1, a red dwarf. This is an extraordinary pair that’s in conflict with the current planetary formation models. The exoplanet NGTS-1b is the first to be discovered with the new Next Generation Transit Survey (NGTS) instrument installed at ESO’s Paranal Observatory in Chile.

The NGTS instrument is an array of twelve 20-centimetre telescopes each designed to detect the transit of an exoplanet in front of its star from the tiny light dimming it causes. The candidates discovered with this technique are subsequently verified with further observations made with other ESO instruments such as HARPS (High Accuracy Radial velocity planet searcher), also used for exoplanets hunting and others, not only in Chile.

About 600 light years from Earth, the star NGTS-1 is a red dwarf with about half the diameter of the Sun while the planet NGTS-1b has a diameter about 30% higher than Jupiter’s with a mass about 20% lower. It’s a hot Jupiter, a type of gas planet called that way because it orbits very close to its star so the face exposed to it can reach very high temperatures. The year on NGTS-1b lasts about 2.6 Earth’s days.

In the past, the exoplanets Kepler-45b and HATS-6b were discovered orbiting red dwarfs but are much less massive than NGTS-1b since their masses are about half Jupiter’s for Kepler-45b and about 30% of Jupiter’s for HATS-6b. The problem is that NGTS-1b seems too massive to form around such a small star like his and comparison with the other two cases is simple because the three stars all have a mass of just over half the Sun’s.

According to our current planetary formation models, when a star forms only a small percentage of the materials that form the dust and gas disk from which a solar system can be born remains available to form planets. The Sun includes over 99% of the mass of the solar system. The previous gas planets uncovered orbiting red dwarfs already seemed extraordinary, the discovery of NGTS-1b was really surprising.

Despite its remarkable size compared to its star, the exoplanet NGTS-1b was difficult to detect due to its star’s dim light. However, red dwarfs are the most common class of stars, about three-quarters of those in the Milky Way, so it’s possible that they can actually have gas giants but we just noticed them thanks to new generation instruments.

Both Dr. Daniel Bayliss, the article’s first author, and Professor Peter Wheatley, who leads the NGTS system operations, stressed the importance of this new instrument in this discovery. New research aimed at the dwarf stars that started in recent years are providing new interesting information to advance our knowledge on the formation and evolution of solar systems.

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