Astronomers have detected five new planets, eight planet candidates, and confirmed three previously reported planets, around nine nearby M-dwarf (red dwarf) stars. Among the new planets, Gliese 180d and Gliese 229Ac are super-Earths located in the conservative habitable zones of their host stars; Gliese 433c is a cold super-Neptune candidate belonging to an unexplored population of Neptune-like planets.

M-dwarfs are the most common stars in our Milky Way Galaxy and the primary class of stars known to host terrestrial planets.

The first terrestrial-mass planet was found around the M-dwarf Gliese 876.

Over the past decade M-dwarfs have been the principle targets for potentially habitable planets because their habitable zones are much closer to the star, and thus the potentially habitable planets have much shorter periods than those orbiting around Sun-like stars.

“Many planets that orbit red dwarfs in the habitable zone are tidally locked, meaning that the period at which they spin around their axes is the same as the period at which they orbit their host star,” said lead author Dr. Fabo Feng, an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Science.

“This is similar to how our Moon is tidally locked to Earth, meaning that we only ever see one side of it from here.”

“As a result, these exoplanets are a very cold permanent night on one side and very hot permanent day on the other — not good for habitability.”

“Gliese 180d is the nearest temperate super-Earth to us that is not tidally locked to its star, which probably boosts its likelihood of being able to host and sustain life,” he added.

Gliese 180d has a minimum mass of 7.49 times that of Earth and an orbital period of 106 days.

The other potentially habitable planet, Gliese 229Ac, is 7.93 times more massive than Earth. It orbits its parent star once every 122 days.

It is the nearest temperate super-Earth to us located in a system in which the host star has a brown dwarf companion.

The brown dwarf in this system, Gliese 229B, was one of the first brown dwarfs to be imaged.

It is not known if they can host exoplanets on their own, but this planetary system is a perfect case study for how exoplanets form and evolve in a star-brown dwarf binary system.

“Our discovery adds to the list of planets that can potentially be directly imaged by the next generation of telescopes,” Dr. Feng said.

“Ultimately, we are working toward the goal of being able to determine if planets orbiting nearby stars host life.”

The Neptune-mass planet Gliese 433c orbits its star at a distance at which surface water is likely to be frozen.

This planet is probably the first realistic candidate for direct imaging of cold Neptunes.

The team also detected three planets Gliese 422b, 433d, and 3082b and seven planet candidates Gliese 173b, 229Ab, 620b, 620c, 739b, 739c, and 911b.

“We eventually want to build a map of all of the planets orbiting the nearest stars to our own Solar System, especially those that are potentially habitable,” said co-author Dr. Jeff Crane, an astronomer at the Observatories of the Carnegie Institution for Science.

The findings appear in two papers in the Astrophysical Journal Supplement Series and the Astronomical Journal.

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Fabo Feng et al. 2020. Search for Nearby Earth Analogs. II. Detection of Five New Planets, Eight Planet Candidates, and Confirmation of Three Planets around Nine Nearby M Dwarfs. ApJS 246, 11; doi: 10.3847/1538-4365/ab5e7c

R.P. Butler et al. 2019. A Reanalysis of the UVES M Dwarf Planet Search Program. AJ 158, 251; doi: 10.3847/1538-3881/ab4905