Habitable moons around cool nearby stars: from science fiction to reality

CARMENES is an instrument designed to find small planets around cool dwarf stars, but it could also detect large satellites that is, moons, of exoplanets. Some of these exomoons may harbor seas of liquid water, favorable places for the development of life.

In Avatar’s universe, Pandora is the fifth moon of Polyphemus, a fictional giant planet around the nearby star Alpha Centauri A; in the Alien saga, Ellen Ripley fought xenomorphs in an infested colony on Acheron, one of the moons of Calpamos, another made-up planet around the real star zeta2 Reticuli; a long time ago, in a galaxy far, far away the Rebel Alliance defeated the Galactic Empire in space battles in orbit of Yavin IV first and, finally, Endor, which were also habitable moons. But do habitable extrasolar moons, or exomoons, exist in reality? And how are they related to Calar Alto?

The story started in 2010, when a team led by the astronomer Nader Haghighipour discovered from Hawai’i a Saturn-mass planet around the “cool” nearby star GJ 1148. The star, also known as Ross 1003 and located at merely 36 light-years, is “cool”  because it is 2400ºC cooler than our star (the surface of the Sun has an approximate temperature of 6000ºC). It is a dwarf star about one third the size and mass of the Sun and only 1,5% its luminosity. GJ 1148 is an M dwarf, the type of star most frequent in our Galaxy.

interior enThe planet that Nader Haghighipour and his team discovered, dubbed GJ 1148 b, had an orbital period (a “year”) of about 41 days and a minimum mass of about 90 Earth masses, very similar to that of Saturn (95.2 Earth masses). However, the data showed a large scatter and a linear trend. Seven years later, another team led by Paul Butler discovered that the linear trend was actually a periodic variation of 530 days, but they did not discuss their possible nature.

As soon as CARMENES, the Calar Alto exoplanet hunter, started operating at the 3.5 m telescope, the German-Spanish team that designed and built it began the investigation of many of these cool nearby M dwarfs, with and without known planets. Of course, GJ 1148 was one of the first targets to look at… and the first one to give a surprise!

GJ 1148 does not have only one exoplanet, but two, both of Saturn-mass like. The presence of two relatively big planets around a small star is rare, as there is only one similar system (GJ 876, aka IL Aquarii, which has two Jupiter-mass planets). The discovery of the new planet, GJ 1148 c, and the refinement of the parameters of the already-known planet, GJ 1148 b (and of the known planets of GJ 876 as well as another five stars), were published in 2018 by a team led by the Bulgarian astronomer Trifon Trifonov.

In this story, Trifonov, a researcher at the Max-Planck-Institut für Astronomy in Heidelberg, Germany, is the main character. But others play a role, too. In late 2019, a small team led by the Spanish astronomer Héctor Martínez-Rodríguez investigated the habitability of moons of all known exoplanets around M dwarfs. Some exoplanets are at the right separation from their stars for water to be liquid on their surfaces; we call it the habitable zone.

GJ 1148 b lies on the habitable zone, but it is expected to be a giant gaseous planet as Saturn. However, a moon around GJ 1148 b, protected by the magnetic field of the planet, may instead harbour a solid surface with liquid water. What  Martínez-Rodríguez and his team found was that most exomoons in the habitable zones of these systems are either ejected or engulfed by their planets, and only those in orbit to large planets, such as GJ 1148 b (and GJ 876 b and c), might survive under severe circumstances.

In early 2020, Trifonov and a team reinforced with experts from Hong Kong, Israel and The Netherlands analysed new data obtained with CARMENES and combined them with all previous available data. They also made an exhaustive dynamical analysis of the star, the two planets and hypothetical moons around GJ 1148 b. Trifonov et al. found that the two planets, which are in eccentric (non-circular) orbits, periodically exchange energy and angular momentum and vary their minimum and maximum separations to their star. Because of it, GJ 1148 b lies outside the habitable zone every few thousand years.

Besides, any moon with the size between those of Titan (main Saturn’s moon and second largest satellite in the Solar System) and Mars would soon decay from its orbit and be engulfed by its planet. In Trifonov’s words, “It is very tempting to assume that GJ 1148 b, a Saturn-mass planet in the habitable zone, may have large exomoons that could be potentially habitable worlds”. But he concluded “Unfortunately, any sufficiently large exomoon that could sustain an ocean or an atmosphere would become a victim of the system’s unique dynamics”. However, if GJ 1148 b does not have a large habitable moon, it may have a myriad of small ones and even a Saturn-like ring.

In spite of CARMENES efforts, exomoons only exist in science-fiction movies… For the moment.

Reference: Trifonov, T., Lee, M.H., Kürster, M. et al, A&A 638, A16 (2020)


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