An unexpected source of water on the TRAPPIST-1 planets

An international team, including scientists from Aix-Marseille Université's Origins Institute and the Laboratoire d'Astrophysique de Marseille (CNRS/Aix-Marseille Université), has recently proposed an original scenario for the formation of planetesimals around the star TRAPPIST-1, even in the initial absence of water ice in the protoplanetary disk*.

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Key facts to remember:

In a previous study, a team from Aix-Marseille Université had already demonstrated that the planets of TRAPPIST-1 were extraordinarily rich in water, accounting for 10% of their total mass, whereas on Earth, water makes up only 0.025% of the mass.
In this new study, scientists from the Origins Institute and the Laboratoire d'astrophysique de Marseille shed light on the phyllosilicates, hydrated minerals, that are thought to be responsible for the formation of these water-rich planets. Phyllosilicates release water vapor into the protoplanetary disk (where the planet is formed) when subjected to temperatures of between 130°C and 330°C.
The water vapor diffuses outwards from the disk and condenses into ice on the phyllosilicate grains, due to the lower temperatures, creating reservoirs of water that can reach up to 25% of the grains' mass over a period of 50,000 years.

The processes involved in planet formation in the TRAPPIST-1 exoplanetary system are still a matter of debate. These planets orbit remarkably close to their star, ten times closer than Mercury is to the Sun. As observed in other star-forming systems, the preferred hypothesis remains that these planets emerged from a protoplanetary disk composed of gas, dust, and ice grains.

Planets extremely rich in water

In an earlier study, a team from Aix-Marseille Université had already demonstrated that the TRAPPIST-1 planets d, e, f, g and h were extraordinarily rich in water**. This represents around 10% of their total mass. By comparison, on Earth, water makes up just 0.025% of mass. This suggests that the planetesimals that formed these planets were exceptionally rich in water. However, observations of young red dwarfs suggest that these stars, although less massive than the Sun, emit intense ultraviolet radiation capable of destroying water molecules by photolysis. In conjunction with the powerful stellar winds emitted by forming stars, it is plausible that the planet-forming environment of TRAPPIST-1 was devoid of ice.

An ice-free formation environment

More recently, a ground-breaking interdisciplinary study bringing together earth sciences and astrophysics, led by researchers from the Laboratoire d'Astrophysique de Marseille and the Origins Institute, has demonstrated the possibility of creating water-rich planetesimals from hydrated minerals, even in an initially ice-free environment. The study highlights phyllosilicates, a class of hydrated minerals containing up to 10% water by mass. These minerals release water vapour into the protoplanetary disk when subjected to temperatures between 130°C and 330°C.

Water-bearing phyllosilicate grains

The water vapour thus produced diffuses towards the outside of the disk, where the temperature is lower, and condenses into ice on the phyllosilicate grains. Thanks to this mechanism, researchers have demonstrated the possibility of generating grains with a water content of up to 25% by mass over a period of 50,000 years. This significant increase in the amount of water on the grains paves the way for the formation of very water-rich planetesimals over this period. The rapid formation of water-rich grains contributes to the creation of TRAPPIST-1 planets d, e, f, g and h, which emerge from planetesimals formed at the water-ice line, a zone in the protoplanetary disk where water vapour condenses to ice.

This new mechanism was likely associated with other previously identified processes, such as the influx of ice from the outermost regions of TRAPPIST-1's protoplanetary disk. Future observations of the TRAPPIST-1 system by the James Webb Space Telescope (JWST) over the next few years will test this theory and confirm, or not, the presence of water on these planets.

Studying Jupiter's Galilean moons to understand TRAPPIST-1

The characteristics of the TRAPPIST-1 exoplanetary system bear striking similarities to those of Galilean moons, whether in terms of the compactness of their orbits, their Laplace resonance configuration, or their composition. Studying the origin of Jupiter's Galilean moons will further our understanding of the genesis of the planets in the TRAPPIST-1 system. At the same time, the characteristics of the TRAPPIST-1 system, analogous to Jupiter's moons, offer valuable insights into the processes at work in our own solar system. An earlier study by this team revealed similarities, suggesting that Galilean moons could form according to a mechanism comparable to that now proposed for the TRAPPIST-1 system.