The star TRAPPIST-1 , a red dwarf located about 39 light-years from the Sun , aroused the interest of professionals and amateurs alike when its accompanying planetary system was discovered. Between 2016 and 2017 , seven small, rocky planets were detected orbiting this star. These planets have sizes ranging from a radius similar to that of Mars to slightly larger than that of Earth. In addition, they orbit very close to TRAPPIST-1, fitting the orbit of the 7 planets within that of Mercury and being more similar to those of the main moons of Jupiter than to those of the planets of the solar system. This planetary system is among the most compact discovered.
TRAPPIST-1 glows reddish, although it emits most of its energy in the infrared. It is also slightly larger than Jupiter, but about 90 times the mass of the gas giant. However, the combination of the small stellar size with the compactness of the system allows several of its planets to be within the star ‘s habitable zone . This habitable zone is the region in which a planet could harbor liquid water on its surface. That is, it starts beyond the zone where it would be too hot and all the water would evaporate in a short time and ends where the temperatures would be so low that all the water on the planet would freeze. Of course, the presence or not of liquid water on the surface of each planet will also depend on many other factors , such as the presence and thickness of its atmosphere or the amount of water that fell on the planet during its formation.
Between 3 and 4 planets of the TRAPPIST-1 system could be within this habitable zone, specifically planets d, e, f and g (ie, the third through sixth planets). All this, as you will understand, immediately aroused great interest in the system, which has been studied in detail since its discovery. This system is currently the one that accumulates the most different planets in the habitable zone of the star , so it would be an excellent subject of study in our search for extraterrestrial and extrasolar life. Over time, different discoveries have increasingly complicated the presence of liquid water on these potentially habitable planets.
The first blow came when we realized that these very compact orbits surely implied that the 7 planets would have their rotation synchronized with their translation, through tidal coupling . This phenomenon, which our Moon also suffers with respect to the Earth, generally means that half of the planet always points towards the star, experiencing a constant day, while the other half points in the opposite direction, in a perpetual night. This would imply that, even being at an appropriate distance in principle, the daytime part of the planet would receive too much heat, evaporating any water that remained in it for a long time and the night part would be too cold, freezing the water in turn. The only hope of finding liquid water on these planets would then lie in a thin band on the border between day and night . Another alternative would be for a sufficiently dense atmosphere to redirect heat with strong air currents between hemispheres, allowing this band of intermediate temperatures to be widened.
A recent study, published in Nature Astronomy in November 2021, tells us about a new impediment to the presence of liquid water on these planets. The 7 planets of the TRAPPIST-1 system are in orbits that exhibit very strong resonances, each of the seven planets being in resonance with each other. That is, for every 8 orbits that the first of these planets completes, the second completes exactly 5, the third exactly 3, the fourth exactly 2, etc. This occurs because of the proximity between the orbits of the planets. At their closest point, planets b and c are just over 600,000 km apart , less than twice the separation between the Earth and the Moon. Therefore, from any of these planets, their companions would appear relatively large in the sky, with the closest ones even larger than the Moon from the Earth’s surface.
These very close orbits will imply that the system will be very unstable against disturbances in the orbits of its planets . If for some reason, such as a large impact with a meteorite, one of the planets falls out of its orbit, it will end up colliding with another of the planets or being thrown out of the system. This has allowed Raymond and his team to put a limit on the amount of material that planets have interacted with since their formation , putting it at less than 5% of the mass of Earth. This small amount of material also implies that the TRAPPIST-1 planets must have formed very quickly, in just a few million years, at least 10 times faster than the rocky planets in our solar system formed.
The implications of this scenario for the presence of water are quite clear, according to the article. For one thing, if the planets formed near their current distance from their star, they accumulated little water during their formation. Also, the bombardment that has taken place since then, which was very limited as you have shown, was also unable to add large amounts of water to any of the planets. Their most optimistic estimates indicate that the outermost planets (g and h) are the only ones that would have received a considerable amount of water during their formation, reaching a maximum of 7 times the amount of water present in the terrestrial oceans.
This amount may seem large, but remember that we calculated that there could be about 25 times more liquid water in our solar system than there is on Earth, all without counting the much larger amount of water ice. On the other hand, due to the tidal coupling that these planets undergo, it is estimated that only a small percentage of the water present during planetary formation would have survived, resulting in a planetary system with much less water than our hopes of finding life would like. .
SN Raymond et al, 2021, An upper limit on late accretion and water delivery in the TRAPPIST-1 exoplanet system, Nature Astronomy 6 , https://doi.org/10.1038/s41550-021-01518-6