The seven rocky planets surrounding the nearby star TRAPPIST-1 could contain a lot of water, suggests a new study. But maybe a bit too much to allow life to appear.
A few months ago, NASA announced the discovery of seven new exoplanets orbiting a dwarf star just 39 light-years away. Confined in a system now called TRAPPIST-1, these new planets seem rocky and have temperatures deemed “friendly” that could allow life to develop. Still, there must be water — at least for the development of life as we know it.
New estimates of the composition of these potentially habitable worlds suggest that yes, they do have water. But maybe a little too much. Two of them contain more than half of their mass in water, reports in the journal Nature Astronomy a team of astronomers.
Surprisingly, this is probably not good news for the appearance of life. “Too much water can be a bad thing,” notes astronomer Cayman Unterborn of Arizona State University.
All TRAPPIST-1 planets were discovered via the “transit method”: several different instruments noticed the tiny luminosity difference of the TRAPPIST star, revealing the presence of planets. The magnitude of these then revealed the size of these planets in question. Astronomers have been able to estimate the masses of the planets — although not precisely — by studying how their transits have varied over time. With the mass and volume information in hand, researchers relied on computer models to get a better idea of the composition of six of the TRAPPIST-1 worlds.
This modeling work suggests that there is a moisture gradient in the TRAPPIST-1 system. The innermost planets, b and c, are probably composed of about 10% water by mass, whereas water represents at least 50% of the mass of planets f and g, the farthest ones. The central planets d and e are somewhere in between. All these worlds are therefore wet, even at the lower end of the gradient. By comparison, the water on Earth is only 0.2% of its mass. The chances of finding life in this system are then very slim.
“In the absence of exposed lands, key geochemical cycles, including the reduction of carbon and phosphorus in ocean reservoirs resulting from continental weathering, will be reduced, limiting the size of the biosphere,” the researchers note. “As such, although these planets may be habitable in the classical definition of the presence of surface water, any biosignature observed from this system may not be entirely distinguishable from abiotic, purely geochemical sources.”
All this water could finally stop some key geological processes that could help life take shape. For example, rocks in the Earth’s mantle often become liquid after moving upward into a lower pressure zone where their melting point is lower. But such a “decompression melting” might not occur on the planets of the TRAPPIST-1 system. The enormous weight of the global oceans would greatly increase the pressures on the mantle.
The planets in orbit around red dwarfs face other challenges of habitability, say researchers. These worlds being in tight orbit, they are “locked”, which means that they always show the same face to their parent star. One side of these planets could be boiling, while the other is icy. This problem could be mitigated by the presence of a thick atmosphere that circulates the heat. But red dwarfs are unstable, and can quickly strip their system of potentially habitable worlds.