Ocean Worlds in the Trappist-1 System

A new evaluation of a nearby star system’s potentially habitable water worlds.

The Trappist-1 system, shown here in an artist's conception, has seven known planets, but only three are thought to be habitable. (NASA/JPL-Caltech)
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An international scientific team led by Patrick Barth from the University of St. Andrews in Scotland evaluated the planetary evolution of three planets in the Trappist-1 system, a family of exoplanets just 40 light years away from Earth. All three planets are considered to be potentially habitable.

The scientists realized that interactions between the planets’ early magma oceans and their atmospheres were critical for understanding how much water might exist today on these worlds, which initially are estimated to have had between 1 and 100 Earth oceans’ worth of water. The team’s modeling revealed three different ways the planets might have evolved: a dry scenario, moderately wet, and wet.

Trappist-1g (1.2 times the mass of Earth, but with a 25 percent lower density) likely followed the wet scenario. Trappist-1f, the next closest planet to its host star, has 0.7 times the mass of Earth, and is likely to be moderately wet or wet. Both planets appear to have ended up covered by thick oceans and a potentially oxygen-rich atmosphere. In fact, the scientific team extrapolated that the ocean on Trappist-1g may be 670 kilometers deep!

The authors further suggested that Trappist-1e, the innermost of the three habitable-zone planets with 0.8 times the mass of Earth and a density about the same or a little higher, likely followed a dry or moderately wet evolutionary pathway, and should be the most similar to Earth. That assessment is in agreement with the Habitable Exoplanets Catalog, which lists Trappist-1e as the most promising of the three planets in terms of potential habitability. In fact, this world ranks 11th among all known exoplanets on the Earth Similarity Index proposed by myself and other researchers.

The new results have to be taken with a grain of salt, though, since the authors did not include the effect of carbon dioxide as a greenhouse gas, nor the effect of large asteroid impacts like the one that created Earth’s Moon. Getting the amount of greenhouse warming right is especially critical for evaluating habitability. Without a greenhouse effect, Earth’s average temperature would be well below the freezing point of water, rather than the balmy 15o Centigrade that we see today. The estimated temperature on Trappist-1d, which was not considered by the authors, is about 10o C. If it does have a greenhouse effect less pronounced than Earth’s, it could also be habitable.

When evaluating the potential for life on the Trappist-1 planets, we have to realize that this is a very different solar system from our own. Conclusions we might draw based on our own planetary neighborhood are unlikely to be valid. To start with, the Trappist-1 host star is barely larger than Jupiter. The solar system is older than our own, perhaps twice as old. Also, all the Trappist-1 planets orbit their star closer than Mercury’s orbit around the Sun, and a year on these worlds lasts only between 1.5 and 18.8 Earth days.

As a result, there’s a good chance that all seven planets are tidally locked, meaning that they permanently show the same side to their host star. This generally bodes badly for life, because one side is roasting, while the other is extremely cold. A thick atmosphere can compensate somewhat by circulating heat from one side to the other. An ocean should help in this regard as well. But there’s no analogous planet in our own solar system that we could study to really understand these effects.

Still, no one can argue that the Trappist-1 system continues to be hugely intriguing. And being only 40 light years away means that we might be able to get more remote sensing data soon, and perhaps one day could even send a robotic probe to visit.

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