Using numerical simulations to identify planets stable for millions of years, a team of researchers concluded that six of the seven roughly Earth-sized planets in the TRAPPIST-1 planetary system are consistent with an Earth-like composition. The exception is TRAPPIST-1f, which has a mass of 25% water.
This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.
TRAPPIST-1 is an ultracool dwarf star in the constellation Aquarius, 38.8 light-years away.
The star is host to a remarkable planetary system consisting of seven transiting planets: TRAPPIST-1b, c, d, e, f, g and h.
All these planets are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods.
“The goal of exoplanetary astronomy is to find planets that are similar to Earth in composition and potentially habitable,” said Dr. Billy Quarles, a postdoctoral researcher at the University of Oklahoma and lead author of a paper reporting the results in the Astrophysical Journal Letters (arXiv.org preprint).
“For thousands of years, astronomers have sought other worlds capable of sustaining life.”
TRAPPIST-1 planets are more tightly spaced than in Kepler planetary systems, which allow for transit timing variations with the photometric observations. These variations tell the researchers about the mass of the planets and the radii are measured through the eclipses.
Mass and radius measurements can then infer the density. By comparing the Earth’s density (mostly rock) to the TRAPPIST-1 planets, the scientists can determine what the planets are likely composed of and provide insight into whether they are potentially habitable.
The TRAPPIST-1 system contains a total of seven Earth-size planets. Three of them — TRAPPIST-1e, f and g — dwell in their star’s so-called ‘habitable zone.’ Image credit: NASA.
“We found that 6 of the 7 planets are consistent with an Earth-like composition,” Dr. Quarles and his colleagues said.
“The exception is planet TRAPPIST-1f, which is likely to have a volatile-rich envelope and has a relatively small dynamical separation with planet TRAPPIST-1g.”
TRAPPIST-1f has the tightest constraints with 25% of its mass in water, which is rare given its radius.
The concern of this planet is that the mass is 70% the mass of the Earth, but it is the same size as our planet. Because the radius is so large, the pressure turns the water to steam, and it is likely too hot for life as we know it.
“We’re continually learning about the planets and will investigate them further in our studies,” Dr. Quarles said.
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B. Quarles et al. 2017. Plausible Compositions of the Seven TRAPPIST-1 Planets Using Long-term Dynamical Simulations. ApJL 842, L5; doi: 10.3847/2041-8213/aa74bf
