Study reveals new data about TRAPPIST-1 planets' compositions and atmospheres

All seven worlds are rocky, and some could hold more water than Earth does.
By Laurel Kornfeld | Feb 07, 2018
A new study that used techniques and simulations developed in just the past year is providing new insights into the compositions and atmospheres of the seven planets in the TRAPPIST-1 system.

Two of the planets were discovered with the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile in 2016. A year later, NASA's Spitzer Space Telescope not only confirmed the presence of the two initial planets but also discovered five more.

Spaced tightly together, the seven planets might be tidally locked to the star, meaning one side always faces the star while the other always faces away from it. All are in orbits closer than Mercury is to our Sun.

Following the latter discovery, NASA's Kepler space telescope undertook a 500-hour study of the TRAPPIST-1 system, which will conclude in March.

Led by Simon Grimm of the University of Bern in Switzerland, a team of scientists produced computer models for each of the planets based on data known about them, including their masses and orbital periods.

They found all the planets to be largely made up of rock with densities that indicate as much as five percent of the mass of several may be composed of water.

Whether that water would be solid, liquid, or gaseous depends on the amount of heat each planet receives from the star, which has just nine percent of our Sun's mass.

TRAPPIST-1e, which has the largest amount of rock, is the one most likely to harbor liquid water on its surface. Slightly more dense than Earth, it could have a dense iron core. Due to its size, density, and the amount of stellar radiation it receives, this planet is considered the most Earth-like of all seven.

Along with TRAPPIST-1c, it does not appear to have a heavy atmosphere, ocean, or layer of ice.

The system's innermost planet, TRAPPIST-1b, likely has a rocky core and a thick atmosphere. TRAPPIST-1d, the lightest of all seven worlds, with just 30 percent the mass of Earth, may be enveloped by volatile substances, either in the form of an atmosphere, ocean, or ice layer.

Any water on the outermost planets, TRAPPIST-1f, g, and h, is likely to take the form of ice. They may have thin atmospheres incapable of holding the heavier molecules present in Earth's atmosphere.

"It is interesting that the densest planets are not the ones that are closest to the star, and that the colder planets cannot harbor thick atmospheres," said Caroline Dorn of the University of Zurich in Switerland.

Findings of the study have been published in the journal Astronomy and Astrophysics.



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