Newly discovered Earth-like worlds are rocky, not gassy

by Mary Caperton Morton
Wednesday, October 19, 2016

In May, a new planetary system was discovered just 40 light-years from Earth, including three Earth-sized planets orbiting around their red dwarf sun in a temperature range that could potentially harbor life. Now, a follow-up study on the system has found that the two innermost planets are primarily rocky with compact atmospheres, as opposed to inhospitable gas giants, like Jupiter.

Researchers led by Julien de Wit of MIT used NASA’s Hubble Space Telescope to catch a glimpse of the atmospheres of two of the planets. They timed the telescope to point at the planets' star, named TRAPPIST-1, just as two of its planets were crossing in front of it, in a rare double transit event.

“For the first time, we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time,” de Wit said in a statement. The team recorded a combined transmission spectrum for the two planets, measuring the changes in wavelength as the amount of starlight from TRAPPIST-1 dipped with each transit.

The changes in starlight did not vary much as both planets crossed their sun, the team reported in Nature. Large changes in starlight during planetary transits indicate the passing body has a large, gaseous atmosphere. Thus, the data indicate that the planets have compact atmospheres, as seen around rocky planets like Earth, Venus and Mars and not seen around gassy planets.

“We can say that these planets are rocky,” de Wit said. So the next question is, he said, what is the makeup of their atmospheres? “The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere. The next step is to try to disentangle all these possible scenarios that exist for these terrestrial planets.”

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