Archive for ultracool dwarfs

Seven temperate rocky planets orbiting the red dwarf TRAPPIST-1

Posted in astronomy with tags , , on February 23, 2017 by Tim Kendall

pia21421The TRAPPIST-1 star, an ultracool red dwarf located 12 parsecs from the Sun, has seven Earth-size planets orbiting it. This artist’s concept appeared on the cover of the journal Nature on Feb. 23, 2017.  It shows the expected physical state of water at the probable temperature of each planet: (courtesy: NASA)

This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory’s Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.The new results were published Wednesday in the journal Nature, and announced at a news briefing at NASA Headquarters in Washington. Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated. Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated – scientists believe it could be an icy, “snowball-like” world, but further observations are needed. “The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey at the University of Liege, Belgium. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”

The paper in Nature is available to subscribers: Gillon, M. et al. Nature http://dx.doi.org/10.1038/nature21360 (2017) and a summary at Nature News here. The planets have radii between about three-quarters that of Earth up to 1.13 times that of Earth, and their derived masses yield densities ranging between 0.6 and 1.2 times that of Earth (image courtesy NASA):

This chart shows, on the top row, artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.

Update: more accurate and precise eccentricities are found, < 0.02 for the six innermost planets, together with a more constrained mass for the seventh, h, in a new preprint by S. Wang et al., 2017 Apr 17. Overall, derived masses for the outer planets d, e, f and g decrease compared to the estimations from the discovery paper.

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