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Three Earth-Sized Worlds Orbiting Low-Mass Star Discovered

Friday, September 30, 2016 14:17
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The discovery of three Earth-sized planets likely orbiting a low-mass star is looking like the real thing.

Astronomers combined the power of the 8-meter Gemini South telescope in Chile with an extremely high-resolution camera to scrutinize the star TRAPPIST-1. Previous observations of the star, which is only about 8% the mass of our Sun, revealed dips in the star’s light output that would be expected if several Earth-sized planets orbited the star. However, the situation would be greatly complicated if, upon closer examination, the star was found to have a yet-unseen stellar companion.

No such companion was found with Gemini, which essentially seals the case for multiple Earth-sized planets orbiting TRAPPIST-1.

Artist’s concept of what the view might be like from inside the TRAPPIST-1 exoplanetary system showing three Earth-sized planets in orbit around the low-mass star. This alien planetary system is located only 40 light years away. Gemini South telescope imaging, the highest resolution images ever taken of the star, revealed no additional stellar companions providing strong evidence that three small, probably rocky planets orbit this star. 

Credit: Robert Hurt/JPL/Caltech.
Steve Howell of NASA’s Ames Research Center led the extremely high-resolution observations using the Differential Speckle Survey Instrument (DSSI), an instrument he has used before at both Gemini telescopes to probe other exoplanetary systems. The new observations reinforced the hypothesis that several Earth-sized planets are responsible for the fluctuations in the star’s brightness. 
“By finding no additional stellar companions in the star’s vicinity we confirm that a family of smallish planets orbit this star,” says Howell. “Using Gemini we can see closer to this star than the orbit of Mercury to our Sun. Gemini with DSSI is unique in being able to do this, bar none.”

The research, led by Howell, is published in the September 13th issue of The Astrophysical Journal Letters.

TRAPPIST-1 is what astronomers call a late M-type star; stars which are small, ultra-cool (compared to most stars), and faint. Late M stars are so faint that the only specimens we can observe are relatively close-by in space and, as the Gemini observations demonstrate, allow astronomers to probe very close to these stars in the search for companions.

“While no current telescope can actually image an Earth-size planet around another star, even if orbiting a nearby star such as TRAPPIST-1, our instrument on Gemini allows us to detect close companion stars and even brown dwarfs.” says Elliott Horch, [Southern Connecticut State University] co-author of the paper. “Such observations validate not only the existence of exoplanets, but their small size as well.”

M stars are of great interest to astronomers today as their diminutive size allows easier detection of small, Earth-size planets. The intrinsic faintness of M stars means that potentially habitable planets will have short orbital periods, on the order of weeks. Such planets will be the targets of detailed study by both ground- and space-based telescopes, studies that will attempt to measure the composition of their atmospheres and see if they are indeed Earth-like beyond just their size.

The discovery of TRAPPIST-1’s likely exoplanet pedigree began late in 2015 with data from the TRAPPIST (the TRansiting Planets and PlanetesImals Small Telescope) project. This work, published in the 12 May 2016 issue of the journal Nature, and led by Michael Gillon, observed TRAPPIST-1 over 62 nights. During that period, the star was found to fluctuate in a manner that is consistent with at least three Earth-sized planets orbiting and periodically eclipsing and blocking part of the star’s light from our view on the Earth. 

While work is still ongoing to refine the total number of planets, two of them appear to orbit in 1.5 and 2.4 days and are so close that they receive four and two times the radiation that our Earth receives from the Sun, respectively. The third planet is more difficult to characterize, having possible orbital periods between 4 to 73 days. However, this third planet’s most likely period, 18 days, would place this world well within the system’s habitable-zone where liquid water could exist on its surface.

The Gemini observations, made with the DSSI instrument, were made during a temporary visit of the instrument at the Gemini South telescope in Chile. “Gemini’s flourishing Visitor Instrument program is producing superb results in all areas of astronomy,” said Chris Davis, a program director at the U.S. National Science Foundation, one of the agencies that funds the International Gemini Observatory and which also provided initial funding for DSSI. “The DSSI observations of the TRAPPIST-1 system of exoplanets is just one example. The instrument team and their collaborators deserve credit for building such a versatile and productive instrument and also for making it available to all of Gemini’s users.”

The DSSI instrument on Gemini provides a unique capability to characterize the environment around exoplanetary systems. The instrument provides extreme-resolution images by taking multiple extremely short (60 millisecond) exposures of a star to capture fine detail and “freeze” the turbulence caused by the Earth’s atmosphere. By combining the images and removing the momentary distortions caused by the Earth’s atmosphere, the final images yield a resolution equal to what the telescope would produce if it was in space. With this technique, called speckle interferometry, astronomers can see details at, or very near, the theoretical limit of the 8-meter Gemini mirror yielding the highest-resolution single telescope images available to astronomers. The available resolution is like being able to separate an automobile’s two headlights at a distance of about 2000 miles.

Contacts and sources:
Dr. Steve B. Howell, NASA Ames Research Center
Dr. Elliott P. Horch, Professor of Physics, Southern Connecticut State University
Peter Michaud, Gemini Observatory


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