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Planet-hunting satellite discovers its first Earth-sized planet

This is an artist’s conception of HD 21749c, the first Earth-sized planet found by NASA’s Transiting Exoplanets Survey Satellite (TESS), as well as its sibling, HD 21749b, a warm sub-Neptune-sized world. Illustration by Robin Dienel, courtesy of the Carnegie Institution for Science

NASA’s planet hunting satellite, the Transiting Exoplanets Survey Satellite (TESS), has made a new discovery in the depths of space. Last month, TESS discovered its first exoplanet. And now it has achieved another milestone, locating its first Earth-sized planet and a larger sibling planet.

The Earth-sized planet, HD 21749c, and its sibling orbit a star slightly smaller than our Sun which is located 53 light-years from Earth. HD 21749c is a rocky planet which orbits its star every eight days, meaning it moves close to its star and has high surface temperatures of up to 800 degrees Fahrenheit.

“For stars that are very close by and very bright, we expected to find up to a couple dozen Earth-sized planets,” lead author and TESS member Diana Dragomir, a postdoctoral researcher in Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research, said in a statement. “And here we are — this would be our first one, and it’s a milestone for TESS. It sets the path for finding smaller planets around even smaller stars, and those planets may potentially be habitable.”

The larger planet is classified as a warm sub-Neptune sized world as is has a mass about 23 times that of the Earth and a radius of about 2.7 times the Earth’s.  Named HD 21749b, it is unusual in that it takes 36 days to complete an orbit which is considerably more than the orbital period of 10 days that was expected for most planets that the mission would find.

In order to find the planets, TESS looked for a small dip in the amount of light given off by the host star, HD 21749, which occurred at regular intervals. That suggested that the light from the star was being blocked by the transition of planets between it and the Earth.

“It’s so exciting that TESS, which launched just about a year ago, is already a game-changer in the planet-hunting business,” Johanna Teske of the Carnegie Institution for Science, second author on the paper, said in a statement. “The spacecraft surveys the sky and we collaborate with the TESS follow-up community to flag potentially interesting targets for additional observations using ground-based telescopes and instruments.”

The findings are published in the journal Astrophysical Journal Letters.

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