NASA’s New Planet-Hunter Finds Its First Earth-Sized World And It’s ‘Potentially Habitable’

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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.

Robin Dienel/Carnegie Institution for Scienc

An Earth-sized world has been discovered in a star system 53 light years from our solar system just days after a study found that life could exist on four close exoplanets.

The first Earth-sized planet discovered using NASA’s Transiting Exoplanets Survey Satellite (TESS) was found orbiting a star along with a much larger, warm planet smaller than Neptune. “It’s a milestone for TESS,” said Diana Dragomir, an exoplanetologist at MIT Kavli Institute and lead author of “TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune”, published in Astrophysical Journal Letters. “It sets the path for finding smaller planets around even smaller stars, and those planets may potentially be habitable.” This is the 10th confirmed planet discovered by TESS.

What did the astronomers find?

Orbiting the K dwarf star HD 21749, the astronomers found HD 21749c, which takes about eight days to orbit the host star and is similar in size to Earth at 89% its diameter. A likely rocky world, it’s thought to have surface temperatures as high as 800°F /427°C. They also found HD 21749b, a warm sub-Neptune-sized world about 23 times Earth’s mass and a radius of about 2.7 times Earth’s, which orbits its host star every 36 days. That one was a bit of a surprise since TESS is not designed to detect planets that take longer than 10 days to orbit their star.

Illustration of NASA’s Transiting Exoplanet Survey Satellite.

NASA’s Goddard Space Flight Center

What is TESS?

Launched atop a SpaceX Falcon 9 rocket on April 18, 2018, TESS is a planet-hunting spacecraft designed to search for Earth-like planets. TESS will observe 400,000 stars across the whole sky to catch a glimpse of a planet transiting across the face of its star. TESS will also enable astronomers to measure the masses, atmospheric compositions, and other properties of many smaller exoplanets for the first time.

To find HD 21749c, astronomers detected 11 periodic dips in the star’s brightness to determine that the star’s light was being partially blocked by a planet about the size of Earth.

NASA’s next planet-hunter, the Transiting Exoplanet Survey Satellite (TESS), successfully launched on a SpaceX Falcon 9 on April 18, 2018. TESS will search for new worlds outside our solar system for further study.

NASA Television

Why is TESS a “game-changer?”

“It’s so exciting that TESS is already a game-changer in the planet-hunting business,” said Johanna Teske, a researcher at Carnegie Institution for Science, and second author on the paper. “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.” Several astronomers from the Carnegie Institution for Science were also involved in the paper, namely Paul Butler, Steve Shectman, Jeff Crane and Sharon Wang.

Part of Las Campanas Observatory after snowfall, with the Magellan telescopes at the right.

General Epitaph – Own work, Public Domain, Wikimedia

Which ground-based telescope was used?

TESS is designed to make a shortlist of targets for large space and ground-based telescopes to investigate further. In this case, Magellan II, a 6.5-meter telescope situated at Carnegie’s Las Campanas Observatory in Chile, was used to follow-up data from TESS. It hosts the Planet Finder Spectrograph (PFS), a tool built by Shectman and Crane using a method pioneered by Butler and his collaborators, which helped confirm that the signal from TESS were indeed planets. It was also used to measure the mass of the Neptune-like HD 21749b, something that wasn’t possible to do for the much smaller HD 21749c. At least, not yet.

The Transiting Exoplanet Survey Satellite (TESS) is NASA’s newest planet hunter.

NASA

How do you measure the mass of an exoplanet?

Wobbly orbits. The mass of a planet is important to help astronomers find out more about its density and chemical composition. The PFS tool uses the “radial velocity” technique, which detects not only the tiny wobbles in the orbit of a planet, caused by the gravity of the host star, but also how the planet’s gravity also affects the star’s orbit. “PFS is one of the only instruments in the Southern Hemisphere that can do these types of measurements,” said Teske added. “So, it will be a very important part of further characterizing the planets found by the TESS mission.”

What will the astronomers do next?

Potentially the first mass measurement of an Earth-size planet found by TESS. “Measuring the exact mass and composition of such a small planet will be challenging, but important for comparing HD 21749c to Earth,” said Wang. “Carnegie’s PFS team is continuing to collect data on this object with this goal in mind.”

Will TESS help find more Earth-sized planets?

Expected to last for at least two years, and as long as a decade, TESS is expected to find over 20,000 exoplanets, hopefully, many Earth-sized. “For stars that are very close by and very bright, we expected to find up to a couple dozen Earth-sized planets,” said Dragomir. “And here we are — this would be our first one, and it’s a milestone for TESS.”

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