TESS finds its first Earth-sized planet

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This is an artist’s conception of HD 21749c. Illustration by Robin Dienel

A NEARBY system hosts the first Earth-sized planet discovered by NASA’s Transiting Exoplanets Survey Satellite, as well as a warm sub-Neptune-sized world, according to a new paper from a team of astronomers that includes Carnegie’s Johanna Teske, Paul Butler, Steve Shectman, Jeff Crane, and Sharon Wang.

Their work is published in the Astrophysical Journal Letters.

“It’s so exciting that TESS, which launched just about a year ago, is already a game-changer in the planet-hunting business,” said Teske, who is 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.”

One such tool, the Planet Finder Spectrograph on the Magellan II telescope at Carnegie’s Las Campanas Observatory in Chile, was a crucial component of this effort. It helped confirm the planetary nature of the TESS signal, and to measure the mass of the newly discovered sub-Neptune.

The PFS–built by Shectman and Crane using a method pioneered by Butler and his collaborators–works using a technique called the radial velocity method, which is currently the only way for astronomers to measure the masses of individual planets. Without known masses, it is very challenging to determine a planet’s density or its general chemical composition.

This method takes advantage of that fact that not only does a star’s gravity influence the planet orbiting it, but the planet’s gravity also affects the star in turn. The PFS enables astronomers to detect these tiny wobbles that the planet’s gravity induces in the star’s orbit.

“PFS is one of the only instruments in the Southern Hemisphere that can do these types of measurements,” Teske added.

“So, it will be a very important part of further characterising the planets found by the TESS mission.”

With an orbit that takes about 36 days to complete, the sub- Neptune, HD 21749b, has the longest period of any of the TESS discoveries published so far.

Because of the technique that TESS employs, it is predicted that most of the planets the mission finds will have orbital periods of fewer than 10 days, so HD 21749b is unusual in this regard. In fact, this also made the detection of the planet in the TESS data an extra challenge.

“There was quite some detective work involved, and the right people were there at the right time,” said lead author Diana Dragomir of MIT’s Kavli Institute for Astrophysics and Space Research.

“But we were lucky, and we caught the signals, and they were really clear. – Carnegie Institution for Science