Jun 25, 2019 | Updated: 07:39 AM EDT

TESS Discovers the First Earth-Sized Planet and a Warm Sub-Neptune-Size World

Apr 15, 2019 02:10 PM EDT

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HD 21749c (IMAGE)
(Photo : Illustration by Robin Dienel, courtesy of the Carnegie Institution for Science) 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.

The Transmitting Exoplanets Survey Satellite of NASA just discovered a nearby system that hosts the first Earth-size planet and also a warm sub-Neptune-size world. The team of astronomers that presented the paper in the Astrophysical Journal Letters include Carnegie's Johanna Teske, Paul Butler, Steve Shectman, Jeff Crane, and Sharon Wang.

The second author on the paper, Teske, said that it is so exciting that barely a year ago that they launched TESS, it is already a game-changer in the planet-hunting business. Teske stated further that the spacecraft surveys the sky and they collaborated with the TESS follow-up community to flag potentially attractive targets for additional observations with the use of ground-based telescopes and instruments.

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

Shectman and Crane built the PFS using a technique pioneered by Butler and his collaborators. It works with the use of a method called the radial velocity method, which currently is the only way for astronomers to measure the masses of different planets. It is challenging to determine a planet's density or its overall chemical composition without known masses.

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

Teske added that the only instrument in the Southern Hemisphere that can do these types of measurements is PFS. As a result, it will be an essential part of further characterizing the planets found by the TESS mission.

Since an orbit takes about 36 days to complete, the sub-Neptune, HD 21749b, has the most extended period of any of the TESS discoveries published so far. Due to the method that TESS uses, there is a prediction that most of the planets on a mission find will have orbital periods of fewer than ten days, so HD 21749b is unusual in this regard and it has made the detection of the planet in the TESS data an extra challenge.

The MIT's Kavli Institute for Astrophysics and Space Research and also the lead author, Diana Dragomir said that there was indeed some detective work involved, and the right individuals were there at the right time. But they were lucky, and they caught the signals.

Dragomir concluded that for the stars that are quite close by and very bright, they anticipated the discovery of a couple of dozen Earth-sized planets. 

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