‘Really Clear Signals’ -TESS Discovery of a Nearby Star System Harboring an Earth-Sized Planet

TESS Exoplanet


“There was quite some detective work involved, and the right people were there at the right time,” said Diana Dragomir,” an exoplanetologist at the University of New Mexico about exoplanets discovered in 2019 that could sustain life using its advanced Transiting Exoplanet Survey Satellite (TESS). “But we were lucky,’ she noted, “and we caught the signals, and they were really clear.” The planetary system called L 98-59 is  located around 35 light-years away from Earth and hosts five new worlds that have characteristics that are also found on planets orbiting the sun. 

The First of Many to Follow?

“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. It sets the path for finding smaller planets around even smaller stars, and those planets may potentially be habitable.”

The nearby system hosts the first Earth-sized planet as well as a warm sub-Neptune-sized world, according to a paper from a team of astronomers that included Carnegie’s Johanna Teske.

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“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.”

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Dragomir wrote in an email to The Daily Galaxy: “The TESS primary mission’s goal was to find hundreds of exoplanets smaller than Neptune around bright stars, but a subset of the exoplanet research community is after an even more ambitious goal: discovering the 40 or so predicted Earth-sized (or smaller!) transiting planets, half of which are expected to orbit M dwarfs, which are stars about half the size of the Sun or smaller. 

“So how has TESS fared so far?” There are already twelve published planets smaller than 1.25 Earth sizes (and at least four more on the way), of which ten orbit M dwarfs,” writes Dragomir. “This last part is not too surprising, as it is much easier to detect small transiting planets around smaller stars than around larger stars; so those are the ones first found with TESS and subsequently validated. Of those ten planets, one is believed to have a temperature amenable to supporting liquid water, which is in line with the 1-3 that are expected.

Mining the TESS Data

“Astronomers continue to scour the TESS data,” Dragomir continued, “so it remains to be seen how well our predictions for the number of Earth-sized planets line up with actual discoveries. Their frequency, whatever we may find it to be, will be vital for planning future exoplanet searches aimed at better detecting and studying these small planets. But for now, TESS has already given us several that are ripe for study, most excitingly with the upcoming JWST which promises to offer, for the first time, a look into the atmospheres of Earth-sized planets.”

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Planet Finder Spectrograph on the Magellan II Telescope

The Planet Finder Spectrographn (PFS) on the Magellan II telescope at Carnegie’s Las Campanas Observatory in Chile, was a crucial component of 2019 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.

Detects tiny wobbles that the planet’s gravity induces 

This method takes advantage of the 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

“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 characterizing the planets found by the TESS mission.”

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The radius of a transiting planet is measured from the depth of its transit in the TESS light curve. An Earth-sized planet passing in front of a Sun-like star causes a 0.01% dip in brightness. Meanwhile, the planet mass is deduced by how much the planet causes the host star to wobble in the PFS data. Together, the density of the planet can be directly measured, which sheds insight into the planet’s chemical composition and thickness of its atmosphere.

With an orbit that takes about 36 days to complete, the sub-Neptune planet, 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.

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A substantial atmosphere 

Its host star has about 80 percent of the mass of our Sun and is found about 53 light-years distant from Earth. HD 21749b has about 23 times Earth’s mass and a radius of about 2.7 times Earth’s. Its density indicates the planet has a substantial atmosphere but is not rocky, so it could potentially help astronomers understand the composition and evolution of cooler sub-Neptune planet atmospheres.

Excitingly, the longer period sub-Neptune planet in this system is not alone. It has a sibling planet, HD 21749c, which takes about eight days to orbit the host star and is much smaller—similar in size to Earth.

Image credit top of page: with thanks to atramateria.com

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Diana Dragomir and Carnegie Institute for Science

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