Artist’s rendering of TESS observing a red dwarf star with orbiting planets.
Artist’s rendering of TESS observing a red dwarf star with orbiting planets, similar to one of its first two potential discoveries. Credit: NASA Goddard Space Flight Center

Astronomers announced last week the discoveries of the first two potential planets detected by NASA’s Transiting Exoplanet Survey Satellite (TESS), a new exoplanet-hunting telescope that launched earlier this year.

The two exoplanet candidates—called Pi Mensae c and LHS 3844 b—have many similarities. Each is slightly larger than Earth, orbits its star in a very short time, is much too hot to support life, and circles a southern hemisphere star that lies less than 60 light-years from Earth.

“It is rewarding to see years of work that the team—engineers, scientists, and support staff—poured into the dream of [TESS] become the reality of discovered planets,” Patricia Boyd, head of TESS’s Guest Investigator Program, said on Twitter.

Over its 2-year mission, scientists expect TESS to discover more than 20,000 new exoplanets, hundreds of which will likely be Earth sized. One coauthor of the Pi Mensae c discovery expressed excitement that TESS’s first discoveries exceed scientists’ expectations:

“This is just the beginning,” Boyd said. “We can’t wait to see what’s next.”

Planets Across the Sky

NASA launched TESS on 18 April 2018, and the telescope began its science operations on 25 July. Like its predecessor, the Kepler Space Telescope, TESS looks for signs that a planet transits its host star, temporarily and repeatedly blocking a small fraction of the star’s light from the telescope’s gaze.

The video below explains how TESS will scan nearly the entire sky for exoplanets and what types of planets it will discover.

YouTube video

A Southern Sky Super-Earth

[pullquote float=”right”]The super-Earth Pi Mensae c orbits a Sun-like star that is a mere 60 light-years away.[/pullquote]TESS’s first potential exoplanet is Pi Mensae c, which astronomers estimated to be approximately twice the size and 4.5 times the mass of Earth. This planet orbits a star that is only slightly more massive and hotter than the Sun and is a mere 60 light-years away. A planet 10 times the mass of Jupiter, Pi Mensae b, was already known to orbit this star.

The super-Earth planet orbits its star in about 6.3 days, which translates to an orbital distance of about 7% of the separation between Earth and the Sun. Given Pi Mensae c’s close proximity to its star, the researchers estimate that the temperature on the surface of the planet would be a scorching 897°C.

This discovery was announced in a preprint of a paper submitted to the Astrophysical Journal Letters on 16 September. A second team independently announced the same exoplanet in a preprint paper a few days later.

The independent findings came as good news to the exoplanet community:

A Red Dwarf Satellite

The second announced exoplanet candidate, LHS 3844 b, is a 1.32-Earth-radius planet that orbits a cool red dwarf star also in the southern sky. The star is about 19% the size and 15% the mass of the Sun and is about half as hot.

Despite LHS 3844 b’s orbital period of 11 hours, the dim red star places the planet’s surface temperature at 532°C. Red dwarf stars are the most common type of star in the galaxy and also will be the most common type of star that TESS will monitor. This exoplanet was announced in a preprint of a paper submitted to the Astrophysical Journal Letters on 19 September.

At only 49 light-years away, LHS 3488 b is one of the closest known exoplanets:

First Two of Many

Each team cross-checked its TESS discovery using transit observations from ground-based telescopes and also gathered spectroscopic observations of the stars to measure the planets’ mass. The exoplanet candidates must be validated through peer review before becoming confirmed detections.

Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, called TESS’s first two discoveries “exciting” and said on Twitter that these “will be the first of many planet candidates discovered by the spacecraft. Over the next few years, it will continue to search new parts of the sky for worlds that orbit stars outside our solar system.”

[pullquote float=”right”]“We do know that planets are out there, littering the night sky, just waiting to be found.”[/pullquote]These two planets were detected using the first round of preliminary science data from TESS, which were made available to scientists on 5 September. These data have not yet been peer reviewed or vetted to remove false-positive signals that might mimic that of a planet.

The discovery teams noted that the exoplanets’ close proximity to their host stars mean that neither is likely to have an atmosphere. However, each planet is an ideal target for future observations, for example, with the oft-delayed James Webb Space Telescope, that may seek to uncover any traces of atmosphere that may exist.

“The team is excited about what TESS might discover next,” TESS deputy director of science Sara Seager said via Twitter. “We do know that planets are out there, littering the night sky, just waiting to be found.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2018), New exoplanet telescope detects its first two planets, Eos, 99, Published on 25 September 2018.

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