Three years on, TESS delivers on discoveries as extended mission continues

by Justin Davenport

Three years ago, on April 18, 2018, NASA’s and the Massachusetts Institute of Technology’s Transiting Exoplanet Survey Satellite, or TESS, launched successfully into a very high Earth orbit aboard the last new Block 4 Falcon 9 before the current human-rated Block 5 entered service.

Since then, it has dutifully observed the southern and northern skies, searching for tell-tale signs of nearby exoplanets crossing between their parent stars and the telescope’s cameras. Its primary mission now complete, TESS is in an extended mission phase that will last until 2022 — at which point it will be eligible for another mission extension provided its systems are still in good shape as intended.

In 2018, once the Falcon 9 placed the telescope on its correct, post-launch trajectory, TESS embarked on a journey to its science orbit over the following month, ending in a flyby of the Moon on May 17, which moved the spacecraft into a 13.7 day, 108,000 by 373,000 km elliptical orbit of Earth.

Specifically, TESS entered a P/2 resonant orbit (2:1 resonance) around the Moon, which minimizes gravitational interferences between the body and the telescope, allows for exposure to consistent temperatures for the spacecraft’s instruments, keeps it out of the Van Allen radiation belts, and gives the telescope an unobstructed view of the stars in both northern and southern hemispheres.

After a three-month checkout, TESS started its all-sky exoplanet survey, beginning with the southern hemisphere star field. In July 2019 TESS turned its eyes to the northern hemisphere, completing its primary mission one year later. 

Whereas the first space telescope designed for exoplanet observations, Kepler, focused on a far from Earth and narrow slice of sky in the Vega and Cygnus constellations for a statistical survey of exoplanets during its primary mission, TESS’s primary mission observed 85% of the sky using a wide-field telescope — with an emphasis on exoplanets orbiting bright stars within 300 light years of Earth. 

Like Kepler, TESS uses the transit method of detection to find exoplanets. The spacecraft holds a precise position and observes a field of stars for anywhere from 27 to 100 days depending on its location in the sky. Astronomers and computers on the ground process these observations and look for minute dips in the starlight signatures from each star, dimmings that could indicate an exoplanet.

Many of the exoplanets TESS has found to date are close enough to Earth to allow follow-up observations with telescopes in space and on the ground. These studies largely seek to both confirm the potential exoplanet as well as to begin the process of understanding its composition.

With its primary mission complete in mid-2020, TESS was approved for an extended mission until September 2022. Presently, the telescope is observing the ecliptic plane of the solar system as well as gaps in the sky not covered during its primary mission.

At the time TESS was launched, there were 3,717 known exoplanets, including 2,652 found by the Kepler telescope. As of April 2021, there are 4,375 known exoplanets, including 122 found and confirmed by TESS, with an additional 2,645 candidate TESS objects of interest, or TOIs, that need follow-up studies for confirmation. 

Some of those confirmed exoplanets include the first planet discovered by TESS: Pi Mensae c, a “hot Neptune” of 4.8 Earth masses orbiting a white F-class star in 6.3 days.

This planet is 60 light years from Earth, well placed for follow-up observations, and its parent star is brighter and more massive than the Sun as opposed to most exoplanet discoveries so far that have been found orbiting red M-class or orange K-class dwarf stars. 

Additional discoveries so far include an exoplanet 40 Earth masses but only three times as large, a gas giant that survived its host star’s evolution through red giant and white dwarf stages, a planet orbiting two stars (like Tatooine in Star Wars), eclipsing binaries, variable stars, brown dwarfs, and supernovae as well as the ability to create crude atmospheric maps of brown dwarfs (failed stars) and perform machine learning to classify variable stars.

In January 2020, NASA announced the TESS discovery of an Earth-sized world in the habitable zone of its star, where liquid water could exist on the surface of the planet if it had an atmosphere. The exoplanet, TOI-700 d, orbits its red dwarf star once every 37 days and is 20% larger than Earth. 

Approximately, 100 light years away, it is well placed for follow-up studies by the James Webb Space Telescope and other observatories.

A major non-exoplanet discovery came in 2020 from the star system TYC7037-89-1, which was found to have three pairs of stars orbiting each other, with all six stars eclipsing their partners as seen from Earth. 

The most-recent major discovery was announced in February 2021. Three “super-Earth” planets were found in TESS data to be orbiting close to a star that is 95% the mass of the Sun. The star, TOI-451, is approximately 400 light years from Earth and is only 120 million years old, placed in a stream of young stars called Pisces-Eridanus.

The exo-system is also well placed for follow-up observations and could reveal a great deal about young planetary systems and their initial evolution — information that can help us understand Earth’s and our solar system’s early evolution as well.

As TESS continues training its four wide-field cameras on the stars, its orbit will allow it to function with very minimal fuel usage, creating the possibility for decades of observations and discoveries.

During the spacecraft’s lifetime, it is expected to discover approximately 20,000 exoplanets.

(Lead image credit: NASA/MIT)

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