- TESS discovers its third exoplanet named HD 21749b.
- It orbits a dwarf star every 36 days, and has a surface temperature of about 150°C.
- It is the longest-period transiting exoplanet discovered so far within 100 light-years.
In April 2018, NASA launched a space telescope, named Transiting Exoplanet Survey Satellite (TESS) to find exoplanets (planets outside our solar system) using the transit method. The spacecraft looks for small dips in distant starlight, whenever a planet or other celestial body passes in front and temporally blocks a portion of the star.
It is the successor of another planet-hunting probe, Kepler, that was designed to find as many new planets as possible. However, TESS is specifically searching for exoplanets relatively close to us. These planets are then easier to study: they can be observed with various telescopes to determine their size, mass, and atmospheric conditions.
Recently, NASA has confirmed that TESS has discovered a new exoplanet, named HD 21749b, orbiting a dwarf star just 53-light years away, in the constellation Reticulum.
Although TESS has already found two exoplanets that are orbiting around their host stars within just hours and days, the third confirmed planet is quite more interesting. Let’s find out why.
A ‘Sub-Neptune’ Exoplanet
Just like other two exoplanets (discovered by TESS), HD 21749b is orbiting a star that is fainter and smaller than our Sun. However, it orbits quite farther away than the other two: the planet takes 36 days to complete one circle around its host star.
In contrast, the first confirmed exoplanet (Pi Mensae c) completes an orbit every 6 days, while the second one (LHS 3884b) takes only 11 hours.
The newly discovered planet is approximately 3 times the size of Earth and 23 times its mass. It has a much cooler surface temperature (150°C) than the other two exoplanets. And, its host star is almost 25 times fainter than what human eyes can see.
An artistic impression of TESS observing HD 21749b | Credit: Goddard Space Flight Center/NASA
HD 21749b is not a rocky planet, but it has a higher density than Neptune. There could be water but there is no hope of this planet being habitable.
Researchers weren’t able to detect the temperature below the surface, but since the exoplanet is not that far, it is feasible to study its internal environment in the future, possibly with NASA’s next-generation space telescope, James Webb, which is planned to be launched in 2021.
Overall, this is the longest-period transiting exoplanet (with coolest surface temperature) discovered so far within 100 light-years.
Image credit: NASA
What’s even more exciting is there could be another planet orbiting the same star. It could be as large as Earth, orbiting its host star every 7.8 days. It has already been spotted by TESS, and if confirmed, researchers could finally study an exoplanet that’s more like Earth (especially in terms of size).
TESS’s 2 Year Mission
Over its two-year mission, the probe will observe almost the entire sky, producing a rich catalog of exoplanets around nearby stars. To do this, it will split the sky into overlapping sectors and analyze each sector for 27 days at a time.
Although the primary objective is to find exoplanets, the probe will record several additional phenomena, such as asteroids, comets, supernova, and eclipsing binaries, while looking into each sector.
The proximity of discovered planets will enable scientists to characterize them through follow-up observations from ground- and space-based telescopes.