An article published in “The Astrophysical Journal Letters” reports the discovery of two exoplanets in the orange star HD 21749’s system, one of which is the first Earth-sized identified thanks to NASA’s TESS space telescope and the other a Mini-Neptune. A team of researchers led by Diana Dragomir already submitted a first version of the article on the exoplanet HD 21749b, the Mini-Neptune also referred to as TOI 186.01, mentioning as candidate TOI 186.02 the rocky exoplanet now referred to as HD 21749c in the new version of the article in which it’s considered confirmed.

Launched on April 18, 2018, the TESS (Transiting Exoplanet Survey Satellite) space telescope took on the great legacy of planet hunter from the Kepler space telescope. Among the stars observed so far there’s HD 21749, of class K and therefore a so-called orange star with a mass of about 83% of the Sun’s and about 53 light years away from the Earth. Analyzing the collected data, the team led by Dr. Diana Dragomir of MIT’s Kavli Institute for Astrophysics and Space Research discovered two interesting exoplanets.

The first confirmed exoplanet was indicated initially as TOI 186.01, where the acronym TOI derives from the English expression TESS Object of Interest, and then as HD 21749b. Its star is shown in the bottom image (NASA/MIT/TESS) together with an artistic concept of the planet.

The exoplanet HD 21749b is a warm sub-Neptune, a gas exoplanet with a radius that was estimated at about 2.7 times the Earth’s for a mass estimated at about 23 times the Earth’s and a year lasting about 36 Earth’s days. Its density, comparable to water’s, indicates a really thick atmosphere, far more than that of Uranus and Neptune. This discovery is even more curious considering the fact that this planet must be wuite warm given its proximity to its star.

When Diana Dragomir’s team wrote the first version of the article there was another exoplanet candidate indicated as TOI 186.02. Meanwhile, the researchers performed further photometric examinations using TESS space telescope’s data and other observations to confirm the existence of the planet now referred to as HD 21749c. It’s a little smaller than the Earth, the first exoplanet of this type discovered thanks to TESS, for a mass that is about 80% of the Earth’s.

It was only a matter of time before the TESS space telescope discovered an Earth-sized planet, unfortunately the rest of the characteristics of HD 21749c are quite different as it’s very close to its star. Its year lasts less than 8 Earth days so even though its star is a little colder than the Sun, it could probably be called a super-Mercury because it must be very hot. The top image shows an artist’s concept of the exoplanet HD 21749c and its star (Image courtesy Carnegie Institution for Science).

Even if the exoplanet HD 21749c is a wasteland it was important to confirm the TESS space telescope’s ability to locate relatively small rocky planets. Each new discovery adds new information on this type of planets and offers new hopes of finding some potentially habitable for follow-up studies.