TESS discovers a dense sub-Neptune exoplanet, plus an Earth-sized world to boot

Two intriguing and newfound planets orbit a small star some 50 light-years away. One is denser than diamond, and the other is the first Earth-sized TESS planet discovered to date.
By | Published: January 10, 2019 | Last updated on May 18, 2023
The discovery and confirmation of the sub-Neptune exoplanet HD 21749b (middle) makes it the third small planet discovered by TESS so far, and many more are likely on the way.
Within just a few months of beginning its primary science mission, NASA’s planet-hunting Transiting Exoplanet Survey Satellite (TESS) already has identified over 50 exoplanet candidates. Now, TESS has discovered and confirmed one of the densest sub-Neptune planets found so far: HD 21749b.

As an added bonus, the newly confirmed world just so happens to be the longest-period confirmed TESS planet discovered to date. Furthermore, it also seems to have a planetary sibling that is surprisingly similar to Earth.

A hefty sub-Neptune

Located about 52 light-years from Earth in the direction of the Southern Hemisphere constellation Reticulum, HD 21749b is a strange world that orbits a bright, orange dwarf star once every 36 days. 

Like many exoplanets discovered so far, HD 21749b sits very close to its star. Specifically, it orbits at a distance of just 0.2 AU (1 AU is the Earth-Sun distance of 93 million miles [150 million km]), so the newfound planet sits twice as close to its host star as Mercury is to the Sun.

Though HD 21749b is only about 3 times the size of Earth, it’s a whopping 23 times as massive. This means that the world has an overall density of 5.7 grams per cubic centimeter. (For reference, that’s significantly denser than diamond and titanium, and about half the density of lead; however, with an average density of 5.5 g/cm3, Earth is also pretty dense.)

By utilizing just three months of data from TESS, researchers have discovered the small, yet massive planet HD 21749b, which orbits a rather Sun-like star.
Importantly, according to the research paper, which is available on the pre-print website arXiv.org, “the density of HD 21749b indicates it is likely surrounded by a substantial atmosphere.” Unfortunately, the author’s also point out the HD 21749b likely is not an ideal target for future atmospheric follow-ups with the upcoming James Webb Space Telescope (JWST).

Finally, HD 21749b is also pretty cool. The world sports temperatures that hover around 300 degrees Fahrenheit (150 Celsius), which is just a bit too hot for liquid water to exist on its surface.

A boiling Earth-sized world

In addition to finding HD 21749b, TESS also recently discovered an Earth-sized world orbiting the same system’s host star. However, since the planet has yet to be confirmed, it still holds the moniker TOI 186.02, which stands for “TESS Object of Interest 186.02.”

At roughly 80 percent the mass and 90 percent the size of Earth, if confirmed, TOI 186.02 would be the very first Earth-sized planet discovered by TESS.

However, just because a planet is about the same size and mass as Earth, it doesn’t mean it’s truly a fitting Earth analog. For example, TOI 186.02 has a very short orbital period of just 7.8 days and little is known about its composition or potential atmosphere. Furthermore, the world experiences temperatures of around 800 degrees Fahrenheit (430 Celsius), making it more akin to Venus than Earth.

No matter whether TOI 186.02 is truly a fitting Earth-like exoplanet or not, these new discoveries show that the multi-planet system HD 21749 is ripe for future study and exploration, so be sure to stay tuned.

The newly discovered planets were announced during this week’s 233rd annual meeting of the American Astronomical Society, held in Seattle, Washington.