A hefty sub-Neptune

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.

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.)