According to the Interactive Extra-Solar Planets Catalogue, humans have spotted some 4,878 planets in 3,604 solar systems other than our own. Termed exoplanets, only few have been found in the habitable zone around their parent star, and none have been confirmed to have life.

There are few things that are standard or typical across the universe, but the exoplanet GJ 367b still manages to stick out as an oddball in Earthling discoveries. Astronomers at the German Aerospace Center's Institute of Planetary Research, using data gathered by NASA's Transiting Exoplanet Survey Satellite (TESS), reported the find in a paper published in Science on Thursday.

© NASA/JPL-CaltechArtist’s rendering of TOI-1231 b, a Neptune-like planet about 90 light-years away from Earth.

For starters, GJ 367b is one of the smallest exoplanets ever spotted, just slightly larger than Mars and just 70% the size of Earth. It's also only about 55% the mass of Earth, making it one of the lightest spotted, too. However, its density is impressively high: according to the scientists, the exoplanet clocks in at an average of 8.106 grams per cubic centimeter. That's not just denser than the Earth, which has an average density of just 5.51 grams per cubic centimeter, but it's also denser than solid iron, which has a density of 7.874 grams per cubic centimeter at room temperature.
However, GJ 367b continues to impress. The tiny planet also orbits a tiny star, a red dwarf that never achieved the size and luminosity of stars like our Sun, so closely that it completes an orbit every eight hours. At that distance, GJ 367b is tidally locked, meaning the same side always faces its star, and is constantly blasted with radiation, according to
GJ 367b's star is relatively close, too, sitting just 31 light-years away.

"From the precise determination of its radius and mass, GJ 367b is classified as a rocky planet," Dr. Kristine Lam, the study's lead author, said in a statement.

"It seems to have similarities to Mercury. This places it among the sub-Earth sized terrestrial planets and brings research one step forward in the search for a 'second Earth,'" she added.
Lam separately told that little is known about how such an odd planet could have formed, but noted that "if the planet is a remnant core of a former gaseous planet, then the gaseous planet should be no bigger than a Neptune-sized planet.
"However, it isn't the first such planet to be spotted, either: an exoplanet designated Kepler 107c, orbiting a star 1,700 light-years away, was found in 2019 to have an even higher density of 12.65 grams per cubic centimeter and is about 1.5 times as wide as the Earth. In its case, however, scientists believe it to be the remnant core of an even larger planet that survived some kind of cataclysmic collision.
The scientists spotted GJ 367b using TESS, a satellite launched in 2018 that took up the mission of the old Kepler space telescope, the first specialized for spotting exoplanets. They then further scrutinized the planet using the High Accuracy Radial Velocity Planet Searcher (HARPS) at the European Southern Observatory's 3.6-meter telescope in Chile.
TESS uses the same method as Kepler in that it looks for stars that "wobble" or regularly dim and brighten - possible signs they have exoplanets orbiting them, which by calculating the star's properties, can then have their own properties largely derived. However, TESS can process 400 times the amount of sky as Kepler, making it a far more effective tool in the search for planets outside our solar system.