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What can Earth-sized exoplanets teach scientists about the formation and evolution of exoplanets throughout the cosmos? This is what a recently posted to the arXiv preprint server hopes to address as an international team of researchers announced the discovery of an Earth-sized exoplanet that exhibits temperatures and a density comparable to those of Earth.

This study has the potential to help scientists better understand the formation and evolution of Earth-sized exoplanets and what this could mean for finding life beyond Earth.

For the study, the researchers used the University of Chicago's to analyze the recently discovered GJ 12 b (aka Gliese 12 b), which is located approximately 40 light-years from Earth and orbiting an inactive M dwarf star, meaning its solar activity is reduced compared to other stars.

GJ 12 b was discovered by NASA's Transiting Exoplanet Survey Satellite (TESS) in 2024 and its temperature and radius were measured at approximately 300 K (26.85°C/80.33°F) and 0.96 Earths, respectively.

Now, researchers used MAROON-X to measure GJ 12 b's mass and eccentricity at approximately 0.71 Earths and 0.16, respectively. For context, Earth's eccentricity is 0.0167, with eccentricity being measured from 0 to 1 as a perfect circle and completely elongated, respectively. Knowing GJ 12 b's mass enabled the researchers to estimate its density at approximately equal to or slightly less than Earth. The researchers note that NASA's James Webb Space Telescope (JWST) could be used to conduct more in-depth observations of GJ 12 b, specifically its atmosphere.

"GJ12b appears to possess an Earth-like or sub-Earth density," the study notes. "A low density could be explained if the planet has a volatile component (such as water mixed within the mantle), an atmosphere, a low iron mass fraction, or some combination of these factors. Its low mass (and thus low surface gravity) may result in it having a diffuse atmosphere that would be highly amenable to JWST observations, but it is also possible that the planet has too low a mass to retain an atmosphere long-term unless it is replenished regularly by tidally induced volcanism."

The researchers compared the planetary and system characteristics of GJ 12 b and its M dwarf to the , the latter of which contains seven Earth-sized exoplanets orbiting an M dwarf star. While GJ 12 b is the only confirmed exoplanet in its system, it orbits within the interior edge of its star's habitable zone, whereas three of the seven Earth-sized exoplanets within the TRAPPIST-1 system orbit within their star's habitable zone.

The reason for the comparison to the TRAPPIST-1 system is that the researchers hypothesize that studying GJ 12 b could prove promising due to its inactivity, whereas the TRAPPIST-1 star's activity makes studying the atmospheres of its exoplanets difficult.

As noted, this study was conducted with the University of Chicago's MAROON-X instrument, which is designed to study Earth-sized exoplanets orbiting within the habitable zones of M dwarf stars and is installed on the International Gemini Observatory. MAROON-X uses the radial velocity method, which measures wobbles that occur between the exoplanet and its star, with the latter frequently being used to estimate planetary characteristics like mass and radius and orbital characteristics.

Most recently, MAROON-X was used to discover , which is located approximately 6 light-years from Earth.

As the number of confirmed exoplanets approaches 6,000, Earth-sized exoplanets like GJ 12 b are helping astronomers better understand the formation and evolution of exoplanets orbiting M dwarf stars, the latter of which are smaller than our sun.