MEarths

Mini-Neptunes, often referred to as "MEarths" in some informal contexts, are a category of exoplanets that occupy a transitional size range between Earth-like rocky planets and gas giants like Neptune. These planets typically have radii between 1.5 and 4 times that of Earth, placing them in a region of the size spectrum that is not well-represented in our own solar system. The term "MEarth" is not a widely standardized classification but is sometimes used colloquially to describe this subset of exoplanets, particularly those discovered by the *MEarth Project*, a ground-based observational program focused on detecting Earth-sized and slightly larger planets orbiting nearby red dwarf stars.

MEarths are of significant interest to astronomers because they may represent a missing link in planetary formation theories. Their atmospheres can vary widely, ranging from thick hydrogen-helium envelopes to hydrogen-dominated atmospheres with significant water vapor or even rocky worlds with minimal atmospheres. Some MEarths may possess conditions suitable for liquid water on their surfaces, depending on their distance from their host star and atmospheric composition. However, many are likely to be inhospitable, with high-pressure atmospheres or extreme surface temperatures.

The discovery of MEarths has been facilitated by advancements in transit photometry, a method that detects planets by measuring the dimming of a star’s light as a planet passes in front of it. Missions such as NASA’s *Kepler* and *TESS* have identified numerous candidates in this size range, while follow-up observations with telescopes like the *Hubble Space Telescope* and the upcoming *James Webb Space Telescope* aim to characterize their atmospheres. Understanding MEarths helps refine models of planetary formation and evolution, particularly in low-mass star systems where such planets are more common.

While not all MEarths are habitable, studying them provides critical insights into the diversity of planetary systems beyond our own. Their prevalence suggests that intermediate-sized planets may be a common outcome of planet formation processes, bridging the gap between terrestrial worlds and gas giants. Future research will likely focus on determining the atmospheric and surface properties of these planets to better assess their potential for habitability or the presence of biosignatures.