Human curiosity has long driven the search for an “Earth 2.0.” Modern astronomy has transformed this question into a scientific search among the billions of planets in our galaxy. The discovery of thousands of exoplanets—planets orbiting stars beyond our Sun—has shown that small, rocky worlds are common, but true twins remain elusive. This search requires specialized metrics and a deep understanding of the complex, interlocking factors that make a planet habitable over billions of years. Identifying the most similar planet requires moving past mere size and distance to analyze the intricate physics and chemistry that allow life to flourish.
How Scientists Measure Earth Similarity
The initial step in this cosmic comparison is to establish a standardized framework for planetary characteristics. Scientists use the Earth Similarity Index (ESI), a quantitative measure designed to rank celestial bodies on a scale from zero to one, with Earth scoring a perfect one. The ESI combines a planet’s physical properties, using parameters like its radius, bulk density, escape velocity, and estimated surface temperature. Planets with an ESI above 0.8 are considered the most Earth-like, though the index describes physical resemblance rather than guaranteed habitability.
A second metric is a planet’s location within the Habitable Zone, often called the Goldilocks Zone. This is the orbital region around a star where a planet’s surface temperature could allow for the presence of liquid water, a solvent necessary for life as we know it. The width and distance of this zone depend entirely on the host star’s size and luminosity. However, orbiting within this zone is not enough, as a planet’s atmosphere and internal dynamics ultimately determine whether water can remain liquid over long stretches of time.
Close, But Not Quite: The Solar System Analogues
Our own solar system provides two striking examples of planets that are physically similar to Earth but failed the long-term habitability test: Venus and Mars. Venus is nearly Earth’s twin in size and mass, but its proximity to the Sun led to a runaway greenhouse effect. Increasing temperatures vaporized surface water, and the resulting water vapor acted as a powerful greenhouse gas, trapping heat in a catastrophic feedback loop. This process, combined with ultraviolet light breaking down the water molecules and hydrogen escaping to space, left behind a searing world with a crushing, carbon dioxide-rich atmosphere.
Mars, conversely, followed a path into the deep freeze, primarily due to its smaller size and the subsequent cooling of its core. This cooling caused the planet’s internal dynamo to shut down, eliminating its global magnetic field. Without a magnetic shield, the solar wind gradually stripped away the Martian atmosphere, leaving it wispy and thin. The loss of atmospheric pressure caused any surface liquid water to either freeze into permafrost or sublimate directly into gas, transforming the once potentially wet world into the cold, arid environment seen today.
The Top Contenders for “Earth 2.0”
The most Earth-like planets discovered to date are exoplanets, and they consistently score the highest on the ESI. One of the most promising candidates is TRAPPIST-1e, which orbits a small, ultra-cool dwarf star about 40 light-years away. This planet is almost perfectly Earth-sized, with a radius about 92% of Earth’s, and has a density that strongly suggests a rocky, terrestrial composition. Its ESI is estimated around 0.95, placing it firmly in the conservative habitable zone of its star system.
Another frontrunner is Proxima Centauri b, the closest known exoplanet to Earth, orbiting our nearest stellar neighbor just over four light-years away. This world has an estimated minimum mass about 1.06 times that of Earth and orbits within its star’s habitable zone, giving it a high ESI of approximately 0.87. However, its host star is a flare star, which frequently erupts with intense radiation that could potentially strip away the planet’s atmosphere, making its habitability uncertain.
Kepler-186f was the first Earth-sized planet found in the habitable zone of another star, discovered by the Kepler Space Telescope. It has a radius about 1.11 times that of Earth, making it slightly larger but still firmly in the rocky planet category. Kepler-186f orbits an M-dwarf star, and its orbit places it near the outer edge of its habitable zone. It receives only about one-third of the stellar energy Earth receives from the Sun. This lower energy input gives it a moderate ESI of about 0.58, suggesting that only a strong greenhouse atmosphere could keep water from freezing.
The Essential Ingredients for Sustained Life
While the ESI and placement in the Habitable Zone identify worlds with the right static properties, sustaining life requires dynamic planetary processes. A global magnetic field, generated by the motion of molten metal in a planet’s core, is a prerequisite for long-term habitability. This magnetic shield deflects high-energy charged particles from the host star’s stellar wind, preventing the gradual erosion and loss of the atmosphere to space. Without this protection, a planet can quickly devolve into a barren world, much like Mars.
A second factor is the presence of plate tectonics, which describes the movement of a planet’s outer crust. On Earth, this geological process serves as a planetary thermostat by regulating the global carbon cycle over vast timescales. Plate tectonics recycles carbon from the atmosphere into the mantle through the subduction of ocean crust, preventing a runaway greenhouse effect like the one that occurred on Venus. This continuous, slow-motion cycling of materials is necessary to maintain a stable, life-supporting climate for billions of years.