The human desire to find a world that mirrors our own fuels much of modern astronomy. Historically, the question of which planet most resembles Earth was confined to the handful of worlds in our own solar system. This search has dramatically expanded with the discovery of thousands of exoplanets, or planets orbiting stars beyond the Sun. Determining a planet’s Earth-likeness involves complex calculations that go far beyond a simple visual comparison. True resemblance is defined by factors like a world’s size, atmospheric composition, and potential to host liquid water.
Defining Planetary Similarity
The scientific community uses two primary concepts to quantify how much a planet resembles Earth. The first is the Habitable Zone, often termed the Goldilocks Zone, which identifies the region around a star where temperatures are theoretically right for liquid water to exist on a planet’s surface. Liquid water is considered a prerequisite for life as we know it, making this zone a fundamental filter for the search. However, simply being in this zone does not guarantee suitability for life, as a planet’s geology and atmosphere also determine its surface temperature.
The second, more quantitative metric is the Earth Similarity Index (ESI), which provides a comparative scale from 0 to 1, where Earth scores a perfect 1.0. This index combines several planetary properties, including the planet’s radius, its bulk density, its escape velocity, and its surface temperature. A higher ESI score indicates a greater physical resemblance to Earth, particularly in terms of rocky composition and the ability to retain an atmosphere. A high ESI only suggests physical likeness; it does not automatically confirm habitability, as the metric does not account for a star’s activity or the planet’s magnetic field.
Comparisons Within Our Solar System
Within our immediate cosmic neighborhood, the two terrestrial planets closest to Earth in size and orbital position are Venus and Mars. Venus is often called Earth’s twin due to its nearly identical size and mass, but its current surface conditions make it an uninhabitable inferno. Its atmosphere is composed almost entirely of carbon dioxide, which created a runaway greenhouse effect that traps heat. This results in surface temperatures of about 462 degrees Celsius and gives Venus a low ESI of approximately 0.44.
Mars offers a different form of resemblance, holding the highest ESI in our solar system besides Earth, at roughly 0.70. The planet possesses a rotational period and axial tilt similar to Earth’s, which results in comparable day-night cycles and seasons. However, Mars is significantly smaller, with a diameter about half of Earth’s and only 11% of its mass, leading to much lower surface gravity. Crucially, the Martian atmosphere is extremely thin, with a surface pressure less than one percent of Earth’s, and fails to create a warming greenhouse effect. This thin atmosphere and low average temperature of about -63 degrees Celsius mean liquid water cannot exist stably on the surface today.
Leading Exoplanet Candidates
The most promising Earth analogs are found outside our solar system, with a number of confirmed rocky exoplanets scoring high on the ESI scale. One of the earliest and most celebrated discoveries was Kepler-186f, which orbits a red dwarf star about 500 light-years away. Discovered in 2014, it was the first confirmed Earth-sized planet found within its star’s habitable zone, though its ESI is a modest 0.58. Because its host star is cooler and dimmer than the Sun, Kepler-186f receives only about one-third of the light energy that Earth does, placing it near the outer, colder edge of the habitable zone.
A compelling multi-planet system is TRAPPIST-1, which hosts seven terrestrial planets orbiting an ultra-cool dwarf star. Of these, TRAPPIST-1e stands out with a remarkably high ESI of approximately 0.95, placing it among the most physically similar worlds known. The planet is slightly smaller and less massive than Earth, and its calculated density suggests a solid, rocky composition. However, its close orbit around a dim star means it completes a year in just six Earth days and is likely tidally locked, creating potential climate challenges.
Proxima Centauri b is another relevant candidate because it is the closest exoplanet to Earth, orbiting our nearest stellar neighbor, Proxima Centauri, just 4.2 light-years away. This world has an ESI of around 0.87 and a minimum mass only slightly greater than Earth’s, orbiting well within its star’s habitable zone. Like the TRAPPIST-1 planets, it orbits a red dwarf, which subjects it to intense stellar flares that could potentially strip away any atmosphere. Its extremely close orbit also suggests it is tidally locked, complicating the existence of a stable, Earth-like climate.
The Current Most Earth-Like World
Based purely on the physical metric of the Earth Similarity Index, the planet currently considered the most Earth-like among confirmed rocky exoplanets is Teegarden’s Star b. This world orbits a small, dim star only 12.5 light-years away and possesses an exceptionally high ESI score of 0.97. Its calculated size and insolation—the amount of energy it receives from its star—are extremely close to Earth’s values, making it a near-perfect match in terms of planetary statistics. However, it orbits an M-dwarf star, meaning the planet is likely tidally locked and potentially exposed to powerful stellar flares. The title of “most Earth-like” constantly evolves as new data refines our understanding and as powerful telescopes discover new candidates.