Within our own solar system, Venus and Mars are the most Earth-like planets by basic physical measures, though neither is remotely hospitable today. Beyond the solar system, a growing catalog of exoplanets includes rocky worlds in habitable zones that score surprisingly well on formal similarity indices. The honest answer, though, is that “most similar” depends heavily on which features you care about, and researchers have developed tools to make that comparison more rigorous than gut feeling.
Venus and Mars, the Obvious Solar System Contenders
Venus is sometimes called Earth’s twin. It is nearly the same size and mass, sits just one orbital slot closer to the Sun, and has a thick atmosphere with active volcanism. On paper, it looks promising. In reality, its surface temperature exceeds 450 °C under a crushing carbon dioxide atmosphere, the product of a runaway greenhouse effect that boiled away any water it once had. Venus tells us a cautionary tale about how a planet of the right size and composition can become utterly uninhabitable if atmospheric chemistry goes wrong.
Mars is the other familiar candidate. It is smaller than Earth and farther from the Sun, with a thin atmosphere that offers almost no insulation or radiation shielding. But Mars was not always this barren. Recent imaging by the Zhurong rover identified deposits along an extended traverse in Mars’s northern lowlands whose sediment characteristics are consistent with ancient ocean coastal deposits, with features that rule out wind-blown or volcanic origins favored elsewhere on Mars.1PubMed Central. Ancient ocean coastal deposits imaged on Mars The growing evidence for a northern ocean bolsters the picture of a Mars that, billions of years ago, had surface liquid water, a denser atmosphere, and possibly Earth-like conditions. What Mars lacks today is what makes Earth special: a thick atmosphere, an active magnetic field, and geological recycling processes that keep the climate stable over eons.
How Scientists Actually Score Similarity
Comparing planets by eye leads to endless debate, so astrobiologists developed a formal metric called the Earth Similarity Index (ESI). It ranges from 1, meaning identical to Earth, down to 0, meaning completely dissimilar. The ESI was designed as a first-pass screening tool based on a planet’s mass, radius, and temperature, using data that is available or potentially available for most discovered exoplanets.2PubMed. A two-tiered approach to assessing the habitability of exoplanets The index is further split into an interior component, which focuses on radius and density, and a surface component, which focuses on escape velocity and surface temperature.3arXiv. Similarity indexing of exoplanets in search for potential habitability: application to Mars-like worlds
One nuance worth knowing: the surface temperature weighting in the ESI formula is set much higher than the interior parameters, which means for gas giants, surface ESI dominates the score. But for rocky planets, the interior component actually plays a larger role in determining the global ESI.4arXiv. Similarity indexing of exoplanets in search for potential habitability: application to Mars-like worlds – Section: III.3 ESI ANALYSIS This matters because it means the index behaves differently depending on what kind of planet you are scoring. A rocky world with an Earth-like density and radius can score well even if its estimated surface temperature has some uncertainty, while a puffier sub-Neptune gets judged almost entirely on its temperature.
The ESI is useful but limited. It says nothing about whether a planet has an atmosphere, water, a magnetic field, or plate tectonics. A high ESI score makes a planet worth studying further. It does not mean the planet is habitable.
The Best Exoplanet Candidates So Far
Thousands of exoplanets have been confirmed, and a handful stand out as strikingly Earth-like in at least some respects. Each has strengths and serious question marks.
Kepler-452b was announced in 2015 and immediately dubbed “Earth’s older cousin.” It orbits a G2-type star, the same spectral class as our Sun, and has the longest orbital period of any small transiting exoplanet confirmed at the time. The host star is slightly larger and older than the Sun, with an estimated age of roughly 6 billion years. The planet’s radius is about 1.6 times Earth’s, and modeling suggests a 49% to 62% likelihood that it has a rocky composition.5The Astronomical Journal. DISCOVERY AND VALIDATION OF Kepler-452b: A 1.6 R⨁ SUPER EARTH EXOPLANET IN THE HABITABLE ZONE OF A G2 STAR The Sun-like host star is a big deal because it means the planet does not face the radiation and tidal locking challenges that plague worlds orbiting smaller, cooler stars. The uncertainty about whether Kepler-452b is actually rocky, however, is a significant caveat. At 1.6 Earth radii, it sits near the boundary where planets start acquiring thick gaseous envelopes rather than remaining solid.
TRAPPIST-1e orbits an M dwarf star roughly 40 light-years away and is one of seven roughly Earth-sized planets packed tightly around it. Among the seven, planet e has attracted the most attention. Climate modeling suggests it avoids a runaway greenhouse effect regardless of its reflectivity, which strengthens the case for habitability based on temperature alone. Researchers have found that Earth-like surface temperatures could be maintained with various atmospheric compositions, including modest levels of carbon dioxide mixed with nitrogen.6Astronomy & Astrophysics. Tidal heating and the habitability of the TRAPPIST-1 exoplanets More recent work has probed how TRAPPIST-1e’s magnetosphere might respond to stellar space weather, including blasts analogous to coronal mass ejections, which could strip the atmosphere if the planet’s magnetic field is too weak or poorly oriented.7Astronomy & Astrophysics. Magnetohydrodynamic simulations preliminarily predict the habitability and radio emission of TRAPPIST-1e TRAPPIST-1e checks many boxes on size and temperature, but the M dwarf host introduces complications addressed below.
Proxima Centauri b is the nearest known exoplanet in a habitable zone, orbiting the closest star to our Sun at just over four light-years away. Its minimum mass is about 1.3 times Earth’s. Proximity makes it a tantalizing target for future study, but its environment is harsh. The planet currently receives roughly 30 times more extreme ultraviolet radiation than Earth and about 250 times more X-rays.8Astronomy & Astrophysics. The habitability of Proxima Centauri b More detailed three-dimensional modeling of the radiation environment has shown that the cosmic-ray intensities at Proxima Centauri b exceed those at Earth by close to an order of magnitude, worse than earlier, simpler models had suggested.9The Astrophysical Journal. On the Comprehensive 3D Modeling of the Radiation Environment of Proxima Centauri b: A New Constraint on Habitability? Whether any atmosphere could survive that bombardment over geological timescales is an open question.
Kepler-1649c is a smaller, quieter candidate. It is 1.06 times the size of Earth, orbits a mid-type M dwarf every 19.5 days, and receives about 74% of the light that Earth gets from the Sun. That gives it an equilibrium temperature of roughly 234 K, placing it firmly inside the habitable zone.10The Astrophysical Journal Letters. A Habitable-zone Earth-sized Planet Rescued from False Positive Status The “rescued from false positive status” part of its discovery story is telling: it was initially dismissed by automated data pipelines and only recovered by human review. That hints at how many similar candidates may still be hiding in existing data.
Why the Host Star Can Make or Break Habitability
Most of the most Earth-sized exoplanets discovered so far orbit M dwarf stars, also called red dwarfs. This is partly a detection bias: small stars make it easier for current instruments to spot small planets. But M dwarfs present serious challenges for habitability that go beyond the planet itself.
Because M dwarfs are so dim, producing less than 2% of the Sun’s luminosity, their habitable zones sit very close in, typically around 0.1 to 0.4 astronomical units. Planets that close face intense high-energy radiation, frequent flares, and blasts of charged particles from stellar winds and coronal mass ejections.11arXiv. Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets This XUV radiation can drive atmospheric expansion and mass escape, meaning a planet that starts with a viable atmosphere might lose it over hundreds of millions of years. UV radiation also influences atmospheric chemistry in complex ways, both driving photochemical reactions that can create ozone and potentially pre-biological molecules, and destroying atmospheric gases that shield the surface.12The Astrophysical Journal. LIVING WITH A RED DWARF: ROTATION AND X-RAY AND ULTRAVIOLET PROPERTIES OF THE HALO POPULATION KAPTEYN’S STAR
Tidal locking is the other major concern. Planets this close to their stars are expected to become gravitationally locked, always showing one face to the star. One hemisphere bakes in perpetual daylight while the other freezes in darkness. Climate models suggest that atmospheric circulation can redistribute heat on such worlds, but the resulting weather patterns differ fundamentally from Earth’s. Tidally locked planets develop strong equatorial winds that drive heat from the dayside to the nightside, and models show relatively small changes in the substellar temperature when the stellar spectrum is shifted toward infrared wavelengths, as it is for M dwarfs.13Monthly Notices of the Royal Astronomical Society. Day and night: habitability of tidally locked planets with sporadic rotation So tidal locking is not automatically fatal, but it creates a climate regime with no real analog on Earth.
This is why Kepler-452b’s G-type host star is so appealing despite the planet’s uncertain composition. A Sun-like star provides gentler radiation, a more distant habitable zone that avoids tidal locking, and a relatively stable luminosity over billions of years. Some researchers have argued that K-type stars, slightly smaller and cooler than the Sun but longer-lived and calmer than M dwarfs, may actually be the ideal hosts for habitable planets.
Plate Tectonics, Atmospheres, and the Factors You Cannot See from Far Away
Size, temperature, and orbital distance are the features we can measure for distant planets. But several factors that are critical to habitability remain invisible at interstellar distances, and their absence from the ESI is a real limitation.
Plate tectonics may be one of the most underappreciated requirements for long-term habitability. On Earth, the cycle of volcanism releasing greenhouse gases and surface weathering pulling them back out of the atmosphere acts as a thermostat that has kept liquid water on the surface for billions of years.14Journal of Geophysical Research: Planets. Climate‐tectonic coupling: Variations in the mean, variations about the mean, and variations in mode Recent modeling has quantified just how important this is: planets with mobile-lid tectonics, the kind Earth has, sustain stable, temperate climates over billions of years through continuous carbon dioxide outgassing from the deep interior. In contrast, planets with a stagnant lid, where the crust does not recycle, fail to offset the gradual brightening of their star and end up frozen despite increasing sunlight.15Earth and Planetary Science Letters. The role of plate tectonic-like behaviour in the long-term climate evolution of Earth We have no way to tell whether an exoplanet has plate tectonics from light-years away, which means even the best candidates come with a major unknown.
Atmospheric composition is another piece of the puzzle that shifts the habitable zone itself. The conventional habitable zone is calculated assuming a roughly Earth-like atmosphere. But if a planet has a thicker nitrogen atmosphere, the zone widens. Modeling with 5 bars of nitrogen background pressure, five times Earth’s, extends the habitable zone in our solar system to roughly 0.9 to 1.7 AU, about 20% wider than the standard conservative estimate.16Monthly Notices of the Royal Astronomical Society. The effect of high nitrogen pressures on the habitable zone and an appraisal of greenhouse states Meanwhile, for planets around M-type stars, the infrared-shifted stellar spectrum interacts differently with carbon dioxide, potentially allowing much higher CO₂ pressures than would be possible around Sun-like stars.17Astronomy & Astrophysics. Atmospheric constraints for the CO2 partial pressure on terrestrial planets near the outer edge of the habitable zone The upshot is that the habitable zone is not a fixed physical boundary but a range that shifts depending on atmospheric details we often cannot observe.
Magnetic fields are the third invisible factor, and the conventional wisdom about them turns out to be partly wrong. The popular narrative says a strong planetary magnetic field is necessary to protect an atmosphere from being stripped away by stellar wind. While a magnetic field does block some forms of atmospheric erosion, such as sputtering and ion pickup, it also opens up escape routes at the polar caps and cusps that increase the overall escape rate. Unmagnetized planets develop induced magnetospheres that provide their own protection from the same types of erosion that intrinsic fields guard against.18Astronomy & Astrophysics. Why an intrinsic magnetic field does not protect a planet against atmospheric escape The relationship between magnetic fields and atmospheric retention is more complicated than the “shield” metaphor suggests, and a strong magnetic field is not the unambiguous advantage it is often portrayed as in popular science coverage.
Planets That Might Be Better Than Earth
The search for the most Earth-like planet rests on an assumption that Earth is the ideal template. Some researchers have questioned that premise. The concept of “superhabitable” worlds refers to planets that might actually support more abundant or more diverse life than Earth does, even though they are not identical to it. Such worlds would tend to be slightly older and more massive than Earth and orbit K-type dwarf stars, which are dimmer and longer-lived than the Sun.19PubMed. Superhabitable worlds The reasoning is that a slightly larger planet retains more internal heat for longer, driving plate tectonics and magnetic dynamo activity further into its lifespan, while a K dwarf’s gentler radiation and longer main-sequence lifetime give biology more time to evolve. Ignoring this possibility has been characterized as an anthropocentric and geocentric bias in how we frame the search for life.20PubMed Central. In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World
An even more radical departure from Earth-centric thinking is the idea of Hycean worlds: planets with massive water oceans beneath hydrogen-rich atmospheres. These are intermediate in size and density between rocky super-Earths and puffy mini-Neptunes. They look nothing like Earth, but the presence of liquid water under a thick, insulating atmosphere could allow habitable conditions across a wider range of orbital distances and planet types than the traditional rocky-planet model permits.21The Astrophysical Journal. Habitability and Biosignatures of Hycean Worlds Follow-up work has confirmed that the ocean conditions on such worlds could plausibly support habitable temperatures, significantly expanding the parameter space in which we should be looking for life.22Monthly Notices of the Royal Astronomical Society. On the ocean conditions of Hycean worlds Whether Hycean worlds actually exist in the predicted form is still uncertain, but they illustrate how much the question “which planet is most similar to Earth” might be the wrong question if the goal is finding life.
Ocean Worlds in Our Own Backyard
While attention naturally goes to distant exoplanets, some of the most intriguing habitable environments in our solar system are not planets at all. Saturn’s moons Enceladus and Titan are considered prime targets for extraterrestrial life, with subsurface liquid water oceans that offer windows into habitability without requiring a planet in a traditional habitable zone.23Experimental Astronomy. Exploration of Enceladus and Titan: investigating ocean worlds’ evolution and habitability in the Saturn system Enceladus famously sprays plumes of water vapor and organic molecules from cracks in its icy surface, giving us a direct sample of its ocean chemistry without even having to drill through the ice. Titan has a dense nitrogen atmosphere, liquid hydrocarbon lakes on its surface, and a subsurface water ocean. Neither moon remotely resembles Earth in overall appearance, but both demonstrate that liquid water and organic chemistry can coexist far outside the classical habitable zone, heated by tidal forces rather than sunlight. If life turns up on Enceladus or Titan, it would reshape the habitability debate far more than finding another rocky planet at the right distance from a Sun-like star.
What It Will Take to Actually Tell These Worlds Apart
For now, most of what we know about exoplanet habitability is modeled rather than observed. We measure a planet’s size from how much starlight it blocks, estimate its mass from how much it tugs on its star, and calculate what temperature it should be given its orbital distance. The atmospheres of these worlds, which hold the real clues, remain largely uncharacterized for rocky planets.
The James Webb Space Telescope has opened a new window. Detecting biosignature gases through transmission spectroscopy is in principle within JWST’s reach, but characterizing rocky or sub-Neptune-sized exoplanet atmospheres has proven to be an intricate task. The emerging consensus is moving away from the idea of finding a single definitive “silver bullet” biosignature gas and toward looking at the full atmospheric context.24PubMed Central. Prospects for detecting signs of life on exoplanets in the JWST era Oxygen alone is not proof of life; methane alone is not proof of life. But certain combinations of gases that should not coexist in chemical equilibrium, maintained over time, could point to biological processes keeping the atmosphere out of balance.
Looking further ahead, the proposed Habitable Worlds Observatory would represent a transformative leap in directly imaging and characterizing Earth-like exoplanets, using advanced coronagraph technology to block out starlight and photograph planets orbiting nearby Sun-like stars.25Astrophysics and Space Science. Advancing European high-contrast imaging R&D towards the Habitable Worlds Observatory Direct imaging would allow scientists to study a planet’s atmosphere without waiting for the planet to pass in front of its star, dramatically expanding the number of worlds that could be characterized. That telescope is still in the planning stages, but the technology development for it is already underway in both Europe and the United States. When it flies, the question of which planet is most similar to Earth may finally have an answer grounded in observation rather than inference.