Solar System Offers Clues to Rocky Exoplanets

In a sweeping new review published in Space Science Reviews, a team of planetary scientists argues that the surest guide to understanding the thousands of rocky exoplanets now being catalogued beyond our Solar System lies in the worlds we already know best. The paper, led by Paul K. Byrne of Washington University in St. Louis alongside Claire Marie Guimond, Peter A. Cawood, Michael J. Way, Doris Breuer, Tilman Spohn, and a broad international collaboration, synthesizes decades of research on Earth, Venus, Mars, Mercury, the Moon, and Jupiter’s volcanic moon Io to construct a comparative framework for interpreting distant terrestrial worlds that no spacecraft will ever visit. Its central conclusion is deceptively simple but profound: rocky planets change, sometimes dramatically, and two worlds of nearly identical size and mass can follow vastly different evolutionary paths.

The exoplanet census has grown remarkably since the first potentially rocky world, the roughly six-Earth-mass CoRoT-7 b, was confirmed in 2009. Today, planets smaller than Neptune appear to be roughly ten times more common than giant planets, and recent reviews suggest such worlds orbit about a third of all Sun-like stars. Yet characterizing them remains extraordinarily difficult. For most exoplanets, astronomers measure only orbital period and radius; only with both mass and radius can bulk density be calculated, and even then inferring whether a planet is genuinely dominantly rock and metal is far from conclusive. The field relies on a rule-of-thumb radius threshold of about 1.6 times Earth’s radius, below which planets are typically classified as rocky, but the authors caution that such “terrestrial” worlds may hold substantial fractions of volatiles relative to Earth, making them terra incognitae to a Solar System geoscientist. Two so-called Earth twins, Kepler-62 f and the questioned Kepler-452 b, have been proposed, with estimated occurrence rates of such temperate twins falling between 0.37 and 0.88 per Sun-like star, though these figures carry considerable extrapolation uncertainty.

The heart of the review is a tour of the Solar System’s rocky bodies and the lessons each offers. Earth, the best-understood planet, is presented not as a static blue marble but as a world whose character has shifted dramatically through time. Its interior—consisting of a solid inner core exceeding 6000 K and at least 330 GPa in pressure, a molten outer core that powers the geodynamo, an ultramafic mantle comprising about 67 percent of the planet’s mass, and a chemically differentiated crust—cooled by some 200 to 300 K since the Hadean. Plate tectonics, the authors emphasize, is likely not the planet’s original operating mode. Instead, Earth probably passed through an early “squishy-lid” regime characterized by shallow intrusive bodies and a warm, deformable lithosphere, before rigid mobile-lid tectonics emerged with the stabilization of Archean cratons between roughly 3.8 and 3.2 billion years ago. Continental crust, with its felsic, granitic composition, appears to require liquid water or some similar agent to lower rock melting temperatures, and today covers only about 40 percent of the surface while constituting nearly 70 percent of total crustal volume.

Earth’s atmosphere and climate have been equally mutable. The Faint Young Sun Paradox arises because the Sun’s luminosity has risen nearly 30 percent since it entered the main sequence, yet geological proxies indicate Earth remained temperate from early in its history, likely sustained by elevated carbon dioxide partial pressures. Reconstructions from fossil raindrop imprints, gas bubbles in ancient basalts, and nitrogen and argon isotopes trapped in 3-to-3.5-billion-year-old quartz suggest late-Archean surface pressures of roughly 0.5 bar or less. The rise of atmospheric oxygen around 2.4 billion years ago, driven by cyanobacterial photosynthesis and organic carbon burial, coincided with the first documented snowball Earth, possibly triggered by the collapse of methane greenhouse warming. Later, large igneous province eruptions such as the Siberian Traps injected 1 to 10 teratonnes of carbon dioxide into the atmosphere, causing the Permian-Triassic mass extinction that eliminated roughly 90 percent of land species. Even Earth’s color may have changed, perhaps from an early orange haze to its familiar blue, a reminder that a living world need not always look the way ours does today.

Venus receives particular scrutiny because, to first order, very little distinguishes it from Earth: a mass of 0.82 Earth masses, a radius of 0.95 Earth radii, and presumably similar bulk composition, yet surface conditions of an astonishing 750 K under 92 atmospheres of pressure, in air that is 93.5 percent carbon dioxide beneath global sulphuric acid clouds. A deuterium-to-hydrogen ratio roughly 100 times Earth’s indicates substantial water loss at some point in the planet’s past. The review highlights two competing scenarios: either Venus never cooled enough to condense its primordial steam atmosphere, or it once possessed genuine oceans before a runaway greenhouse desiccated the world. Which is correct has profound implications for interpreting Earth-size exoplanets, since the former would suggest that worlds close to their stars are reliably post-runaway greenhouses, while the latter would imply stellar distance matters less than assumed and that climate catastrophe can strike even a clement world. Notably, the authors warn that as an ocean evaporates into a warming atmosphere, water reaching stratospheric altitudes can be photodissociated, allowing hydrogen to escape and atmospheric oxygen to rise—potentially fooling remote observers into inferring a habitable, even inhabited, world when the opposite is true.

Mars, at 0.53 Earth radii and 0.11 Earth masses, illustrates the fate of smaller worlds. Its bimodal hypsometry, expressed as the hemispheric crustal dichotomy, may reflect degree-one mantle convection, crustal growth feedback, or a gigantic ancient impact rather than the buoyant felsic rocks that produce Earth’s two-tier topography. The planet’s geology is written largely in its past: the Noachian eon saw the construction of the Tharsis and Elysium volcanic rises, the Valles Marineris canyon system, and extensive fluvial activity, but with no evidence of subduction or transform faults, Mars has operated under a stagnant-lid regime for essentially its entire history. Its small size gave it a high surface-to-volume ratio, rapid interior cooling, and a thick lithosphere; critically, its mantle and crust, only 15 percent the size of Earth’s combined silicate reservoirs, exhausted their degassable volatile budget long ago, leaving the atmosphere unable to be replenished after solar wind stripping. Whether early Mars sustained long-term temperate conditions or was mostly an ice-house world with punctuated warm intervals remains actively debated, with three-dimensional climate models supporting scenarios from persistent lakes and even a northern ocean to a cold, icy highlands state.

Mercury and the Moon, the smallest rocky bodies of the inner Solar System, share histories of early volcanism followed by global contraction. Mercury’s crust was built by voluminous effusive eruptions that largely ceased around 3.5 billion years ago as secular cooling put the lithosphere into compression, producing a worldwide network of thrust faults and several kilometres of radial contraction that continues today. Its outsize core, roughly 0.8 of its body radius, may result from formation in a highly reduced inner disk or from mantle-stripping impacts; intriguingly, elevated potassium-thorium and potassium-uranium ratios suggest the innermost planet is nonetheless somewhat volatile-rich. The Moon, coalesced from debris of the proto-Earth–Theia collision, crystallized an anorthositic flotation crust from its magma ocean around 4.35 billion years ago, with later basaltic mare volcanism concentrated between 3.8 and 3.2 billion years ago and persisting to at least 2 billion years in the radiogenic Procellarum KREEP Terrain, as confirmed by Chang’e-5 and Chang’e-6 sample returns. Io, the Solar System’s most volcanically active body, demonstrates a different heat source entirely: tidal dissipation driven by its eccentric orbit within the Laplace resonance with Europa and Ganymede sustains surface heat flux 15 to 40 times Earth’s, with eruption temperatures up to 1600 K and a resurfacing rate of about 1.5 centimetres per year.

The review’s final section confronts the exoplanet classes with no Solar System analogues. Super-Earths, planets of roughly 1 to 1.6 Earth radii below the empirically observed “radius valley,” may be bare rock worlds like LHS 3844 b, or remnant cores of formerly puffier planets that lost primordial hydrogen envelopes—a scenario under which some rocky planets effectively form billions of years after their systems establish, thermally blanketed for eons by their departed atmospheres. Tidally locked planets, especially common around M-dwarf stars, could host hemisphere-scale differences in volatile deposition and volcanic activity. Lava worlds, whose daysides reach rock-melting or even rock-evaporating temperatures, sustain long-lived magma oceans heated more strongly from above than within, offering potential insight into bulk rock compositions through thermal emission spectroscopy. Other exotic outcomes include long-lived internally heated planets driven by tidal dissipation, radiogenic abundance, or magnetic induction heating; super-Mercuries with outsize iron cores; low-density “super-Ganymedes” cloaked in thick water or ice layers; and worlds whose bulk compositions diverge from chondritic norms due to protoplanetary disk processes.

The authors distill these comparisons into rules of thumb tempered by caution. Relatively large, ancient planets are more likely to retain moderate atmospheres, ongoing volcanism, and perhaps magnetic fields and liquid-water conditions, while smaller worlds cool, contract, and lose geological activity faster—yet tidal and induction heating complicate even this elementary guidance, as does the stochastic influence of giant impacts, orbital arrangement, and formation location. The starkest lesson remains the Earth–Venus contrast: two worlds within a single mass class with wildly divergent histories, raising the uncomfortable possibility that Venus’s fate could one day befall Earth, either through catastrophic volcanic outgassing or under a steadily brightening Sun. The team closes with a charge to the field: planetary evolution is not a predictable linear path but a wending, stochastic trail, and anyone interpreting a rocky exoplanet from a handful of bulk measurements should be circumspect and prepared to be surprised. That, they argue, is the core lesson the Solar System offers the search for other worlds.