Planets are gravitationally rounded bodies that orbit stars and dominate their orbital neighborhoods, but that tidy definition conceals an enormous range of objects. Our own solar system contains rocky worlds with thin atmospheres, gas giants with no solid surface, and ice giants hiding exotic phases of water under crushing pressure. Beyond our Sun, thousands of confirmed exoplanets have expanded the category even further, revealing worlds that orbit two stars, worlds hot enough to vaporize rock, and worlds drifting through interstellar space with no star at all. Understanding planets means grappling with how they form, what they are made of, and why no two planetary systems look quite alike.
What Counts as a Planet
The question sounds simple, but astronomers argued about it for years before reaching even a partial consensus. In 2006, the International Astronomical Union adopted a formal definition for the first time, establishing three criteria for objects in our solar system: a planet must orbit the Sun, be massive enough for gravity to pull it into a roughly spherical shape, and have “cleared the neighborhood” around its orbit of other debris.1Social Studies of Science. The Problem with Pluto That third criterion is what demoted Pluto. The Kuiper Belt is crowded with icy bodies, and Pluto shares its orbital zone with many of them.
Whether “clearing the neighborhood” is a good dividing line remains contested. One approach puts the criterion on firmer mathematical footing by calculating whether a body’s mass is large enough to gravitationally scatter or absorb all the debris in its orbital zone within a reasonable timeframe. If the body’s mass exceeds a threshold value for its distance from the star, it qualifies; if not, it doesn’t. By that measure, the eight recognized planets in our solar system pass easily, and Pluto falls short by a wide margin.2The Astronomical Journal. A Quantitative Criterion for Defining Planets The debate hasn’t gone away, though. Some planetary scientists argue that geology should matter more than orbital dynamics, and that any geologically complex world deserves the label.
How Planets Form
Planets build themselves inside disks of gas and dust that surround newborn stars. The leading explanation for rocky and icy planets is called core accretion: tiny grains of dust stick together, grow into pebbles, then boulders, and eventually coalesce into bodies large enough for their gravity to attract more material. For gas giants, the process goes a step further. Once a solid core reaches roughly ten times the mass of Earth, it begins pulling in vast quantities of hydrogen and helium from the surrounding disk, rapidly ballooning into something resembling Jupiter or Saturn.3Astronomy & Astrophysics. Planet formation models: the interplay with the planetesimal disc
Core accretion works well for planets at moderate distances from their star, but struggles to explain gas giants found on very wide orbits, sometimes 30 to 50 times the Sun-Earth distance. At those distances, the disk is thin and cold, and building a core large enough to grab gas would take longer than the disk survives. An alternative mechanism, disk gravitational instability, may fill the gap. In this scenario, a massive region of the gas disk collapses directly under its own gravity, forming a giant planet in one dramatic step rather than through slow accumulation. Recent models show this process can produce gas giants on wide, slightly elliptical orbits around stars of various masses, though it becomes less effective around the smallest stars.4The Astrophysical Journal. Formation of Giant Planets by Gas Disk Gravitational Instability on Wide Orbits around Protostars with Varied Masses. II. Quadrupled Spatial Resolution and Beta Cooling
Why Planets Don’t Stay Where They Form
A planet born at a certain distance from its star rarely stays there. Gravitational interactions between the young planet and the gas disk cause the planet to spiral inward or, less commonly, outward. Smaller planets experience what researchers call Type I migration, where the disk’s gravity tugs on them asymmetrically. Larger planets that have carved a gap in the disk undergo Type II migration, which was long thought to simply carry the planet inward at the same rate the gas itself drifts toward the star.5The Astrophysical Journal. Evolution of Migrating Planets Undergoing Gas Accretion
That picture has gotten more complicated. If Type II migration always dragged giant planets inward at the disk’s drift speed, most gas giants would end up as “hot Jupiters” parked close to their stars. Yet most known gas giants orbit at moderate or large distances. Hydrodynamic simulations show that gas continues to leak through even a deeply carved gap, and the resulting density distribution around the planet can actually stall or reverse its inward drift. Where the disk’s density drops off steeply, the torques from the inner and outer disk no longer push the planet inward so readily.6The Astrophysical Journal. Retention of Long-period Gas Giant Planets: Type II Migration Revisited Separate work confirms that while the planet’s migration rate stays proportional to the disk’s viscosity, it is not simply equal to the unperturbed gas drift speed, meaning planets can migrate more slowly than the classic theory predicted.7Astronomy & Astrophysics. Toward a new paradigm for Type II migration Migration explains a great deal of the orbital architecture we see: it is why some giant planets sit scorchingly close to their stars and others remain far out.
What Planets Are Made Of Inside
A planet’s interior tells you its history. Earth’s iron core, for example, separated from its rocky mantle early in the planet’s life, in a process that was shaped by how much water was present. Machine-learning simulations trained on quantum-mechanical data suggest that water promotes magnesium partitioning into the metallic core during differentiation, while silicon and iron prefer the silicate mantle. A self-consistent model of this process yields a bulk Earth water content of about 0.23 percent by weight, equivalent to roughly ten times the mass of water currently in Earth’s oceans.8PubMed Central. Earth’s core-mantle differentiation shaped by water Most of Earth’s water, in other words, is locked deep inside the planet rather than on its surface.
Once a core forms, its ongoing cooling and solidification can drive a magnetic field. Earth’s geodynamo is powered by convection in the liquid outer core: as the inner core grows, lighter elements are released upward, stirring the conducting fluid and generating the magnetic field that shields the atmosphere from the solar wind.9Physics of the Earth and Planetary Interiors. Geomagnetic field and the growth of the Earth’s inner core: Past, present and future Not every planet manages this. Mars had a global magnetic field early in its history but lost it, likely because its smaller core cooled and solidified too quickly to sustain convection.
Ice Giants and Exotic Water
Uranus and Neptune are the solar system’s least understood planets. Classified as ice giants, they are smaller than Jupiter and Saturn but far more massive than Earth, with interiors thought to contain enormous quantities of water, ammonia, and methane under extreme pressure. At the conditions deep inside these worlds, water enters a state that doesn’t exist on Earth’s surface: superionic ice, where oxygen atoms lock into a crystal lattice while hydrogen ions flow freely through it like a liquid. Density-functional calculations have revealed that the form of superionic ice previously assumed to dominate at depth is actually less stable than a different crystal phase, one that appears at pressures above roughly a million times atmospheric pressure.10PubMed. Superionic to Superionic Phase Change in Water: Consequences for the Interiors of Uranus and Neptune
Recent work using first-principles free-energy calculations to derive the entropy of water under ice-giant conditions has produced a striking result: the interiors of Uranus and Neptune may be 15 to 30 percent colder than earlier models assumed. Colder interiors increase the likelihood that carbon, squeezed out of methane at high pressure, precipitates as diamond and literally rains downward through the mantle, a phenomenon sometimes called “diamond rain.”11The Astrophysical Journal. Ab Initio Entropy Calculations of Water Predict the Interiors of Uranus and Neptune to Be 15%–30% Colder than Previous Models It is one of the more dramatic illustrations of how alien planetary interiors can be.
Atmospheres Under Pressure
A planet’s atmosphere is not a permanent fixture. Atmospheric escape, driven by solar radiation and the solar wind, can strip volatiles from a world over billions of years. This process has been important for every terrestrial planet in our solar system. Mars lost much of its early atmosphere partly because it lacked a strong magnetic field. Venus may have lost an ocean’s worth of water to space. Escape is especially significant for low-mass exoplanets, where weaker gravity makes it easier for gas molecules to reach escape velocity.12Annual Review of Earth and Planetary Sciences. Atmospheric Escape from Solar System Terrestrial Planets and Exoplanets
Venus illustrates the other extreme: an atmosphere that stayed and ran away. Climate modeling shows that if the sunlight hitting an Earth-like planet increases to about 1.4 times the current flux at Earth’s orbit, a runaway greenhouse effect can evaporate the oceans entirely. That threshold is close to the solar flux Venus received early in the Sun’s history, even before the Sun brightened to its current luminosity.13Icarus. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus Venus and Earth started with similar sizes and compositions, but a relatively small difference in distance from the Sun may have tipped Venus into a catastrophic feedback loop that boiled away its water and left it with a crushing carbon dioxide atmosphere.
The Habitable Zone and Oceans Beyond It
The habitable zone is the band of orbital distances around a star where a rocky planet could sustain liquid water on its surface. Updated climate models place the inner edge for our solar system at about 0.99 AU (Earth sits at 1.0 AU, uncomfortably close) and the outer edge at about 1.70 AU.14The Astrophysical Journal. HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES The zone shifts depending on the star: cooler K and M dwarf stars have narrower, closer-in habitable zones, while hotter F-type stars have wider, more distant ones.15PubMed. Habitable zones around main sequence stars
But liquid water doesn’t require sunlight if you have another heat source. Jupiter’s moon Europa almost certainly has a global ocean beneath its icy shell, kept liquid by tidal heating from Jupiter’s gravity flexing the moon’s interior. Simulations show that tidal heating in Europa’s rocky mantle produces patterns of heat flow that ocean convection carries up to the ice shell, thinning it more at the poles than the equator.16AGU Advances. Europa’s Ocean Translates Interior Tidal Heating Patterns to the Ice‐Ocean Boundary The ice shell’s thickness depends on how strong the tidal heating is and how viscous the ice is. With moderate tidal heating of around 10 to 20 milliwatts per square meter, the shell could be under 90 kilometers thick; at higher rates, it thins to under 40 kilometers.17Planetary and Space Science. Europa’s structural conditions for the existence of subsurface ocean and the absence of metallic core-driven magnetic field When tidal heating exceeds a certain threshold, additional heat goes entirely into melting the ice rather than warming the ocean further.18Journal of Geophysical Research: Planets. Tidally Heated Convection and the Occurrence of Melting in Icy Satellites: Application to Europa Europa is a reminder that habitability may extend well beyond any star’s traditional habitable zone.
Free-Floating Planets
Not every planet orbits a star. Planet-formation theory predicts that gravitational interactions in young planetary systems should fling some planets out into interstellar space, and microlensing surveys have been finding them. When a massive object passes between a distant star and Earth, its gravity bends and briefly brightens the star’s light. Free-floating planets produce extremely short microlensing events lasting hours to less than a day, with very small angular Einstein radii.
Two ultra-short events detected by the OGLE survey illustrate the range. One, lasting just 0.155 days, was consistent with an Earth-mass object; the other, lasting about 0.9 days, pointed to something closer to Jupiter’s mass.19Astronomy & Astrophysics. Two new free-floating or wide-orbit planets from microlensing An even shorter event, OGLE-2016-BLG-1928, holds the record for the shortest microlensing event ever identified and was attributed to a roughly Earth-mass rogue planet.20The Astrophysical Journal Letters. A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event More recently, simultaneous ground- and space-based observations of another event broke a key degeneracy and directly measured the mass of the lensing object at roughly one-third of Jupiter’s mass. Comparison with simulations suggests it formed in a protoplanetary disk like a normal planet and was later ejected.21PubMed. A free-floating-planet microlensing event caused by a Saturn-mass object Population statistics from these surveys hint that free-floating planets may actually outnumber stars in the Milky Way.
Exoplanet Surprises
Exoplanet discoveries have repeatedly overturned expectations. One major surprise has been the sheer variety of worlds that exist. The James Webb Space Telescope can now take transmission spectra of Earth-sized planets around small, cool stars, opening a window into whether these worlds hold atmospheres at all.22Nature Astronomy. A JWST transmission spectrum of the nearby Earth-sized exoplanet LHS 475 b One recent result upended a widely held assumption: TOI-561 b, an ultrahot super-Earth with a dayside temperature expected to exceed 3,000 K if it were a bare rock, turned out to have a thick volatile atmosphere that cools its surface well below that threshold. The finding contradicts the popular hypothesis that intensely irradiated rocky planets lose their atmospheres entirely, and suggests that magma oceans can retain substantial reservoirs of gas.23The Astrophysical Journal Letters. A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561 b
Many exoplanets orbiting close to small stars are expected to be tidally locked, with one hemisphere permanently facing the star and the other in perpetual darkness. Modeling these worlds reveals they develop extreme day-night temperature contrasts, and the transition to large contrasts happens at much smaller values of the relevant atmospheric parameters than researchers had predicted based on earlier hot-Jupiter work. The atmosphere acts as a heat engine, and its inefficiency limits how much warmth can be transported to the night side.24PubMed Central. TEMPERATURE STRUCTURE AND ATMOSPHERIC CIRCULATION OF DRY TIDALLY LOCKED ROCKY EXOPLANETS Interestingly, the overturning circulation that rises on the dayside and sinks on the nightside turns out to dominate heat transport in the rocky-planet case, carrying more heat than the large-scale winds that tend to get more attention in models.25PubMed Central. The rotational and divergent components of atmospheric circulation on tidally locked planets
Orbital Stability and Chaos
Planetary systems look orderly, but they are not immune to chaos. In our own solar system, the orbits of the inner planets are technically chaotic on very long timescales. Overlapping secular resonances introduce unpredictability, and calculations suggest that Mercury, on a timescale of about a trillion years, could eventually collide with Venus or fall into the Sun.26Annual Review of Astronomy and Astrophysics. Chaos in the Solar System In the outer solar system, the chaos is even slower but still present, arising from three-body resonances among the giant planets. This long-term instability has practical consequences: it limits how far into the future we can reliably predict planetary positions.
For exoplanetary systems, orbital stability is a pressing question. Many discovered systems have planets packed much more tightly than ours. The overlap of first-order orbital resonances between neighboring planets sets a boundary: when two planets orbit close enough that their resonances overlap, large-scale chaotic motion kicks in, eventually leading to dramatic changes in orbits and potential ejections.27The Astrophysical Journal. FIRST-ORDER RESONANCE OVERLAP AND THE STABILITY OF CLOSE TWO-PLANET SYSTEMS Resonance overlap criteria have become a practical tool for assessing whether a newly discovered pair of planets can remain stable over the age of their star.
Saturn’s Surprisingly Young Rings
Saturn’s rings look like a permanent feature, but mounting evidence suggests they are temporary. Data from the Cassini mission, combined with modeling of how micrometeoroid bombardment pollutes and erodes ring material over time, points to rings that are only a few hundred million years old, a small fraction of the solar system’s roughly 4.5-billion-year history. Models starting with an initial ring mass of about one to three times the mass of Saturn’s small moon Mimas reach pollution levels matching what Cassini observed within that timeframe.28Icarus. Constraints on the initial mass, age and lifetime of Saturn’s rings from viscous evolutions that include pollution and transport due to micrometeoroid bombardment
The rings also do not have an indefinite future. Initially massive rings spread and lose mass quickly through viscous evolution, then converge on their current mass over billions of years. But once viscosity weakens, micrometeoroid bombardment takes over as the dominant driver of change, and it erodes the rings efficiently. The implication is that the rings have a finite lifetime much shorter than the age of the solar system.29Space Science Reviews. The Age and Origin of Saturn’s Rings We happen to live during a window in solar system history when Saturn has spectacular rings. A civilization visiting a billion years from now might find only a faint remnant.
Comets, Asteroids, and the Origins of Life
Planets do not exist in isolation. The small bodies orbiting among them, comets and asteroids, have played an outsized role in planetary evolution. During the period of heavy bombardment roughly 4.5 to 3.8 billion years ago, impacts delivered enormous quantities of organic molecules to Earth’s surface. For plausible early atmospheric conditions, intact cometary organics were arriving at a rate of at least a million to ten million kilograms per year, a flux that declined with a half-life of about a hundred million years.30PubMed. Cometary delivery of organic molecules to the early Earth Whether that delivery was essential for the origin of life or merely supplemented what was already forming on Earth’s surface remains an open question, but it underscores how interconnected planetary bodies and small-body populations really are.
Looking for Industrial Pollution on Other Worlds
If other civilizations have modified their planets, we might be able to detect the evidence. Artificial greenhouse gases like certain fluorinated compounds and sulfur hexafluoride have no significant natural sources on Earth, and they absorb infrared light at distinctive wavelengths. Modeling shows that a combination of these gases at modest concentrations on a planet in the TRAPPIST-1 system could be detected by JWST’s instruments in as few as five to 25 transits, depending on concentration. At certain levels, these artificial gases are actually more detectable in the mid-infrared than standard biosignatures like oxygen or methane.31The Astrophysical Journal. Artificial Greenhouse Gases as Exoplanet Technosignatures
Nitrogen dioxide offers another angle. It is a common byproduct of combustion and industrial activity, and it absorbs strongly in the ultraviolet and visible spectrum. On planets around cooler stars, where fewer short-wavelength photons break it down, NO₂ would accumulate to higher levels. A 15-meter space telescope observing an Earth-like planet around a Sun-like star at 10 parsecs could detect present-day Earth-level NO₂ with a signal-to-noise ratio of about five within roughly 400 hours. Historically, Earth’s NO₂ levels were about three times higher than today, meaning such an instrument could spot a civilization at a stage roughly 40 years behind our own.32The Astrophysical Journal. Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology Clouds and aerosols complicate things, and non-detections would still be scientifically useful as upper limits. But the fact that routine atmospheric characterization of exoplanets could, in principle, turn up signs of technology is one of the more quietly thrilling developments in planetary science.