Planetary Science

How Planets Form

Planets form within protoplanetary disks, the rotating disks of gas and dust that surround newly formed stars. The process begins when microscopic dust grains collide and stick together through electrostatic and van der Waals forces, gradually building larger aggregates. As these aggregates grow to centimeter and meter scales, further growth becomes challenging because collisions at higher velocities tend to fragment the bodies rather than merge them. This is known as the meter-size barrier, and overcoming it likely involves a combination of processes including streaming instabilities, where particles concentrate in dense clumps through aerodynamic interactions with the gas disk.

Once solid bodies reach kilometer scales, they become planetesimals, objects large enough for gravity to play a significant role in further growth. Planetesimals grow by colliding and merging with each other in a process called accretion. The largest planetesimals grow fastest because their stronger gravity captures a larger share of surrounding material, a process called runaway accretion. Eventually a few dominant bodies, called protoplanets or planetary embryos, emerge and sweep up remaining planetesimals in their orbital zones. The terrestrial planets in our solar system formed through collisions between roughly Mars-sized embryos, with the final giant impact that formed the Moon occurring about 50 million years after the solar system began forming.

Gas giant planets like Jupiter and Saturn formed through a different process. According to the core accretion model, a solid core of roughly 10 to 15 Earth masses accumulated in the outer solar system where ices of water, ammonia, and methane added to the available solid material. Once this core reached a critical mass, it began rapidly accreting hydrogen and helium gas from the surrounding disk, growing to hundreds of Earth masses in a relatively short period. This process must occur before the gas disk disperses, which happens within roughly 3 to 10 million years of the star formation, placing a strict time constraint on gas giant formation.

Terrestrial Planets

The terrestrial planets, Mercury, Venus, Earth, and Mars, are characterized by solid rocky surfaces, relatively thin or absent atmospheres, and compositions dominated by silicate rocks and iron. Despite these similarities, the four planets differ dramatically in their surface conditions, atmospheric properties, and geological activity, demonstrating how small differences in size, distance from the Sun, and initial conditions can lead to very different outcomes over billions of years.

Mercury, the smallest and innermost planet, has virtually no atmosphere and a heavily cratered surface that has changed little in billions of years. Its unusually large iron core, comprising about 60 percent of the planet total mass, may be the result of a giant impact early in solar system history that stripped away much of its rocky mantle. Venus, similar in size and mass to Earth, has a thick carbon dioxide atmosphere with surface pressures 90 times that of Earth and surface temperatures of about 460 degrees Celsius, hot enough to melt lead. This extreme greenhouse effect makes Venus the hottest planet in the solar system despite being farther from the Sun than Mercury.

Earth is unique among the terrestrial planets in having liquid water on its surface, a nitrogen-oxygen atmosphere, plate tectonics that continuously recycle the crust, and a strong global magnetic field generated by its convecting liquid iron outer core. Mars, about half Earth diameter, once had a thicker atmosphere and liquid water on its surface, as evidenced by dried river channels, lake beds, and mineral deposits that form only in the presence of water. Mars lost most of its atmosphere over billions of years, likely because it lacks a global magnetic field to protect against solar wind stripping, and its smaller size allowed it to cool more rapidly, reducing volcanic outgassing.

Giant Planets and Ice Giants

Jupiter and Saturn are classified as gas giants, composed primarily of hydrogen and helium in proportions similar to the Sun. Jupiter, the most massive planet in the solar system at 318 Earth masses, has a complex banded atmosphere driven by internal heat, with the Great Red Spot being a storm system larger than Earth that has persisted for at least several centuries. Saturn is best known for its spectacular ring system, composed of countless particles of ice and rock ranging from dust grains to house-sized boulders, extending from about 7,000 to 80,000 kilometers above the planet equator but averaging only about 10 meters thick.

Uranus and Neptune are classified as ice giants because a substantial fraction of their mass consists of heavier volatile compounds, often called ices, including water, ammonia, and methane, in addition to hydrogen and helium. Uranus is tilted nearly on its side, with an axial tilt of 98 degrees, likely the result of a giant impact during formation. Neptune, the most distant planet, has the strongest sustained winds in the solar system, exceeding 2,000 kilometers per hour, despite receiving very little solar energy. Both ice giants have ring systems and families of moons, though far less studied than those of Jupiter and Saturn due to only a single spacecraft flyby each, Voyager 2 in 1986 and 1989 respectively.

Moons and Planetary Satellites

The solar system contains over 200 known moons orbiting the planets, ranging from tiny irregular captured asteroids to worlds larger than Mercury. The largest moons, including Jupiter Ganymede (the largest moon in the solar system), Saturn Titan (the only moon with a dense atmosphere), and Neptune Triton (likely a captured Kuiper Belt object), are geologically complex worlds in their own right, with diverse surfaces, subsurface oceans, and in Titan case, rivers and lakes of liquid hydrocarbons.

Several moons in the outer solar system harbor subsurface liquid water oceans beneath their icy crusts, maintained by tidal heating from gravitational interactions with their parent planets. Jupiter Europa has a global saltwater ocean beneath a kilometers-thick ice shell, with plumes of water vapor erupting through cracks in the ice. Saturn Enceladus vents jets of water ice and organic molecules from its south polar region, with the Cassini spacecraft having directly sampled this material and detected molecular hydrogen, a potential energy source for microbial life. These ocean worlds are considered among the most promising places to search for extraterrestrial life in the solar system.

Planetary Atmospheres and Climate

A planet ability to retain an atmosphere depends on its mass (and therefore gravitational pull) and its temperature (which determines the speed of gas molecules). Small, hot worlds like Mercury cannot retain any significant atmosphere because gas molecules move fast enough to escape the planet weak gravity. Larger, cooler worlds like Jupiter retain even the lightest gases, hydrogen and helium, because the escape velocity is high and the thermal velocities are comparatively low.

Atmospheric composition is shaped by volcanic outgassing, chemical reactions, photochemistry driven by stellar radiation, atmospheric escape processes, and in the case of Earth, biological activity. The greenhouse effect, where certain atmospheric gases absorb and re-emit infrared radiation and trap heat near the surface, plays a critical role in determining surface temperatures. Earth greenhouse effect raises the average surface temperature by about 33 degrees Celsius above what it would be without an atmosphere, making liquid water possible. Venus experienced a runaway greenhouse effect at some point in its history, where rising temperatures caused surface water to evaporate, adding more water vapor (a potent greenhouse gas) to the atmosphere, which raised temperatures further until all surface water was lost.

Studying planetary atmospheres across the solar system and on exoplanets provides a natural laboratory for understanding climate physics. Comparing Earth, Venus, and Mars, three rocky planets with very different atmospheric outcomes, helps scientists understand the conditions that make a planet habitable and the processes that can push a planet toward uninhabitable extremes. Observations of exoplanet atmospheres through transit spectroscopy are beginning to characterize the compositions of worlds around other stars, searching for biosignature gases like oxygen and methane that might indicate biological activity.