Introduction
The Solar System's eight planets fall into two sharply contrasting families separated by the asteroid belt at ~2.7 AU. The four inner terrestrial planets — Mercury, Venus, Earth, and Mars — are small, dense, rocky worlds with thin atmospheres (where they exist at all), solid surfaces, and masses ranging from 0.055 to 1.0 Earth mass. The four outer giant planets — Jupiter, Saturn, Uranus, and Neptune — are enormous, low-density worlds composed predominantly of gas and ice, with masses from 14 to 317 Earth masses and no well-defined solid surface. This division is not coincidental: it reflects the fundamental chemistry of planet formation in a protoplanetary disc, specifically the role of the snow line — the distance (~2.7 AU in the early Solar System) beyond which water ice could condense from the disc gas.
Inside the snow line, only refractory materials (silicates, metals) could condense as solid particles, limiting the total amount of solid building material available. Terrestrial planet embryos therefore grew to modest sizes — insufficient to gravitationally capture large quantities of hydrogen and helium from the nebular gas. Beyond the snow line, the addition of water ice (and later ammonia and methane ices at greater distances) dramatically increased the surface density of solid material, allowing planetary cores to grow rapidly to ~10–20 Earth masses. At this threshold — the runaway accretion threshold — the core became massive enough to gravitationally capture and retain the abundant H/He disc gas, growing to hundreds of Earth masses in perhaps a few million years via core accretion. This process had to complete before the protoplanetary disc dissipated (typically within 1–10 million years of stellar formation), placing a tight time constraint on giant planet formation.
Comparative planetology — the systematic comparison of planetary bodies to understand their similarities and differences — has been transformed in recent decades by two parallel revolutions. First, spacecraft exploration has provided close-up data from every planet and dozens of moons in the Solar System. Second, the discovery of more than 5,700 exoplanets around other stars has revealed that many aspects of our Solar System that were once considered generic are in fact unusual: the absence of planets between Earth and Neptune in size (no super-Earths or mini-Neptunes in our Solar System), the relatively ordered orbital architecture, and Jupiter's current position are all features that set our system apart from the majority of known planetary systems. Understanding why our Solar System is the way it is — and what that means for habitability — is one of the central questions of 21st-century planetary science.
Key Terms
A planet composed primarily of silicate rocks and iron-nickel metal, with a well-defined solid surface. The Solar System's terrestrial planets — Mercury, Venus, Earth, Mars — all formed inside the snow line where only refractory solids could condense. They share a basic internal structure (metallic core, silicate mantle and crust) but differ enormously in size, atmosphere, and geological activity. The term extends to moons and dwarf planets with similar compositions (e.g., Earth's Moon, Io, Pluto). Mean densities of terrestrial planets range from 3,900 kg/m³ (Mars) to 5,500 kg/m³ (Earth), with uncompressed densities of 3,700–4,100 kg/m³ indicating broadly similar bulk compositions.
A planet composed predominantly of hydrogen and helium, with a mass large enough (>~50 Earth masses) to have captured substantial nebular gas during formation. In the Solar System, Jupiter (~317 M⊕) and Saturn (~95 M⊕) are gas giants. Their interiors consist of a gaseous outer envelope, a layer of liquid metallic hydrogen (where the pressure exceeds ~1 Mbar and hydrogen becomes a pressure-ionised conductor), and a likely rocky/icy core. The metallic hydrogen layer drives the powerful magnetic fields of both planets. Gas giant formation is time-critical: it must occur while the protoplanetary disc still contains nebular gas (within the first ~5 Myr of Solar System formation).
A planet with a mass of ~14–17 Earth masses composed predominantly of "ices" — water, methane, and ammonia in supercritical or ionic form at high pressures — surrounding a rocky core, with a relatively thin H/He envelope. In the Solar System, Uranus and Neptune are ice giants. They are distinct from gas giants in mass, composition, and internal structure: they lack a deep metallic hydrogen layer, and their unusual, off-centre magnetic fields suggest a conducting fluid layer rather than a metallic hydrogen dynamo. The term "ice giant" reflects their formation beyond the methane and ammonia snow lines, not necessarily the presence of solid ice at depth.
The leading theory for giant planet formation, in which a solid planetary core grows by accreting planetesimals until it reaches the critical mass (~10–20 Earth masses) needed to gravitationally capture nebular H/He gas in a runaway process. The model successfully explains the correlation between stellar metallicity and giant planet occurrence (more solids → easier core formation), the presence of heavy-element enrichment in giant planet envelopes, and the gross division between terrestrial and giant planets at the snow line. The competing disc instability model (gravitational fragmentation of the disc) may contribute to forming gas giants at large separations, but core accretion is favoured for most Solar System giants.
The inward or outward movement of a forming planet due to angular momentum exchange with the surrounding protoplanetary gas disc. Type I migration affects low-mass planets embedded in the disc and is generally inward; Type II migration occurs when a giant planet opens a gap in the disc and migrates with the gap on the disc's viscous timescale. Disc migration is the mechanism required to explain hot Jupiters — gas giants found within 0.1 AU of their host stars, far inside where they must have formed beyond the snow line. In our Solar System, the "Grand Tack" model invokes Type II inward migration of Jupiter to ~1.5 AU, followed by outward migration driven by resonance with Saturn, which may explain the low mass of Mars and the depletion of material in the inner Solar System.