In our solar system, the four inner planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial or telluric planets. They are predominantly rocky, with dense metallic cores largely composed of iron. Scientific models suggest that Mars and Venus may once have hosted conditions similar to present‑day Earth, potentially supporting life. The term “terrestrial” derives from the Latin word “terra,” meaning land. By contrast, the outer four planets—Jupiter, Saturn, Uranus, and Neptune—are Jovian, or gas giants. They are massive bodies composed mainly of hydrogen and helium, and the name reflects their resemblance to the archetypal giant, Jupiter. Though often called gas planets, their interiors are largely in a liquid‑metal or super‑critical state rather than true gas.
Our solar system formed from a rotating cloud of gas and dust known as a solar nebula. The nebular hypothesis, the prevailing model of planet formation, posits that all planets coalesced from the same primordial material. The Sun and the Jovian planets are dominated by hydrogen and helium, whereas the terrestrial planets are rich in heavier elements such as silicon, iron, and copper. Despite these compositional differences, every planet in the system is approximately spherical; the slight flattening of the terrestrial planets’ poles results from their slower rotation rates.
Nearly all planets orbit the Sun in elliptical paths, as described by Johannes Kepler’s laws. Mercury’s orbit is uniquely inclined, deviating 7° from the ecliptic plane that hosts Earth’s orbit, whereas Jupiter’s orbit is only about 1° out of alignment. These orbital similarities underscore common dynamical characteristics shared by both terrestrial and Jovian bodies.
Both classes of planets feature a layered interior: a central core surrounded by a mantle. Terrestrial planets possess a solid crust that caps the mantle, while Jovian planets lack a true solid surface. Models suggest that the cores of gas giants comprise a mixture of rock, metal, and metallic hydrogen. Each planet is enveloped by an atmosphere, with Jovian planets exhibiting distinct cloud strata of varying colors. Although the gas giants have no solid “surface,” their atmospheric layers act as an effective outer shell.
Dynamic weather patterns are a hallmark of all planets in the solar system. High‑contrast bands, storms, and spots—visible even from Earth—indicate vigorous atmospheric circulation. On the gas giants, storms can span planetary scales; Jupiter’s Great Red Spot, for instance, is roughly twice the diameter of Earth and can trap material from deeper layers into higher cloud decks. Terrestrial planets also experience weather, but its intensity is comparatively modest. Magnetic fields are common: the Jovian planets generate powerful dipoles, while several terrestrial planets—including Earth—maintain magnetic shields that shape auroras by deflecting charged particles from the solar wind.
