One star. Eight worlds. A shared origin.
Explore the planets, build an orbit, and follow the nebular model from cloud to planetary system.
Inner and outer planets
| Property | Inner terrestrial | Outer giants |
|---|---|---|
| Members | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Material | Rock and metal dominate | Jupiter/Saturn: H–He dominated; Uranus/Neptune: larger fractions of heavier compounds |
| Surface | Solid rocky surfaces | No accessible solid surface like Earth's |
| Size and density | Smaller; generally higher bulk density | Larger; generally lower bulk density |
| Orbits | Smaller semi-major axes and shorter years | Larger semi-major axes and longer years |
| Rings and moons | No rings; zero to a few moons | All have rings and multiple moons |
“Ice giant” describes bulk composition, not a planet made of ordinary frozen ice. The asteroid belt lies mainly between Mars and Jupiter. Pluto is a dwarf planet in the trans-Neptunian region, not a ninth member of the eight-planet sequence.
Predict an orbital period.
For a small body orbiting a one-solar-mass star, with a in AU and T in years: T² ≈ a³. At a = 4 AU, T = √64 = 8 years. Eccentricity changes the speed distribution around the orbit, but not this period at fixed a.
What is T, in years, for a = 9 AU around a one-solar-mass star?
Hint and solution
T = a³ᐟ² = 9³ᐟ² = 27 years. For a star of mass M in solar masses and negligible orbiting mass, use T² = a³/M.
Investigate and explain
Choose e = 0.6 in Kepler’s laboratory. Compare the two equal-time sectors near perihelion and aphelion. Why are their shapes different while their areas are equal? Then double a and check that T increases by 2³ᐟ², not by 2.