Planetary Science for CSIR NET Earth Science: Solar System, Orbits and Key Formulas

Planetary science is one of the smallest units in the CSIR NET Earth Science syllabus, yet it appears in most papers. Questions draw on a few physical laws and a set of facts about the solar system, which makes it ideal for quick, reliable marks.

What the syllabus covers

• Origin of the solar system: the solar nebula hypothesis, condensation sequence, the frost line, accretion of planetesimals.
• Planets and small bodies: terrestrial versus giant planets, moons, asteroids, comets, Kuiper Belt and Oort Cloud.
• Orbital mechanics: Kepler’s laws, Newton’s law of gravitation, escape velocity, orbital velocity.
• Meteorites: chondrites, achondrites, iron and stony-iron meteorites; chondrules and CAIs as records of the early solar system.
• The Earth–Moon system: origin of the Moon (giant-impact hypothesis), tides, lunar surface and highlands versus maria.
• Planetary surfaces and atmospheres: impact cratering, volcanism and atmospheres of Venus and Mars.

Formulas you must know

1. Kepler’s third law: T² ∝ a³. For bodies orbiting the Sun, T (years)² = a (AU)³.
2. Newtonian form: T² = 4π²a³/GM.
3. Escape velocity: v_e = √(2GM/R), about 11.2 km/s for Earth.
4. Circular orbital velocity: v_o = √(GM/r), which equals v_e/√2 at the same radius.
5. Surface gravity: g = GM/R². It scales with density × radius for bodies of similar composition.
6. Equilibrium temperature: T ∝ (1 − α)^1/4 d^−1/2, where d is the distance from the Sun.

Concepts examiners love

• Why terrestrial planets are rocky and giant planets are gas- and ice-rich: the frost line and the condensation sequence.
• Carbonaceous chondrites as the most primitive meteorites, close to the Sun’s composition apart from volatile elements.
• Crater counting to estimate the relative age of a surface: more craters means an older surface.
• Venus’s runaway greenhouse versus Mars’s thin atmosphere and loss of water.
• Evidence for the giant-impact origin of the Moon: its small iron core and an oxygen-isotope composition matching Earth’s.

One-week revision plan

• Days 1–2: solar system origin and the planets.
• Days 3–4: Kepler’s laws and orbital calculations, solving at least 15 numericals.
• Day 5: meteorites and the Earth–Moon system.
• Days 6–7: planetary surfaces and atmospheres, then a timed set of previous year questions.

Common mistakes

• Forgetting to convert units: AU and years work for Kepler’s simplified law, but SI units are needed for the Newtonian form.
• Confusing escape velocity with orbital velocity.
• Mixing up the meteorite classes and what each one tells us.