ASTR 200


Updated 2019-09-18

Comparative Planetology

This is the study of similarities and differences of “planets”, including moons, asteroids and comets.

Basic Patterns

Terrestrial planets: inner planets that are small, rocky, solid surface, has few moons

Jovian planets: large size and mass, low density, now solid surface, farther from the sun, cooler, many moons or rings.


The sun’s gravity regulates the orbit of the planets. The emitted energy is the primary factor which determines the temperature of the planets. If provides nearly all of the visible lights in the solar system. The high-energy particles streaming out from the Sun affect planetary atmospheres and magnetic fields (e.g. Aurora borealis)

Each Planet in Detail


Mercury is a small rock with lots of crater impacts orbiting close to the sun. It rotates 3 times for every 2 times it orbits the Sun. Protation=58.65 days, Porbit=87.92 days.

This is because Mercury is close to the Sun and is tidally locked (like how our Moon is tidally locked to Earth).


Venus is closest to Earth in terms of the geometric properties. It has a thick atmosphere with runaway greenhouse effect. The atmosphere is compoased of CO2 and sulfric acid.

Radar is needed to map out the typography of the surface. It has no detectable magnetic field.


Never heard of it before.


Mars is Earth-like in some ways that it may have had oceans in the past. The atmosphere is 95% CO2 and is quite thin. The northern hemisphere has more geological activity like volcanic activity, and has lower elevations and few craters. The southern hemisphere has high elevation, many craters, and no volcanic activity.


Jupiter has more mass than all the planets put together. It is still 1000 times less massive than the Sun. Some of its moons are sizes of terrestrial planets.

The structure consists of rocky core, ice and metallic hydrogen (compressed). Jupiter generate heat by radial contraction.


Saturn has flat rings which is due to tidally disrupted satellites. The planetary structure is similar to Jupiter.


Uranus is much smaller than Jupiter and saturn and are mostly made of ice. It has 4 rings and 10 moons


Similar to Uranus

Asteroid Belt

Between Mars and Jupiter. Objects found here are mostly rocky.

Kuiper Belt

Beyond Neptune. Pluto is a member of Kuiper Belt.

Comets and Asteroids

They hold clues to formation of planets. Asteroids are rocky and mostly found in asteroid belt.

Comets are generally icy and produce long trails if they’re in the inner solar system. Comet reservoirs are the Kuiper belt and Oort cloud (which is even further out).

Different Planetary Configurations