Comparative Planetology

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

Basic Patterns

• All planets orbit the Sun in the same orientation (counter clockwise)
• All planets almost in the same plane.
• The sun contains 99.9% of the system’s mass.
• Inner planets are rocky, outer planets are icy or gas-rich.
• Crater production (e.g. impact basins) was larger in the past.

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.

Exceptions

• Uranus and Pluto have significant tilt
• Venus rotates “backwards”
• Triton orbits Neptune “backwards”
• Earth is the only terrestrial planet with relative large moon.
• Pluto and other large trans-neptunian (in the Kiper belt) objects are not considered as planets.

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

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. P_rotation=58.65 days, P_orbit=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

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.

Earth

Never heard of it before.

Mars

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

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

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

Uranus

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

Neptune

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

• Hot Jupiters: jupiter mass objects very close to the star
• Multi “super earth” systems: 2-7 plents each of several Earth radii
• Very distant giant planets at 50-200au from their stars.