Giant Planets Lecture Notes

Characteristics of the Giant Planets

• Giant planets are named due to their substantial mass, ranging from 15 Earth masses (Uranus/Neptune) to 300 Earth masses (Jupiter), and their large physical size.
• They lack solid surfaces; what is visible are the cloud layers in their atmospheres.

Composition of the Giant Planets

• Jupiter and Saturn are primarily composed of hydrogen and helium and are referred to as gas giants.
• Uranus and Neptune have more water and are called ice giants.

Jupiter (♃)

• Jupiter is the fifth planet from the Sun.
• It is named after the Chief Roman God.
• Jupiter is the largest planet in the solar system.
• 71% of all the matter in the solar system (excluding the Sun) is Jupiter.
• Its distance from the Sun is 5.2 AU.
• A day on Jupiter lasts less than 10 hours.
• Jupiter has the second most moons in the solar system, with 95 confirmed moons.

Cloud Patterns on Jupiter

• Jupiter exhibits strong dark and light bands known as belts and zones.
• These bands are also seen on other giant planets, but are not as distinct as on Jupiter.
• The Great Red Spot is a long-lasting giant storm on Jupiter.
• It is over twice the size of Earth and has persisted for at least 400 years.

Magnetic Fields

• Magnetic fields are generated by the motion of electrically conducting liquids within the planets.
• The orientation of their magnetic fields is at an angle to the rotation axis.

Magnetospheres

• Magnetospheres are extensive; Jupiter’s extends 6 AU.
• They interact with the solar wind.
• This interaction produces auroras, similar to Earth’s Northern Lights, and strong radio waves.

Jupiter's Auroras

• The bright spots around Jupiter’s poles are due to the interaction between Jupiter’s moons and its atmosphere.
• Charged particles stream from the atmospheres of Jupiter’s moons towards the planet.

Saturn (♄)

• Saturn is the sixth planet from the Sun.
• It is named after the Roman Titan of wealth and agriculture.
• Saturn is the second-largest planet in the solar system.
• It is the only planet less dense than water.
• Saturn is 9.6 AU from the Sun.
• It has a prominent and easily visible ring system.
• It has the most confirmed moons in the solar system: 146.

Saturn’s Rings

• Ring particles obey Kepler’s Laws.
• The ring system is complex, consisting of thousands of ringlets.
• Gaps within the rings are not empty.
• The brightness or darkness of rings reflects the amount of material present.
• The ring system is extremely thin, approximately 100 m thick, despite being 75,000 km across.

Ring Stability

• Rings do not last forever; they are destroyed by collisions and sunlight.
• Shepherd moons help stabilize the rings.
• Earth lacks a ring because it does not have shepherd moons to contain the material.

The Ice Giants: Uranus and Neptune

• Uranus and Neptune are smaller and contain more water.
• They are referred to as ice giants.

Uranus (⛢ or ♅)

• Uranus is the seventh planet from the Sun.
• It is named after the Greek Deity of the Sky and is the only planet named after a Greek God.
• Uranus was the first planet discovered by modern astronomers.
• It is 19.2 AU from the Sun.
• Uranus is unique for rotating on its side.
• It has 27 confirmed moons.

Discovery of Uranus

• While Jupiter and Saturn were known to the ancients, Uranus was too faint to be distinguished from other stars.
• It is best seen when Earth is closest to Uranus and with minimal light pollution.
• Discovered in 1781 by William Herschel while stargazing with a homemade telescope.

Naming Controversy of Uranus

• The tradition in astronomy allows the discoverer to name the planet.
• William Herschel initially named the planet Georgium Sidus (George’s Star) after King George III of England.
• This name was not well-received outside Britain.

Proposed Names of Uranus

• Proposed names included Neptune (Roman God of the Sea) and Uranus (Latinized name of the Greek Deity of the Sky).
• Johann Elert Bode proposed Uranus.
• The naming controversy lasted 70 years until Uranus was accepted.

Uranus' Extreme Axial Tilt

• Its 84-year orbit and 98° axial tilt result in extreme seasons.

Neptune (♆ or )

• Neptune is the eighth planet from the Sun.
• It is named after the Roman God of the Sea.
• Neptune was the second planet discovered by modern astronomers.
• It was the first planet discovered using physics.
• Neptune is 30.0 AU from the Sun.
• It is not visible to the unaided eye.
• Neptune has 14 confirmed moons.
• It is the coldest planet in the solar system, with a temperature of -353°F (-214°C).

Discovery of Neptune

• Neptune was discovered because Uranus deviated from its predicted orbit.
• Urbain Le Verrier predicted its existence.
• The gravity of Neptune was tugging on Uranus.
• It was found in 1846 by Johann Galle.
• Galileo had previously seen Neptune but did not recognize it as a planet.

Naming Controversy of Neptune

• Disputes arose over who should name the planet: the predictor (Urbain Le Verrier) or the discoverer (Johann Galle).
• Le Verrier initially wanted to name it Neptune, but later wanted to name it “Le Verrier” after himself.
• Galle suggested Janus, the Roman God of Beginnings.
• Ultimately, Neptune was chosen as the name.

Models of the Interior

• Jupiter and Saturn: gases compress into liquid at depths of a few 1,000 km.

• High pressure and temperature form liquid metallic hydrogen.

• Cores are composed of liquid water and rock.

• Uranus and Neptune: have more water and ices (ammonia, methane) and lower pressures compared to Jupiter and Saturn.

Origins of the Giant Planets

• Jupiter and Saturn formed from the accretion disk while hydrogen/helium were still present.
• The solar wind later blew out these gases.
• Uranus and Neptune formed later by the merger of icy smaller bodies.
• Uranus and Neptune have more heavy elements.
• Uranus and Neptune likely formed closer to the Sun but were pushed further out by Jupiter and Saturn.
• They even swapped order on the way out.

Under the Clouds: Uranus/Neptune

• Unlike Jupiter and Saturn, the highest clouds are composed of methane ice.
• Their bluish color is due to the scattering of light by methane.

Additional Giant Planet Characteristics

• All giant planets have rapid rotation, causing them to be squished instead of perfectly round.
• Strong Coriolis forces result in many storms and high winds; Saturn's winds reach 1600 km/hr.
• They have different axial tilts relative to the ecliptic.
  • Earth: $23.5°$
  • Jupiter: $3°$
  • Uranus: $98°$, which results in extreme seasons