Solar System Planets Characteristics

Why This Matters

When you study planetary characteristics in space physics, you're not just memorizing facts about eight worlds—you're learning to decode the physical laws that govern everything from orbital mechanics to atmospheric retention. Every characteristic you'll encounter connects back to fundamental principles: gravitational dynamics, thermodynamics, electromagnetic interactions, and planetary differentiation. The exam will test whether you understand why Mercury can't hold an atmosphere while tiny Titan can, or how a planet's rotation rate connects to its magnetic field strength.

Don't approach this as a list of isolated facts. Instead, think of each planet as a natural laboratory demonstrating specific physics concepts. When you see a question about Venus's extreme temperatures or Jupiter's powerful magnetosphere, you're being tested on your ability to connect observable characteristics to underlying mechanisms. Know what concept each planetary feature illustrates, and you'll be ready for both multiple choice and FRQ challenges.

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Gravitational Dynamics: Mass, Size, and Their Consequences

A planet's mass determines its gravitational pull, which cascades into nearly every other characteristic—from atmospheric retention to internal differentiation.

Planetary Sizes and Masses

• Jupiter is 318 times Earth's mass—this enormous mass creates gravitational forces strong enough to influence asteroid trajectories and retain light gases like hydrogen
• Escape velocity scales with mass and radius—smaller planets like Mercury ($v_{escape} \approx 4.3 \text{ km/s}$) lose atmospheric gases more easily than Earth ($v_{escape} \approx 11.2 \text{ km/s}$)
• Mass drives internal heat retention—larger planets maintain molten cores longer, enabling sustained geological activity and magnetic field generation

Density and Gravity

• Terrestrial planets average 4-5.5 g/cm³—their rocky, iron-rich compositions contrast sharply with gas giants at 0.7-1.6 g/cm³
• Surface gravity depends on both mass and radius—Saturn, despite being 95 Earth masses, has surface gravity only slightly higher than Earth's due to its enormous radius
• Density reveals internal structure—a planet's mean density indicates the proportion of heavy elements versus volatiles, a key diagnostic for planetary classification

Moons and Ring Systems

• Gravitational capture and accretion explain moon formation—Jupiter's 95 known moons demonstrate how massive planets dominate their orbital neighborhoods
• Ring systems exist within the Roche limit—Saturn's rings persist where tidal forces prevent particle aggregation into moons ($d_{Roche} \approx 2.44 R_p \left(\frac{\rho_p}{\rho_m}\right)^{1/3}$)
• Moon-planet interactions drive tidal heating—Io's extreme volcanism results from gravitational flexing by Jupiter, not radioactive decay

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Orbital Mechanics: Motion Around the Sun

Kepler's laws and gravitational physics govern how planets move, and orbital parameters directly affect climate, seasons, and energy received from the Sun.

Orbital Characteristics

• Orbital period follows Kepler's third law—$T^2 \propto a^3$, so Mercury's 88-day year versus Neptune's 165-year orbit reflects their vastly different semi-major axes
• Eccentricity measures orbital elongation—Mercury's high eccentricity (0.206) causes significant variation in solar radiation, while Venus's near-circular orbit (0.007) keeps conditions more stable
• Inclination affects seasonal patterns—most planets orbit within a few degrees of the ecliptic, but Pluto's 17° inclination contributed to its reclassification debates

Rotation Periods and Axial Tilts

• Rotation period varies from 10 hours (Jupiter) to 243 days (Venus)—faster rotation correlates with stronger magnetic field generation through the dynamo effect
• Axial tilt drives seasonal intensity—Earth's 23.5° tilt creates moderate seasons, while Uranus's 98° tilt causes poles to face the Sun directly during solstices
• Retrograde rotation occurs on Venus and Uranus—Venus rotates clockwise when viewed from above, likely due to a massive ancient impact event

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Composition and Structure: What Planets Are Made Of

The solar nebula's temperature gradient during formation determined which materials condensed where, creating the fundamental terrestrial/giant planet divide.

Composition and Internal Structure

• Terrestrial planets formed inside the frost line—only refractory materials (metals, silicates) could condense in the hot inner solar system, producing small, dense worlds
• Gas giants formed beyond the frost line—where water ice and other volatiles condensed, enabling rapid core growth and massive hydrogen/helium envelope capture
• Differentiation separates materials by density—all planets developed layered structures with heavy elements sinking to form cores, driven by gravitational potential energy release

Surface Features and Geology

• Impact craters record bombardment history—Mercury and the Moon preserve ancient craters, while Earth's surface constantly erases them through erosion and tectonics
• Volcanism indicates internal heat—active volcanism on Earth and Io versus extinct volcanism on Mars reveals different thermal evolution paths
• Tectonic activity requires sustained internal heat—Earth's plate tectonics is unique among terrestrial planets, driven by mantle convection and radioactive decay

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Atmospheric Physics: Retention, Composition, and Climate

Whether a planet keeps an atmosphere depends on the balance between gravitational binding energy and thermal energy of gas molecules—the Jeans escape mechanism.

Atmospheric Properties

• Atmospheric retention requires sufficient gravity—a molecule escapes when its thermal velocity exceeds escape velocity; this is why hot, low-mass planets lose atmospheres fastest
• Venus's 92-bar $CO_2$ atmosphere creates a runaway greenhouse effect with 465°C surface temperatures—demonstrating how composition trumps distance from Sun
• Mars's thin 0.006-bar atmosphere cannot support liquid surface water today, though evidence suggests a denser past atmosphere before solar wind stripping

Temperature Ranges

• Solar flux decreases with distance squared—Mercury receives 6.7 times more solar energy per unit area than Earth, while Neptune receives only 1/900th
• Greenhouse effect amplifies temperature—Venus is hotter than Mercury despite being farther from the Sun, proving atmospheric composition matters more than proximity
• Extreme temperature swings indicate thin atmospheres—Mercury's 430°C day to -180°C night variation occurs because no atmosphere retains or distributes heat

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Electromagnetic Properties: Magnetic Fields and Protection

Planetary magnetic fields arise from the dynamo effect—convection of electrically conducting fluid in a rotating planet's interior generates self-sustaining magnetic fields.

Magnetic Fields

• Earth's magnetic field strength is approximately 25-65 microtesla—generated by convection in the liquid iron outer core combined with planetary rotation
• Jupiter's magnetosphere is the largest structure in the solar system—its rapid rotation and metallic hydrogen interior create a field 20,000 times stronger than Earth's
• Mars lacks a global magnetic field today—only crustal remnants remain, indicating its core solidified and dynamo action ceased billions of years ago

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Quick Reference Table

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Self-Check Questions

1. Comparative analysis: Both Venus and Mars have $CO_2$-dominated atmospheres, yet their surface conditions are drastically different. What two factors explain this difference, and how do they connect to atmospheric physics principles?

2. Concept identification: Which two planets best demonstrate how axial tilt affects seasonal patterns, and what specific outcomes result from their different tilts?

3. Mechanism application: If a planet has no magnetic field but once did, what does this tell you about changes in its internal structure? Use Mars as your example.

4. Compare and contrast: Jupiter and Saturn are both gas giants, yet Saturn's density is lower than water's while Jupiter's is higher. Explain this difference using the relationship between mass, radius, and gravitational compression.

5. FRQ-style synthesis: An exoplanet is discovered with Earth's mass but orbits much closer to its star. Predict what would happen to its atmosphere over time, citing at least two physical mechanisms from this guide.