Interactive physics simulator
Planets of the Solar System
Explore all eight planets orbiting the Sun in a live, to-scale orrery. Click any planet to inspect its physical data — mass, radius, surface gravity, orbital period, and distance from the Sun — with all values computed in real time from accurate orbital mechanics.
Solar System Orrery
Click any planet for detailed data. Drag the speed slider to accelerate time. Toggle labels and orbits with the panel controls.
Live Orbital Data
- Planet
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- Distance (AU)
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- Orbital Period
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- Orbital Speed
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- Surface Gravity
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- Type
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- Moons
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- Year Elapsed
- 0.00 yr
What is a Planet?
A planet is a celestial body that (1) orbits a star, (2) has sufficient mass for self-gravity to create a roughly spherical shape (hydrostatic equilibrium), and (3) has gravitationally "cleared the neighbourhood" around its orbit. Our Solar System contains eight planets in two broad categories: four inner terrestrial planets (Mercury, Venus, Earth, Mars) and four outer giant planets — two gas giants (Jupiter, Saturn) and two ice giants (Uranus, Neptune).
Planetary motion is governed by Kepler's Three Laws. The third law is most useful for comparing planets: T² ∝ a³, where T is the orbital period (years) and a is the semi-major axis (AU). Orbital speed is given by v = 2πa / T. Surface gravity follows g = GM / R².
The Eight Planets
| Planet | Type | AU | Period | Moons |
|---|---|---|---|---|
| Mercury | Terrestrial | 0.387 | 88 d | 0 |
| Venus | Terrestrial | 0.723 | 225 d | 0 |
| Earth | Terrestrial | 1 | 1.0 yr | 1 |
| Mars | Terrestrial | 1.524 | 1.9 yr | 2 |
| Jupiter | Gas Giant | 5.203 | 11.9 yr | 95 |
| Saturn | Gas Giant | 9.537 | 29.5 yr | 146 |
| Uranus | Ice Giant | 19.19 | 84.0 yr | 28 |
| Neptune | Ice Giant | 30.07 | 164.8 yr | 16 |
Key Formulas
T² ∝ a³ (Kepler's 3rd Law)
T = orbital period (yr), a = semi-major axis (AU)
v = 2πa / T (Orbital speed)
g = GM / R² (Surface gravity)
G = 6.674 × 10-11 N·m²/kg²
vesc = √(2GM / R) (Escape velocity)
Planet Comparison
| Planet | Mass (M⊕) | Radius (km) | Surface g (m/s²) |
|---|---|---|---|
| Mercury | 0.055 | 2,440 | 3.7 |
| Venus | 0.815 | 6,052 | 8.9 |
| Earth | 1.000 | 6,371 | 9.8 |
| Mars | 0.107 | 3,389 | 3.7 |
| Jupiter | 317.8 | 71,492 | 24.8 |
| Saturn | 95.16 | 60,268 | 10.4 |
| Uranus | 14.54 | 25,559 | 8.9 |
| Neptune | 17.15 | 24,764 | 11.2 |
Planet Types
Terrestrial (rocky) planets:
- Mercury — heavily cratered, no atmosphere, extreme temp swings.
- Venus — runaway greenhouse effect, hottest planet at ~465 °C.
- Earth — liquid water, oxygen atmosphere, plate tectonics.
- Mars — thin CO₂ atmosphere, Olympus Mons (largest volcano).
Gas/Ice Giants:
- Jupiter — Great Red Spot storm, 95 moons including Ganymede.
- Saturn — spectacular ring system, lowest density (would float on water).
- Uranus — tilted 98°, rotates retrograde on its side.
- Neptune — fastest winds (~2,100 km/h), Great Dark Spot.
Solved Examples
The orbital period of Mars is 1.88 Earth years and its semi-major axis is 1.52 AU. Verify Kepler's Third Law (T² ∝ a³).
- Kepler's Third Law: T² = k × a³, where k is the same for all planets orbiting the Sun.
- Calculate T²: (1.88)² = 3.534 yr²
- Calculate a³: (1.52)³ = 3.512 AU³
- T² / a³ = 3.534 / 3.512 ≈ 1.006 ≈ 1 ✓ (matches Earth's ratio)
Answer: T² / a³ ≈ 1 (in units yr² / AU³) — Kepler's Third Law verified for Mars.
Calculate the orbital speed of Earth around the Sun. (Radius = 1.5 × 10¹¹ m, T = 3.156 × 10⁷ s)
- Orbital speed: v = 2πr / T
- v = 2π × 1.5 × 10¹¹ / 3.156 × 10⁷
- v = (9.425 × 10¹¹) / (3.156 × 10⁷)
- v ≈ 2.985 × 10⁴ m/s ≈ 29.85 km/s
Answer: Earth's orbital speed ≈ 29.8 km/s
Jupiter's mass is 1.898 × 10²⁷ kg and its radius is 7.149 × 10⁷ m. Find the surface gravity.
- Surface gravity: g = GM / R²
- g = (6.674 × 10⁻¹¹ × 1.898 × 10²⁷) / (7.149 × 10⁷)²
- g = (1.267 × 10¹⁷) / (5.111 × 10¹⁵)
- g ≈ 24.8 m/s²
Answer: Jupiter surface gravity ≈ 24.8 m/s² (about 2.53g)
Common Mistakes
- Confusing rotation (spin on own axis = day) with revolution (orbit = year).
- Thinking the Sun is at the center of circular orbits — orbits are ellipses (Kepler's 1st Law).
- Confusing mass and weight — Jupiter's mass is 317 Earth masses, but its surface density is low (1,326 kg/m³).
- Saying Venus is the closest planet to the Sun — Mercury is. Venus is second.
- Assuming farther planets are always colder — Venus is hotter than Mercury due to the greenhouse effect.
Practice Questions
1. List the eight planets in order from the Sun.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
2. What is the difference between a terrestrial planet and a gas giant?
Terrestrial planets (Mercury, Venus, Earth, Mars) have solid rocky surfaces and are smaller and denser. Gas giants (Jupiter, Saturn) are huge, mostly hydrogen and helium with no solid surface. Ice giants (Uranus, Neptune) are intermediate with water, ammonia, and methane ices.
3. Which planet has the longest day in the Solar System?
Venus — its rotation period (243 Earth days) is longer than its orbital period (225 Earth days), and it rotates retrograde (opposite to most planets).
4. Which planet has the most moons?
Saturn — with over 140 confirmed moons, it holds the record as of recent discoveries.
5. What is the Goldilocks zone?
The Goldilocks (or habitable) zone is the range of orbital distances from a star where liquid water can exist on a planet's surface — not too hot, not too cold. Earth is well within the Sun's Goldilocks zone.
Quick Summary
- Eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Terrestrial planets are small, rocky, and dense; gas/ice giants are large and low-density.
- Kepler's Third Law: T² ∝ a³ — farther planets have much longer orbital periods.
- Orbital speed: v = 2πa/T; inner planets move faster than outer ones.
- Surface gravity: g = GM/R² — varies enormously across planets.
- Venus is the hottest (greenhouse effect); Mercury has the most extreme temperature swings.
- Saturn's rings are made of ice and rock; Uranus is tilted 98° and rotates retrograde.
Frequently Asked Questions
How many planets are in the Solar System?
Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a "dwarf planet" by the IAU in 2006.
What defines a planet?
According to the IAU (2006), a planet must: (1) orbit the Sun, (2) have sufficient mass for gravity to make it roughly spherical, and (3) have "cleared the neighborhood" around its orbit of other debris.
What is a terrestrial planet?
A terrestrial (rocky) planet is a dense, solid-surfaced world primarily made of silicate rocks and metals. The four terrestrial planets in our Solar System are Mercury, Venus, Earth, and Mars.
What is a gas giant?
A gas giant is a large planet composed mainly of hydrogen and helium with no solid surface. Jupiter and Saturn are gas giants; they are much larger and less dense than terrestrial planets.
Why do the outer planets take longer to orbit the Sun?
By Kepler's Third Law (T² ∝ a³), the farther a planet is from the Sun, the longer its orbital period. Neptune, at 30 AU, takes about 165 Earth years to complete one orbit.
What is a planet's orbital period?
The orbital period is the time a planet takes to complete one full revolution around the Sun. Earth's orbital period is 365.25 days (1 year); Mercury's is 88 days; Neptune's is 165 years.
Which planet is hottest and why?
Venus (~465 °C surface temperature) is hotter than Mercury, despite being farther from the Sun. Venus's thick CO₂ atmosphere creates a runaway greenhouse effect that traps heat.
What is the Great Red Spot?
The Great Red Spot is a persistent high-pressure anticyclonic storm on Jupiter, larger than Earth, that has been observed for at least 350 years.
What are the rings of Saturn made of?
Saturn's rings are composed mainly of ice particles (water ice), rocky debris, and dust ranging in size from micrometers to several meters, extending up to 282,000 km from the planet.
Does any planet rotate in a different direction?
Yes — Venus rotates retrograde (east to west), so the Sun rises in the west on Venus. Uranus has an axial tilt of 98°, essentially spinning on its side.
What is the habitable zone?
The habitable zone (Goldilocks zone) is the orbital range around a star where liquid water can exist on a surface. Earth sits squarely within the Sun's habitable zone (roughly 0.95–1.37 AU).
How is a planet's surface gravity calculated?
Surface gravity g = GM/R², where G is the gravitational constant (6.674 × 10⁻¹¹ N·m²/kg²), M is the planet's mass, and R is its radius. Jupiter's surface gravity is about 24.8 m/s² (2.5g).