The Solar System — The Sun, the Planets, and Their Order

Lecture 2 Overview: Our Neighborhood, the Solar System

Now that we’ve grasped the scale of the universe (Lecture 1), let’s tour our own home: the solar system. It’s a single star (the Sun) and the family of planets, moons, and small bodies bound to it by gravity.

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1. The Sun — 99.8% of the Solar System’s Mass

The Sun is a star that accounts for about 99.86% of the solar system’s total mass. It generates energy through nuclear fusion, converting hydrogen into helium.

• Composition: about 74% hydrogen + about 24% helium
• Surface temperature: about 5,500°C / core: about 15 million °C
• Energy source: hydrogen fusion (about 600 million tons of hydrogen fused every second)

The Sun’s overwhelming gravity is what keeps every planet locked in orbit. For more on the mechanics behind this, see Intro to Gravity and Orbits.

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2. Two Types of Planets

Category	Rocky (terrestrial)	Giant gas/ice (Jovian)
Planets	Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus, Neptune
Size	Small and heavy (dense)	Large and light (low density)
Surface	Solid rock	No solid surface (gas, ice)
Moons/rings	Few	Many (well-developed rings)

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3. The Planets at a Glance

Sun — Mercury — Venus — Earth — Mars — [Asteroid Belt]
— Jupiter — Saturn — Uranus — Neptune — [Kuiper Belt]

- Mercury: the smallest and closest to the Sun
- Venus: a thick carbon dioxide atmosphere, the hottest planet (about 460°C)
- Earth: liquid water and life; axial tilt gives us seasons
- Mars: a red desert, with traces of past water
- Jupiter: the largest planet, home to the Great Red Spot (a giant storm)
- Saturn: famous for its spectacular rings
- Uranus: tipped on its side / Neptune: the strongest winds in the solar system

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4. Moons and Small Bodies

• Moons: bodies orbiting a planet. Earth’s Moon helps stabilize its tides and seasons.
• Asteroids: found mainly in the asteroid belt between Mars and Jupiter.
• Comets: “dirty snowballs” of ice and dust that grow a tail as they approach the Sun.
• Dwarf planets: Pluto, Ceres, and others. Pluto was reclassified from planet to dwarf planet in 2006.

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5. Why Pluto Is No Longer a Planet

In 2006, the International Astronomical Union (IAU) formally defined what counts as a “planet.”

Three conditions for a planet:
1. It orbits the Sun
2. It's massive enough to be round
3. It has cleared its orbital neighborhood of other objects

→ Pluto fails condition 3, so it was reclassified as a "dwarf planet."

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Key Takeaways

1. The Sun is a hydrogen-fusing star that makes up about 99.86% of the solar system’s mass.
2. Planets split into rocky worlds (the inner four) and giant gas/ice worlds (the outer four).
3. Small bodies cluster in the asteroid belt at the boundary and the Kuiper Belt further out.
4. Moons, asteroids, comets, and dwarf planets fill out the rest of the solar system.
5. Pluto became a dwarf planet because it fails the third condition of the planet definition — clearing its orbital neighborhood.

Next lecture: How stars are born and how they die. We’ll follow a star’s life from its birth in a nebula, through the main sequence, to an ending as a supernova, white dwarf, neutron star, or black hole.