Astronomy
The planets — the 5 questions people ask most
From "how many planets are there" to "why is Mars red," here are the five most common questions about our solar system, answered plainly.
1. How many planets are there in the solar system?
Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union, joining four other officially recognized dwarf planets — Ceres, Haumea, Makemake, and Eris.
| # | Planet | Known for |
|---|---|---|
| 1 | Mercury | Closest to the Sun, smallest planet |
| 2 | Venus | Hottest planet due to a runaway greenhouse effect |
| 3 | Earth | The only known planet with life |
| 4 | Mars | The Red Planet, home to the solar system's tallest volcano |
| 5 | Jupiter | Largest planet, a gas giant with dozens of moons |
| 6 | Saturn | Famous for its extensive ring system |
| 7 | Uranus | An ice giant that rotates on its side |
| 8 | Neptune | The windiest planet, farthest from the Sun |
2. Which planet is closest to the Sun?
Mercury — the smallest planet in the solar system, and also the fastest, completing a full orbit in about 88 Earth days. Despite being closest to the Sun, it isn't the hottest planet; that title goes to Venus, whose thick atmosphere traps heat far more effectively.
3. Why is Mars called the Red Planet?
Mars's surface is covered in iron oxide — essentially rust — giving it its distinct reddish color, visible even to the naked eye from Earth. Mars is also home to Olympus Mons, the largest known volcano in the solar system, roughly three times the height of Mount Everest, and has been the primary target of the search for past microbial life due to strong evidence of ancient liquid water on its surface.
4. What is the biggest planet in the solar system?
Jupiter, a gas giant so massive that its mass alone is more than twice that of every other planet in the solar system combined. It's famous for the Great Red Spot, a giant, centuries-old storm larger than Earth, and for having dozens of moons — including Ganymede, the largest moon in the solar system.
5. Why does Saturn have rings?
Saturn's rings are made almost entirely of ice particles, ranging from tiny grains to chunks the size of a house, with a small amount of rocky debris mixed in. They're thought to be the remnants of a moon, comet, or asteroid that wandered too close and broke apart under Saturn's gravity. Saturn isn't the only ringed planet — Jupiter, Uranus, and Neptune all have much fainter rings — but Saturn's are by far the most visible and extensive.
How do I spot planets in tonight's sky?
Unlike stars, planets don't twinkle — they shine with a steadier light, since they're close enough to appear as tiny discs rather than pinpoints. Venus and Jupiter are usually the brightest points of light in the sky after the Moon, often visible near sunrise or sunset. Because planet visibility shifts through the year as they orbit the Sun, the most reliable way to know exactly what's up on a given night is a planetarium app or observatory listing — but as a rule of thumb, Jupiter and Saturn are each visible for several consecutive months at a time when in season, while Mars and Venus cycle in and out of view more quickly. Check our monthly night sky guide for what constellations frame the sky this month, and pair it with a dark night — check tonight's moon phase for how much moonlight you'll be working against.