The solar system consists of the Sun at its center, with planets, asteroids, comets, satellites, and other bodies orbiting it. Eight planets revolve around the Sun.
Basic Knowledge
In order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Remember them as “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.”
① Terrestrial planets (Mercury, Venus, Earth, Mars): small, dense, and rocky.
② Jovian planets (Jupiter, Saturn, Uranus, Neptune): large, low-density, and composed of gases and ice.
Mercury and Venus orbit inside Earth's orbit and are therefore inferior planets; Mars and planets farther away are superior planets.
Inferior planets can be seen only in the morning or evening (morning star and evening star), not at midnight.
The Sun is a giant star that makes up about 99.8% of the solar system's mass and produces energy through nuclear fusion (hydrogen → helium) inside it.
Solar system (a collection of celestial bodies within the Sun's gravitational region)
Terrestrial planets (small, dense, and rocky; Mercury, Venus, Earth, Mars)
Jovian planets (large, low-density, and made of gas/ice; Jupiter, Saturn, Uranus, Neptune)
Inferior and superior planets (planets orbiting inside / outside Earth's orbit)
Morning star and evening star (Venus seen in the morning / evening)
Sun (the solar system's only star; shines through nuclear fusion; radius about 70万km)
Further Study (Background and Significance)
The Sun's surface temperature is about 6000K, and its core temperature is about 15 million K. Through nuclear fusion, about 4 million tons of mass are converted into energy every second (E=mc²).
In 2006, Pluto was reclassified as a dwarf planet by a decision of the International Astronomical Union (IAU). Other dwarf planets include Eris, Ceres, Makemake, and Haumea.
The nearest star system to the Sun is Proxima Centauri, about 4.2 light-years away. Light takes 4.2 years to travel that distance. More than 5000 exoplanets (planets orbiting other stars) had been discovered as of 2024, and some are being studied as possible locations for life.
The outer solar system's Kuiper Belt (outside Neptune) and the Oort Cloud (even farther away) are considered sources of comets.
- Eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Terrestrial = Mercury, Venus, Earth, Mars (rocky and dense)
- Jovian = Jupiter, Saturn, Uranus, Neptune (gas/ice and low density)
- Inferior planets = Mercury and Venus (not visible at midnight)
- Morning star and evening star = Venus
- The Sun makes up 99.8% of the mass
- Pluto became a dwarf planet in 2006
Points of Caution (Easily Confused)
① Do not reverse the characteristics of terrestrial and Jovian planets (size, density, and composition). ② Inferior planets are not visible at midnight. ③ Venus is the second planet closest to the Sun, but its surface temperature is 460℃, hotter than Mercury's (427℃) because of the greenhouse effect. ④ Pluto is a dwarf planet, not a planet.
Practice
- List all the planets of the solar system in order from closest to farthest from the Sun.
- List all the planets classified as terrestrial planets.
- What is Venus called when it appears in the western sky in the evening?