Planets
The term planet originates from the Greek word planēt, meaning “wanderer.” In modern usage, Merriam-Webster defines a planet as “any of the large bodies that revolve around the Sun in the solar system.”
In 2006, the International Astronomical Union (IAU) provided a formal definition of a planet:
- It must orbit the Sun.
- It must be massive enough to be nearly round in shape.
- It must have cleared its orbit of other debris.
Objects that satisfy the first two conditions but not the third are termed dwarf planets. This decision led to Pluto’s reclassification from a planet to a dwarf planet.
Based on this definition, there are eight officially recognized planets in our solar system.
Inner Planets (Terrestrial Planets)
These are the four planets closest to the Sun—Mercury, Venus, Earth, and Mars. They are rocky, dense, and have solid surfaces.
Mercury
- Nearest planet to the Sun.
- Smallest planet in the solar system.
- Surface marked by craters, similar to Earth’s Moon.
- No atmosphere and no moons.
Venus
- Second planet from the Sun.
- Sixth largest planet.
- Dense carbon dioxide atmosphere with sulfuric acid clouds.
- Experiences a runaway greenhouse effect → hottest planet in the solar system.
- Rotates retrograde (east to west).
Earth
- Third planet from the Sun.
- Fifth largest planet.
- Only planet known to support life.
- Atmosphere rich in nitrogen and oxygen.
- One natural satellite – the Moon.
Mars
- Fourth planet from the Sun.
- Seventh largest planet.
- Known as the “Red Planet” due to iron oxide dust.
- Thin CO₂ atmosphere; evidence of past water flows.
- Two small moons: Phobos and Deimos.
Outer Planets (Giant Planets)
These do not have solid surfaces. Instead, they consist of swirling gases around dense cores.
Jupiter (Gas Giant)
- Fifth planet from the Sun.
- Largest planet in the solar system.
- Distinctive features: Great Red Spot, strong magnetic field.
- 90+ moons, including Ganymede, the largest moon in the solar system.
Saturn (Gas Giant)
- Sixth planet from the Sun.
- Second largest planet.
- Famous for its bright ring system of ice and dust.
- 80+ moons, including Titan, larger than Mercury and with a dense atmosphere.
Uranus (Ice Giant)
- Seventh planet from the Sun.
- Third largest planet.
- Rotates on its side (axial tilt of ~98°).
- Atmosphere contains methane, giving it a pale blue color.
Neptune (Ice Giant)
- Eighth and farthest planet from the Sun.
- Fourth largest planet.
- Atmosphere rich in methane, giving it a deep blue color.
Dwarf Planets
The IAU recognizes five official dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.
Ceres
- Largest object in the asteroid belt (between Mars and Jupiter).
- Only dwarf planet located in the inner solar system.
Pluto
- Once considered the ninth planet of the solar system.
- Reclassified as a dwarf planet in 2006.
- Diameter ~2,377 km.
- Has five moons, including Charon.
Haumea
- Located in the Kuiper Belt.
- Distinctly oval-shaped due to rapid rotation.
- Among the fastest rotating large objects in the solar system.
Makemake
- Slightly smaller than Pluto.
- Second-brightest object in the Kuiper Belt (after Pluto).
Eris
- More massive than Pluto, though slightly smaller in size.
- Its discovery triggered the debate leading to Pluto’s demotion.
- Pluto’s reclassification was finalized in 2006 by IAU.
What Happened to Pluto?
For decades, Pluto was taught in textbooks as the ninth planet of the solar system. However, in 2006, the IAU formally reclassified it as a Dwarf Planet. While Pluto orbits the Sun and is completely spherical, it failed the third criterion: it has not “cleared its neighbourhood.” Pluto is located in the Kuiper Belt, a region beyond Neptune, where it shares its orbital zone with thousands of other icy, rocky bodies of similar size.
