What Is a PLANET? A Beginner's Guide

What is a PLANET?

At the simplest level, a PLANET is a natural object in space that orbits a star, is large enough that its own gravity pulls it into a roughly spherical shape, and has removed most other objects from its immediate orbital neighborhood. This definition comes from modern astronomical guidelines and helps distinguish planets from smaller objects such as asteroids, comets, and dwarf planets. Our Solar System contains eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune — each with its own unique properties.

Key characteristics of a PLANET

Understanding what makes an object a PLANET involves three basic traits:

• Orbits a star: A PLANET travels around a star (in our case, the Sun). Objects that orbit stars are part of planetary systems.
• Sufficient mass for roundness: Gravity causes a sufficiently massive body to pull itself into a near-spherical shape. Smaller, irregularly shaped objects do not meet this criterion.
• Cleared its orbit: Over time, a PLANET dominates its orbital zone by either consuming, ejecting, or capturing smaller bodies, leaving a relatively clear path.

How planets differ from related objects

Not everything that orbits a star is a PLANET. A useful comparison:

• Dwarf planets: These are round and orbit a star but have not cleared their orbital region (Pluto is the best-known example).
• Moons (natural satellites): These orbit planets, not stars directly. Earth's Moon orbits Earth, not the Sun.
• Asteroids & comets: Smaller, often irregular bodies that may orbit in belts or on elongated paths.

Variety among planets

PLANETs come in different sizes, compositions, and environments. Broad categories include rocky terrestrial planets like Earth and Mars, and giant planets like Jupiter and Neptune that are mostly gas or ices. Some planets have thick atmospheres and strong magnetic fields; others are airless and cratered. Surface conditions and climate vary widely depending on distance from their star, composition, and geological activity.

Why the definition matters

Classifying an object as a PLANET helps astronomers communicate and organize knowledge about planetary systems. The modern definition clarifies debates that arose when objects such as Pluto were discovered and when thousands of exoplanets (PLANETs orbiting other stars) began to be found. Clear categories help with scientific study, mission planning, and education.

PLANETs beyond our Solar System

Since the 1990s, astronomers have discovered thousands of exoplanets. These PLANETs range from rocky Earth-sized worlds to super-Jupiters far larger than anything in our Solar System. New discoveries constantly expand our understanding of what a PLANET can be, showing that planetary systems are diverse and often different from our own.

How we study PLANETs

Studying planets uses a mix of methods:

• Telescopes: Ground-based and space telescopes observe reflected light, thermal emission, and the effects of PLANETs on their stars.
• Space missions: Probes and orbiters visit PLANETs to gather close-up data (for example, missions to Mars and the gas giants).
• Indirect detection: Techniques like the transit method and radial velocity reveal exoplanets by observing how they dim their star’s light or tug on the star.

Common beginner questions

• Is Pluto a PLANET? Under the modern definition, Pluto is classified as a dwarf planet because it has not cleared its orbit.
• Can a PLANET be habitable? Habitability depends on many factors, such as the presence of liquid water, a suitable atmosphere, and stable conditions. Earth is the only known habitable PLANET so far.
• How many PLANETs are there? In our Solar System there are eight. Beyond that, thousands of exoplanets have been confirmed, and many more candidates await confirmation.

Practical tips for beginners

Start exploring PLANETs by observing the night sky: Jupiter and Saturn can be seen with the naked eye, and a small telescope or binoculars will reveal details like Jupiter’s moons. Read about recent discoveries to follow how definitions and knowledge evolve. Remember that the word PLANET covers a huge variety of worlds — learning about them opens a window into how planetary systems form and change.

Final note

PLANET is a rich and evolving concept in astronomy. From the familiar world under our feet to alien exoplanets orbiting distant stars, each PLANET teaches us more about the universe and our place within it.