The word planet comes from the Greek for “wanderer,” a nod to how these bodies drift across the starry background. Today, we define a planet as any large natural body orbiting a star that does not generate light through nuclear fusion. This definition explicitly excludes comets, asteroids, meteoroids, and moons. In our solar system, there are eight recognized planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Which bodies count as planets today?
The eight planets split into two distinct groups based on composition and distance from the Sun. The first four, Mercury through Mars, are terrestrial planets. They are rocky and lack thick atmospheres. Why? The Sun’s intense heat likely prevented volatile substances in the original solar nebula from condensing into these inner bodies.
The outer four, Jupiter through Neptune, are giant or Jovian planets. They formed in colder regions where gases could condense. This allowed them to grow massive and trap huge atmospheres made mainly of hydrogen and helium. None have a solid surface you could stand on. Their gaseous layers fade into liquid interiors.
Why was Pluto demoted to a dwarf planet?
Pluto spent decades recognized as the ninth planet. It fits neither the terrestrial nor the giant category. It is much smaller than the others and resembles a giant comet nucleus. In 2006, the International Astronomical Union (IAU) reclassified it as a dwarf planet. This move reflected the conclusion that Pluto is simply a very large member of the Kuiper belt.
Other bodies in the outer solar system, like Eris, rival or exceed Pluto in size. This discovery helped cement the new classification. The term minor planet is also used, though it usually refers to asteroids located mostly between the terrestrial and giant planets.
The distinction isn’t just about size; it’s about where things formed and what they made of themselves.
Astrologers still track planetary positions against the 12 zodiac constellations, but the scientific definition has shifted. The line between planet and dwarf planet remains a useful way to map the solar system’s structure. What might surprise you is how many similar bodies still hide in the cold outer edges of our system, waiting to be counted.
