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what makes a planet a planet

A “planet” is defined by three main rules: it must orbit the Sun, be big enough to be (roughly) round, and be gravitationally dominant in its path so it clears its neighborhood of similar‑sized objects.

What Makes a Planet a Planet?

Quick Scoop

If you zoom out and look at our solar system today, only eight objects officially get to wear the “planet” badge: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. That wasn’t always the story—Pluto used to be in the club until astronomers tightened the rules in 2006.

In 2006, the International Astronomical Union (IAU) voted on a formal definition of “planet,” and that decision is still shaping textbooks and online debates in 2026. It was a huge moment in space “culture,” with arguments in classrooms, on forums, and across social media that still flare up whenever Pluto’s name is mentioned. The controversy actually helped a lot of people learn what a planet is in the first place.

The 3 Official Criteria (IAU Definition)

To qualify as a planet in our solar system, an object must:

  1. Orbit the Sun
  2. Be (roughly) round
  3. Clear its neighborhood

Let’s break that down.

1. It must orbit the Sun

  • The object has to go around the Sun directly, not around another planet.
  • If it orbits a planet (like our Moon or Jupiter’s moons), we call it a moon , no matter how big or round it is.

This is why big moons like Ganymede and Titan, which are larger than Mercury, are still “moons,” not planets—they orbit Jupiter and Saturn, not the Sun.

2. It must be massive enough to be round

  • The object needs enough mass and gravity to pull itself into an approximately spherical shape.
  • This state is called hydrostatic equilibrium : gravity pulls inward equally in all directions, smoothing out big bumps over time.

Smaller objects like most asteroids are irregular, lumpy rocks because they don’t have enough mass for gravity to reshape them into a ball.

3. It must clear its neighborhood

This is the trickiest but most important part.

  • The planet must be gravitationally dominant in its orbit.
  • Over time, it either:
    • Sweeps up smaller objects,
    • Slingshots them away,
    • Or locks them as moons.
  • There should be no other objects of comparable size sharing similar orbits, aside from its own satellites.

This rule is what knocked Pluto out of the classic “planet list” and into the “dwarf planet” category.

Why Pluto Is Not a Planet (Under This Definition)

Pluto checks some boxes—but not all:

  • Orbits the Sun? Yes.
  • Round? Yes, it’s massive enough to be spherical.
  • Cleared its neighborhood? No.

Pluto lives in the Kuiper Belt , a region beyond Neptune filled with icy bodies and debris. There are many other objects of similar size sharing that zone, and Pluto’s gravity is not strong enough to dominate and “clean up” that area.

So in 2006, Pluto was reclassified as a dwarf planet : round, orbiting the Sun, but not gravitationally dominant. It now shares that label with other large Kuiper Belt objects like Eris and others highlighted in astronomy discussions.

Planets vs Other Space Objects (Quick HTML Table)

Here’s a compact look at how the IAU definition separates planets from other categories:

html

<table>
  <thead>
    <tr>
      <th>Type</th>
      <th>Orbits</th>
      <th>Shape</th>
      <th>Clears Neighborhood?</th>
      <th>Example</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Planet</td>
      <td>Sun</td>
      <td>Roughly spherical</td>
      <td>Yes</td>
      <td>Earth, Jupiter</td>
    </tr>
    <tr>
      <td>Dwarf planet</td>
      <td>Sun</td>
      <td>Roughly spherical</td>
      <td>No</td>
      <td>Pluto, Ceres</td>
    </tr>
    <tr>
      <td>Moon (natural satellite)</td>
      <td>A planet or dwarf planet</td>
      <td>Often spherical if large</td>
      <td>Not applicable (orbits another body)</td>
      <td>Moon, Europa, Titan</td>
    </tr>
    <tr>
      <td>Asteroid</td>
      <td>Usually Sun</td>
      <td>Irregular</td>
      <td>No</td>
      <td>Vesta, Eros</td>
    </tr>
    <tr>
      <td>Kuiper Belt object</td>
      <td>Sun</td>
      <td>Irregular or spherical</td>
      <td>No</td>
      <td>Many small icy bodies beyond Neptune</td>
    </tr>
  </tbody>
</table>

Ongoing Debates and New Ideas

Astronomers and space fans still argue about what makes a planet a planet , especially as we discover thousands of planets around other stars (exoplanets).

Some key debate points:

  • IAU definition is Sun‑specific
    The official wording is framed for objects that orbit “the Sun.” For exoplanets, scientists usually apply the same three concepts but replace “Sun” with “star.”
  • Should “round = planet”?
    Some researchers propose that anything large enough to be round and orbit a star should be called a planet, even if it hasn’t fully cleared its orbit. This would put Pluto and many others back into the planet club.
  • Cultural vs scientific definition
    In popular culture and on forums, people often treat Pluto as an “emotional planet”—many grew up with nine planets and feel attached to that list. Scientifically, though, the clearing‑the‑neighborhood rule keeps the classification tied to orbital dynamics rather than nostalgia.

On forums like r/explainlikeimfive and r/Astronomy, people often share simplified versions of the rules, sometimes joking that “you know a planet when you see it,” but then circle back to the same three criteria: star‑orbiting, round, and orbit‑clearing.

Why the Definition Changed in the 2000s

The 2006 change did not come out of nowhere. It was driven by new discoveries and the need for consistency.

  • Astronomers discovered multiple Pluto‑like bodies in the outer solar system (e.g., Eris), some roughly similar in size.
  • If Pluto remained a planet, many more objects would also qualify, potentially giving us dozens of “planets.”
  • The IAU wanted a definition that:
    • Reflected an object’s role in shaping its region , not just size.
    • Was simple enough to teach and use.

So they settled on the three‑part definition: orbit, roundness, and neighborhood clearing.

“Latest News” Angle in 2020s Context

Even in the mid‑2020s, “what makes a planet a planet” is a trending topic every time:

  • New exoplanets are found that challenge simple categories (e.g., super‑Earths or mini‑Neptunes).
  • Space agencies keep publishing kid‑friendly explainers and interactive content reminding people of the IAU’s criteria and why Pluto is a dwarf planet.
  • Online discussions frequently revisit whether the current definition should be updated as we learn more about planetary systems across the galaxy.

So while the official rulebook hasn’t changed since 2006, the conversation around “what makes a planet a planet” is very much alive.

TL;DR – What Makes a Planet a Planet?

A planet, in the official IAU sense, is a body that:

  • Orbits the Sun.
  • Is massive enough that gravity makes it roughly spherical.
  • Has cleared its orbital neighborhood of similarly sized objects, making it the dominant body on its path.

Anything that fails the last part—but still orbits the Sun and is round—gets labeled a dwarf planet , which is where Pluto currently lives.

Information gathered from public forums or data available on the internet and portrayed here.