Pluto and the Dwarf Planets

For most of the twentieth century, Pluto was the ninth planet, the small, cold outpost at the edge of the solar system.

Then astronomers began finding other worlds out there like it, and the question of what counts as a planet suddenly needed a real answer.

The result was a new category: the dwarf planets.

Here is what that term means, why Pluto was moved into it,

what the first close-up look at Pluto revealed and how it compares with its fellow dwarf planets,

from Ceres in the asteroid belt to distant Eris.

What Makes a Dwarf Planet?

In 2006, the International Astronomical Union, the organization responsible for naming and classifying objects in space, adopted a formal definition.

To be a planet, an object must meet three conditions:

1. It orbits the Sun.
2. It has enough mass for its own gravity to pull it into a nearly round shape.
3. It has cleared the neighborhood around its orbit, meaning it is by far the dominant body in its zone.

A dwarf planet meets the first two conditions but not the third, and it is not a moon of another world.

Pluto is round and orbits the Sun,

but it shares its region with a multitude of other icy bodies in the Kuiper Belt,

so it falls into the dwarf planet category.

How Pluto Lost Its Planet Status

Clyde Tombaugh discovered Pluto in 1930 at Lowell Observatory in Arizona,

after months spent comparing photographs of the same star fields taken on different nights and looking for a dot that moved. Pluto was hailed as the ninth planet,

even though it turned out to be much smaller than first thought, smaller even than Earth's Moon.

In the 1990s, astronomers began finding many other icy objects orbiting beyond Neptune,

confirming that Pluto was part of a large population rather than a lone planet.

The turning point came with the discovery of Eris, a distant world about the same size as Pluto and somewhat more massive.

If Pluto was a planet, Eris had to be one too, and many more might follow.

The 2006 definition resolved the question, and Pluto was reclassified as a dwarf planet.

The decision was not unanimous,

and some planetary scientists still argue for a broader definition based on a body's shape and geology rather than its orbit.

For now, though, the official classification stands.

Pluto Up Close: What New Horizons Found

Until 2015, the best images of Pluto were a few blurry pixels.

That July, NASA's New Horizons spacecraft flew past after a journey of more than nine years and transformed Pluto into a real, astonishingly varied world.

A Frozen Heart

The most famous feature is a bright, heart-shaped region.

Its western half, named Sputnik Planitia, is a vast basin filled with nitrogen ice about 1,000 km (600 miles) across.

The ice slowly churns in huge cells, like a very sluggish lava lamp,

and no impact craters have been found on it, which means its surface is geologically young.

Mountains, Haze and a Hidden Ocean?

New Horizons also found mountains of water ice rising several kilometers high,

glaciers of nitrogen ice flowing into lowlands and a thin atmosphere with layers of blue haze.

Many scientists think Pluto may hide an ocean of liquid water deep beneath its icy shell,

although that has not been confirmed. For a world so small and so far from the Sun,

Pluto turned out to be remarkably active.

Pluto and Charon: A Double World

Pluto's largest moon, Charon, is about half its width, which is enormous for a moon.

The two are so close in size that they orbit a shared balance point in the space between them,

and each keeps the same face turned toward the other at all times.

Some astronomers describe them as a double dwarf planet.

Pluto also has four much smaller moons, named Nix, Hydra, Kerberos and Styx.

Pluto itself takes about 248 years to orbit the Sun, on a stretched,

tilted path that ranges from about 30 to about 49 times Earth's distance from the Sun.

Meet the Other Dwarf Planets

Five bodies have been officially recognized as dwarf planets by the International Astronomical Union. Here is how they compare:

A few details stand out. Ceres is the only dwarf planet in the inner solar system.

It was discovered in 1801 and counted as a planet for several decades before being reclassified as an asteroid.

NASA's Dawn spacecraft orbited it starting in 2015 and found bright deposits of salts, left behind by salty water that seeped up from below.

Haumea spins so fast, once in about four hours, that it has been stretched into an elongated shape,

and it is surrounded by a thin ring.

Eris follows such a far-flung orbit that one trip around the Sun takes more than 550 years.

How Many Are Out There?

The official list is short, but it almost certainly is not complete.

Many astronomers think several other distant bodies, such as Gonggong, Quaoar, Sedna and Orcus, are large enough to be round and would qualify.

Some estimates suggest there could be dozens or even hundreds of dwarf planets in the outer solar system, waiting to be found or confirmed.

Deciding whether a faint, distant object is truly round is difficult,

which is why the official count grows slowly even as the list of candidates gets longer.

Frequently Asked Questions

Could Pluto become a planet again?

Only if the International Astronomical Union changed its definition.

The debate has never fully ended, and some scientists continue to argue for a broader definition,

but under the current rules Pluto remains a dwarf planet.

Is a dwarf planet just a big asteroid?

No. The key difference is shape.

A dwarf planet is massive enough for gravity to pull it round, while most asteroids and small Kuiper Belt objects are irregular lumps.

Dwarf planets can also have atmospheres, moons and complex geology.

Can you see Pluto through a telescope?

Yes, but it is a real challenge.

Pluto is very faint, so you need a fairly large amateur telescope, dark skies and a detailed chart. Even then,

it looks like a tiny star.

Ceres is much easier and can often be found with binoculars when it is well placed.

Small Worlds, Big Discoveries

Losing its planet title did not make Pluto any less interesting.

If anything, it revealed that Pluto belongs to a whole family of small, surprising worlds.

As telescopes and spacecraft keep probing the outer solar system, the story of the dwarf planets is only getting richer.