Is Pluto still a planet, or do astronomers call it a dwarf planet?

When it was discovered in 1930, Pluto was considered to be the ninth planet of the Solar System. It’s quite tiny, though: about half the size of our own Moon.

(Pluto is bottom left)

But over time, astronomers began to realise just how many similar objects there are out there beyond Neptune, in what is called the Kuiper Belt (which is kind of similar to the Asteroid Belt between Mars and Jupiter). Dozens, maybe hundreds or even thousands of them.

So if we are calling Pluto a planet, putting in the same category as Earth, Mars, etc., then there’s potentially a vast number more which should be added to the traditional list of planets. That’s not very convenient or clear.

So in 2006 the International Astronomical Union decided to define exactly what we mean by the word ‘planet’. An upshot of that was that Pluto didn’t fully qualify and came to be redefined as a ‘dwarf planet’. That is,

• it’s a round object and it’s orbiting the Sun just like a planet,
• however, it’s very small, more like the size of a moon or asteroid,
• and as such, it’s not big enough (gravitationally) to ever carve out an orbit all to itself like any big planet would.

Pluto is now just one of several dwarf planets that have been identified, including Haumea, Makemake, Eris and Ceres.

A couple of interesting facts about Pluto:

Pluto’s 248-year elliptical orbit around the Sun sometimes brings it closer to the Sun than Neptune’s orbit.

Pluto has a surprisingly big satellite (moon), called Charon. In effect, it’s a binary system.

Why does Pluto have mountains?

 

The reasons are mean, the place where pluto lives has its own rules, it is very cold here, water ice becomes bedrock.

It gets frozen harder and meaner than stone on Earth. It is a place of giants mountains, jagged blocks of this water ice-They are not pushed up by colliding continents.

Frozen nitrogen, a great flat sea of it.

A glacier. A thing called Sputnik Planiti. A moving sheet of softer ice-puts great heft on the edges of its basin.

It shoves against the water ice crust-A thing that is unyielding, cracks and pushes colossal blocks miles into cold air.

There are other peaks.

Great mean mounds on the plains, these things are likely cryovolcanoes-They spew a slushy, icy mix from gut of Pluto, no hot lava.

Why is Pluto no longer considered a planet?

 The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).

The definition of 'planet' was made more strict. It is as following:

1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.

2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.

3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).

Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.

Objects like Pluto are called Dwarf Planets.

Why is Pluto no longer considered a planet?

 The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).

The definition of 'planet' was made more strict. It is as following:

1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.

2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.

3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).

Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.

Objects like Pluto are called Dwarf Planets.