Sunday Science: Pluto and the Dwarf Planets

The sizes of the five current dwarf planets compared to the moon. Credit: the-dialogue.com/

Over the summer, Sunday Science has tackled the planets of the Solar System. It's remarkable the number of emails I've had asking me to include Pluto in the list - even though it was demoted to  "dwarf planet" status more than 10 years ago.

What is a dwarf planet?

There are five officially recognised dwarf planets in our Solar System: Pluto, Eris, Ceres, Haumea and Makemake, but astronomers predict there could be hundreds more. (Check out the end of this post for a pretty cool animation.)

The fundamental difference between a planet and a dwarf planet is to do with what else is in its orbit around the Sun.

​Because Pluto shares its orbital neighbourhood with other dwarf planets and other objects in an area of space called the Kuiper belt, it's not classified as a planet.

The International Astronomical Union (IAU) has three criteria for a full-sized planet:

1. It is in orbit around the Sun.
2. It has sufficient mass to assume hydrostatic equilibrium (in other words, it can form a nearly round shape).
3. It has "cleared the neighbourhood" around its orbit.

​So, Pluto and the other dwarf planets fail on the last point. There are too many other objects sharing this orbital path.

What do we know about Pluto?

From https://xkcd.com/1551/. Click to enlarge.

Pluto was predicted by Percival Lowell and discovered in 1930 by Clyde Tombaugh. It's the largest of the dwarves but it's still tiny compared to the other planets - and its diameter is two-thirds of the Earth's moon.

Pluto is comprised of around one-third water in the form of ice and the rest of the planet is rock.

​It also has a highly elliptical orbit and that means it sometimes is closer to the Sun than Neptune.

Pluto sometimes has an atmosphere when it's closer to the Sun and its surface ice thaws to form a thin nitrogen-rich atmosphere.

What about the other dwarf planets?

Of the five dwarf planets, Ceres is the odd one out because it is located between in the asteroid belt between Mars and Jupiter. So, it's the closest dwarf planet to the Sun - and also the smallest with a mere 590-mile diameter.

Haumea has an elongated shape (a bit like a rugby ball), which is believed to be the result of its rapid rotation. It's named after the Hawaiian Goddess of childbirth (which is a nice contrast to Pluto - the Roman God of the underworld) and has two moons. 

Makemake​ was first observed in 2005 and is named after the god of fertility in Rapa Nui mythology. It's also the second brightest object in the outer Solar System and, despite sharing a lot of Pluto's characteristics, it does not have an atmosphere.

Eris is named after the Greek Goddess of discord and strife, which is quite fitting for a dwarf planet that's shrouded in controversy. When it was first discovered in 2005, Eris was thought to be larger than Pluto and it was submitted as the tenth planet in our Solar System. However, its discovery was one of the reasons Pluto was demoted in 2006 to dwarf planet status.

A highly dubious Lego scale model. If Ironman is the Earth, his helmet is the planet Mercury and the Lego dot is Pluto.

Have we visited the Dwarf Planets?

Yes. NASA's New Horizons probe flew past Pluto in 2015. The probe took multiple snaps of Pluto, including images of its icy mountains and a new view of its largest moon Charon.

​New Horizons is currently in hibernation mode before it prepares to fly past an icy body on the edge of our Solar System called 2014 MU69 on 1st January 2019.

Extra reading and watching

Here's an animation I took from this fascinating Reddit post. The visualisation is from the simulator Universe Sandbox, which is based on the list of 'possible' dwarf planets from Caltech professor Mike Brown.

You can try this simulation yourself in Universe Sandbox by selecting home > Open > Solar Sys > Solar System All "Possible" Dwarf Planets (the author of this video also turned on view > Orbits).

And here's quite an extensive video all about Pluto:

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What is Sunday Science?

Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
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So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.