Dwarf Planets

Today let’s talk about the smaller planets in our solar system: The dwarfs. These are rocky terrestrial-like planets that orbit the furthest away (with one exception).

Jump straight to:

What is a Planet?

To this day, the definition of planet is up in the air. A planet is widely accepted as something that orbits a star, has a large gravitational force (but is still less than a star’s), and has a clear orbit path. Our knowledge of planets is still growing, and so our understanding of what a planet is will change as well.

What Kind of Planets Are There?

Just like stars, planets come in many types, our solar system being very rich in planet diversity. Planets are classified into four groups: terrestrial planets, gas giants, ice giants and dwarf planets. From that they can be classified even further. Think about which class you would place each planet in our solar system.

Dwarf Planets

The cutest of all planet types! Dwarfs are very small terrestrial planets. None are big enough to hold a permanent atmosphere, so they are said to have a transient atmosphere, or just no atmosphere at all.

We’ll go over known dwarf planets in our solar system, all in order from closest to furthest from the sun.

Side note: These dwarf planets are nearly impossible to look at with a telescope, with the exception of Pluto.



  • Radius: 473 km
  • Semi-major axis: 414 million km
  • Eccentricity: 0.076
  • Orbit: 4.6 Earth years
  • Sidereal day: 9 hours
  • Surface temperature: roughly -100 °C
  • Gravitational constant: 0.27 m/s²
  • Tilt: 4°
  • Moons: 0

Ceres lives in the asteroid belt unlike the other dwarfs. If you were to weigh to asteroid belt, Ceres makes up 25% of the mass. Not only that, but this little planet stands out among its neighbours since it’s so much larger than them.

Structure of Ceres

Image Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Justin Cowart

Ceres is just like a terrestrial planet, hypothesised to be made of a non-dense core and mantle. This core is most likely solid, but the mantle is composed of later upon layer of water ice and salts.

The surface is fairly smooth, unlike its asteroid neighbours. It is believed that the salt in the ice below the surface may have smoothed out larger impacts, leaving only smaller craters.

Looking at Ceres through a telescope.

You will need at least binoculars to see Ceres, as it is very faint. This tiny planet will look just like regular background stars; you will struggle to see any detail of the surface, unfortunately.

NASA, ESA, J. Parker (Southwest Research Institute)

Don’t fret! Although the results seem disappointing, observing Ceres is a difficult task. Even if all you see is a tiny white dot, it is still an amazing feat!

If it makes you feel any better, the image on the left is one of the clearest images of Ceres taken by Hubble.



  • Radius: 1,190 km
  • Semi-major axis: 5.9 billion km
  • Eccentricity: 0.2488
  • Orbit: 248 Earth years
  • Sidereal day: 6 days and 9 hours
  • Surface temperature: roughly -230 °C
  • Gravitational constant: 0.62 m/s²
  • Tilt: 119.5°
  • Moons: 5

The most well known of all dwarf planets is Pluto. Although it used to be considered a planet, we have demoted its status to dwarf.

Pluto is incredibly tiny, with a fifth of Earth’s radius. And since it’s so small, its size meant that Pluto’s rotational velocity is extremely slow, hence it completes one plutonian day in 6 Earth days.

Pluto, for a long time, was considered as the furthest planet in our solar system. Its distance from the sun is, on average, 40 x larger than Earth’s, meaning a year is about 248 Earth years. With the exception of Ceres, all other dwarf planets reside past Neptune, in the Kuiper belt. For this reason, they are often called trans-neptunian objects.

Pluto’s Structure

Pluto’s core makes up a large portion of the planet. This core is most likely made of rock, since Pluto doesn’t have a magnetic field. Covering the core lies a layer of water ice, sandwiched between the core and an icy surface.

Close up of Tombaugh Regio/ The Heart. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Pluto’s surface is filled with rocky mountains, and icy materials such as water ice, methane ice and carbon monoxide ice. It also has a heart shaped planed which contains all these ices, and on the left edge of the heart these ices behaved like glaciers.



Charon, Styx, and Pluto’s Other Moons

Although Pluto is small, its moon game is strong. 4 moons orbit Pluto: Nix, Styx, Hydra and Kerebos. Pluto’s 5th moon, Charon, doesn’t orbit in the way the others do.

Charon is approximately half as wide as Pluto (606 km radius, 50.9% of Pluto’s radius). In fact, Charon is so massive that both planet and moon orbit each other. They are tidally locked so they always face each other.


The rest of the moons are further away and much smaller than Charon. Styx, the smallest of the 4, has been estimated to be 7 km wide and Hydra, the largest, is roughly 55 km wide.

Massive objects like planets and stars have enough gravity to pull them into a spherical shape. Because these four moons are so small, they never had enough gravitational force to pull them into a nice spherical shape. Nix, Styx, Hydra and Kerebos are all irregularly shaped blobs of rock.


Looking at Pluto Through A Telescope

Pluto is impossible to find in an urban area; the light pollution is too high.

On average, this tiny planet has an apparent magnitude of 15. In an earlier post, I mentioned something called limiting magnitude (the faintest thing your telescope can see). You can find it with this equation:

Limiting Magnitude = 2 + 5log(aperture)

If we sub in 15 and arrange for the aperture, we get the minimum aperture needed as 35 cm (13.5 inches). That’s huge!

To cut it short: you can see Pluto if you’re rich, and live in a rural area.


An artist’s impression of Haumea. I can’t credit them yet because I can’t find them!
  • Radius: 816 km on average
  • Semi-major axis: 6.47 billion km
  • Eccentricity: 0.191
  • Orbit: 284 Earth years
  • Sidereal day: 4 hours
  • Surface temperature: under -220 °C
  • Two: Hi’iaka and Namaka

Deeper within the Kuiper belt lies Haumea, an ellipsoid dwarf planet rotating rapidly with two tiny moons.

6.5 billion km away from the sun, this dwarf planet takes nearly 300 years to make a complete orbit! Since it’s so far out, most heat from the sun doesn’t reach Haumea, and its surface temperature lies very close to absolute zero.

Why is A Haumean Day so Short?

From what we’ve seen so far, larger planets tend to have shorter days because they spin faster, due to conservation of angular momentum.

Haumea doesn’t follow this trend. It takes only 4 hours to spin around 360°. From what astronomers have gathered about Haumea, the fast rotation is most likely due to a rogue object colliding into a larger and perhaps more spherical version of Haumea.

The planet rotates abnormally quickly. The rotational velocity of Haumea elongated it into its current shape: an ellipsoid.


Structure of Haumea

Haumea probably is made up of a large rocky core and a thin layer of ice. There’s very little material other than ice, however Haumea has a red spot which is higher in organic compounds than anywhere else on the dwarf.

Hiʻiaka, Namaka and Haumea’s Ring

Very recently a ring was observed around Haumea, made up of small particles and debris. This is the first ring system ever discovered around a dwarf planet.

A thin ring of particles surrounds Haumea, possibly debris from a collision that formed Haumea and its two moons. IAA-CSIC / UHU

Haumea also has two moons: Hi’iaka and Namaka. Both were named after Hawaiian Goddesses, as was Haumea. Although astronomers know very little about how these moons came to be, we believe they were a part of Haumea before a collision had split the three.

Hi’iaka has an abnormally high amount of water ice. Most Trans-Neptunian Objects will have a mix of multiple ices such as dry ice, nitrogen ice, and water ice, but Hi’iaka is dominated by pure water ice. This moon is the largest of the two.

Namaka also has a large amount of water ice. It lies closer to Haumea, orbiting incredibly eccentrically at times. Namaka’s eccentricity is highly variable due to heavy interference from Hi’iaka.

Looking at Haumea Through A Telescope

No way José!


Artist’s impression on Makemake.
  • Radius: Somewhere near 725 km
  • Semi-major axis: 6.84 billion km
  • Eccentricity: 0.156
  • Orbit: 305 Earth years
  • Sidereal day: 22.5 hours
  • Surface temperature: unknown but near absolute zero
  • Moons: 1 unconfirmed

Makemake is the smallest dwarf planet we’re covering today. Although it is tiny, it does have a moon. Makemake orbits the sun from such a large distance that very little heat reaches the dwarf, so temperatures don’t stray far from absolute zero (-273.15°C).

Structure of Makemake

A colour image of Makemake. Credit: NASA/ESA

We have no clue what Makemake’s interior structure is like. It’s probably similar to Haumea, in that there’s a rocky core and a thin layer of ice.

Makemake’s surface is slightly red, either due to icy or crystalline structures like frozen methane and ethane or organic materials. We don’t know yet.

Looking at Makemake Through a Telescope

You won’t see it, even if you went to the least light polluted area in your country.


One of the best pictures of Eris, and its little moon Dysnomia.  Credit: NASA/ESA, M.Brown.
  • Radius: Roughly 1163 km
  • Semi-major axis: 10.2 billion km
  • Eccentricity: 0.441
  • Orbit: 558 Earth years
  • Sidereal day: 25.9 hours
  • Surface temperature: unknown but near absolute zero
  • Moons: 1 (called Dysnomia)

Lastly lies Eris, our furthest planet in the solar system.  A staggering 10 billion km away on average from the sun, Eris get barely any heat.

Structure of Eris

Eris is too small and far away for our telescopes to take any measurements on its interior. In fact, all we know about Eris’ structure is that there’s methane ice in the atmosphere.

Distance to Sun of Eris, compared with Pluto. Eris, at times, is closer than Pluto! Cool, right?

Eris orbits very eccentrically; at times it will be beyond even the Kuiper belt, and at times it will be close to Neptune. At it approaches its furthest distance from the sun (called the aphelion) Eris’ atmosphere of methane freezes. As it gets closer, the methane sublimes.

Looking at Eris Through A Telescope

No chance!

How Did The Dwarf Planets Form?

Although most dwarf planets can’t hold any atmosphere at all, Pluto can (but only just)! NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

All of the planets in our solar system were initially small bits of debris that clumped together to eventually make a rocky core.

The dwarf planets, unlike the others, didn’t pull in enough mass during their formation; They were a lot smaller than the other planets right from the start.

Their small cores resulted in a weak gravitational force, and so couldn’t pull in and keep an atmosphere either. The dwarfs that can pull in gas, do not keep hold of it for too long.

In the end, what we end up with is a small planet with little or no atmosphere.




The planet series is not over yet. Upcoming will be a more mathematical approach to planets, and even stuff on exoplanets! So keep an eye out for more!

That is all this week, happy stargazing!

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