Dwarf Planets

Dwarf Planet Facts

If you learned your solar system facts more than about ten years ago the chances are you’ve never heard of dwarf planets. You’ll probably be surprised to know that Pluto, classed as a true planet for decades, is now officially a dwarf.

And you might be amazed to hear that there are now five solar system objects officially recognised as dwarf planets, with dozens more that almost certainly earn the title and hundreds of possible ones.

The dwarf planets are mostly invisible from Earth, even with a good telescope; they’re too small and too far away. Only two can occasionally be seen directly. To see any of the others you’ll have to rely on shots from orbiting telescopes and space probes.

dwarf planets

The dwarf planets are interesting, though. They’ve only been studied since the early 1990s, but already we’ve learned that the solar system is very different from what we always thought.

What is a dwarf planet?

Until the late 1970s astronomers thought Pluto was larger than Mercury, although even with the most powerful telescopes it showed up as little more than a fuzzy, distant dot.

pluto-dwarf-planet

Then, in 1978, Pluto’s moon Charon was discovered. This was interesting for many reasons, and one of them was that it allowed the planet’s mass to be accurately measured.

Much to everyone’s surprise Pluto turned out to be tiny – about one-twentieth of Mercury’s size, and only a fifth as large as our own Moon. It was still classed as a planet, but its small size raised some doubt.

Then, in 1992, more objects started to be discovered beyond the orbit of Neptune, and some of them were almost as large as Pluto. Now astronomers started to wonder if Pluto really was a planet – and, if it was, how many other planets were there?

Finally, in 2005, Eris was discovered. It’s far out beyond Pluto, around three times as far from the Sun, but it’s 27% more massive. Either more planets had to be added to the list or a new category had to be created.

eris-dwarf-planet-artist-impression

In the end the International Astronomical Union decided that Pluto had some significant differences from the other planets, and more in common with the new objects that were being found.

A new category, dwarf planet, was created and Pluto was transferred to it. This upset some astronomers but it makes a lot of sense.

A dwarf planet is similar to a planet in some ways:

  • It orbits the Sun
  • It’s massive enough that its own gravity has pulled it into a roughly spherical shape
  • It’s not a satellite of another object apart from the Sun

There’s one fact that makes dwarf planets different, though:

  • It has not cleared the neighbourhood around its orbit

Dwarf planets are too small for their gravity to clear their orbits of debris and small objects. A line had to be drawn somewhere, and that’s it.

Where are they found?

Most dwarf planets are also Trans-Neptunian Objects – they’re beyond the orbit of Neptune. Three of the five known ones – Pluto, Haumea and Makemake – are in the Kuiper Belt, between 30 and 50 times further from the Sun than we are.

Kuiper-belt

Eris, the largest, is in the scattered disk – a loose collection of objects thrown out of the Kuiper Belt by the gravity of the gas giants.

Eris has a very eccentric orbit – its distance from the sun varies from 37.8 to 97.6 million kilometres – and right now it’s close enough to us to be seen with a good amateur telescope.

The other dwarf planet that’s visible from Earth is Ceres, and it’s the only one that’s inside Neptune’s orbit. It’s in the asteroid belt between Mars and Jupiter, and although it’s much smaller than Eris it’s close enough to often be seen with an inexpensive telescope – and sometimes even with the naked eye.

Ceres-dwarf-planet

How many are there?

The simple answer is, nobody knows. These five are officially recognised:

Name Diameter
Ceres 946km
Pluto 2372km*
Haumea 1,240km*
Makemake 1,430km*
Eris 2,326km*

*Diameter is approximate

Many astronomers are happy to expand this list, though. Some recognise eleven dwarf planets; others think we can already be sure about even more.

Almost 400 more objects might meet the definition and it’s estimated there are 200 in the Kuiper Belt and perhaps as many as 10,000 in the entire solar system. The list of recognised ones is likely to keep growing for decades.

Already astronomers are finding out interesting things about dwarf planets. Jupiter’s moon Triton isn’t a dwarf planet, because it’s in orbit around Jupiter – but it used to be. Astronomers now believe it used to be in the Kuiper Belt, but was drawn in towards the inner solar system then captured by the giant planet’s gravity.

Examining Triton could tell us a lot about the dwarf planets beyond Neptune. These objects might be small, but they’re certainly not boring.