Planets

Planet Facts

Want to learn all about the eight planets in our solar system?

There are eight planets in the solar system – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. You’ve probably seen at least a couple of them when you’ve looked at the sky, although they can be hard to tell apart from stars.

All of them except Uranus and Neptune can be seen with the naked eye, and with binoculars or an inexpensive telescope you can see all eight.

Using a telescope will also let you see more detail on the closer planets – you can watch Venus change phases like the moon, going from a full circle to a crescent, or pick out Jupiter’s four largest moons.

On a clear night you can even see the giant planet’s Great Red Spot – a huge circular storm that’s been raging since at least 1830 – or observe Saturn’s rings. With luck it’s even possible to see the ice caps on Mars.

great red spot

What is a planet?

You might be wondering why we say there are eight planets. After all most of us were taught that there are nine, and you may have noticed that Pluto is missing from the list.

That’s because astronomers no longer class Pluto as a planet. They used to, but then they started to discover other objects about the same size as Pluto – and far smaller than the other planets.

To end the confusion, astronomers came up with a clear definition of a planet. To qualify, an object has to meet these tests:

  • Orbiting a star or the remains of one
  • Massive enough for its own gravity to pull it into a sphere
  • Has cleared its orbit of asteroids and other debris

Eight objects in our solar system meet all of these tests and are classed as planets; Pluto only passes the first two, so it’s not a planet. Instead it’s a dwarf planet.

Where are the planets?

Diagrams of the solar system don’t give an idea of how far apart the planets really are. If they did, even the largest planets would appear as tiny dots. If the sun was the size of a basketball then Earth would be a small pea, 30 metres away.

Jupiter would be a ping-pong ball 170 metres away. The distances are huge. That’s why, without a telescope, Jupiter looks like a bright star – it’s at least 630 million kilometres from Earth.

Jupiter facts

Distance of the planets from the Sun

distance of planets from the sun

Types of planet

Not all planets are like Earth. Within the solar system there are two groups, with four planets in each; the inner, or terrestrial planets and the outer, or giant, planets. The ones in each group are very different.

The terrestrial planets are Mercury, Venus, Earth and Mars. These are all relatively small, but dense. As far as we can tell all of them have cores of metal – mostly iron – surrounded by rock.

The giant planets are a lot larger, as the name suggests, but much less dense. They do contain some rock but most of their mass is lighter material. Until recently they were all classed as gas giants, because it was believed they were mostly gas around a solid core, but now astronomers split them into two types:

Jupiter and Saturn are gas giants. They have small, solid cores of rock and metal, surrounded by a layer of hydrogen so dense that it’s become metallic. Outside this is a deep atmosphere that’s mostly hydrogen and helium.

Uranus and Neptune are ice giants. Again the core is small and made of rock and metal but it’s surrounded by a thick layer of frozen water, methane and ammonia. Outside of that is the atmosphere, mostly hydrogen, helium and methane.

What about Planet X?

Some people believe there could be other planets out there, beyond the orbit of Neptune. The main evidence for this is that Uranus and Neptune’s orbits seem slightly different from what they should be, as if the gravity of another planet is affecting them.

It’s just possible that there is at least one more planet out there – the solar system is huge, and we’ve only explored a small fraction of it – but most astronomers don’t believe in the so-called “Planet X”.

Telescopes have spent a long time looking at its predicted location, and have found nothing, so the eight known planets are probably it.

Explore lots more fascinating facts about our planets via the links below.