Adam Roxburgh of Balclutha Primary School asks :-

Is Pluto a planet or not? It has moons, so is it a planet? How do the moons hide? Are the moons hiding behind Pluto?

Haritina Mogosanu, the Publicity Officer for the Royal Astronomical Society of New Zealand and the Education Coordinator with the KiwiSpace Foundation, responded.

In 2006 professional astronomers downgraded Pluto to the status of a 'dwarf planet' after spotting several other similar-sized objects, using very large telescopes.

These include objects named Haumea, Makemake, and Eris; the latter seems to be larger than Pluto itself and, like Pluto, it has a moon. Many moons are bigger than Pluto: our Moon, Jupiter’s Io, Ganymede, Callisto and Europa, Saturn’s moon Titan, and Neptune’s Triton.

Having moons is not a condition for a celestial object to be classified as a planet: for example, Mercury and Venus have no moons, whereas there are several large asteroids (orbiting between Mars and Jupiter) which do have moons. To qualify as being a ‘proper’ planet a body has to be large enough to be spherical (due to self-gravity), which Pluto is; and it must dominate its neighbourhood and sweep it clear of other debris – which is not really what Pluto does. For example, of the 246 years it takes Pluto to orbit the Sun, twenty of those years it slips inside Neptune’s orbit. Besides, Pluto circuits the Sun in a tilted plane that is quite different from that of the eight major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune). This is another reason to reject it from fully-fledged planet status.

Pluto has at least five moons. You need quite a good telescope to be able see Pluto itself, and the smaller moons have been discovered only by using very large ground-based telescopes, plus the Hubble Space Telescope. As they orbit around Pluto, each may seem to hide behind it and then emerge again; this is simply because we are looking at Pluto from Earth. Our Moon does the same thing – sometimes you can see it at night, sometimes you can’t – but if you lived in space above the North or South Pole then you would be able see it all the time, appearing to go in circles around our own planet.

Don’t feel too sorry for Pluto. It is probably the most beloved of all the dwarf planets, and definitely a favourite amongst junior astronomers.