Pluto…

I still think of Pluto as our ninth planet even though it was downgraded to dwarf planet status in 2006. It was only discovered in 1930 and since a year on Pluto is 248 years on Earth, it has not even completed one orbit of the Sun since its discovery. I include Pluto because I learned two methods for remembering the names and order of the planets. The mnemonic I first learned was Many Volcanoes Erupt Mouldy Jam Sandwiches Under Normal Pressure for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. This was useful but not a patch, logically, when compared to My Very Easy Method Just Speeds Up Naming Planets – that is much more elegant.

But Pluto is now just a Kuiper Belt object – one of several large rocks that sit in a band of rocky objects just over 49 times as far from the Sun as we are. The average distance from the Sun to the Earth is 150,000,000 km or 1 AU (one astronomical unit). Pluto has a diameter about two thirds that of our moon. It has an elliptical orbit so it is sometimes closer to the Sun than Neptune – it was for about 20 years between 1979 and 1999. That won’t happen again for another 230 years. So far, five moons of Pluto have been discovered. The first was Charon found in 1978. Charon has a diameter just over half that of Pluto, which is unusually large in proportion to the object it is orbiting. The other four are called Styx, Nix, Kerberos and Hydra, which I find rather pleasing given that I used to enter Classical History competitions as a boy.

Pluto is a rocky object with water ice, methane ice and even some nitrogen ice on its surface. Because of its elliptical orbit it is sometimes much closer to the Sun (as close as only 30 AU), this causes the frozen surface briefly to sublime, changing from a solid into a gaseous atmosphere. In 2006 NASA launched the New Horizons mission to explore Pluto and other Kuiper Belt objects. It took nine years to get there but has now taken some amazing photographs and collected other data too. The science instruments on board confirmed that Pluto is about two thirds rock and one third ice. Pluto has a low density so its mass is about one sixth the mass of Earth’s moon.

There are two other notable dwarf planets in our Solar System. One is Ceres, found in the asteroid belt between Mars and Jupiter, and is about one fourteenth as massive as Pluto. It was recently visited by NASA’s Dawn spacecraft. The largest known object in the Solar System not to have been visited by a spacecraft is Eris which is also a Kuiper Belt object and about 25% more massive than Pluto, although having a slightly smaller volume.

Questions…

1. What determines how long a day is on Earth?
2. How long does the Earth take to orbit the Sun once?
3. What is a moon?
4. At its closest, how far is Pluto from the Sun in km, expressed in standard form?
5. If your mass is 45 kg on Earth, what would your mass be on Pluto?
6. Would you expect to weigh more standing on Pluto or Eris? Explain your answer.