How spaced out are planets? How big are they compared with the Earth? The Solar System How spaced out are planets? How big are they compared with the Earth?

Q Which is the 3rd planet from the Sun? Which is the 5th planet from the Sun? Which is the planet which you would expect to be coldest? Which is the planet which you would expect to be hottest?

A Earth – “Third rock from the Sun.” Jupiter Neptune – it is furthest away. (I will accept Pluto but it is not a true planet and is sometimes closer to the Sun than Neptune!) Mercury – because it is so close. In fact Venus is the hottest because of its thick atmosphere.

The Planets are not at all even spaced. There is a large gap between Mars and Jupiter. This gap is filled by the asteroid belt. Sometimes these can cause us problems here on Earth!

Model Solar System Sun Mercury 1.2 cm Venus 2.4 cm Earth 3.3 cm Mars Jupiter 17.3 cm Saturn 31.7 cm Uranus 63.9 cm Neptune 100.0 cm

Sun Mercury 1.2 cm 58 million km Venus 2.4 108 Earth 3.3 150 Mars 5.0 228 Jupiter 17.3 778 Saturn 31.7 1430 Uranus 63.9 2870 Neptune 100.0 4500

Planets are all much smaller than the Sun.

Jupiter is the biggest Planet Jupiter, Saturn, Uranus, Neptune in front of the Sun to scale.

Earth and Saturn to scale.

Mercury 0.40 400 Venus 0.70 450 Earth 1.00 20 Mars 1.50 -20 Jupiter Planet Distance (AU) Distance (millions of km) Temperature (degrees Celsius) Mercury 0.40 400 Venus 0.70 450 Earth 1.00 20 Mars 1.50 -20 Jupiter 5.0 -150 Saturn 9.5 -170 Uranus 19.0 -200 Neptune 30.0 -210 Plot a graph of distance (millions of km) against temperature (degrees C)

What is an “AU” in astronomy? Why is Venus hotter than Mercury, even though it is further from the Sun? What would happen to liquid water on the surface of Venus? What would happen to liquid water on the surface of Mars? Using your answers to 3 and 4 above, why is the Earth the only planet in the Solar System suitable for complex life?

Chart of temperature (up the side) against distance (along the bottom Go from – 210 to +400 on the temperature axis. Go from zero to 30 on the distance axis.