# Life on other planets

We live on the Earth. It has an atmosphere, a surface gravity of 10 N/kg and an average surface temperature in the temperate zones of about 20 oC. It is 150 million kilometres from the Sun (an average star in the middle of its life).

If we are to find life as we know it then the conditions would have to be every similar to those on our own planet, Earth.

Lets look at what would happen if the conditions changed.

 Planet Weight of the 50kg pupil Mercury 190 N Venus 440 N Earth 500 N Mars 190 N Jupiter 1300 N Saturn 600 N Uranus 520 N Neptune 690 N Pluto 14.5 N

The table shows how the weight of a 50 kg pupil would change if they went to some of the other planets. If the weight is too large they would start having problems standing up as their muscles and bones would be under greater stress than on Earth. Just think how you would feel standing on the surface of Jupiter with a surface gravity pull of over ten times that on the Earth.

On the other hand the surface gravity on Pluto is so low that you would have to be careful about bumping into the ceiling of your space station if you walked around with springy steps.

Creatures on planets with a high gravity pull would have developed with short fat legs while those on planets with a low gravity pull would have legs that were long and spindly!

## Temperature

The temperature on the surface of a planet is really affected by two things:
(a) How much energy reaches the planet from its 'Sun'
(b) The atmosphere of the planet

We will only consider (a) here.

The radiant energy in the form of light and heat reaching every square metre of a planet's surface depends on:
(a) How far the planet is from the star and
(b) How much energy the star is giving out

(a) For an 'average' star is like our Sun with a surface temperature of 6000 oC life as we know it exists at 150 million kilometres from it – the position of Earth.

If we move closer to the Sun the temperature on the planets surface rises. On Mercury (a mere 58 million km from the Sun) the average daytime temperature is a searing 350 oC. Quite impossible for life. At the other extreme is Pluto at the outer reaches of the Solar System – million km from the Sun. Here the Sun looks like a bright star and the highest temperature on the surface of the planet is –233 oC, cold enough to freeze air and only 40o above absolute zero! No life would be possible here.

(b) At 150 million km from our Sun life is possible, but if we were orbiting the bright star Rigel (in the constellation of Orion) at that distance things would be very different. Rigel is emitting about 200 000 times as much radiation as the Sun and its surface temperature is a staggering 12 000 oC. The surface temperature of Earth would rise to make all life impossible.

If our Sun were to become a red giant it would expand to swallow up the Earth. The surface temperature of the Sun would fall to 3000 oC but this would still destroy all life on our planet. Finally in the last stages of its life the Sun would shrink to a black dwarf and what was left of our planet would become a lifeless, frozen ball in the blackness of space.

A very useful resource for data on astronomy is the online encyclopaedia (checked November 2012)

http://www.daviddarling.info/encyclopedia/ETEmain.html

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB

© Keith Gibbs 2020