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.
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)