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We are,(to a remarkable degree, the right distance from the right sort of star, one e
5 of ten billion and we wouldn't be here now./ We are also fortunate to orbit where we
that is big enough to radiate lots of energy, but not so big as to burn itself out
swiftly
t 1s a curiosity bf physics that the larger a stor the more rapidly it burns. Had our sun
Ocen ten times as massive、it would have evhonsted itself after ten million years instead
of
do. 1o0 much nearer and evervthing on Farth would have boiled away. Much rarther
away and everything would have frozen.
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m 1978, an astrophysicist named Micheel Hart made some calculations and
Concluded that Earth would have been uninhabitable had it been just 1 percent rartner
That's not much, and in fact it wasn't enough.
percent
10 from or 5.percent closer to the Sun.
The figures have since been refined and made a little more generous
5
nearer and I5 percent farther are thought to be more accurate assessments 1oI om
zone of habitability
- but that is still a narrow belt.
To appreciate just how narrow, you have only to look at Venus. Venus 1s only ©10
15 twenty-five million miles closer to the Sun than we are. The Sun's warmth reaches it
just two minutes before it touches us.
In size and composition, Venus is very like
Earth, but the small difference in orbital distance made all the difference to
(3)how it
turned out. It appears that during the early years of the solar system Venus was only
slightly warmer than Earth and probably had oceans. But those few degrees of extra
20 warmth meant that Venus could not hold on to its surface water, with disastrous
consequences for its climate. As its water evaporated, the hydrogen atoms escaped
into space, and the oxygen atoms combined with carbon to form a dense atmosphere
of the greenhouse gas CO2. Venus became stifling. Although people of my age will
recall a time when astrononmers hoped that Venus might harbor life beneath its padded
25 clouds, possibly even a kind of tropical vegetation, we now know that it is much too
fierce an environment for any kind of life that we can reasonably conceive of. Its
surface temperature is a roasting 470 degrees centigrade (roughly 900 degrees
Fahrenheit), which is hot enough to melt lead, and the atmospheric pressure at the
surface is ninety times that of Earth, or more than any human body could withstand
We lack the technology to make suits or even spaceships that would allow us to visit
Our knowledge of Venus's surface is based on distant radar imagery and som。
disturbing noise from an unmanned Soviet probe that was dropped hopefully into the
