The figures have since been refined and made a little more generous,
5 percent nearer and 15 percent farther are thought to be more accurate assessments for our zone of habitability—but that is still a narrow belt.
The discovery of extremophiles in the boiling mudpots of Yellowstone and similar organisms found elsewhere
made scientists realize that actually life of a type could range much farther than that-even, perhaps, beneath the icy skin of Pluto.
What we are talking about here are the conditions that would produce reasonably complex surface creatures.)
To appreciate just how narrow, you have only to look at Venus.
Venus is only 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 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 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 astronomers hoped that Venus might harbor life beneath its padded clouds, possibly even a kind of tropical verdure,
we now know that it is much too fierce an environment for any kind of life that we can reasonably conceive of.