So far space scientists have discovered about seventy planets outside the solar system,
out of the ten billion trillion or so that are thought to be out there, so humans can hardly claim to speak with authority on the matter,
but it appears that if you wish to have a planet suitable for life, you have to be just awfully lucky,
and the more advanced the life, the luckier you have to be.
Various observers have identified about two dozen particularly helpful breaks we have had on Earth,
but this is a flying survey so we'll distill them down to the principal four. They are:
Excellent location. We are, to an almost uncanny degree, the right distance from the right sort of star,
one that is big enough to radiate lots of energy, but not so big as to burn itself out swiftly.
It is a curiosity of physics that the larger a star the more rapidly it burns.
Had our sun been ten times as massive, it would have exhausted itself after ten million years instead of ten billion and we wouldn't be here now.
We are also fortunate to orbit where we do.
Too much nearer and everything on Earth would have boiled away.
Too much farther away and everything would have frozen.
In 1978, an astrophysicist named Michael Hart made some calculations
and concluded that Earth would have been uninhabitable had it been just 1 percent farther from or 5 percent closer to the Sun.
That's not much, and in fact it wasn't enough.