Published on:

This planet and its moon are so peculiar, maybe there are none like it

My Times column on the Lucky Planet theory:

We may be unique and alone in the Universe, not because we are special but because we are lucky. By “we”, I mean not just the human race, but intelligent life itself. A fascinating book published last week has changed my mind about this mighty question, and I would like to change yours. The key argument concerns the Moon, which makes it an appropriate topic for a bank holiday Moonday.

David Waltham, of Royal Holloway, University of London, is the author of the very readable Lucky Planet, which argues that the Earth is probably rare, perhaps even unique, as planets go. He is also a self-confessed “moon bore” who has made important discoveries about how the Moon formed.

Ever since Copernicus, the “mediocrity principle” has been scientific orthodoxy: that our planet is not the centre of the Universe; it’s just one of (as we now estimate) a thousand billion billion spherical objects of similar size orbiting fiery suns just like ours.

But in that case, as the nuclear physicist Enrico Fermi famously asked, where is everybody? Why no faint radio messages from our distant neighbours in space? There should be enormous numbers of planets that have been around for longer than us, long enough surely to get to the point of transmitting some interstellar Muzak. Yet not a peep.

Dr Waltham points out that planets where life fails to survive cannot give rise to sentient life forms, so we are bound to find ourselves on one that has managed to be just right. Precisely because we are afflicted with this severe case of observational bias, the mediocrity principle need not follow. We can be misled by what we can see around us into thinking our case is typical, when actually it might be almost impossibly rare. We might be neither special nor commonplace, just lucky.

And indeed, there does seem to be a long string of coincidences behind our existence. The pressure of anti-gravity in our universe happens to be very, very small — not quite big enough to blow the Universe apart before stars could form. Phew. The relative strength of nuclear and electrical forces is just right to allow carbon to be one of the commonest elements, and carbon’s capacity to form lots of bonds is crucial to life. Cheers. The strength of molecular bonds is just right to allow chemistry to happen at our distance from the Sun. Hooray.

Then there’s the climate. Although there were probably at least four times when the Earth came close to freezing altogether or overheating irreversibly, it somehow recovered each time, unlike on Venus or Mars, and for the last half billion years the weather has been astonishingly benign. Periodic catastrophes, caused by volcanoes or meteorites, have set the evolution of life back, but not often enough to prevent intelligence emerging eventually: another stroke of luck.

Spookily, the slow waxing of our Sun’s strength over four billion years should have produced a ten-degree rise in average temperature, but it has not because it has been almost precisely matched by a slow decline in our greenhouse effect as carbon dioxide became progressively scarcer. This has kept the temperature in a small range for a very long time — long enough once again to allow the emergence of intelligent life. (The recent uptick in carbon dioxide levels as a result of fossil fuel burning is still small in comparison.)

Waltham posits three possible explanations for these great strokes of good fortune: God, Gaia and Goldilocks. God does not show His workings; Gaia says living things themselves somehow unwittingly control the thermostat; and Goldilocks says it’s just an almighty fluke that we’ve managed to keep things neither too hot nor too cold, but just right.

This is where the Moon comes in and delivers the verdict decisively to Goldilocks. It is most unusual for a small planet to have such a huge moon — almost a double planet. It probably came about after a collision between two planets, a chunk of the larger one being ejected into space, where it first formed rings like Saturn’s, but these then coalesced into a big satellite.

At that point the Moon was only about 20,000 miles away, or one tenth the distance it is now. Our day was five hours long. The ocean’s tides, caused by attraction of the Moon, themselves slowed down the Earth’s rotation and caused the Moon to move steadily outwards.

All this was a stroke of luck because the Moon stabilised the rate and angle of our spinning such that we got a fairly long day and regular seasons to keep warming the poles and preventing the irreversible growth of ice. What Waltham has discovered, however, is that this was an even bigger lucky strike than we used to think.

Had the Earth’s day been a few minutes longer just after the collision, or the Moon’s diameter a few miles greater, then the Earth would have had an unstable spin and life would have been repeatedly wiped out by chaotic climate change. If the day had been shorter or the Moon smaller, then we would have had more and longer ice ages, because too little heat would have reached the poles through air currents.

Very few planets indeed could have collided with an object the right size to produce such a moon and even fewer of them would have ended up with a Goldilocks moon that was just the right size. Since life cannot control the Moon’s orbit, Gaia cannot explain this piece of luck. The Moon therefore shows decisively just how hard it will be to find another planet of sufficiently stable climate to spawn life that could last long enough to develop intelligence.

Waltham has persuaded me that we are “perhaps the luckiest planet in the visible universe”, the only one among billions of billions to have thrown six after six whenever the dice were rolled. Whichever planet achieved this would have thinking beings on it who would think they were special, whereas really they were just lucky. And they would be alone, or very nearly so.

The moral? Remember the famous David Low cartoon from after the fall of France in 1940, showing a defiant Tommy on a sea-lashed rock, with the message: “Very well, alone.”

By Matt Ridley | Tagged:  rational-optimist  the-times