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From the Wall Street Journal, my latest Mind and Matter on stability, the moon and
This month saw the discovery of the first small and "rocky"
planet like ours outside the solar system, Kepler 10b, orbiting a
star more than 500 light years away. This month also saw terrible
floods in part of Australia. Here I intend to link these two news
stories. But don't worry-I have not gone astrological on you. The
link is not a causal one.
Kepler 10b is not much like Earth. It weighs nearly five times
as much, orbits a similar star at a much closer range-closer than
Mercury is to the sun-and is hot enough on its surface to melt
iron. So it is unlikely to hold life, at least of a kind we could
empathize with. But it does confirm that other stars than ours have
rocky planets in their solar systems, and given the gazillions of
stars out there, it makes it probable that Earth-like planets,
tepid enough to melt ice but not boil water, probably exist.
It therefore surely follows that the universe must be teeming
with intelligent life. After all, life got going here pretty early
in the planet's story and has now had time to evolve brains big
enough to send radio signals into space. Given that many stars are
much older than the sun, many planets ought to have achieved the
same result a long time ago. In which case, as the physicist Enrico
Fermi famously asked, "Where are they?" Why have we not picked up
signals from other civilizations?
Somewhere in this chain of logic there must be a flaw; otherwise
the airwaves would be crammed with alien chat shows. It was the
Brazilian cosmologist Carlos Frenk who told me of a flaw that his
colleagues see. Life started early in Earth's history, yes, but it
took a long time for us to achieve big enough brains to get
technology started. This was because evolution takes time and needs
The Earth has had its share of instability. The most
sophisticated life forms kept getting knocked out by catastrophes,
of which the near-total extinction of all life at the end of the
Permian period and the extinction of the dinosaurs at the end of
the Cretaceous are only two recent examples. But actually, the
Earth is an astonishingly stable planet. For four billion years, it
mostly has maintained not just a similar climate and a similar
orbit but a similar rotation pattern, too.
We have the moon to thank for this, Mr. Frenk says. Our
unusually large satellite helps to keep us on an even keel. Without
it, we would get occasionally tumbled by the influence of Jupiter.
But the moon's birth (blasted from our own surface by an asteroid
collision) was a highly improbable event. A bigger collision and
the Earth would have been smithereens; a smaller one and the debris
would have fallen back to Earth rather than coalesce as a single
moon. So here's the flaw in the Fermi paradox: The birth of a moon
just the right size to stabilize a rocky planet's orbit and
rotation is very improbable.
And Australia? Australia has an unusually unstable climate. It
can be sure of getting terrible floods and droughts, but it cannot
be sure when. This made it unsuitable for early agriculture, which
is why aborigines sensibly did not invent it (as their New Guinea
neighbors did when their climate became more stable after the end
of the last ice age).
This circumstance made it difficult for Australian
hunter-gatherers to form the dense demographic concentrations that
gave rise to technological civilization through specialization and
the exchange of ideas. If Earth's climate had remained as unstable
as Australia's, and as unstable as it was all over the planet
during the last ice age, then the radio would never have been
invented and the Earth would still be silent. Stability is key.