Worth knowing...
It's not too surprising that the Earth is so hospitable to life — we
would never have evolved enough to wonder about it if it weren't. But
when you start to look at all of the things that have come together in
just the perfect way to support plants and animals, it gets a little
mind-boggling. It's not just about air, water, and distance from the
sun. Here are the nine ingredients for a habitable planet.
Whenever astronomers announce the discovery of an "Earth-like planet,"
they're usually talking about planets with roughly Earth-like gravity
that are close enough to their star to be warm but not hot and that may
(or may not) have liquid water. But there's a whole lot beyond those
three must-have ingredients that makes Earth special. Here's an
(incomplete) roundup:
- The moon: The Earth has a slight
tilt and teeters like a top as it spins, which can cause drastic shifts
in climate over the course of thousands of years. But because of the
moon's stabilizing effect on our orbit, our climate is a lot more
steady. Plus, the moon causes the tides, and some biologists think life began in tidal pools.
- Stable rotation:
There's no reason to think that a planet without a stable rotation
would be completely inhabitable (actually, some people think such "eyeball planets"
could be our best bet for aliens), but the regularity and frequency of
day and night on this planet go far to prevent extreme temperatures and
encourage life.
- A magnetic field: Our planet is blessed
with a strong, stable magnetic field, which staves off the cosmic rays
and solar flares that would otherwise fry the planet every now and then.
It's also tied up with the next must-have feature on the list ...
- Dynamic geology: The cloud of gas and dust
that eventually coalesced into the Earth contained enough radioactive
elements to keep the core of the planet churning merrily for billions of
years. Without that motion, there wouldn't be a magnetic field at all.
- Atmosphere:
Of course, we can't discount the importance of the ozone layer. Back in
the very early days of life, plant-like organisms unknowingly made way
for animal life by filling the atmosphere with oxygen. That
high-altitude layer of gas shielded early animals from lethal radiation.
- Isolation:
Venus and Mars are close to the Earth, but our solar system as a whole
is in the middle of nowhere. Because we're far from the major spiral
arms of the Milky Way, we're in a lot less danger of running afoul of
some greater star's gravitational pull (among other hazards).
- Long-lived sun: Our sun is a yellow dwarf,
a relatively rare type of star that's both small and stable. It also
has a long life, and probably won't start to fizzle out for another five
billion years or so. Larger stars generally burn hotter and die sooner,
while smaller stars have a tendency to spit out enormous plumes of
radiation.
- Plenty of time: The sun has about five billion
years left, and the Earth itself is around four and a half billion
years old. But life only arrived in that last half-billion. We're here
because our planet was sturdy enough to last until the first signs of
life appeared.
- Gas-giant neighbors: Sure, the sun and moon are great, but there's another soldier on the battlefield to help make Earth habitable: Jupiter. In general,
gas giants tend to clump up near their home stars. But because they're
toward the outside of our solar system, their intense gravity
conveniently catches wayward asteroids and comets, making events like the one that killed the dinosaurs a rarity.
These are just a few of the items on the long list of
features that make Earth more habitable than the average planet. And the
fact that the list of essential features is so long might suggest that
life is incredibly rare in the universe, or even that ours is the only
planet that satisfies all of the necessary conditions. In reality, there
are two concepts at play here, although they seem to be mutually
exclusive. Call it the "rare Earth" hypothesis versus the "mediocrity principle"
hypothesis. The first says that the conditions that have led to life on
Earth are so finicky and uncommon that there probably aren't a lot of
other planets that meet all the criteria. The second says that if you
choose something at random from a set of many other objects, it's more
likely to be a middle-of-the-road example of the group than
representative of one of the group's extremes. In other words, there is
probably another Earth out there just because it's statistically more
likely that our planet is like other planets than unlike them.
The problem with the mediocrity principle is that the Earth isn't
chosen randomly from all of the planets in the universe. In fact, our
planet might be one of the extremes, if only the extremes can actually
support life. Marveling that the planet that we happen to live on also
happens to be very well suited to life is a bit like marveling that
polar bears live in a place where their white fur can blend in with the
snow — they didn't "happen" to evolve in a snowy environment just like
we didn't "happen" to show up in a place that's well-suited for life.
Still, the foundational belief behind the mediocrity principle is
probably true: There's nothing about this particular planet or this
particular star that makes it better for supporting life than others
that have the exact same hospitable circumstances.
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