Note from Dan: It’s Christmas time, so this will be the only post you get from me until Dec. 26th. Merry Christmas to all the citizens of BipolarNation!

Here’s your holiday gift from me.

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The Copernican Principle said that even if the odds of there being life on other planets are small, the sheer amount of stars, planets, and galaxies is more than enough to compensate for it.

For example, according to this principle, even if just .01% of the stars out there host life, that still means there are billions of billions of life forms out there.

But what ARE the odds of life on another planet? What were the odds of life on Earth, as it were? According to something called the Rare Earth hypothesis, the answer is simple: very low.

In fact, there are so many variables that go into creating a planet or object that is capable of sustaining advanced, complex life as we know it, a lot of people ask the question: if there’s so much life out there, why can’t we see it?

Much of the content from this article is derived from the Rare Earth hypothesis article at Wikipedia.

I synthesized it down for your reading pleasure. With no further ado, here are 14 reasons the Earth is extremely rare.

1. You need a rocky planet/moon to live on

You can’t walk on Jupiter. It’s more likely for life to be on a gas planet’s rocky moon, or more simply, a rocky planet. This knocks out gas planets, which are a majority of planets we know about, and knocks out all the star systems that have no rocky planets or moons.

2. The Galactic Habitable Zone

Our sun has prime real estate in our galaxy. Most of the Universe is “dead zone,” i.e., located in a spot that can’t support life as we understand it. The habitable zones are usually found in specific spots relative to the center of galaxies: we’re living in one.

If you’re too far from the center of the galaxy, there won’t be much metal (non-helium and non-hydrogen elements) to build a terrestrial planet. However, if you’re too close to the center of the galaxy, the radiation from the black hole at the center is too intense.

It’s estimated that 5% of stars in the Milky Way are in the habitable area. That cuts out the other 95% for you math majors.

3. The Local System Habitable Zone

Your planet has to be close enough to a star to have liquid water (i.e., not ice), and not so close that water evaporates. The size of this zone, of course, varies with the star.

So now we need a rocky planet (#1) located in the habitable zone around its star (#3), which is in turn located in the habital zone around the center of its galaxy (#2). You get the point.

4. An Agreeable Central Star

If the star your planet orbits is too large, there might be too much radiation for anything to develop other than underground microbes. Large stars also have a shorter shelf life, giving less time for life to develop before the star goes turns into Marlon Brando. You need stars that age gradually and give out consistent energy.

If the star is too small, it will shrink the habitable zone and the planet will be so close to the star that it will get in a tidal lock orbit with it. This means the same side would always face the star; the lack of variation doesn’t help life develop as much as variation does. It would be really hot on one side, icy cold on the other. Planets that are closer to the star are also more susceptible to solar flares.

Stars that are “just right,” like our sun, are rare.

5. A Solar System With the Right Structure to Absorb Bolide Impacts

Our solar system is set up nicely: big, gravity-hogging gas giants on the outside suck in a lot of the asteroids and comets (or “bolides”) that could devastate the smaller, inner rocky planets. Simple enough, although there are some downsides to this - gas giants can mess things up, too. See next:

6. Stable orbit

If any gas giants interfere with the inner rocky planet’s orbit, it would obviously be bad: a gas giant with a collapsing orbit could also pull the rocky planet into its star or be ejected out into space. So while you probably need big outer planets for some purposes, they have to be the right kind, and stay out of the rocky planet’s way.

7. A rocky planet of the right size

Too small, no atmosphere; too big, and its gravity will pull in too many bolides.

8. A large moon

That the Earth has the kind of moon it does could actually be the result of a theoretical odd turn of events. Notice that none of the other rocky planets have satellites, except Mars, which sucked a couple of asteroids into orbit.

Our moon (”Luna”) is the largest moon in the solar system relative to the size of its planet. It’s hard for a planet as small as Earth to capture a moon as big as Luna, so many have hypothesized that the Moon is actually the result of a collision with a young Earth and an object the size of Mars (”Giant impact hypothesis“).

Long story short, Luna was formed after this impact. Though it started out as a close-talker to Earth, it is slowly slipping away from us. It still has the gravitational power to cause the Earth’s tides, which led to tidal pools, something scientists say was critical in evolving life.

This giant impact also gave the Earth something else: an axial tilt.

9. The Axial Tilt that’s Just Right

When that theoretical big object hit the Earth, it got us spinning so fast we just had five-hour days, and changed the direction the Earth was spinning. Our spin has slowed, but we still spin off-axis.

And guess what? The axial tilt is “just right.” Too much tilt, and the variation in seasons will be extreme. No tilt, and there would be too little variation. Earth’s moderate variation between seasons is because its axial tilt isn’t flat or extreme. As you can guess, a moderate variation like this is friendly to supporting life.

10. A magnetic field

A magnetic field is handy because it protects us from solar wind and other space hibbity jibbity. We have a magnetic field because our planetary core made of molten iron. So a planet could use that kind of stuff in its core.

There are also some essential elements that are radioactive with a half life that lasts long enough to sustain the magnetic field. If the magnetic field sputters, it won’t give life much time to develop, so you need specific elements present for a strong magnetic field.

11. Plate tectonics

This is a new idea. Plate tectonics help run the magnetosphere (see above). This ties in to #8, as it’s possible the Giant Impact from Luna kick-started plate tectonics.

12. The right atmosphere

Your atmosphere and planet need carbon to support carbon-based life forms like us (but not too much, or else it gets all Al Gore). Life as we know it also needs to metabolize oxygen - the planet’s gotta have oxygen (which also helps build Ozone, which protects us from UV rays).

Protection from those rays is key, because nucleic acids and proteins absorb them. They need not to. Once these proteins are sufficiently protected, photosynthesis can occur. On Earth, oxygen levels from photosynthesis reached their present level and started the Cambrian explosion.

13. The right amount of bolide impacts

That’s right - even though #5 says you need gas giants on the outside of a solar system to absorb asteroids, too LITTLE asteroid impacts can actually slow down evolution.

Huge bolide impacts can be evolutionary “pumps,” and the global change can challenge life into finding new ways of adapting and becoming complex.

14. See any aliens?

There’s a theory that an advanced civilization would reach something called Technological Singularity. To understand Technological Singularity, look at the progress of humankind. We haven’t followed a steady slope of technological progress by any means. Instead, our progress looks like a J-Curve, following an exponential rate of acceleration.

Once you reach the high point in the J-curve, the ability of technology increases by even more leaps and bounds until you reach Technological Singularity, which is where the rate of improvement approaches infinity.

An advanced civilization would not have to be that far ahead of us to reach singularity. They’d just have to start earlier. With singularity, they could theoretically be capable of traveling the distances we view as insurmountable, and if they wanted to let us know about their presence, we’d know.

Now, just because we don’t see it doesn’t mean it’s not there. But there is a possibility that humankind, if not alone in the Universe, is at least the most advanced species out there. If all we have to compete with are moon microbes and single-cell molecules, we may essentially be alone in the Universe, and the first “self-aware” species.

Naturally, the Copernican principle says “Sure, the Earth is rare - but there’s so many objects out there, there’s BOUND to be another one.” Makes sense. It’s up to you to decide what you think. Keep in mind that with Rare Earth, all of these variables have to go TOGETHER for another Earth to exist. Drop just one - like the kind of star, or twenty degrees of axial tilt - and you don’t have another Earth.

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