The Circumstellar Habitable Zone (CHZ)

What do you need in order to have a planet that supports complex life? First, you need liquid water at the surface of the planet. But there is only a narrow range of temperatures that can support liquid water. It turns out that the size of the star that your planet orbits around has a lot to do with whether you get liquid water or not. A heavy, metal-rich star allows you to have a habitable planet far enough from the star so  the planet can support liquid water on the planet’s surface while still being able to spin on its axis. The zone where a planet can have liquid water at the surface is called the circumstellar habitable zone (CHZ). A metal-rich star like our Sun is very massive, which moves the habitable zone out further away from the star. If our star were smaller, we would have to orbit much closer to the star in order to have liquid water at the surface. Unfortunately, if you go too close to the star, then your planet becomes tidally locked, like the moon is tidally locked to Earth. Tidally locked planets are inhospitable to life.

Here, watch a clip from The Privileged Planet: (Clip 4 of 12, full playlist here)

But there’s more.

The Galactic Habitable Zone (GHZ)

So, where do you get the heavy elements you need for your heavy metal-rich star?

You have to get the heavy elements for your star from supernova explosions – explosions that occur when certain types of stars die. That’s where heavy elements come from. But you can’t be TOO CLOSE to the dying stars, because you will get hit by nasty radiation and explosions. So to get the heavy elements from the dying stars, your solar system needs to be in the galactic habitable zone (GHZ) – the zone where you can pickup the heavy elements you need but not get hit by radiation and explosions. The GHZ lies between the spiral arms of a spiral galaxy. Not only do you have to be in between the arms of the spiral galaxy, but you also cannot be too close to the center of the galaxy. The center of the galaxy is too dense and you will get hit with massive radiation that will break down your life chemistry. But you also can’t be too far from the center, because you won’t get enough heavy elements because there are fewer dying stars the further out you go. You need to be in between the spiral arms, a medium distance from the center of the galaxy.

Like this:

Here, watch a clip from The Privileged Planet: (Clip 10 of 12, full playlist here)

The GHZ is based on a discovery made by astronomer Guillermo Gonzalez, which made the front cover of Scientific American in 2001. That’s right, the cover of Scientific American. I actually stole the image above of the GHZ and CHZ (aka solar habitable zone) from his Scientific American article (linked above).

These are just a few of the things you need in order to get a planet that supports life.

Here are a few of the more well-known ones:

• a solar system with a single massive Sun than can serve as a long-lived, stable source of energy
• a terrestrial planet (non-gaseous)
• the planet must be the right distance from the sun in order to preserve liquid water at the surface – if it’s too close, the water is burnt off in a runaway greenhouse effect, if it’s too far, the water is permanently frozen in a runaway glaciation
• the solar system must be placed at the right place in the galaxy – not too near dangerous radiation, but close enough to other stars to be able to absorb heavy elements after neighboring stars die
• a moon of sufficient mass to stabilize the tilt of the planet’s rotation
• plate tectonics
• an oxygen-rich atmosphere
• a sweeper planet to deflect comets, etc.
• planetary neighbors must have non-eccentric orbits

By the way, you can watch a lecture with Guillermo Gonzalez explaining his ideas further. This lecture was delivered at UC Davis in 2007. That link has a link to the playlist of the lecture, a bio of the speaker, and a summary of all the topics he discussed in the lecture. An excellent place to learn the requirements for a suitable habitat for life.

UPDATE: Welcome, visitors from Post-Darwinist! Thanks for the link Denyse! New visitors may be interested in this post, which is a jumping off point for all of posts on science and faith issues.

Everyone who isn’t Christopher Hitchens or Richard Dawkins already knows about the standard fine-tuning argument. But have you ever considered what it takes to make a planet that is capable of supporting the minimal requirements of living systems? The area of science that specializes in answering this question is called astrobiology. Let’s take a look!

I will be working from a lecture (with Q&A) delivered in October 2007 at California State University – Fresno, by two of my favorite scholars, Jay Wesley Richards and Guillermo Gonzalez.

The Copernican Principle

Richards introduces the idea of the Copernican Principle. This principle states that the progress of science will show that there is nothing special (designed) about man’s place in the universe.

The minimal requirements for life

I’ve written about this before here, but basically life requires a minimum amount of encoded biological information to allow it to replicate itself. The only element in the periodic table that allows you to encode information is carbon. Carbon is the hub of large molecules which form the paper and text of biological information. No carbon = no life.

Secondly, you need some environment in which to form molecules around the carbon, such as amino acids and proteins. That environment is liquid water. And you need the liquid water to be at the surface the planet where you want life to exist.

The requirements of a habitable planet

Here are just a few of the requirements mentioned in the lecture.

• a solar system with a single massive Sun than can serve as a long-lived, stable source of energy
• a terrestrial planet (non-gaseous)
• the planet must be the right distance from the sun in order to preserve liquid water at the surface – if it’s too close, the water is burnt off in a runaway greenhouse effect, if it’s too far, the water is permanently frozen in a runaway glaciation
• the solar system must be placed at the right place in the galaxy – not too near dangerous radiation, but close enough to other stars to be able to absorb heavy elements after neighboring stars die
• a moon of sufficient mass to stabilize the tilt of the planet’s rotation
• plate tectonics
• an oxygen-rich atmosphere
• a sweeper planet to deflect comets, etc.
• planetary neighbors must have non-eccentric orbits

Note that these requirements are connected. If you mess with one, some of the others will be thrown out of tune. For more habitability requirements, see this article by Gonzalez and Richards.

What are the probabilities that we will get these conditions?

Richards explains that the question of whether this is designed is like winning the lottery. Your chance of winning depends on two things:

1. the odds of getting all the conditions correct
2. the number of tries that you get

If the odds of winning are 1 in a million, you could still win by buying a million tickets with all the different numbers. In the universe, there are only about 10^22 possible solar systems. So if the odds of getting a habitable planet are 1 in 10^9, you’ll get tons of life. But what if the odds are 1 in 10^40? Then you’re not likely to win.

But this is not the argument that these two are making, because even though there are a lot of factors needed for a habitable planet, we still can’t say for certain how likely it is that each of these conditions will obtain. Therefore, we can’t make the argument except by estimating the odds of getting each condition.

Although you could use very generous estimates, it would still be guessing, and you can win a debate by guessing. So are we stuck?

How to make a design argument using habitability

Gonzalez explains why you can still make an argument for design by arguing that the coorelation between habitability and measurabiliy is intentional. (By measurability, he really means the ease of making scientific discoveries). And you do this by correlating the conditions for sustaining life with the conditions for allowing scientific discoveries.

Gonzalez gives two examples:

1. Solar eclipses require that the sun and moon have certain sizes and certain distances from the sun. The surface of the Earth is the optimal location in our solar system for observing solar eclipses. We were able to make many valuable discoveries due to this fine-tuning, not the least of which was confirming the theory of general relativity, which was cruicial to the science of cosmology.
2. The location of our solar system is fine-tuned within two spiral arms of a spiral galaxy. We escape from radiation and other dangers, but to also allow use to capture heavy elements that are needed to make a suitable Sun and humans bodies, too. But the same conditions that allow life also allow us to make scientific discoveries, such as star formation theory and cosmic microwave background radiation measurements, which was needed in order to confirm the creation of the universe out of nothing (the big bang).

Spooky. And what until they list off a half-dozen more examples in their book “The Privileged Planet”. It’s downright terrifying!

Conclusion

Richards sums up the argument with an illustration. He asks why scientists construct observatories high up on mountains. The answer is in order to avoid “light pollution” from nearby cities, which ruin the ability of scientists to observe the stars and make discoveries. And this is what we see with our planet and solar system. No one builds a planet that can be used to make scientific discoveries in a place that doesn’t support life. It turns out that the very places in the universe that are good for making observations are also the best places for supporting life.

Further study

I would recommend checking out the documentary DVD, if you find the book too scary. There is also a university lecture DVD with both authors, filmed at Biola University. If you want to see the DVD online for FREE, then click here (narrated by John-Rhys Davies). Awesome! Go science!