What conditions support the minimum requirements for complex life?

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.

Circumstellar Habitable Zone
Circumstellar Habitable Zone

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:

Galactic Habitable Zone
Galactic Habitable Zone and Solar Habitable Zone

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.

4 thoughts on “What conditions support the minimum requirements for complex life?”

  1. Looking at the night sky is like being able to visualize a love song. I think that the fact that our planet is located in the optimal position for scientific observations is one of the most beautiful discoveries of our time. It speaks to Intelligent Design. Just the thought that God positioned man just perfectly, so we could view his handiwork. We not only have the perfect position where habitability is optimized, but where we have a clear view of the universe God has set us in. A perfect view of His creation. It is so beautiful to think that God wants us to seek truth always and then gives us the tools we need to find Him. It’s so beautiful that science is finding him, and that the arguments for Intelligent Design are strengthened all the time with every new discovery. It is so beautiful to see our faith come alive in science, to know that we are probably alone in the galaxy, but loved and not abandoned. To be able to look back with our telescopes, back into time, to see the work of His hands and think about our uniqueness in the cosmos. It saddens me when I hear Christians talk about an unknowable God. I know they mean it as compliment to His greatness, but I firmly believe he wants us to know Him, and to know Him intimately. The Christian walk is about a personal relationship with the God, but if you don’t take the time to study apologetics how can you truly have a personal connection with Him. When you want to have an intimate relationship with someone, you invest a large amount of your time trying to understand everything about them. Most importantly you’d want to know what that person’s goals and plans are and how you would fit into those. God wants us to know His ways, His plan and His design. He shows us because he loves us and He allows us the opportunity of seeking Him and responding to his overtures. Science has so much more to discover and Christians should be actively following these discoveries. When you are working on developing a loving relationship, especially a new one, you spend time trying to figure out everything about the other person, you spend time understanding their background, you spend time making observations, you sometimes even spend time just watching them contentedly. Yes you try and please them, yes you hang out, but you learn more not by just merely hanging out, but by actively seeking and communicating. As Christians we need to use the tools that we have in Cosmology and Biology, we need to use reason and research. We can’t just skip the homework, we can’t have emotion without knowledge, you need the ontological argument for starters just because you can’t profess to love someone if you can’t even prove they exist! After that the study of epistemology becomes important so that you can argue analytically, and those truths find their home in the sciences. Just like it would be a mistake to be lazy in love, so is it a mistake to be lazy with apologetics. Think about it. The best part is that with Him there is promise that all the effort will be rewarded. He has put the whole universe us at our disposal and says, “Seek and you will find.” How beautiful is that?


    1. I don’t think it’s possible to love something that you don’t know about and don’t want to know about. I think the tendency is to try to identify God with our feelings and needs, instead of trying to figure out what he is really like out there, using objective tools to investigate his existence and character.


      1. Agreed, also I realize that I came at this from a more female point of view, and men need apologetics even more so than females, to keep them engaged. Men need constant stimulus in a relationship and that is what apologetics supplies in their relationship with God. This is also why when you engage Christian women on apologetics, in general, they think it’s sufficient to give you the, “all you need is a relationship with Christ..,” lovey-dovey stuff. Women can become quite content there, but apologetics is essential for living out the great commission which is where we all need to be once we have accepted Christ for ourselves. The Christian faith is not just a walk with Christ, but a mission.


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