The Galactic Habitable Zone

Imagine a scene in space some 4.57 billion years ago.  A glowing, smoldering disc of hydrogen and helium (with trace amounts of other heavier elements) is spinning rapidly and bulging at the center.  And then suddenly, the center ignites - a shock wave pushes most of the lighter gasses away from the center, leaving countless smaller rocky bodies and dust.  And at the center, a star is born.

This is, of course, a scene played out countless billions of times in our universe with minor variances.  But for us, at least, those minor variances were pretty important.  How?  Location, location, location.

First, The Sun was born in a region of the galaxy with the right amount of heavier elements - known as metals in astronomy.  Without enough, the small rocky bodies that would eventually collide and form the inner planets wouldn't have formed around the Sun^[1].  If the Sun had formed further out in the galaxy, there wouldn't have been enough of the metals to form large enough bodies of rock to hold onto a thick enough atmosphere to hold liquid water.

Too much closer and you have other problems.  The closer you get to the galactic core, the tighter the stars are packed together.  All of these stars would push, pull, tug, and twist on the Oort Cloud - a vast collection of comets and small ice/rock bodies that lie well beyond the orbit of Neptune.  And with the increased metalicity nearer the galactic core, there would be a lot more of them.  Out there, they're not much of a menace - kind of like that crazy uncle that lives several states away.  The problem is the fact that with all the gravitational disturbances from closer, more highly packed stars would send a lot more of those crazy uncles into the solar system, crashing drunkenly through our front door, drinking all of our beer, running up our long-distance phone bill, and barking at our dog.  Or, cosmologically speaking, they would crash much more frequently into our planet, each potentially unleashing the force of several megatons of TNT; what the impact didn't kill, the flaming debris launched into near orbit and settling down across the globe certainly would inconvenience.  That is, of course, if the huge amounts of radiation from the much higher concentration of super nova events and the super-massive black hole at our galaxy's center didn't hinder them from evolving in the first place.

Second, the Sun's stellar nursery^[2] gave it the necessary angular momentum to orbit the center of the galaxy in a highly circular position - that is, no matter where it is in it's rotation around the core, it's almost exactly the same distance from the core.  This is important because it means our solar system rarely crosses through the dense, dangerous major spiral arms, because they revolve at almost the same rate.  The spiral arms give birth to vast quantities of new stars; they are also full of deadly radiation.  So, it's a good idea to steer clear of them as well - which, fortunately for us, we do.

1 - The current model of stellar evolution predicts that the planets formed by accretion - dust and ice particles began to stick together, growing in size. When these clumps grew to a size of a kilometer, their gravity attracted them to each other, letting them grow faster, and larger, in size.  It is often referred to as runaway planet growth.

2 - Most likely, the Sun and it's protoplanetary disk formed in a stellar nebula; a ball of dust and gas several light years in size.  For further reading on this theory, click here.

The background image on this page is courtesy of Yeshe Fenner/Space Telescope Institute.