Counting Habitable Planets

View from a water world
Artist's concept of the exoplanet Kepler-186f, which orbits in its star's habitable zone. NASA/Kepler/Danielle Futselaar

NASA's Kepler Telescope is a planet-hunting instrument specifically designed to search for worlds orbiting distant stars. During its primary mission, it uncovered thousands of possible worlds "out there" and showed astronomers that planets are quite common in our galaxy. However, does that mean that any of them are actually habitable? Or better yet, that life actually exists on the surface?

Planet Candidates

While data analysis is still underway, the initial results from the Kepler mission revealed 4,706 planet candidates, some of which were found orbiting their host star in the so-called "habitable zone". That's a region around the star where liquid water could exist on the surface of a rocky planet.

Before we get too excited about this, we must first realized that these detections are indications of planet candidates. Slightly more than a thousand have actually been confirmed as planets. Obviously, these and other candidates need to be studied very carefully to understand what they are and whether they can support life.

Let's assume that these objects are planets. The numbers reported above are encouraging, but on the surface they don't seem that impressive considering the vast number of stars in our galaxy.

That is because Kepler did not survey the entire galaxy, bur rather only one four-hundreth of the sky. And even then, this initial data set is likely to only find a small fraction of the planets that are out there.

As additional data is accumulated and analyzed, the number of candidates could jump ten fold. Extrapolating out to the rest of the galaxy, scientists estimate that the Milky Way could contain upwards of 50 billion planet, 500 million of which could be in the habitable zone.

And of course this is only for our own galaxy, there are billions upon billions more galaxies in the universe. Unfortunately, they are so far away, it is unlikely that we will ever know if life exists within them.

However, these numbers need to be taken with a grain of salt. Since not all stars are created equal. Most of the stars in our galaxy exist in regions that may be inhospitable to life.

Finding Planets in the "Galactic Habitable Zone"

Normally when we use the words "habitable zone" we are referring to a region of space around a star where a planet would be able to sustain liquid water. Meaning the planet is neither too hot, or too cold. But, it also  has to contain the needed blend of fundamental elements and compounds to provide the necessary building blocks for life.

As it so happens, finding a star that is fit to host a solar system and have said system support life may prove quite the trick. You see, beyond all the previously stated requirements about warmth and such, the planet must first contain an appreciable amount of heavy elements in order to construct a world fit for life.

But this must also be balanced against the fact that you don't want excessive amounts of very high energy radiation (i.e. x-rays and gamma-rays) as they would seriously hinder development of even basic life. Oh, and you probably don't want to be in a really high density region, because there would be lots of stuff to bump into and stars exploding and, well, just lots of stuff that you don't want.

You may be wondering, so what? What does this have to do with anything? Well, in order to satisfy the heavy element condition, you have to be reasonably close to the galactic center (i.e. not near the edge of the galaxy). Fair enough, there is still quite a lot of galaxy to choose from. But in order to avoid high energy radiation from nearly continuous supernovae you want to steer clear of the inner third of the galaxy.

Now things are tightening up a bit. Now we get to the spiral arms. Don't go near those, way too much going on. So that leaves the regions between the spiral arms that are more than a third of the way out, but not too close to the edge.

While controversial, some estimates put this "Galactic Habitable Zone" at less than 10% of the galaxy. What's more is that, by its own determination, this region is decidedly star poor; most of the galaxies stars in the plane are in the bulge (inner third of the galaxy) and in the arms. So we may only be left with 1% of the galaxy's stars. Maybe less, much less.

So How Likely Is Life in Our Galaxy?

This, of course, brings us back to Drake's Equation -- a somewhat ridiculous, yet fun tool for estimating the number of alien civilizations in our galaxy. The very first number on which the equation is based is simply the star formation rate of our galaxy. But it pays no mind to where these stars are forming; an important element considering most of the new stars born reside outside the habitable zone.

Suddenly, the wealth of stars, and therefore potential planets, in our galaxy seems rather small when considering the potential for life. So what does this mean for our search for life? Well, it is important to remember that however difficult it may appear for life to emerge, it did so at least once in this galaxy. So there is still hope that it could, and has, happened elsewhere. We just have to find it.

Edited and updated by Carolyn Collins Petersen.