How Many Habitable Planets are Out There?

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

One of the most profound questions we can ask about our universe is whether or not life exists "out there". More popularly put, many people wonder if "they" have visited our planet? Those are good questions, but before scientists can answer those, they need to search out worlds where life might exist.

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?

LombergA1600-full_blue.jpg
This Kepler Space Telescope image shows our position in the galaxy and the target area the telescope used to search out extrasolar planets across 3,000 light-years of space. The small blue circle on Earth shows the approximate extent that our radio, TV, and telecommunications signals have reached in just over a century since radio was first used. Galaxy painting by Jon Lomberg. NASA/Kepler

Planet Candidates

While data analysis is still underway, results from the Kepler mission have revealed thousands of planet candidates. More than three thousand have been confirmed as planets, and some of them are 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.

The numbers are encouraging, but they only reflect a small part of the sky. That is because Kepler did not survey the entire galaxy, but rather only one four-hundredth of the sky. And even then, its data only indicate a small fraction of the planets that could possibly exist throughout the galaxy.

As additional data is accumulated and analyzed, the number of candidates will increase. Extrapolating out to the rest of the galaxy, scientists estimate that the Milky Way could contain upwards of 50 billion planets, 500 million of which could be in their stars' habitable zones. That's a LOT of planets to discover!

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, if conditions were ripe for life in our neighborhood of the cosmos, chances are good it can happen elsewhere, given enough materials and time.

It's important to remember, however, that these numbers need to be taken with a grain of salt. Not all stars are created equal and 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 scientists use the words "habitable zone" they 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. 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. In other words, it has to have materials to make a surface or at the very least, a welcoming place for life to form.

However, having a "Goldilocks spot" that just right also has to recognize that the planet isn't bombarded with excessive amounts of very high energy radiation (i.e. x-rays and gamma-rays). Those would seriously hinder the development of even basic life forms such as microbes. In addition, the planet probably shouldn't be in a very star-crowded region, since gravitational effects may prevent conditions from being conducive to life. That's why it's not very likely there are worlds at the hearts of globular clusters, for example.

A planet's place in the galaxy may also affect its ability to bear life. In order to satisfy the heavy element condition, a world should be reasonably close to the galactic center (i.e. not near the edge of the galaxy). However, the inner parts of the galaxy could well be populated with supermassive stars about to die. And, in order to avoid high energy radiation from nearly continuous supernovae, that region could be dangerous for planets with life.

The Galactic Habitable Zone

So, where does that leave the search for life? The spiral arms are a good start, but they can be populated by a lot of supernova-prone stars or clouds of gas and dust where new stars are forming. 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.

Milky Way Galaxy
An artist's concept of what our galaxy looks like from outside. Note the bar across the center and the two main arms, plus smaller ones. NASA/JPL-Caltech/ESO/R. Hurt

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 that can support life-bearing planets. Maybe less, much less.

So How Likely Is Life in Our Galaxy?

This, of course, brings us back to Drake's Equation -- a somewhat speculative, 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 doesn't take into account 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.