An Idiot's Guide to Classifying Planets

Since the Kepler Space Telescope’s launch in 2009, it has discovered over 1,000 confirmed exoplanets and returned information on upwards of 4,000 candidates. With the mountain of incoming data growing ever larger, astronomers must discern which of these worlds actually merits further investigation.

Now, a team of astronomers at the University of Washington’s Virtual Planet Laboratory has devised a “habitability index” for categorizing the alien worlds so far uncovered in the ongoing search for life beyond Earth.

Written by professors and lead investigators Rory Barnes and Victoria Meadows with contributions from research assistant Nicole Evans, the paper will publish in the forthcoming issue of The Astrophysical Journal.

“Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme,” said Barnes, “so that as we move into a time when hundreds of [planets] are available, we might be able to say, ‘OK, that’s the one we want to start with.'”

In the traditional search for extraterrestrial life, researchers have abided by the notion of a “Goldilocks Zone,” a band of space around a host star in which an Earth-like planet is neither too close nor too far away to sustain liquid water on its surface.

“That was a great first step, but it doesn’t make any distinctions within the habitable zone,” Barnes said. “Now it’s as if Goldilocks has hundreds of bowls of porridge to choose from.”

The new index proposes to rank planets with a simple numerical value based on a set of criteria input by the researchers, lending more nuance to the process of profiling each planet.

Our own solar system hosts four rocky worlds and four gas giants, as well as many moons, comets, asteroids, and dwarf planets. Of these objects, only Earth brims and breathes; a fertile oasis that traces a careful orbit through our Sun’s habitable zone.

The search for life-sustaining worlds elsewhere, however, is a tricky one.

Stars make themselves known. The belching, molten leviathans of fire and gas rocket their light like indomitable beacons across the cold canto of outer space, flickering at us from millions of light years away. They tear each other apart. They generate winds that blast through oceans of dust. They collapse on themselves and die in spectacular explosions.

Planets, on the other hand, are far less conspicuous. They emit no light at all. Were it not for the brilliant incandescence of the suns they orbit, planets would be little more than dim rocks hurtling through the vacuum of the cosmos, indiscernible within the darkness. But space teems with these dark worlds, whether we see them or not.

Astronomers believe that there must be a minimum of 100 to 200 billion planets in our galaxy, which is to say nothing of the inferences we can make based on what we have observed about stars. If we take into account the likelihood that most stars actually do host planets, a more reasonable estimate would be closer to the order of 10 trillion planets within the Milky Way alone.

Now consider that the Universe holds hundreds of billions of galaxies; the odds of other Earth-like planets existing are too immense to comprehend. So why don’t our telescopes pluck them of the sky like fish from open waters?

Quite simply, space is just that big, and things are just that far apart.

Take Alpha Centauri, our nearest stellar neighbor at 4.3 light years away. If you were to shrink Earth down to a speck of dust, and the Sun to the size of a walnut, Alpha Centauri would lie 640 kilometers, or 400 miles, away. Now triple that distance, and you’ll find the closest habitable speck of dust.

In order to detect these faraway worlds, astronomers look for telltale dips in a star’s brightness, which may indicate that a planet has passed in front of it and temporarily blocked its light.

Instruments like the Kepler Space Telescope can observe very rudimentary properties of these transits by how much the star’s light dims, how long the transit lasts, and the amount of time between transits, for example. But these criteria alone are not enough to indicate whether or not a transiting planet could harbor life.

“We haven’t talked about the mass of the [host] star, the radius of the star, and the luminosity of the star–things we absolutely need to have for the planets in question,” Barnes told Discovery.

More sophisticated spacecraft like NASA’s Transiting Exoplanet Survey Satellite and the James Webb Space Telescope–both of which are set to launch by 2019–will be capable of measuring such intimate details as the atmospheric composition of alien worlds.

Out of the 1,000 confirmed planets so far discovered by the Kepler Space Telescope, Barnes and his colleagues have encountered 250 that appear to be valid candidates for habitability.

According to their proposed index, a planet’s capacity to bear life can be assessed on the basis of four factors: rockiness, how much radiation it receives from its host star, how effectively its surface reflects radiation back into space, and the eccentricity of its orbit.

The time and effort required to determine whether or not a planet is truly suitable for life can be substantial. By ranking planets using the parameters described above, scientists can dedicate valuable resources to investigating the validity of only the most probable candidates.

“This innovative step allows us to move beyond the two-dimensional habitable zone concept to generate a flexible framework for prioritization that can include multiple observable characteristics and factors that affect planetary habitability,” said Meadows.