It's distinctly possible that the first truly Earth-like planet in another star system will be discovered by a non-scientist.

Well, that's not quite true: the process of discovery is more complicated than that. However, volunteers working with exoplanet data from NASA's Kepler telescope recently identified 42 planet candidates orbiting relatively nearby stars. Of those, 20 potentially lie within the habitable zone of their systems, meaning the basic conditions could be right for liquid surface water. One of these worlds, known as PH2 b, is definitely a Jupiter-sized planet clearly within its star's habitable zone. While the planet itself is unlikely to harbor conditions suitable for life, perhaps it has moons that would.

These volunteers were from Planet Hunters, part of the Zooniverse family of citizen science projects. Several Zooniverse efforts have yielded a number of scientific papers, proving that real science can arise from crowdsourcing. The latest Planet Hunter paper, which will be published in the highly ranked Astrophysical Journal, credited more than 40 citizen scientists, a collaboration that does credit both to them and to the professional scientists they worked with.

These projects excel in distributing tasks that cannot be automated with current technology, but which are sufficiently time-consuming that professional scientists (a category including graduate students) aren't able to devote enough attention to them. Based on the successes of the Zooniverse and related projects, it's clear that with a simple Web-based tutorial and a well-designed user interface, non-scientists can perform real research-related tasks.

Citizen science, thanks to the number of participants and cross-checking of results, is both efficient and incredibly successful. (In my teaching days, I had my Astronomy 101 students participate in the Galazy Zoo project for identifying galaxy types, and the Ice Hunters, a search for small objects in the outer Solar System.)

Mass transit, citizen-science style

The Kepler telescope locates planets by monitoring a patch of the sky 115 square degrees in extent. (For comparison, the full Moon is about 0.2 square degrees.) The telescope watches for slight variations in the light of stars that could indicate a transit: the small eclipse created when a planet passes in front of the star. The duration and depth (how much light is blocked) of the transit reveals information about the size of the planet and its orbit. This pattern is known as a light curve.

Transits are easiest to spot for large planets orbiting close to their host star. That's because those planets create the largest eclipses and undergo multiple transits in a short period of observation. By contrast, alien astronomers looking for Earth will only see it transit the Sun once every 365 days, and it will block only a tiny amount of the Sun's light when it does. Thus, it's a tricky task to discover planets orbiting sufficiently far from their host star to harbor liquid water.

The Planet Hunters project allows volunteers to look through various light curves, sorting out possible planet transits from other natural variations in starlight. These might arise from dim binary companions, brown dwarfs, or fluctuations in the star's output due to stellar weather. The premise of Planet Hunters is that human beings can sometimes be better at distinguishing between the different types of light curves than automated computer routines.

As with other citizen science projects, Planet Hunters makes sure multiple sets of eyes look at each candidate light curve. That ensures double- and triple-checking of results, helping to avoid false positives. Finally, professional exoplanet astronomers look at the light curves identified by the citizen scientists and make the ultimate decision. This process allows researchers to have a lot of confidence in the identifications—and the volunteers turn out to be very good at their work.

Looking for the sanctuary moon

The Planet Hunters identified 42 exoplanet candidates, including 33 with at least three transits—the more transits we can observe, the more reliable the identification as a planet, and the better the estimates of orbital characteristics. Forty of the potential exoplanets have orbits longer than 100 days, and 9 may have orbital periods greater than 400 days, placing them farther out than most previously identified worlds.

Based on their light curves, these candidates are mostly giant planets: Neptune-sized or larger. However, 15 out of the 42 could lie in their star's habitable zone, based on a simple model of planetary temperature. (Five more may also be in the habitable zone, but the observations are only based on two transits, too few to make precise orbital measurements.) If these resemble the giant worlds in our Solar System, then they may have very planet-like moons: an ocean moon like Europa, but with a surface uncovered by ice. That possibility is pleasing to Star Wars fans, as well as scientists.

Of the candidates, one stands out as a definite detection: PH2 b, determined to be a planet to 99.92 percent confidence thanks to follow-up observations using the Keck telescope. (The "PH" in the name is in honor of the Planet Hunters who discovered it.) PH2 b is about 10 times the diameter of Earth, placing it firmly in the giant planet category. It orbits its host star in approximately 282.5 Earth days. The researchers assumed a Jupiter- or Neptune-like atmosphere, and as a result determined it orbits within its system's habitable zone. If it has moons, they might possess liquid water.

Of course, habitability as we currently understand it is a function both of location and of history. After all, Venus lies within the Solar System's habitable zone, but harbors no liquid surface water; Mars, which lies outside the habitable zone, has evidence of past surface water, but is dry now. Nevertheless, with larger numbers of exoplanets in the database, we will have better statistics to assess the probability of liquid water in coming years.

Thanks to citizen science projects like Planet Hunters, you could play a significant part in the discovery of habitable worlds.

Submitted to Astrophysical Journal, 2013. ArXiv: 1301.0644 (About DOIs).