When I first reviewed the Planet Hunters Web site in January 2011, the project was in its infancy. The site and its navigation had some bugs to work out. But it had a compelling yet seemingly quixotic premise: That a group of volunteers, peering in Web browsers at graphs of stars’ brightness based on public data from NASA’s Kepler planet-hunting telescope, might be able to discover planets that Kepler’s own search algorithms may have missed.
Two years later, that concept has been borne out beyond any skeptic’s wildest imaginings. In September 2011, Planet Hunters announced its first two planet candidates, and soon after announced several more. The project’s first confirmed discovery—a planet circling a binary star in a quadruple star system—came in fifth on CNN’s list of the top 10 science stories of 2012.
In January 2013, the project announced a second confirmed planet—a Jupiter-sized world orbiting in the so-called habitable zone of a sunlike star—as well as 42 new planetary candidates, including 15 in their respective stars’ habitable zones. These worlds—ranging in size from about 2.5 Earth radii up to slightly larger than Jupiter—are too large to support life as we know it, presumably being gas giants, they may well have large moons.
Planet Hunters volunteer Kian Jek was recently awarded the Chambliss Amateur Achievement Award, the American Astronomical Society’s most prestigious award given annually to an amateur astronomer, for his work on behalf of the project. Kian, one of the two hunters credited with Planet Hunters’ initial confirmed discovery, is one of a small cadre of skilled volunteers that have supported the Planet Hunters science team—who, although professionals, also volunteer their time to work on this project— by vetting and cataloguing potential planetary candidates, modeling stellar and planetary systems, keeping tabs on exotic variable stars such as “heartbeat binaries” and dwarf novae, as well as tracking unlisted eclipsing binary systems in which a pair of stars orbit each other in our line of sight, each eclipsing the other in turn.
PC Planet Hunting
I’ve participated in a number of “citizen science” online astronomy projects over the years, but none has sparked my imagination like Planet Hunters, which lets anyone with a computer and an Internet connection take part in one of modern science’s great quests: the search for planets orbiting other stars. On the Planet Hunters site, you can look for signs of these so-called exoplanets in public data from NASA’s Kepler mission. If you’re among the first to report a new planet, you get credit for the find and in some cases can have your name appear as a co-author on the discovery paper.
Planet Hunters is a collaboration between Yale University and the Zooniverse, a Web hub that hosts a number of citizen science projects. It got started with astronomy projects, the first being Galaxy Zoo, in which the public was enlisted to classify galaxies in images from the Sloan Digital Sky Survey; it’s since added others such as Moon Zoo and Solar StormWatch. Nearly half of the 14 Zooniverse projects are astronomy related; of the others, one of them, Cell Slider focuses on identifying cells for cancer research; others are related to tracking wildlife, climate science, and studying the ancient Greeks. Although Planet Hunters isn’t officially connected to the Kepler mission, there are close ties and cooperation between the two.
150,000 Points of Light
Kepler, a space telescope, was launched in March 2009, tasked with “…exploring the structure and diversity of planetary systems….” (by discovering them), looking in particular for Earth-sized planets, and worlds in a star’s habitable zone. After Kepler completed its basic mission in 2012, the mission was extended for another 3 years.
Kepler uses the “transit method” for planet hunting, searching for tiny dips in a star’s brightness caused by the passage (transit) of a planet in front of the star. Kepler repeatedly (every 29 minutes) images the same star field near the constellation Cygnus showing more than 150,000 stars, using a photometer to precisely measure each star’s brightness. These readings generate light curves—plots showing variations in a star’s luminosity over time. A transit shows as a string of data points descending below the star’s light curve. Kepler uses search algorithms to find transits in its data—so far it’s credited with more than 100 exoplanet discoveries, and has published a list of more than 2,700 planet candidates.
But Kepler monitors a huge variety of stars: some of constant brightness, others that flicker erratically or pulsate like clockwork. Eclipsing binaries—two stars that orbit each other and periodically eclipse one another—often show transits similar to those from planets. Although Kepler’s planet search algorithms are very good at detecting prospective planets, they don’t catch everything, and the human eye has been shown to be better at detecting anomalies in some pattern-recognition tasks than a computer. That’s where Planet Hunters comes in. Having multiple participants view each image greatly improves the odds of not missing a world.
Keep Reading: The Planet Hunters Site
In order to access Planet Hunters’ features, as well as get credit on the site for any discoveries, it’s necessary to register: a free, painless process that allows access to the other Zooniverse sites as well. After a video and tutorial, you’re ready to hunt.
Planet Hunters runs on Firefox, Safari, Chrome, Opera, and Internet Explorer 9 (it doesn’t run with IE8). The site is also optimized for use with Safari on an iPad, although there is no Planet Hunters mobile app.
Regardless of browser, to search for planets, you click on the Classify link. A graph showing a star’s light curve over a roughly month-long period appears, as well as some facts about the star. Vertical and horizontal sliders let you zoom in on the graph. You’re asked several questions: whether the star is quiet or varies in brightness; whether there are any transits (if so, you’re asked to mark them); and whether you want to comment on the star. You can “collect” the light curves; most hunters have at least one collection of possible transits they’ve found. Once you’re done, Planet Hunters serves you up another light curve. This process is well designed and by and large works without a hitch.
Site navigation has greatly improved since I first reviewed the site. That said, there still are some issues. For example, you can comment on a light curve if you see something notable, and see other people’s comments for the current quarter, but you can’t read comments on the star from previous quarters of data (except the first). This may become an issue in cases where a light curve shows an apparent planetary transit, and it would be helpful to see what others have said about the light curve.
Another issue is with the pages listing planetary candidates. You can view light curves of stars (either that you’ve classified, or an overall list) in which Planet Hunters has found potential planets, but sometimes you will see the same star listed multiple times. Many of these (unofficial) candidates have since been claimed as official candidates by the Kepler project, and one list of candidates stored on an Excel spreadsheet downloadable from the site doesn’t seem to be integrated with the rest of the candidates on the Web page. And now that Planet Hunters has started claiming official candidates in published and submitted papers, it may be confusing to distinguish between these and the informal candidates.
With the announcement of 42 official Planet Hunters candidates came the assurance that the candidate pages would be updated to reflect these additions. Participants can hope that the project will iron out the other kinks in the candidate pages when this is done.
Planet Hunters isn’t for everyone. Transits are rare: for one to be seen, a star has to have planets close to it, and the planetary system must be exactly edge-on to us so that a planet would pass in front of the star. Numerous planets have already been found in Kepler data, including most of those easiest to find. Looking at hundreds of graphs isn’t everyone’s idea of fun. But many of the light curves—particularly of variable stars—are remarkable in their own right, having an almost artistic beauty.
I’ve been a very active participant in the site nearly since its inception, and have classified many thousands of light curves. I’ve come across many interesting objects, including a few unlisted eclipsing binary stars, several light containing transits that were later announced as candidates by the Kepler team or other researchers rather than Planet Hunters, and I even among the people cited for having reported one of the 42 potential planets that were recently announced.
The Search for Other Earths
As of the beginning of 2013, the nearly 200,000 registered Planet Hunters users had collectively classified 16 million stellar light curves comprising nearly two years of data. Between the Kepler project itself, Planet Hunters, and other research teams, roughly 2,900 planets (counting both confirmed planets and candidates) have been found. Probably the great majority of shorter-period planets (that orbit their stars in 100 days or less) have already been reported.
Now that there are several years of data, though, the possibility of finding longer-period planets, including ones in their stars’ habitable zones, has become greater. This was borne out in January 2013 not only by the Planet Hunters announcement that included 1 confirmed planet and 15 planet candidates in the habitable zone, but by the 461 planetary candidates announced by the Kepler project the same day, some of which were in their stars’ habitable zones, including four worlds just a little larger than Earth.
The idea that the Sun is a star, and that other stars may have their own planets, goes back at least to the ancient Greeks. The Copernican revolution cemented this concept, but not without backlash: In 1600, Italian philosopher and monk Giordano Bruno was burned at the stake as a heretic; among his offenses was the assertion that there were “…countless suns and countless earths all rotating around their suns.” It wasn’t until the late 20th Century that the search for exoplanets took off in earnest. The first was discovered in 1995; more than 800 have since been confirmed.
Planet Hunters gives you an exceptional opportunity to take part in one of the most exciting scientific adventures of our time. It doesn’t require an astronomy degree, or even a telescope. All you need is a computer, a questing spirit, and a lot of patience and free time. There are a lot of planets out there, maybe even Earth-like worlds, and someone has to find them. Why not you?
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