The Kepler mission has only been taking data for two years. In that time, its rate of discovery has been staggering: over 2,300 planet candidates, with another 61 confirmed planets. Those numbers are even more impressive if you consider that Kepler can only detect planets around a small fraction of the stars that it's observing.

Kepler works by watching for the shadow cast when a planet passes between its host star and the Earth. That means the plane of a planet's orbit has to be aligned so that it passes between us and the star. If the orbital plane is tilted, we won't be able to detect it with this method.

But now, researchers have demonstrated that it's possible to spot a few objects that Kepler can't otherwise see directly. While searching for hints of moons orbiting exoplanets, they found that one of the planets spotted by Kepler was being tugged around by another planet—one that orbited in a slightly different plane, and was otherwise undetectable using this method.

The work was done by a project called HEK, or Hunt for Exomoons with Kepler. As a moon swings around an exoplanet, it will alternately pull the planet forward or tug it back relative to its orbit. These subtle changes should show up in the Kepler data: the planet may not show up in front of its star when we expect it to, or it may spend more (or less) time between the star and Earth than we'd predict. These are called Transit Timing Variations (TTVs) and Transit Duration Variations (TDVs).

When the team looked at the data that Kepler has made public, they found something unusual with the planetary candidate KOI-872.01 (KOI stands for "Kepler Object of Interest"). It had some of the largest timing variations ever detected—about two-hour variations in an orbit that takes a bit under 34 days. But there was no sign of any transit duration variations, which should be present if there was a moon. All of which suggests that the planet was being pulled around by another planet Kepler hadn't detected.

To determine whether a planet could account for the complex variations observed in the Kepler data, the researchers turned to a massive amount of computer modeling. They tested companions with orbital periods anywhere from one day to 10 years. After refining any solutions that looked promising, they then modeled the orbital dynamics for a billion years to determine whether the possible solution was stable. That left them with only two solutions, one of which was a much better fit for the Kepler data.

(To make sure this approach was reliable, they scrambled the Kepler data and tried to find any matches to it using the same procedure. After a thousand trials, they came up blank.)

The original planetary candidate, now termed KOI-872b, is several times more massive than Jupiter. The new one, KOI-872c, has an orbital period of 57 days and is about 1.3 times the mass of Saturn. Its orbital plane has to be inclined by at least one degree compared to that of KOI-872b, but it can't be more than 4.5° off, or the fit to the Kepler data would be thrown off.

The orbital models produced by the authors is exact enough that it can be used to predict future variations in the transit timings. The next time the Kepler team releases more data to the public, we should be able to see whether the HEK team has gotten it right.

The story behind the HEK team is nearly as interesting as its latest findings. They're not officially part of the Kepler project, and the major co-investigators are scattered around a variety of institutes. One of them, Allan Schmitt, is simply a member of planethunters.org, part of the Galaxy Zoo collection of citizen science projects. (Schmitt has an AOL e-mail address and his institution is listed as "Citizen Science" in the paper's credits.)

The group has no official funding, despite its large computational needs. So they've tried to crowdource the money for a $10,000 cluster using the science-focused donation site Petridish.org. They've already overshot that goal by $2,000, and one $2,500 donor will get the chance to name the cluster once it's built.

All of this, of course, relies on the data being available in the first place. The official Kepler team gets first crack at it, but they're victims of their own success: Kepler is producing so much data that they can't analyze it all in detail within the time that NASA allows them exclusive access. When that time limit is up, the data is simply made public for everyone to use. The HEK team has clearly put it to good use.

Science, 2012. DOI: 10.1126/science.1221141 (About DOIs).