Last month, when astronomers with the Kepler space telescope released a list of 1,235 possible planets orbiting other stars, one particular candidate, KOI 326.01, especially stood out. Scientists, journalists, and the general public couldn’t help it: In a population of planetary candidates dominated by sizzling, Jupiter-sized gas giants—which are much easier to spot—here was the closest thing yet to our very own planet. It was just about the size of Earth, even a little smaller, and had a temperature around 138 degrees—rather warm for human tastes, but still a place where liquid water could rain down from clouds into oceans, and where life as we know it could possibly exist. A clever but perhaps overambitious monetary calculation valued the planet at exactly $223,099.93. Alas, KOI 326.01’s 15 minutes of fame must now end. Additional analysis of the planet’s star now suggests that the planet is a lot larger, and most likely a lot hotter, than previously thought. “The details of the planet need to be hammered out, but this certainly means that this is not an Earth-size planet in the habitable zone,” where liquid water could exist, says Natalie Batalha, a Kepler team member.
The road to demotion began when a DISCOVER fact-checker, Mara Grunbaum, asked for some additional information about the planet. In response, Batalha and her colleagues dug up images of the sky near KOI 326.01—and almost immediately found a problem. The planet’s sun, known as KIC 9880467, is located close to another star (see above). In a reference catalog characterizing the stars in the probe’s field of view, KIC 9880467 is listed as brighter than its neighbor. But as you can easily see in the above image, that is not the case. That simple error messes up the calculations of the planet’s temperature and size. Kepler finds planets by detecting tiny dips in a star’s brightness during transits---when a planet crosses in front of it. When the Kepler team analyzed the combined light from the two stars, they assumed KIC 9880467 accounted for most of the brightness. Now they have to chalk up almost all that light to the neighboring star. In fact, while Batalha is still confident KOI 326.01 exists, she is no longer sure which of the two nearby stars it is orbiting. In either case, the calculations indicate that the planet is somewhat warmer and a lot larger than the previous estimate. And if it is orbiting the bright, neighboring star, as Batalha suspects, the planet’s temperature will soar. More analysis is required, but it’s safe to say that KOI 326.01 should no longer be considered potentially habitable. It is important to note that this is not a failure on the part of the Kepler team. Since before the launch in March 2009, Kepler principal investigator William Borucki has warned not to read too much into individual planets---that's why they're called "planet candidates." Kepler is primarily a statistical mission. Its goal is to determine what percentage of stars contains Earth-sized planets. One candidate does not particularly help the cause, but by probing more than 156,000 stars and coming up with thousands upon thousands of candidates, scientists hope to definitively determine how common worlds generally like ours are. Even without KOI 326.01, the data so far suggest that about 10 percent of stars harbor Earth-sized worlds (with a diameter between 50 percent and 125 percent of Earth's). Astronomers will refine that percentage as the telescope observes more transits over the next year and a half. In addition, the Kepler team’s paper [PDF] from last month contains plenty of caveats about the planetary candidates. It notes that KOI 326.01 is only a “moderate probability candidate,” with about a 20 percent chance of being a false positive, as compared to hundreds of others considered strong candidates (and that's not even counting the possibility of an erroneous catalog). The paper also notes that the characteristics of KIC 9880467 are derived from a basic analysis of the star’s color; other stars, including the neighboring one, are more definitively classified with data on their temperatures and surface gravity. Batalha concedes that in retrospect the Kepler team should have included a special note about KOI 326.01, considering the planetary candidate would surely grab attention. “Because this is such an interesting target, it probably would have been good to have a paragraph in the paper reiterating that it was a weaker candidate,” she says. But she adds that it might be good for the public to see the way science works: “We’re seeing the scientific method playing out in real time.”
