It’s a good thing that planets outside our Solar System get catalog designations instead of proper names, or space scientists would now be scraping the barrel for “Ralph” or “Tigger.” That’s because on Wednesday, NASA announced that the Kepler space telescope had hit the “motherload” of exoplanets, confirming 715 new planets in 315 star systems. It used a new statistical technique that the space agency says has removed a bottleneck that has plagued the analysis of the Kepler data.
From the time the unmanned Kepler probe was launched in 2009 until its at-least-temporary decommissioning last year, it surveyed about 150,000 stars. Kepler was able to detect possible planets orbiting around them, as the planets cut off part of the starlight when passing in front of the star. Using this technique, Kepler detected thousands of planetary candidates and hundreds were confirmed.
Unfortunately, there’s a problem. Confirming these candidates has been a slow, laborious process because planets aren’t the only things that orbit stars. Many times, other small, dim stars are orbiting the star under observation, which produce false positives, so astronomers need to sort out which objects orbiting a particular star are planets and which are other stars.
NASA’s answer to this conundrum was to employ a statistical method called “multiplicity.” Unlike when Kepler started work, we now know much more about exoplanetary systems, and this data allows scientists to come to some very helpful conclusions. For example, if planets are randomly distributed in star systems, we shouldn’t see very many. Instead, we see thousands of possible planets and very often more than one is orbiting a particular star.
Furthermore, many star systems are similar to our own Solar System. Their orbits are in a single plane like a pancake, rather than than in intersecting orbits like the popular image of an atom. In other words, these multiple planet systems are very stable and that’s where the clever bit comes in. Star systems made up of more than one star are unstable and are unlikely to have multiple planets mixed in with the orbiting stars, and that gives NASA a faster way of confirming which candidates are planets.
NASA compares this technique to lions and lionesses. When you see a pride of lions, discounting cubs, it’s very likely made up of one lion and a number of lionesses. If you see one cat, the odds are even that it’s a lion or a lioness. If you see two, they may be a lion and a lioness or two lionesses, but less likely to be two lions. But if you see three or more, the odds are very great that its one lion and the rest are lionesses. According to NASA, the same goes for stars and planets. If you’ve got more than one object orbiting a star and at least one is a planet, the odds are they’re all planets.
A research team at NASA's Ames Research Center in Moffett Field, California co-led by planetary scientist Jack Lissauer verified the technique by studying Kepler data gathered from May 2009 to March 2011. The result is not only 715 new planets, but 94 percent of them are smaller than Neptune, which is a great advance on earlier work that tended to find giant planets.
Among these are four orbiting in their stars’ habitable zones where liquid water can exist and where life could be present. One of these is Kepler-296f, which is twice the size of Earth and orbits a star half the size and five percent as bright as the Sun. However, NASA points out that it may be a mini-Neptune with a thick hydrogen-helium atmosphere.
According to NASA, this new statistical technique is already giving scientists new insights into how star systems form, and could result in hundreds more exoplanets being confirmed.
The Kepler results will be published in The Astrophysical Journal on March 10.