The galaxy could be stuffed with large moons that orbit alien worlds, according to an analysis of data from NASA's Kepler space telescope. Such moons are thought to be the most likely places to find alien life, so groups are clamouring to find the first.
Astronomers have raked in thousands of exoplanets, but their smaller moons have proved harder to come by. Kepler looks for exoplanets by watching how the light from a star dips as a planet passes in front, known as a transit. Moons should produce a smaller, secondary dip, but that dip's timing varies because an orbiting moon can transit before, after or at the same time as its parent planet.
A team led by David Kipping at the Harvard-Smithsonian Center for Astrophysics is currently searching for exomoons by modelling all the positions one could be in and looking for similar light signals in the Kepler data. But it's such a computationally intensive strategy that they have had to crowdfund their own supercomputer and borrow NASA's to crunch the numbers.
Now Michael Hippke of the Institute for Data Analysis in Neukirchen-Vluyn, Germany, has road-tested a simpler method. While he didn't find any clear exomoon signals, the results suggest an abundance of moons the size of Jupiter's Ganymede, the largest in our solar system.
More moons than planets?
This technique, called the orbital sampling effect, was dreamed up last year by René Heller of McMaster University in Ontario, Canada, but Hippke is the first to try it with real data. It works by overlaying all of the transit data for an exoplanet and looking for signs of an extra dip on both sides, to catch the moon in as many positions as possible.
"The downside is this dip could be caused by other reasons," says Hippke, such as sunspots on the star. The upside is its speed: he was able to process 4000 Kepler planets in just a few months on an ordinary computer. For that reason he thinks this method is best used to highlight planets that deserve a closer look. The most promising planet he found, Kepler-264 b, has hardly been examined.
"It is much faster than the technique we use," says Kipping, who expects to have crunched through only 300 planets by the end of 2015. He agrees the new method could guide further searches, but thinks it is unlikely to find an exomoon. "Personally I would not believe a detection solely based on these quick and dirty methods," he says. "We want the first confirmed exomoon to be an extremely solid, clear case."
Hippke also used the method to look at the statistics of exomoons in general. His results suggest that, on average, planets with orbits of between 35 and 80 days host a single moon the size of Ganymede.
This is great news for alien hunters. Kepler has seen plenty of planets in these short orbits, and if they orbit a red dwarf star, their moons would be right in the habitable zone, where water is liquid. The Hubble Space Telescope recently showed that the real Ganymede has a subsurface ocean, and the same technique could one day identify water hidden within exomoons. "There might be more habitable exomoons out there than planets," says Hippke.
Journal reference: Arxiv: http://ift.tt/1Ajt8nF
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