THE galaxy could be stuffed with large moons that orbit alien worlds, according to an analysis of data from NASA鈥檚 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鈥檚 timing varies because an orbiting moon can transit before, after or at the same time as its parent planet.
聯Astronomers have raked in thousands of exoplanets, but smaller moons have proved harder to come by聰
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A team led by at the Harvard-Smithsonian Center for Astrophysics is searching for exomoons by modelling all the positions one could be in and looking for similar light signals in the Kepler data. But it鈥檚 such a computationally intensive strategy that they have had to and 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鈥檛 find any clear exomoon signals, the results suggest an abundance of moons the size of Jupiter鈥檚 Ganymede, the largest in our solar system.
This technique, called the orbital sampling effect, was by 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 ().
鈥淭he downside is this dip could be caused by other reasons,鈥� 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.
More moons than planets?
鈥淚t is much faster than the technique we use,鈥� 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. 鈥淧ersonally I would not believe a detection solely based on these quick and dirty methods,鈥� he says. 鈥淲e want the first confirmed exomoon to be an extremely solid, clear case.鈥�
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 (see 鈥�Ganymede鈥檚 secret subsurface sea鈥�). 鈥淭here might be more habitable exomoons out there than planets,鈥� says Hippke.
Ganymede鈥檚 secret subsurface sea
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Lights, camera, ocean! Observations by the Hubble Space Telescope have confirmed the presence of a subsurface ocean on Jupiter鈥檚 moon Ganymede. The method used could help in the hunt for life outside our solar system.
It has long been thought that Ganymede harboured a secret sea, but evidence had been inconclusive. Now Joachim Saur at the University of Cologne, Germany, and his colleagues have used Hubble to investigate by studying the moon鈥檚 aurora.
This is produced by Ganymede鈥檚 magnetic field. But interactions with Jupiter鈥檚 magnetic field mean the aurora rocks back and forth over a roughly 5 hour period. If Ganymede has a salty subsurface ocean, though, it would reduce this rocking effect. That鈥檚 because an electrically conductive salty ocean generates its own magnetic field in the presence of an internal magnetic field.
Saur鈥檚 team watched Ganymede for 7 hours, and saw this smaller rocking effect. 鈥淲e cannot explain these observations without an ocean,鈥� says Saur (Journal of Geophysical Research: Space Physics, ).
The technique could help find subsurface oceans on planets or even moons outside the solar system. Our telescopes are not powerful enough to see these auroras yet, but it is only a matter of time, says Saur. 鈥淥ne day that鈥檚 going to come.鈥�
This article appeared in print under the headline 鈥淭he hunt is on for habitable moons鈥�
