Radio waves can help find alien moons around other stars
Sen—With more than 1800 planets so far discovered orbiting stars other than the Sun, astronomers are now turning their minds to seeking out even more elusive worlds, their moons.
It might sound an impossible task. But then it was not long ago that finding exoplanets seemed a similarly tough challenge.
Today one team of scientists suggest these planetary satellites might be detected by looking for a physical relationship similar to one that astronomers have observed between a planet and one of its moons in our own Solar System.
The planet is Jupiter and the interaction between its magnetic field and the largest and innermost of its four main Galilean Moons, Io, produces radio wave emissions. Io is a world covered with active volcanoes and these create a highly charged layer of its atmosphere termed an ionosphere.
As it orbits Jupiter, this ionosphere interacts with Jupiter’s magnetosphere, a layer of charged plasma, to create a frictional curent which causes radio signals to be broadcast.
Physicists from the University of Texas at Arlington believe that a search for similar radio emissions in alien solar systems could yield the first exomoons. They suggest in The Astrophysical Journal that detailed calculations based on the Jupiter-Io dynamic should be used to look for satellites orbiting exoplanets.
An image of Io in front of planet Jupiter, taken by the New Horizons probe in early 2007, as it was racing to a rendezvous with distant Pluto. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center
Exomoons would hold a special fascination for astrobiologists because some of those orbiting our own large gasball planets Jupiter and Saturn are being seen as possible homes for alien life. It could exist in the watery oceans that exist beneath the surfaces of Jupiter’s moon Europa and Saturn’s moon Enceladus, for example.
Zdzislaw Musielak, professor of physics at the UT Arlington College of Science, is co-author of the new paper, entitled Detection of Exomoons Through Observation of Radio Emissions.
Professor Musielak told Sen: “Rocky moons orbiting gaseous planets is a scenario well-known from our Solar System and often used by astronomers as an example of a planet-moon system at other stars.
“However, in order for an exomoon to support life other conditions are required. First, such an exomoon has to be large enough to be able to maintain its own atmosphere. Second, it must be located in a star’s habitable zone where temperatures are high enough to keep water in its liquid state.
“One possible scenario is that the exomoon orbits a Jupiter-like exoplanet, which is located in the habitable zone. Moreover, if the size of such exomoon is large enough (Earth-like size or slightly smaller) to have its own atmosphere, then there is potential for life.”
Professor Musielak told us: “Another interesting scenario is to have a rocky exomoon orbiting a Jupiter-like exoplanet outside of the habitable zone, like the Jupiter-Europa system in our Solar System. As a result of tidal heating between Jupiter and the moon, the latter receives enough energy to keep water in the liquid state, at least under the moon’s surface that is mainly rocky and icy.
“Likely there are many similar systems around other stars. What is truly interesting about such system is that the exomoon does not have to be too big, does not have to be located inside star’s habitable zone, and yet it may still have physical conditions for supporting life!”
Professor Musielak says that traditional methods of finding exoplanets are less useful when it comes to trying to spot exomoons instead. NASA’s Kepler space telescope detected exoplanets by measuring dips in a star’s brightness as one passed in front of it, an event called a transit. Measuring the extra difference due to an exomoon’s presence has not reliably been achieved.
Joaquin Noyola, a PhD graduate student in Musielak’s research group, is lead author of the new paper. He says that it will not be necessary for an exomoon to have volcanic activity for there to be an ionosphere surrounding it.
He said: “Larger moons—such as Saturn’s largest moon, Titan—can sustain a thick atmosphere, and that could also mean they have an ionosphere. So volcanic activity isn’t a requirement.”
The scientists also looked at a rippling of the plasma that had been observed in Jupiter’s magnetic field, due to the interatction with Io. They say that this rippling, termed Alfvén waves, could be used to help fidn exomoons.
They also suggest two nearby exoplanets which might be examined for evidence of exomoons. One is called Gliese 876b, which is about 15 light-yeast away. The other is Epsilon Eridani b, only 10.5 light-years distant. Current powerful telscopes on Earth might be able to detect exomoons orbiting these planets, the researchers suggest.
Co-author and PhD student Suman Satyal said: “Most of the detected exoplanets are gas giants, many of which are in the habitable zone. These gas giants cannot support life, but it is believed that the exomoons orbiting these planets could still be habitable.”