Radio wave emissions rising out of an interaction between a planet and its moon may help physicists discover the moons of distant exoplanets, says a new study.
Some of these exomoons could even sustain life, the scientists said while picking possible candidates in Gliese 876b, which is about 15 light years away, and Epsilon Eridani b, which is about 10.5 light years away.
Finding planets outside our solar system is by itself no small task. By virtue of their small size compared to the star they orbit and the fact that they do not emit light, planets are found mostly by indirect means.
Scientists hunting for life beyond Earth have discovered more than 1,800 planets (exoplanets) outside our solar system, but so far, no exomoons.
The physicists were looking at radio wave emissions arising from the interaction of Jupiter's magnetic field and its moon Io. Io has a charged upper atmosphere that is likely created by the moon's extremely active volcanoes.
During its orbit, Io's ionosphere interacts with Jupiter's magnetosphere (a layer of charged plasma that protects the planet from radiation) to create a frictional current that causes radio wave emissions. These are called "Io-controlled decametric emissions". Similar interactions on distant planets could indicate a moon and can be picked up by using sensitive radio telescopes.
While other moons may lack volcanic activity that produces an ionosphere, like Saturn's largest moon Titan they can sustain a thick atmosphere, which could include ionosphere, scientists point out to say volcanic activity is not the only criteria.
The UT Arlington College of Science team, which published their findings in the 10 August issue of The Astrophysical Journal believes that while most of the exoplanets detected are gas giants and cannot support life, by virtue of being in the habitable zone their moons could be ideal candidates for supporting life.
The paper also suggests looking for Alfvén waves that are produced by the Io and Jupiter magnetosphere interaction and says those waves also could be used to spot exomoons in similar situations.
Methods to find exoplanets
Current land-based telescopes -- such as the National Science Foundation-supported Long Wavelength Array -- can be used to detect exomoons in closer planetary systems.
Until the launch of the planet hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. The method relies on the fact that a star orbited by a planet is tugged by the gravitational pull of its smaller companion. These slight but periodic movements affect the star's normal light spectrum, or colour signature. Using highly sensitive spectrographs, planet hunters can track a star's spectrum, searching for periodic shifts towards the red, blue, and back again.
Transit photometry detects distant planets by measuring the minute dimming of a star as an orbiting planet passes between it and the Earth. NASA's Kepler telescope, for example, measures changes in brightness from a star to identify transits by an orbiting planet. Checking for a moon in such transits hasn't been possible so far.
Microlensing is the only known method capable of discovering planets at great distances from the Earth. Whereas radial velocity searches look for planets in our immediate galactic neighborhood, up to 100 light years from Earth, and transit photometry can potentially detect planets at a distance of hundreds of light-years, microlensing can find planets orbiting stars near the centre of a galaxy, thousands of light-years away.