Astronomy: Radio waves key to tracking down elusive exomoons

Posted Aug 24, 2014 by Robert Myles
Hardly a week passes these days without scientists and astronomers reporting on the discovery of a new exoplanet — a planet outwith our solar system.
Hundreds of planets have been found orbiting other stars. Image: An artist s concept of the planet H...
Hundreds of planets have been found orbiting other stars. Image: An artist's concept of the planet HR 8799b
NASA, ESA, and G. Bacon/STScI)
Changed days from 30 years ago when, although it was clear our Milky Way galaxy was brimming with stars, the only solar system definitely known to exist was our very own.
Yet, back then, it seemed inconceivable, illogical as Mr Spock might say, that the Sun could be the only star in the cosmos shepherding a flock of planets.
Confirmation that ours was not the only solar system in the firmament came in 1994 when Dr. Alexander Wolszczan, a radio astronomer at Pennsylvania State University, reported what he called "unambiguous proof" of extra-solar planetary systems. Scientists accepted Wolszczan’s assessment and the following year saw the first discovery of a planet orbiting a star similar to our Sun. In 1995, a team of Swiss scientists announced having found a rapidly orbiting world located in extremely close proximity to the star 51 Pegasi in the constellation Pegasus just over 50 light years from Earth.
The initial trickle of exoplanetary discoveries has since become a fast moving stream as ever more powerful telescopes like NASA’s Kepler Space Telescope focus on solar systems far, far away. That stream of exoplanetary discoveries threatens to become a tumult once new observatories like the ESA’s Planck Space Telescope and NASA’s James Webb Space Telescope become fully operational.
But all the recent exoplanetary excitement of recent years — over 1800 exoplanets have now been catalogued — disguises that, until now, despite most of the planets in our own Solar System having moons, not one exoplanet has been confirmed as having a planetary companion.
That may be about to change as physicists from the University of Texas at Arlington have a theory that following a trail of radio wave emissions may lead them to discovering that elusive confirmed first exomoon.
The physicists’ findings were recently published under the title “Detection of Exomoons Through Observation of Radio Emissions,” in the Astrophysical Journal and use as their template how radio wave emissions result from the interaction between Jupiter’s magnetic field and one of its moons, Io. The researchers suggest using detailed calculations based on the Jupiter/Io dynamic to seek out radio emissions that could e the footprint of a moon or moons orbiting a distant exoplanet.
Commenting, Zdzislaw Musielak, professor of physics in the UT Arlington College of Science and co-author of the new paper, said,“This is a new way of looking at these things,” adding, “We said, ‘What if this mechanism happens outside of our solar system?’ Then, we did the calculations and they show that actually there are some star systems that if they have moons, it could be discovered in this way.”
Science fiction buffs are no strangers to the idea of life being perfectly at home on a moon, rather than a planet. Such a concept has fuelled fantasy fiction from the Ewoks of Star Wars to the verdant vistas of James Cameron’s Avatar.
But back down to Earth, or at least close enough in cosmic terms, right here in our own Solar System, astrophysicists consider Saturn’s moon Enceladus and one of Jupiter’s moons, Europa, as being feasible candidates for supporting life based on their atmospheric composition, potential for water and distance from the Sun.
But when it comes to exomoons, existing methods applied in the dogged hunt for exoplanets just won’t hunt, according to Musielak.
To take an example, NASA’s Kepler telescope looks for exoplanets by measuring the changes in brightness from a star to identify planetary transits — orbiting planets passing between the star and Kepler. So far, distinguishing whether a moon is also part of that transit hasn’t been possible.
The team from UT Arlington used earlier theories concerning using radio wave observations to discover exoplanets but applied it in a new way. Their “test subject” was Io, Jupiter’s third largest moon, or more precisely, Io’s ionosphere.
The ionosphere of Io consists of a charged upper atmosphere most likely created by the moon’s extremely active volcanoes.
This global view of Jupiter s moon  Io  was obtained during the tenth orbit of Jupiter by NASA s Gal...
This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Io, which is slightly larger than Earth's Moon, is the most volcanically active body in the solar system.
NASA/JPL/University of Arizona
As Io orbits its parent planet, the moon’s ionosphere interacts with Jupiter’s magnetosphere, a layer of charged plasma protecting the planet from radiation. Radio wave emissions result from the frictional current produced by that interaction.
The technical term for these radio emissions is “Io-controlled decametric emissions” and the researchers conclude discovering similar emissions near exoplanets could be crucial in predicting the existence of other moons.
A moon being volcanically active is not a prerequisite to a moon possessing an ionosphere when modelling such planet/moon pairings, according to Joaquin Noyola, a Ph.D. graduate student at UT Arlington, and part of Musielak’s research group.
“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,” Noyola said.
A schematic of a plasma torus around an exoplanet  which is created by the ions injected from an exo...
A schematic of a plasma torus around an exoplanet, which is created by the ions injected from an exomoon's ionosphere into the planet's magnetosphere.
University of Texas at Arlington
The researchers also considered so-called Alfvén waves produced by Io and Jupiter’s magnetosphere interaction and believe these could be used to spot exomoons. Alfvén waves are the rippling of the plasma in a magnetic field, first described by Swedish electrical engineer, plasma physicist and 1970 Nobel Prize for Physics winner, Hannes Alfvén in the early 1940s.
Focussing on known exoplanets rather than examples of planet/moon pairings in Earth’s backyard, the UT Arlington team zoomed in on two exoplanets where they are “cautiously optimistic” that astronomical observers could apply their study’s calculations when searching for exomoons the size of our moon aided by future, more sensitive radio telescopes.
The prime candidates in these early stages of exoplanetary research are Gliese 876b, at about 15 light-years distant, relatively close to Earth, and the even closer Epsilon Eridani b, just 10.5 light-years away.
According research paper co-author, Suman Satyal, a Ph.D. graduate student, existing telescopes, such as the National Science Foundation-supported Long Wavelength Array, could be used to detect exomoons in closer planetary systems.
Bigger moons hold out a better prospect for detection, says Satyal. Such larger moons are likely to be found orbiting gas giant planets, Saturn’s moon Titan and Jupiter’s moon, Ganymede, being examples in our own solar system.
“Most of the detected exoplanets are gas giants, many of which are in the habitable zone,” said Satyal. “These gas giants cannot support life, but it’s believed that the exomoons orbiting these planets could still be habitable.”