Radio Emissions: An Exomoon Detection Technique?

by Paul Gilster on August 13, 2014

Here’s an interesting notion: Put future radio telescopes like the Long Wavelength Array, now under construction in the American southwest, to work looking for exomoons. The rationale is straightforward and I’ll examine it in a minute, but a new paper advocating the idea homes in on two planets of unusual interest from the exomoon angle. Gliese 876b and Epsilon Eridani b are both nearby (15 light years and 10.5 light years respectively), both are gas giants, and both should offer a recognizable electromagnetic signature if indeed either of them has a moon.

The study in question comes out of the University of Texas at Arlington, where a research group led by Zdzislaw Musielak is looking at how large moons interact with a gas giant’s magnetosphere. The obvious local analogue is Io, Jupiter’s closest moon, whose upper atmosphere (presumably created by the active volcanic eruptions on the surface) encounters the charged plasma of the magnetosphere, creating current and radio emissions.

The researchers calls these “Io-controlled decametric emissions,” and they could be the key to an exomoon detection if we can find something similar around a nearby gas giant like those named above. Io’s atmosphere may be volcanic in origin, but we know from the example of Titan that moons in greatly different configurations can also have an atmosphere. The interactions with the magnetosphere are what is important. “We said, ‘What if this mechanism happens outside of our solar system?’” says Musielak. “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.”

plasma_torus

Image: 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. Credit: UT Arlington.

We’ve often speculated about the habitability of a moon orbiting a gas giant, but neither of the planets named above, Gliese 876b and Epsilon Eridani b, is within the habitable zone of its respective star. The former has a semimajor axis of 0.208 AU, beyond the HZ outer edge for this M4V-class red dwarf. Epsilon Eridani b is likewise a gas giant (about 1.5 times Jupiter mass) with an orbital distance of approximately 3.4 AU, again outside the K2V primary’s habitable zone. So early work on these two planets would not be related to the habitability question but would serve as a useful test of our ability to detect exomoons using electromagnetic interactions.

I wrote David Kipping (Harvard-Smithsonian Center for Astrophysics) this morning to ask for his reaction to the electromagnetic approach to exomoon detection. Kipping heads The Hunt for Exomoons with Kepler, which uses techniques involving planetary transits and the signature of exomoons within. He called this work “…an inventive idea which could discover exomoons not detectable with any other technique,” and went on to point out just where electromagnetic methods might be the most effective.

Magnetospheres are more extended for gas giants on wide orbits, like Jupiter. So I would expect this technique to be most fruitful for cold Jupiters, whereas the transit technique is better suited for planets at the habitable-zone distance or closer. The complementary nature of these detection techniques will allow us to find moons around planets at a range of orbital separations.

Adding more tools to our inventory can only help as we proceed in our search for the first exomoon. Let me quote Kipping’s further thoughts on the method:

In order to make a detection with this method, the moon must possess an ionosphere and so some kind of atmosphere. Io has a tenuous atmosphere because of intense tidal friction leading to volcanism and subsequent sulphur dioxide outgassing, but we don’t really know how common such a scenario is. Alternatively, a moon may be able to retain an atmosphere much like the Earth does, but in the Solar System only Titan satisfies this criteria.

The host planet must have a strong magnetosphere. For Jupiter-sized planets, this is reasonable but Neptunes and mini-Neptunes dominate the planet census and if such objects have moons, their magnetospheres are unlikely to be strong enough to produce an observable radio signal via interaction with a moon’s ionosphere.

For these reasons, an absence of a radio signal would not necessarily mean that there were no moons, unlike the transit technique which can make more definitive statements.

The technique is most useful for nearby planetary systems, within a few parsecs, but then again these are likely the most interesting systems to explore!

Unlike the transit method, this technique does not require the orbital inclination of the planetary system to be nearly aligned to our line of sight – a significant advantage.

The best-case quoted sensitivities, 0.25 to 0.75 Earth radii, are comparable to the best-case sensitivities with the transit method.

This new exomoon work reminds me of Jonathan Nichols’ thinking on radio telescopes and exoplanet detection. An astronomer at the University of Leicester, Nichols proposed at a Royal Astronomical Society meeting in 2011 that a radio telescope like the Low Frequency Array (LOFAR), now under construction across France, Sweden, the Netherlands, Great Britain and Germany, could detect the radio waves generated by the aurorae of gas giants, emissions that we can detect from Jupiter and Saturn in our own system. Nichols believes we might use such methods to find planets up to 150 light years away. See Exoplanet Aurora as Detection Tool for more.

The paper is Noyola et al., “Detection of Exomoons Through Observation of Radio Emissions,” The Astrophysical Journal Vol. 791, No. 1 (2014), p. 25 (abstract). The paper on aurora detection is Nichols, “Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions,” Monthly Notices of the Royal Astronomical Society, published online July 1, 2011 (abstract / preprint).

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{ 10 comments… read them below or add one }

Tulse August 13, 2014 at 16:56

Although the two discussed planets are beyond the HZ, that definitely doesn’t mean that an exomoon wouldn’t be habitable. At the very least, tidal heating could produce a subsurface ocean, and the “milder” temperatures of being closer to the HZ than Europa or Enceladus might make such a circumstance all the more likely (less heating would be required). I’d also be curious if tidal heating could actually impact on an atmosphere — does anyone know if a tidally heated exomoon just outside the habitable zone could get a substantially heated surface, enough to support surface liquid water?

Michael August 13, 2014 at 17:22

Same topic but more up to date: http://arxiv.org/abs/1308.4184
The Astrophysical Journal, Volume 791, Issue 1, article id. 25, 5 pp. (2014)

In the Jupiter-Io system, the moon’s motion produces currents along the field lines that connect it to Jupiter’s polar regions. The currents generate, and modulate radio emissions along their paths via the electron-cyclotron maser instability. Based on this process, we suggest that such modulation of planetary radio emissions may reveal the presence of exomoons around giant planets in exoplanetary systems. A model explaining the modulation mechanism in the Jupiter-Io system is extrapolated, and used to define criteria for exomoon detectability. A cautiously optimistic scenario of possible detection of such exomoons around Epsilon Eridani b, and Gliese 876 b is provided.

Andrew Palfreyman August 14, 2014 at 0:53

It sounds slightly odd to say “in this year 2014, no exomoons were known”. Positively Victorian. Perhaps my children will grin about that, in a few years.

Michael M August 14, 2014 at 2:30

Interesting is the size of the power~ 4 terra watts! travelling from Jupiter to Io, that makes a lot of electromagnetic noise. If we could eventually harness that power by say having better conductors from polar regions on Jupiters moons it would make a huge difference to our expansion into the outer solar system and would provide enormous power for interstellar transmissions.

http://arxiv.org/ftp/astro-ph/papers/0209/0209070.pdf

Tom Mazanec August 14, 2014 at 8:20

Could someone please add this detection method to the Wikipedia Exomoon article? I tried to, but kept getting messed up with the editing system.

ljk August 14, 2014 at 10:40

Links to very recent Centauri Dreams articles on exomoon detection methods and evidence for actual such objects to add to our database:

http://www.centauri-dreams.org/?p=30670

http://www.centauri-dreams.org/?p=30424

http://www.centauri-dreams.org/?p=29719

A CD search exomoon will also turn up articles discussing their chances for being places friendly to life.

Note that the venerable Kepler satellite is also hunting for exomoons and found an Earth-size world just two months ago:

http://www.cfa.harvard.edu/HEK/

I am just glad to see scientists thinking outside the box when it comes to finding alien worlds. Who knows what else we may discover in the process?

As just one example of scientific serendipity from space history, celestial gamma ray objects were found by our early Vela satellites that were actually looking for nuclear bomb tests by the Soviets over the far side of the Moon!

David Hart August 14, 2014 at 17:11

Whilst the original paper talks about a targeted search of known nearby exoplanets, and Kipping talks about targeted search of all nearby stars for exomoons of so-far undetected exoplanets, an all-sky survey may detect nearby sunless or rogue exoplanets Jupiter-sized and upwards that have close-in moons. Nothing in the methodology requires the exoplanet-exomoon to be in a planetary system with a sun.

Joëlle B. August 15, 2014 at 12:49

@David Hart

Good thinking. I found some further info to look into, if anyone is interested in the idea of rogue planets and their prospective satellites.

Abstract: ‘The Survival Rate of Ejected Terrestrial Planets with Moons’

“During planet formation, a gas giant will interact with smaller protoplanets that stray within its sphere of gravitational influence. We investigate the outcome of interactions between gas giants and terrestrial-sized protoplanets with lunar-sized companions. An interaction between a giant planet and a protoplanet binary may have one of several consequences, including the delivery of volatiles to the inner system, the capture of retrograde moons by the giant planet, and the ejection of one or both of the protoplanets. We show that an interesting fraction of terrestrial-sized planets with lunar-sized companions will likely be ejected into interstellar space with the companion bound to the planet. The companion provides an additional source of heating for the planet from tidal dissipation of orbital and spin angular momentum. This heat flux typically is larger than the current radiogenic heating of the Earth for up to the first few hundred million years of evolution. In combination with an atmosphere of sufficient thickness and composition, the heating can provide the conditions necessary for liquid water to persist on the surface of the terrestrial-mass planet, making it a potential site for life. We also determine the possibility for directly detecting such systems through all-sky infrared surveys or microlensing surveys. Microlensing surveys in particular will directly measure the frequency of this phenomenon.” (Debes, Sigurdsson 2007, http://iopscience.iop.org/1538-4357/668/2/L167/ )

Another paper worth looking at in relation to the above: ‘Formation of irregular and runaway moons/exomoons through moon-moon scattering’ http://arxiv.org/abs/1407.2619

Joëlle B. September 3, 2014 at 17:53

Formation, Habitability, and Detection of Extrasolar Moons

Abstract: “The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon’s stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1 – 0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.”(René Heller et al. 2014)

http://arxiv.org/abs/1408.6164

ljk September 12, 2014 at 10:55

Are aliens hiding on MOONS? Hunting for ET on planets’ satellites may be our best chance at first contact, claims expert

Dr David Kipping of the Harvard-Smithsonian Center for Astrophysics says our best chance of finding life may be on exomoons

He is working on a mission to study distant moons for signs of habitability using the Kepler telescope, reports All About Space magazine

Moons around extrasolar gas giants could potentially be havens for life

In an upcoming study Dr Kipping and his team will look at hundreds of exoplanets for signs of moons

They will then discern how common different moons are in the Milky Way
And it’s hoped that one day habitable exomoons will be found

By GILES SPARROW FOR ALL ABOUT SPACE

PUBLISHED: 05:47 EST, 11 September 2014 | UPDATED: 09:44 EST, 11 September 2014

Full article here:

http://www.dailymail.co.uk/sciencetech/article-2751823/Is-best-chance-finding-alien-life-MOON-Hunting-ET-satellites-distant-planets-best-bet-contact-claims-expert.html

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