Authors: Rene Heller and Rory Barnes
First Author’s Institution: Leibniz Institute for Astrophysics, Germany
Some exoplanets seem to have walked directly out of the best science fiction movies. We’ve discovered a planet consisting purely of water (GJ1214b) and one with two suns (Kepler 16b). Do the movies “Waterworld” and “Star Wars” ring a bell? Others seem far more odd than even the most creative science fiction writer could have imagined. Some worlds scrape their host star every periastron passage. You had better make sure you’re on the opposite side of the planet at that time of the year! Other worlds exist in darkness without a star at all.
Considering the question of habitability for planets like these seems like a joke. But what if we stopped looking at these extreme worlds and turned our eyes to their moons instead? Perhaps their moons are less extreme. And given that our own Jupiter hosts 67 moons, surely they’re more abundant. Can such extreme planets host habitable moons? The 36-page paper written by Heller and Barnes, which appeared in the January issue of Astrobiology, attempts to address this question.
The photometric precision of Kepler now makes the detection of Earth-sized exomoons possible. If, during an exoplanet transit, an exomoon also passes in front of the host star, there will be an added dip in the observed light. See this video for a great demonstration. Another popular method for detecting exomoons is transit timing variations. The exact timing of a transit will vary if there are secondary bodies (either other planets or moons) in the system.
This paper does not address the observational difficulties of detecting exomoons (although it is a pretty cool puzzle to solve), but rather the theoretical question of what makes an exomoon habitable. The authors consider the physical and orbital parameters, which may affect the conditions for life. They argue that it will be possible to constrain an exomoon’s habitability based on the data available solely at the time they’re discovery is confirmed. Let’s look at a few of the factors that come into play.
Arguments Against Habitability
If a moon crosses through the radiation belt of a planet it will undoubtedly experience harmful radiation. Such ionizing radiation could prevent complex molecules from forming or even strip away a moon’s atmosphere. In this process, known as sputtering, particles are literally ejected from the surface due to the incoming radiation. As an example, the incident particles on Jupiter’s satellites are the heavy ions O+, S+, and H+. This causes an outgoing flux of H2O, OH, O2, H2, O, and H – molecules and elements necessary for life are constantly being stripped away from the planet.
An oversupply of energy will lead an exomoon into a runaway greenhouse effect. On Earth, the thermal equilibrium temperature of incoming and outgoing radiation is 255K. However, the mean surface temperature is actually 289K. This additional heat is due to the greenhouse effect, where radiation is absorbed by gases in the atmosphere and re-radiated in all directions. This is dependent on many different factors: the number of greenhouse gases, the albedo effect of the clouds, the amount of liquid surface water, and the energy distribution of the host star.
For an earth-like planet or moon, the calculated threshold before the atmosphere becomes a runaway greenhouse is approximately 300 W/m2. Such energy sources include 1) stellar illumination 2) stellar reflected light from the planet 3) thermal radiation from the planet and 4) tidal heating. If the total incident energy from all sources is less than the calculated threshold the exomoon will be habitable.
Heller & Barnes derive the stellar and planetary contributions of the illumination on a tidally locked moon. This allows them to compute the surface flux on the moon at any given point on the moon’s surface. A typical orbit-averaged flux is between 300 and 400 W/m2 . While exomoons are at the threshold of the greenhouse effect, this indicates that climates on exomoons are very similar to freely rotating planets (i.e. planets that do not suffer from being tidally locked to a star).
Consider Jupiter’s moon Io. With over 400 active volcanoes, it is the most geologically active object in the solar system. Why? Because one side of Io is constantly being pulled toward Jupiter while the other side is constantly being pulled toward the other Galilean satellites. This generates friction, or tidal heating. Take a paper clip and bend it back and forth repeatedly. The heat you feel beneath your fingers is the result of tidal heating. This additional energy source may cause the exomoon to experience the runaway greenhouse effect. Here the surface would boil away, leaving the exomoon uninhabitable.
Arguments For Habitability
While discussed previously as leading to a runaway greenhouse effect – energy is a necessary condition for life. It is a classic example of the goldilocks effect, while we can’t have too much, we also can’t have too little. As long as the added energy from illumination (both from the host star and the planet), thermal radiation, and tidal heating do not cause a runaway greenhouse effect then the planet will remain habitable.
The earth is embedded within a protective bubble – the magnetic field – that prevents solar radiation from reaching our surface (and also sparks the northern lights). If an exomoon had such a magnetic field, it would certainly protect it from the harmful radiation belt of its host planet. How likely is an exomoon to have a magnetic field? We’re not sure. The only moon in the solar system with one is Ganymede. The origin of its magnetic field is still unknown.
Most Hot Jupiters are tidally locked to their host stars – one hemisphere will permanently face the star, while the other will experience eternal darkness. An exomoon will in turn be tidally locked to its planet (as our moon is locked to us – leading to only one visible side), but not to the star. The exomoon will effectively rotate around the star, allowing the entire surface to experience day and night. It will also be able to experience seasons. This is certainly a favorable condition for life!
What does this all add up to?
An equation of course!
Heller and Barnes estimated a theoretical model that calculates when an exomoon is habitable. In analogy with the circumstellar habitable zone, they define a circumplanetary “habitable edge.” Moons must exist outside this edge in order to be habitable. If they exist within it, they risk running into a greenhouse by stellar and planetary illumination or tidal heating. There is no constraint on the outer edge. The final result is derived from combing the limits for the runaway greenhouse as well as the energy flux budget.
These are only the highlights of Heller and Barnes’ 36-page paper. Estimating an exomoon’s habitability with this model requires a well parametrized system. In theory, however, we should be able to use this model to assess if a moon is habitable based only on its orbital parameters. With the next generation of telescopes we will be able to look for spectroscopic signatures of life.
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