While some scientists are looking for habitable planets outside our solar system, other researchers are solving a similar question for the satellites of these planets.
The so-called exomoons have not yet been found outside our solar system, and this could take decades. In a new research paper, scientists theorize whether liquid water could exist on a Mars-sized satellite of the gas giant.
We are talking about Jupiter's moon called Ganymede. It is the largest satellite in the solar system, 5/6 the size of Mars.
NASA confirmed in 2015 the presence of a liquid ocean on Ganymede after observing auroras through the Hubble telescope, which appear to fluctuate less than they should have given Jupiter's magnetic field. The space agency said it was likely related to the salt ocean beneath the surface of Ganymede.
Whether it is possible to speak of this satellite as a potential exo moon is an ambiguous question. Scientists have looked at energy sources such as stellar radiation (which varies with distance from the star), the stellar reflected light that Jupiter casts on Ganymede, the planet's own thermal radiation, which affects the heating of the satellite by changing the planet's gravitational attraction. This tidal heating would be most pronounced if Ganymede had an eccentric orbit, similar to Jupiter's volcanic moon Io.
“It is known that the tidal heating coefficient decreases if the moon is molten inside, because the lava creates its own reverse mechanism when the heating decreases and the moon cools below the surface. This is called the "thermostat tidal effect", "- said co-author Rene Heller, an astrophysicist at the Institute for Solar System Research. Max Planck in Germany.
“We are investigating for the first time the interaction of all possible exolunar heat sources depending on different distances to the star,” he added. "In fact, we are even considering two possible types of stars: our Sun and a red M-type dwarf star."
For a sun-like star, the authors found that any moon around a gas giant beyond three astronomical units (three distances from Earth to the Sun) would have a sufficiently large flow of energy to stop the tidal thermostat effect. But if the moon is unstable enough, it can have global volcanism - just like what we see on Io.
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Heller described this situation as "dangerous" for living organisms.
“They can have a lot of liquid water on the surface, but at the same time they can be covered with destructive volcanoes,” he wrote. "However, we agree that such moons can be habitable, provided the right amount of tidal warming is correct, and we are showing how far these moons should be to their planets."
M-dwarfs are a common target for exoplanet searches because they are smaller and dimmer, making it easier to see planets passing through their surfaces. But it is more difficult for exoluns to determine how suitable they are for life in such a system.
"The moons cannot be stable in those zones of stellar systems that are theoretically suitable for the origin of life," Heller said.
The best examples for heated bodies in our own solar system are the moons: Jupiter - Io and Europa, and Saturn's Enceladus. While there is ample evidence that there could be a huge ocean under the ice surface of Europa and Enceladus, Heller pointed out that his research is more focused on the habitability of the satellite's surface. The best example, he says, might be Titan, Saturn's moon, which has a much warmer surface. Titanium has a thick orange atmosphere as well as liquid hydrocarbon lakes.
“Basically, we have the opportunity to observe large moons around low-mass planets, and I think that the first exoloon will be unlike anything we know in the solar system,” Heller said.
“It could be a moon like Mars around a planet like Earth, or Earth around Neptune at a distance from their star, which could be similar to the distance from Mercury to the Sun. (there are a lot of options). There will probably be something incredible at first glance, like a planet around a pulsar or hot Jupiter. I'm really curious to know what this object will be like,”says the German astrophysicist.
Although several new telescopes have emerged over the next decade to “hunt” exoplanets, Heller says they are not optimized for exoons. The search for exoluns is financially risky, and the likelihood of success is highly questionable, which means that this project is likely to remain a low priority for the astronomical community.
The James Webb Space Telescope, a multifunctional telescope that will launch in 2018, is expected to view only a few exoplanets, so its chances of finding an exo moon are low, Heller says. The Transiting Exoplanet Survey Satellite, which also launches next year, will observe only a very few transiting planets.
“These planets will be so close to their stars that any moon around the planet will be immediately thrown out of the system by stellar gravitational disturbances,” Heller said.
The chances may be increased by the European supertelescope CHEOPS (CHaracterising ExOPlanets Satellite), which is currently under construction.
“I know that some of the CHEOPS science team is actively working on strategies for exploring moons around planets in wider orbits,” Heller said. But he added that PLATO (PLAnetary Transits and Oscillations of stars), a project that will start around 2024, is likely to be the "suitable tool" for this work. It will conduct a targeted search for planets, similar to the Kepler space telescope, but around brighter stars.