Indications of a volcanically active exomoon

A rocky extrasolar moon (exomoon) with bubbling lava can turn a planet 550 light-years away from us. This is suggested by an international team of researchers led by the University of Bern on the basis of theoretical predictions corresponding to observations. The exo-Io seems to be an extreme version of the moon Io of Jupiter.

The moon of Jupiter Io is the most volcanically active body of our solar system. There is today evidence that an active moon outside our solar system, an exo-Io, could be hidden in the WASP-49b exoplanet system. "It would be a dangerous volcanic world with a molten lava surface, a lunar version of nearby super-lands like 55 Cancri-e," says Apurva Oza, postdoctoral fellow at the Institute of Physics at the University Bern and PRN collaborator. PlanetS, "a place where Jedis will die, dangerously familiar to Anakin Skywalker." But the object that Oza and his colleagues describe in their work seems to be even more exotic than the science fiction of Star Wars: the eventual exomoon could orbit around a burning giant planet, which would run to turn around the host star in less than three days … A scenario located 550 light-years away from the unobtrusive constellation of Lepus, under the brilliant constellation of Orion.

Sodium gas as indirect evidence

Astronomers have not yet discovered a rocky moon beyond our solar system and it is on the basis of circumstantial evidence that researchers in Berne conclude that exo-Io is present: sodium gas has been detected at WASP 49-b at an abnormally high altitude. "The sodium-free gas is so far away from the planet that it is unlikely that it will be emitted solely by a global wind," Oza said. The observations of Jupiter and Io in our solar system, made by the international team, as well as mass loss calculations show that an exo-Io could be a very plausible source of sodium to WASP 49-b . "Sodium is where it should be," says the astrophysicist.

Tides keep the system stable

Already in 2006, Bob Johnson of the University of Virginia and the late Patrick Huggins of the University of New York in the United States had shown that a large amount of sodium in an exoplanet could indicate a moon or a ring of hidden material. Ten years ago, researchers in Virginia calculated that such a compact system of three bodies: a star, a giant planet close together and the moon, can be stable for billions of years. Apurva Oza was then a student in Virginia and after his doctorate on the atmospheres of moons in Paris, decided to resume the theoretical calculations of these researchers. He is now publishing the results of his work with Johnson and his colleagues from Astrophysical Journal.

"The huge tidal forces in such a system are the key to everything," says the astrophysicist. The energy released by the tides towards the planet and its moon maintains its stable orbit, warming it and simultaneously making it volcanic. In their work, the researchers were able to show that a small rocky moon could reject more sodium and potassium in space by this extreme volcanism than a large gaseous planet, especially at high altitudes. "The sodium and potassium lines are quantum treasures for us astronomers because they are extremely bright," says Oza, "the street lights that illuminate our streets with a yellow haze resemble the gas we detect. now in the spectrum of a dozen or so exoplanets.

"We need to find more clues"

The researchers compared their calculations with these observations and found five candidate systems in which a hidden exomoon can survive destructive thermal evaporation. For WASP 49-b, the observed data can be better explained by the existence of an exo-Io. However, there are other options. For example, the exoplanet could be surrounded by a ring of ionized gas or non-thermal processes. "We have to find more clues," Oza admits. The researchers therefore rely on other observations with terrestrial and spatial instruments.

"While the current wave of research is gearing towards habitability and biosignatures, our signature is a signature of destruction," says the astrophysicist. Some of these worlds could be destroyed in a few billion years due to extreme mass loss. "The interesting part is that we can monitor these destructive processes in real time, like fireworks," said Oza.