The possible exomoon would orbit a hot giant planet, which in turn would race once around its host star in less than three days.
Theoretically, large amounts of sodium at an exoplanet could point to a hidden moon or ring of material, and ten years ago, researchers at Virginia calculated that such a compact system of three bodies: star, close-in giant planet and moon, can be stable over billions of years.
Artist's composition of a volcanic exo-Io undergoing extreme mass loss. The hidden exomoon is enshrouded in an irradiated gas cloud shining in bright orange-yellow. Credit: © University of Bern, Illustration: Thibaut Roger
Sodium gas as circumstantial evidence
Astronomers have not yet discovered a rocky moon beyond our solar system and it's on the basis of circumstantial evidence that the researchers behind a new paper conclude that the exo-Io exists: Sodium gas was detected at the WASP 49-b at an anomalously high-altitude.
The neutral sodium gas is so far away from the planet that it is unlikely to be emitted solely by a planetary wind. Observations of Jupiter and Io in our solar system along with mass loss calculations show that an exo-Io could be a very plausible source of sodium at WASP 49-b. sodium is right where it should be" says the astrophysicist.
Tides keep the system stable
"The enormous tidal forces in such a system are the key to everything," explains astrophysicist Apurva Oza from the University of Bern. The energy released by the tides to the planet and its moon keeps the moon's orbit stable, simultaneously heating it up and making it volcanically active. In their work, the researchers were able to show that a small rocky moon can eject more sodium and potassium into space through this extreme volcanism than a large gas planet, especially at high altitudes.
"Sodium and potassium lines are quantum treasures to us astronomers because they are extremely bright," says Oza, "the vintage street lamps that light up our streets with yellow haze, is akin to the gas we are now detecting in the spectra of a dozen exoplanets."
The researchers compared their calculations with these observations and found five candidate systems where a hidden exomoon can survive against destructive thermal evaporation. For WASP 49-b the observed data can be best 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 need to find more clues," Oza admits. The researchers are therefore relying on further observations with ground-based and space-based instruments.