Hereās to all the moons weāll never see. As gas-giant planets move from distant orbits towards their stars, they run the risk of devouring their own potentially habitable moons.
In recent years, the flood of exoplanet discoveries has raised speculation about their possible moons. Astronomers think that they should exist and may be the most likely places to find alien life. But dedicated searches have not yet revealed any.
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That may be because they donāt survive, suggest of the California Institute of Technology and his colleagues. Planets similar to Jupiter and Saturn could swallow up satellites like Io and Europa before we have a chance to see them.
We know that giant planets tend to form far from their stars and then migrate inwards. In extreme cases, they end up orbiting very close to their stars as so-called hot Jupiters. But for each moon near a planet, thereās a particular point in the migration where it becomes endangered.
Over time, the orbits of moons tend to with respect to their planet. āIt traces out a flower pattern, an ellipse moving with time,ā Spalding says. In turn, the planetās orbit shortens as it moves towards its star.
This orbital dance can stretch and drag the moonās path dangerously. If the migration continues, itās only a matter of time before it veers too close to the planet. Eventually, the planetās gravity will tear the moon apart ā or bring the two bodies into a collision from which the moon canāt walk away.
āThe further youāve migrated, the more moon space you sweep clean,ā Spalding says.
The death of exo-Io
To illustrate the theory, Spalding considered what would happen to Jupiterās closest moon, Io, in an alternative history of our solar system.
If Jupiter started where it is now ā 5.2 times as far from the sun as Earth is ā and then migrated inward, Io would be destroyed at 0.6 Earth distances from the sun. The moon Europa would follow soon after.
In a different solar system, the path of a gas giant from the cold outer reaches of space to the habitable zone ā where it is warm enough for liquid water ā could have a sad irony. The trip could eliminate moons that by the end of the journey would have had the right conditions for life.
The moons weāre looking for
The thought scares of Harvard University, who is currently racing to find the first confirmed exomoon.
āYour first thought is, oh no ā does that mean weāre not going to have any moons left over by this mechanism?ā he says.
But he thinks that the bigger project of looking for exomoons wonāt take too big a hit. The easiest moons to find are larger bodies located further from their planets. These were probably captured or formed later in the planetās lifetime ā like Neptuneās moon Triton or Earthās own moon ā after the risky migration was done. The moons in Spaldingās simulations are both in the most danger and the hardest to see.
āThose moons are , to paraphrase Star Wars,ā Kipping says.
Journal reference: , The Astrophysical Journal, accepted
Image credit: IAU/L. CalƧada
