Migrating giants planet might eat their own life-friendly moon

  Here’s to all the moons we’ll 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.

That may be because they don’t survive, suggest Christopher Spalding 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, there’s a particular point in the migration where it becomes endangered.

Over time, the orbits of moons tend to shift with respect to their planet. “It traces out a flower pattern, an ellipse moving with time,” Spalding says. In turn, the planet’s orbit shortens as it moves towards its star.

This orbital dance can stretch and drag the moon’s path dangerously. If the migration continues, it’s only a matter of time before it veers too close to the planet. Eventually, the planet’s gravity will tear the moon apart – or bring the two bodies into a collision from which the moon can’t walk away.

“The further you’ve migrated, the more moon space you sweep clean,” Spalding says.

The death of exo-Io
To illustrate the theory, Spalding considered what would happen to Jupiter’s closest moon, Io, in an alternative history of our solar system.

If Jupiter started where it is now – 5.2 times as far from the sun as Earth is – 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 – where it is warm enough for liquid water – 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 we’re looking for
The thought scares David Kipping of Harvard University, who is currently racing to find the first confirmed exomoon.

“Your first thought is, oh no – does that mean we’re not going to have any moons left over by this mechanism?” he says.

But he thinks that the bigger project of looking for exomoons won’t 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 planet’s lifetime – like Neptune’s moon Triton or Earth’s own moon – after the risky migration was done. The moons in Spalding’s simulations are both in the most danger and the hardest to see.

“Those moons are not really the moons we’re looking for, to paraphrase Star Wars,” Kipping says.