When it comes to the early days of our solar system, Jupiter holds a dubious reputation. In some ways, the giant served as Earth’s protector, its gravity launching dangerous debris away from the rocky planets. At the same time, Jupiter may have hurled material inward as well, crashing hydrogen-rich asteroids and planetary embryos, or planetesimals, into crowded young terrestrial planets.
Now, researchers suggest that in doing so, Jupiter and other gas giants may have contributed something else crucial to rocky worlds: water.
The most massive worlds may have shepherded water-rich debris from the outer solar system to fall on the rocky worlds. And new research suggests the delivery of the liquid, a key ingredient for life as we know it, may not have been luck. Instead, all planetary systems fortunate enough to host a gas giant in their outskirts should automatically have water-rich material falling on their rocky inner planets.
After the gas giants have fully developed, the debris they hurl inward can be dangerous. But during a key phase of their birth, they toss hydrogen-rich material that winds up locked into Earth's crust and mantle, emerging later to bond with oxygen and become water.
"In the process of formation, they send this big pile of planetesimals all over the place, and some bash into the terrestrial planets," said Sean Raymond, an astronomer who studies how planets evolve at France's University of Bordeaux and lead author of a study published in the journal Icarus. By modeling the role of gas giants in the early solar system, Raymond found that giant planets of varying sizes unavoidably hurled water-rich material into the inner system, where rocky worlds could potentially hold it as liquid water on their surfaces.
Water, of course, is a key ingredient for the evolution of life as we know it on Earth. So when it comes to hunting worlds beyond the solar system, rocky worlds capable of hosting the precious liquid are thought to be the best hunting grounds for extraterrestrial life. Since the 1980s, researchers have struggled to determine how water arrived on Earth. Today, carbon-rich asteroids are the leading suspect.
In the young solar system, collisions were frequent and orbits crossed one another, and the early asteroids were still easily affected by close encounters with other planets, whose gravity tossed them toward rocky worlds."I think it's a very interesting story, and one that's fundamental if you're trying to understand how you make habitable planets," said astrochemist Conel Alexander, who studies primitive meteorites from those asteroids.
About 4.5 billion years ago, a cloud of gas leftover from the formation of the sun birthed the planets. The gas hung around for millions of years, influencing the motion of the planets and their rock-rich components. Rising temperatures meant that hydrogen, a building block for water, was trapped in ice in the colder regions of the solar system, far out of Earth's reach.
It seemed our planet was destined to be a dry and barren wasteland. So what happened?
'A ridiculously simple concept'
In recent years, models of our solar system have shown that the gas giants most likely underwent an intricate dance before ending up in their current spots. Neptune and Uranus probably formed closer to the sun than they are today. Eventually, they moved outward, trading places along the way. Known as the Nice model, this process is thought to have spurred the Late Heavy Bombardment, a spike of icy impacts about 600 million years after the solar system formed.
Saturn and Jupiter may have undergone an even more harrowing journey, plowing through the young asteroid belt on their way into the inner solar system before reversing course and heading back outward. Along the way, they also sent asteroids crashing toward the Earth. This is known as the Grand Tack model, which Raymond was helping formulate in 2008.
Around that time, Raymond first became intrigued with how Jupiter may have shaped water delivery in the early solar system. But his modeling was stymied by a minor programing issue he couldn't seem to shake. It took the arrival of post-doctoral researcher Andre Izidoro, nearly a decade later, to solve the problem.
"Izidoro found a bug I'd had for years in half an hour," Raymond says ruefully. "I was really happy that he found it so we could actually do the project."
Under the new model, as a gas giant grows larger, consuming more material, its increasing gravity destabilizes nearby protoplanets. The drag of the still-present nebula gas affects how the debris moves through the solar system, sending a fraction of them inward towards the inner solar system. Some of that material became trapped in the asteroid belt, populating it with the carbon-rich asteroids whose water content is so similar to Earth's.
Originally, Raymond says, the carbon-rich asteroids were scattered across a region spanning from 5 to 20 times the Earth-sun distance. "It must have covered the whole solar system," he says.
But Alexander, who studies carbon-rich asteroids, suspects that the region was smaller, with most of the suspects forming just outside of Jupiter's orbit. Still, he thinks Raymond's model does a good job of explaining how water-rich material was delivered to Earth, calling the hypothesis “perfectly reasonable.”
"This is the best way to get these volatiles into the terrestrial planet forming region," Alexander says.
The model leaves several questions hanging, such as why so little of the wealth of mass of the early solar system is present today. "That's a key part that needs to be connected," Raymond admits.
Still, he says the model helps fill in several gaps, including why Earth's water matches the composition of asteroids of the outer belt more than the drier asteroids of the inner belts.
"It's a ridiculously simple consequence of Jupiter and Saturn growing," he says.
Hunting water-rich worlds
Before Raymond's model, researchers thought it was the unusual dance of the outer planets that sent water into the inner solar system and kept Earth from a dry future. If that were true, it would be bad news for other worlds, where the gas giants may have remained wallflowers who never moved far from where they started.
The new model suggests that any gas giant would send wet material hurling inward as a consequence of their formation. While massive Jupiter-sized worlds were the most effective, Raymond found that any sized gas giant could trigger the growth. That’s good news for researchers hunting watery planets outside our solar system.
In our own solar system, the model shows that ices from the outer solar system snowed down on the Earth in three waves. The first came as Jupiter swelled up. The second was triggered during Saturn's formation. And the third would have occurred when Uranus and Neptune migrated inward before being blocked by the other two and sent back to the outskirts of the solar system.
"I think the coolest thing is that it basically implies for any exo-solar system where you have giant planets and terrestrial planets, those giant planets would send water inward to the terrestrial planets," said David O'Brien, a researcher at the Planetary Science Institute who studies planet formation and the evolution of the early solar system. "That opens up a lot of possibilities for habitable planet studies."
Unfortunately, so far we don’t have many similar systems to compare to. Most of the known exoplanets have been identified with NASA's Kepler mission, which O'Brien said is most sensitive to planets with orbits smaller than Earth's and has difficulty detecting gas giants in the outer system. Small rocky planets are also more challenging to observe. That doesn't mean they aren't there—it just means we haven't spotted them yet.
But if such systems exist, Raymond's research suggests that the rocky worlds should be rich with what we consider the liquid of life. "If there's terrestrial planets and giant planets, those giant planets probably gave the terrestrial planet some water," O'Brien says.