Planets

Objects Destabilized by Jupiter or Saturn May Have First Brought Water to Earth

Gas giants forming in the early moments of our solar system may have propelled so-called planetesimals in the asteroid belt toward Earth, carrying water with them.

Nick Purser/Getty Images

A lingering question among researchers is: How did water first appear on Earth?

Earth's neighborhood was quite dry during to the early moments of the universe, according to some studies, which means that somehow, water was brought from other parts of the solar system, such as the outer areas of the asteroid belt lying between Mars and Jupiter. Other studies have found that water on Earth matches the water found in the asteroid belt. But debate remains about how that water was delivered to Earth.

A new paper published in the journal Icarus suggests Earth's water indeed came from small bodies called planetesimals, which formed in the asteroid belt and the regions around Jupiter and Saturn that lie beyond the belt. The findings were based on computer simulations of the early universe.

"In simple terms, the growing giant planets are like toddlers throwing their food on the walls and the floor," Sean Raymond, lead author of the study and an astronomer at the University of Bourdeaux, told Seeker.

As planets formed during the early moments of our solar system, Raymond explained, the gas giants destabilized the orbits of nearby planetesimals, stretching their paths from circles to ellipses, which eventually pushes them across the orbits of Jupiter or Saturn.

"The planetesimals rarely collide with the giant planets, but they do come pretty close and get huge gravitational kicks,” Raymond said. “They are flung all over the solar system, and a fraction are deposited on new, stable orbits in the asteroid belt, preferentially in the outer [region of the] belt. Some of them are kicked inward past the asteroid belt to where the rocky planets are growing, seeding them with water."

A model of how our solar system could have looked early in its formation. At left is the young sun (a protosun), with particles of gas (the nebula) and chunks of rock (the planetesimals) nearby. This zone is approximately where the inner solar system exists today. Most of the water in the solar system lay beyond the "frost line." | University of Hawaii Institute for Astronomy

Raymond acknowledged that estimates of the exact amount of water delivered to Earth vary, depending on variations in the structure of the disk of planet-building material surrounding the sun. He said, though, the process of water-rich planetesimals migrating near Earth is "unavoidable."

Raymond said there is ongoing work to determine why the Earth's solar system looks so different from other solar systems. For example, elsewhere large gas giants orbit very close to their parent star. Imagine Jupiter or Saturn right next door to the sun. And other systems host "super-Earths," or planets that are between the size of Earth and Neptune. These findings suggest that giant planets may migrate positions over time and affect the orbits of other planets, perhaps kicking out Earth-sized planets from a solar system altogether, in some cases.

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Dave Kornreich, an assistant professor and physicist at Humboldt State University in California, pointed out that telescopic improvements may soon help astronomers see solar systems like our own.

"I wouldn't be so quick to call our solar system an oddball," he wrote on Cornell University's "Ask An Astronomer" web page in 2015. "It's true that many of the planets we've found around other stars are very large and very close to their stars, unlike our solar system, but we are now at the point where we are starting to see multiple planet systems that look more and more like our solar system. As technology improves, we will most likely start to see systems just like ours."

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