New research published in Nature Astronomy suggests that giant impacts in the late stages of rocky planet formation could help young planets retain their primordial atmospheres, challenging the long-held assumption that the immense heat from such collisions drives atmospheres away.
The study, by Darius Modirrousta-Galian, a former Yale postdoc now at Shanghai Jiao Tong University, and Jun Korenaga, a professor of Earth and planetary sciences at Yale, focuses on the end of a rocky planet's accretion process, after the protoplanetary disk has largely dissipated. This is when large planetesimals collided with growing planets such as Earth, releasing enormous amounts of energy.
The researchers argue that a giant impact has two effects that do not weigh equally on the atmosphere. While the entire accreted mass contributes to the increase in gravitational binding energy, only part of the impact energy raises mantle temperature. "Mantle melting consumes energy as latent heat, while the impactor's metallic core transfers little energy to the mantle and remains superheated at depth," the authors explain. "Giant impacts therefore increase gravitational binding energy more than mantle thermal energy, shifting planets towards atmospheric retention."
"Giant impacts are a key part of how rocky planets grow," said co-author Korenaga. "Planets the size of Earth or larger are expected to form through collisions between smaller planetary bodies. These collisions add mass and release enormous amounts of heat, which can drive atmospheric escape. Their effect on a planet's atmosphere depends on how those two changes work together."
The findings could help explain the abundance of super-Earths and sub-Neptunes, the most common types of exoplanets detected and two types not represented in our Solar System. The researchers suggest sub-Neptunes could have grown larger through more massive impacts, which not only added mass but allowed them to retain larger primordial hydrogen and helium atmospheres. Super-Earths, by contrast, would not have experienced those same massive impacts, did not gain enough mass to retain their atmospheres in the same way, and lost them due to stellar radiation.
"Larger, atmosphere-bearing planets emerge preferentially through collisional growth," the researchers write. "By contrast, smaller planets without such impacts tend to lose their primordial atmospheres under sustained stellar irradiation. At the population level, these results suggest that sub-Neptunes have, on average, more massive rocky interiors than super-Earths."
"Why nature produces both types in such abundance is a fundamental puzzle in exoplanetary science," said Modirrousta-Galian. "We set out to find an explanation."
The results also bear on the prospects for habitability. While primordial hydrogen and helium atmospheres did not themselves create habitable conditions, some research indicates they were essential for the development of a world that eventually became habitable, through interactions with a young rocky planet's magma ocean. These interactions may have been a significant source of Earth's water and could be on other rocky worlds too.
"This overturns the decades-old canonical view that giant impacts almost always lead to complete atmospheric loss, demonstrating instead that these collisions can play a crucial role in the formation of sub-Neptunes," Korenaga said.
Analysis
Why This Matters
- Changes the scientific picture of how rocky planets, including Earth, gained and kept their atmospheres during the violent final stages of formation.
- Offers a plausible explanation for why super-Earths and sub-Neptunes are so common among detected exoplanets, a puzzle not represented in our own Solar System.
- Has implications for habitability: primordial atmospheres interacting with magma oceans may have contributed to Earth's water, and the same could apply to other rocky worlds.
Background
During the late stages of planet formation, a young planet's atmosphere had already been accreted, and the protoplanetary disk had largely dissipated. Large planetesimals still collided with the growing planet, releasing enormous heat that planetary scientists assumed would drive off the light primordial hydrogen and helium atmospheres. This study complicates that simple picture by accounting for how impact energy is distributed inside the planet and how the added mass strengthens the planet's gravitational hold on its atmosphere.
Key Perspectives
Darius Modirrousta-Galian and Jun Korenaga: argue that giant impacts increase gravitational binding energy more than mantle thermal energy, helping planets retain their primordial atmospheres and explaining why sub-Neptunes end up with puffy atmospheres while super-Earths lose theirs.
Previous planetary science view: held that the massive heat from giant impacts should drive primordial hydrogen and helium atmospheres away, a decades-old canonical view the new research says is overturned.
What to Watch
- Whether future mass and radius measurements of super-Earths and sub-Neptunes confirm the prediction that sub-Neptunes have, on average, more massive rocky interiors.
- Further modelling of how primordial atmospheres interact with magma oceans, and whether those interactions are a significant source of water on rocky worlds.
- Follow-up work on the role of giant impacts in the emergence of habitable conditions on young planets.
Checked
12 claims in this story checked against 1 source: 12 supported.
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- SupportedGiant impacts may help rocky planets retain their atmospheres, the study suggests.Universe Today: “new research suggests that ... giant impacts could've actually helped young rocky planets retain their atmospheres.”
- SupportedThe research appears to overturn the long-held view that giant impacts doom primordial atmospheres.Universe Today: “These results appear to overturn the long-held idea that giant impacts spelled doom for planetary atmospheres”
- SupportedThe research, published in Nature Astronomy, suggests late-stage giant impacts could help young planets retain primordial atmospheres despite the heat they generate.Universe Today: “The work is titled " Retention of primordial atmospheres through giant impacts ," and it's published in Nature Astronomy.”
- SupportedThe authors say, “Mantle melting consumes energy as latent heat, while the impactor's metallic core transfers little energy to the mantle and remains superheated at depth.”Universe Today: “"Mantle melting consumes energy as latent heat, while the impactor’s metallic core transfers little energy to the mantle and remains superheated at depth."”
- SupportedThe authors say, “Giant impacts therefore increase gravitational binding energy more than mantle thermal energy, shifting planets towards atmospheric retention.”Universe Today: “"Giant impacts therefore increase gravitational binding energy more than mantle thermal energy, shifting planets towards atmospheric retention," the researchers explain.”
- SupportedCo-author Jun Korenaga said, “Giant impacts are a key part of how rocky planets grow,” and explained that collisions add mass and heat, with their atmospheric effect depending on how those changes work together.Universe Today: ““Giant impacts are a key part of how rocky planets grow,” said co-author Korenaga. “Planets the size of Earth or larger are expected to form through collisions between smaller planetary bodies. These collisions add mass and release enormous amounts of heat, which can drive atmospheric escape. Their effect on a planet’s atmosphere depends on how those two changes work together.””
- SupportedThe study is by Darius Modirrousta-Galian, a former Yale postdoc now at Shanghai Jiao Tong University, and Jun Korenaga, a Yale professor of Earth and planetary sciences.Universe Today: “Modirrousta-Galian is a former Yale postdoc who is now at Shanghai Jiao Tong University, and Korenaga is a professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences.”
- SupportedThe researchers suggest sub-Neptunes could have grown larger through more massive impacts and retained larger primordial atmospheres.Universe Today: “This research shows that sub-Neptunes could have grown larger through more massive impacts, and those impacts not only added mass but allowed them to retain larger atmospheres.”
- SupportedThe researchers write, “Larger, atmosphere-bearing planets emerge preferentially through collisional growth. By contrast, smaller planets without such impacts tend to lose their primordial atmospheres under sustained stellar irradiation. At the population level, these results suggest that sub-Neptunes have, on average, more massive rocky interiors than super-Earths.”Universe Today: “"Larger, atmosphere-bearing planets emerge preferentially through collisional growth," the researchers explain. "By contrast, smaller planets without such impacts tend to lose their primordial atmospheres under sustained stellar irradiation. At the population level, these results suggest that sub-Neptunes have, on average, more massive rocky interiors than super-Earths."”
- SupportedModirrousta-Galian said, “Why nature produces both types in such abundance is a fundamental puzzle in exoplanetary science. We set out to find an explanation.”Universe Today: ““Why nature produces both types in such abundance is a fundamental puzzle in exoplanetary science,” said Modirrousta-Galian. “We set out to find an explanation.””
- SupportedPrimordial hydrogen and helium atmospheres did not themselves create habitability, but interactions with a young planet's magma ocean may have been a significant source of Earth's water and could be on other rocky worlds too.Universe Today: “Primordial H/He atmospheres themselves didnt create habitability. But some research shows that they're essential for the development of a world that is eventually habitable, through interactions with a young rocky planet's magma ocean. These interactions may have been a significant source of Earth's water and could be on other rocky worlds, too.”
- SupportedKorenaga said, “This overturns the decades-old canonical view that giant impacts almost always lead to complete atmospheric loss, demonstrating instead that these collisions can play a crucial role in the formation of sub-Neptunes.”Universe Today: ““This overturns the decades-old canonical view that giant impacts almost always lead to complete atmospheric loss, demonstrating instead that these collisions can play a crucial role in the formation of sub-Neptunes,” Korenaga said.”
