March 1, 2024

Super-Earths and sub-Neptune explained by planetary migration •

In the great tapestry of the cosmos, planets orbit their stars in patterns that have long captivated astronomers. Traditionally, planets in systems like ours are understood to maintain stable orbits.

However, emerging evidence suggests a more dynamic scenario in the early phases of planetary systems. Some planets may move away from their initial locations, moving inward or outward, in a process known as planetary migration.

The enigmatic valley of lightning

This phenomenon may shed light on a long-standing enigma in astronomy: the peculiar scarcity of exoplanets about twice the size of Earth, a phenomenon known as a radius valley or gap.

Although the universe appears to be full of planets smaller and larger than this specific size, those that fall in this middle ground are remarkably rare.

Remo Burn, exoplanet researcher at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, and lead author of this study, recalls: “Six years ago, a reanalysis of data from the Kepler space telescope revealed a shortage of exoplanets with sizes around two earth rays.”

This observation, which baffled researchers, aligned with predictions made by Burn and others, suggesting an underlying pattern in planetary formation and evolution.

Christoph Mordasini, collaborator at the National Center for Research Competence (NCCR PlanetS) and leading figure at the University of Bern, reflects on the genesis of this hypothesis during his tenure at MPIA.

This collaboration between the MPIA and the University of Bern has been fundamental in the exploration of these cosmic phenomena.

The role of planetary migration

The prevailing theory to explain trough lightning postulates that planets may lose portions of their atmospheres due to stellar irradiation, particularly losing volatile gases such as hydrogen and helium. However, Burn emphasizes that this theory does not fully explain the role of planetary migration.

The concept of planetary migration, recognized for more than four decades, suggests that planets can change position within their solar systems under certain conditions.

The extent of this migration and its impact on the structure of planetary systems are crucial to understanding the formation of the lightning valley.

The gap in planet sizes delineates two distinct classes of exoplanets: rocky worlds known as super-Earths and the larger gaseous sub-Neptunes.

“We don’t have this class of exoplanets in the Solar System,” Burn notes, highlighting a gap in our understanding of the structure and composition of these distant worlds.

Contribution of sub-Neptunes

Despite uncertainties about their precise nature, it is generally accepted that sub-Neptunes have much thicker atmospheres than their rocky counterparts.

The question remains: do these differences in atmospheric thickness contribute to the radius gap, or indicate divergent formation paths for these two types of planets?

Julia Venturini of the University of Geneva, a key participant in the PlanetS collaboration and leader of a pivotal 2020 study, offers a compelling conclusion.

“Based on simulations we already published in 2020, the most recent results indicate and confirm that the evolution of sub-Neptunes after their birth contributes significantly to the observed radius valley.”

From icy beginnings to atmospheric giants

It is assumed that the Sub-Neptunes, born in the frigid outskirts of their solar systems where they receive minimal heat from their stars, undergo a significant transformation as they migrate closer to their stars.

The heat thaws their icy compositions, leading to the formation of thick water vapor atmospheres.

This transformative process increases the planets’ radii, since observations used to measure planetary sizes cannot distinguish between the planet’s solid core and its dense atmospheric layer.

At the same time, this research also explores the phenomenon in which rocky planets appear to “shrink” due to atmospheric loss, further contributing to the scarcity of planets with sizes about twice the radius of Earth.

Double dynamics of planetary migration

This dual mechanism of atmospheric addition to sub-Neptune and atmospheric loss from rocky planets highlights a dynamic interplay in planetary evolution.

Thomas Henning, director of the MPIA, praises the Bern-Heidelberg group’s theoretical research for significantly improving our understanding of the formation and composition of the planetary system.

The current study is the culmination of years of meticulous preparatory work and ongoing refinements to the physical models.

These models intricately simulate the birth and evolution of planets, incorporating the formation of atmospheres, gas and dust disk processes around young stars, and radial migration.

Uncovering the mysteries of water

A main focus of this study is the behavior of water under the various pressures and temperatures found on planets and their atmospheres.

“The center of this study was the properties of water at the pressures and temperatures that occur inside planets and in their atmospheres”, explains Remo Burn.

Recent advances in understanding the behavior of water across a wide spectrum of conditions have enabled realistic simulations of sub-Neptunes, elucidating the development of their extensive atmospheres in warmer environments.

Henning finds it remarkable how molecular-level physical properties can influence large-scale astronomical processes, such as the formation of planetary atmospheres.

Aquatic worlds and the search for life

Christoph Mordasini, contributing to the discussion, speculates that extending these results to colder regions where water remains liquid could suggest the existence of water worlds with deep oceans.

These planets, potentially harboring life, would be prime candidates for biomarker searches due to their significant sizes.

However, the journey of discovery is far from over. Although the simulations closely align with the observed size distributions and accurately position the radius gap, discrepancies remain.

For example, too many icy planets are estimated to be too close to their central stars. However, these inconsistencies are not seen as setbacks, but rather as opportunities to deepen our understanding of planetary migration.

Implications of planetary migration

Future observations, especially with powerful telescopes like the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT), promise to refine these simulations.

By determining the composition of planets based on their sizes, these observations will offer a crucial test of the theoretical models discussed, opening new avenues of exploration in the quest to understand the cosmos.

In summary, this important investigation into the dynamic processes of planetary migration and atmospheric evolution, particularly with regard to sub-Neptune and the radius valley, highlights a significant leap forward in our understanding of planetary systems.

By meticulously unraveling the behavior of water under extreme conditions and examining the impact of migration patterns, scientists have illuminated the intricate mechanisms that shape the size and composition of planets.

This synergy of theoretical insights and advanced simulations refines our models of the cosmos and offers new hope for investigating the potential for life on distant worlds, marking a pivotal moment in the ongoing quest to understand the vast and complex universe.

The full study was published in the journal Nature Astronomy.


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