Super-Earths and sub-Neptunes explained by planetary migration

In the grand tapestry of the cosmos, planets orbit their stars in patterns that have long captivated astronomers. Traditionally, it’s understood that planets in systems like our own maintain stable orbits.

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

The enigmatic radius valley

This phenomenon could shed light on a longstanding enigma in astronomy: the peculiar scarcity of exoplanets about twice the size of Earth, a phenomenon referred to as the radius valley or gap.

While the universe seems teeming with planets smaller and larger than this specific size, those that fall within this middle ground are notably rare.

Remo Burn, an exoplanet researcher at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, and the 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 radii.”

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

Christoph Mordasini, a collaborator from the National Centre of Competence in Research (NCCR PlanetS) and a figurehead at the University of Bern, reflects on the genesis of this hypothesis during his tenure at MPIA.

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

The role of planetary migration

The prevailing theory to explain the radius valley posits that planets might lose portions of their atmospheres due to stellar irradiation, particularly losing volatile gases like hydrogen and helium. However, Burn emphasizes that this theory does not fully account for the role of planetary migration.

The concept of planetary migration, recognized for over four decades, suggests that planets can shift positions within their solar systems under certain conditions.

The extent of this migration and its impact on the structure of planetary systems are crucial in understanding the formation of the radius 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 do not have this class of exoplanets in the Solar System,” Burn notes, highlighting a gap in our understanding of these distant worlds’ structure and composition.

Contribution of sub-Neptunes

Despite uncertainties about their precise nature, it’s generally agreed 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 do they indicate divergent formation pathways for these two types of planets?

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

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

From icy beginnings to atmospheric giants

Sub-Neptunes, born in the frigid outskirts of their solar systems where they receive minimal warmth from their stars, are hypothesized to undergo a significant transformation as they migrate closer to their stars.

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

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

Concurrently, this research also explores the phenomenon where rocky planets appear to ‘shrink’ due to atmospheric loss, further contributing to the scarcity of planets with sizes around twice that of Earth’s radius.

Dual dynamics of planetary migration

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

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

The current study is the culmination of years of meticulous preparatory work and continuous refinements to 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.

Unraveling the Mysteries of Water

A key focus of this study is the behavior of water under the varied pressures and temperatures found within planets and their atmospheres.

“Central to this study were the properties of water at pressures and temperatures occurring inside planets and their atmospheres,” explains Remo Burn.

Recent advancements in understanding water’s behavior across a broad 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 astronomical processes on a grand scale, such as the formation of planetary atmospheres.

Water worlds and the search for life

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

These planets, potentially harboring life, would be prime candidates for the search for biomarkers due to their significant sizes.

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

For example, an excess of ice planets are calculated to be too close to their central stars. Yet, these inconsistencies are not viewed as setbacks but rather as opportunities to deepen our understanding of planetary migration.

Implications of planetary migration

Future observations, particularly 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 for the theoretical models discussed, opening new avenues for exploration in the quest to understand the cosmos.

In summary, this important research into the dynamic processes of planetary migration and atmospheric evolution, particularly concerning sub-Neptunes and the radius valley, underscores a significant leap 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 planet sizes and compositions.

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

The full study was published in the journal Nature Astronomy.

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