There’s a mysterious gap in the size distribution of super-Earths
Explore how planetary migration offers a solution to the enigmatic gap in super-Earth sizes and the mysteries of the “radius valley.”
In the vast cosmic ballet of stars and planets, a perplexing cosmic conundrum has finally met its match. Researchers at the Max Planck Institute for Astronomy (MPIA) have unraveled the mystery behind the scarcity of exoplanets precisely twice the size of Earth, according to a study published on February 9.
Imagine a celestial dance where planets, like disciplined performers, follow stable orbits around their central star. However, in the cosmic theater, some rebellious planets opt for an early departure from their cosmic cradle, migrating either inward or outward during their early evolution.
This planetary migration might just be the cosmic answer to a long-standing puzzle – the elusive radius valley or gap, a celestial rarity observed among exoplanets about twice the size of Earth.
Six years ago, data from the Kepler space telescope hinted at a shortage of exoplanets with sizes hovering around two Earth radii. Remo Burn, an exoplanet researcher at MPIA, recalls the revelation that set the stage for their cosmic investigation.
Working alongside other research groups, Christoph Mordasini, from the National Centre of Competence in Research (NCCR) PlanetS, predicted the existence of this gap even before it was observed.
Losing a part of the original atmosphere due to irradiation
The proposed explanation for this celestial anomaly involved exoplanets losing a part of their original atmosphere due to irradiation from the central star, particularly volatile gases like hydrogen and helium.
However, Burn emphasized that this explanation overlooked the influence of planetary migration – a concept established for about four decades, allowing planets to traverse planetary systems over time.
Within this cosmic gap, two distinct types of exoplanets emerge – super-Earths, rocky planets more massive than our home planet, and sub-Neptunes, slightly larger than super-Earths and possessing more extended atmospheres. The Solar System lacks this sub-Neptune class, adding an air of mystery to its structure and composition.
Enter Julia Venturini from Geneva University, a key member of the PlanetS collaboration. Building on simulations published in 2020, Venturini’s latest results affirm that the evolution of sub-Neptunes post-birth significantly contributes to the observed radius valley.
The icy birthplaces of these planets, receiving minimal warmth from their stars, result in sizes missing from the observed distribution. As these icy sub-Neptunes migrate closer to their stars, the ice thaws, forming a thick water vapor atmosphere. This process shifts planet radii to larger values, confounding observations that measure planetary sizes.
The study relies on physical models and simulations tracing planet formation and evolution, considering gas and dust disks, atmospheric emergence, gas mixing, and radial migration.
Crucially, the study delves into the molecular intricacies of water at different pressures and temperatures inside planets and atmospheres. Only recently acquired detailed knowledge allows realistic simulations of sub-Neptunes’ behavior, unveiling the manifestation of extensive atmospheres in warmer regions.
MPIA Director Thomas Henning highlights how properties at the molecular level influence large-scale astronomical processes, showcasing the interplay of science across scales. Christoph Mordasini contemplates expanding results to cooler regions, suggesting the potential existence of water worlds with deep oceans, offering exciting prospects for life beyond Earth.
While the current work is a crucial milestone with a close match between simulated and observed size distributions, inconsistencies linger. Researchers view these discrepancies as opportunities to deepen their understanding of planetary migration.
Telescopes like the James Webb Space Telescope and the under-construction Extremely Large Telescope hold the promise of assisting further by determining planet composition based on size. As these cosmic detectives continue their work, the cosmos opens its vast expanse to reveal the secrets of celestial migration and the intricate dance of planets around distant stars.
The complete study was published in the journal Nature Astronomy on February 9.
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