Beyond Earth: ‘Super-Earths’ breakthrough could help narrow search for habitable planets

By Isobel Williams via SWNS

Simulations have solved the exoplanet puzzle - explaining the mysterious gap in the size distribution of "super-Earths".

And the breakthrough could help scientists to focus on which planets to search for life on.

New research suggests that some planets might depart from their birthplaces during their early evolution by migrating inward or outward.

Such planetary migration could explain an observation that has puzzled researchers for several years, the relatively low number of exoplanets with sizes about twice as large as Earth, known as the radius valley or gap.

Dr. Remo Burn, an exoplanet researcher at the Max Planck Institute for Astronomy (MPIA) in Germany, said: “Six years ago, a reanalysis of data from the Kepler space telescope revealed a shortage of exoplanets with sizes around two Earth radii.”

Two different types of exoplanets inhabit the size range surrounding this gap.

On one hand, there are rocky planets, which can be more massive than Earth and are hence called super-Earths.

On the other hand, astronomers are increasingly discovering so-called sub-Neptunes in distant planetary systems, which are, on average, slightly larger than the super-Earths.

Dr. Burn added: “However, we do not have this class of exoplanets in the Solar System.

“That’s why, even today, we’re not exactly sure about their structure and composition.”

Despite being unsure about these planets’ structure and composition, astronomers broadly agree that these planets possess significantly more extended atmospheres than rocky planets.

This is due to the fact that as these presumably icy planets migrate closer to the star, the ice thaws, eventually forming a thick water vapor atmosphere.

This process results in a shift in planet radii to larger values, as observations cannot differentiate between solid planets and dense atmospheres.

At the same time, as already suggested in the previous picture, rocky planets ‘shrink’ by losing their atmosphere, also contributing to this gap.

To get their results, published in Nature Astronomy, the team used physical models that trace planet formation and subsequent evolution.

They encompass processes in the gas and dust disks surrounding young stars that give rise to new planets.

These models include the emergence of atmospheres, the mixing of different gases, and radial migration.

Dr. Julia Venturini from Geneva University said: “Based on simulations we already published in 2020, the latest results indicate and confirm that instead, the evolution of sub-Neptunes after their birth significantly contributes to the observed radius valley.”

Professor Thomas Henning from MPIA added: “The theoretical research of the Bern-Heidelberg group has already significantly advanced our understanding of the formation and composition of planetary systems in the past.

“The current study is, therefore, the result of many years of joint preparatory work and constant improvements to the physical models.

“It’s remarkable how, as in this case, physical properties on molecular levels influence large-scale astronomical processes such as the formation of planetary atmospheres.”

The team plans to expand their research to eradicate any inconsistencies. They note that telescope observations will assist them in determining the composition of planets depending on their size.

Professor Christoph Mordasini from the University of Bern, Switzerland, said: “If we were to expand our results to cooler regions, where water is liquid, this might suggest the existence of water worlds with deep oceans.

“Such planets could potentially host life and would be relatively straightforward targets for searching for biomarkers thanks to their size.”