Simulations provide potential explanation for mysterious gap in super-Earth size distribution

Usually, planets in advanced planetary systems, such as the Solar System, follow stable orbits around their central star. However, there are many indications that some planets may leave their birthplaces early in their evolution by migrating inwards or outwards.

This planetary migration could also explain an observation that has perplexed researchers for several years: the relatively small number of exoplanets about twice the size of Earth, known as the radius valley or gap. Conversely, there are many exoplanets smaller and larger than this size.

“Six years ago, a new analysis of data from the Kepler space telescope revealed a shortage of exoplanets with a size around two Earth radii,” says Remo Burn, an exoplanet researcher at the Max Planck Institute for Astronomy. (MPIA) in Heidelberg. He is the lead author of the paper reporting the results presented in this paper, now published in Natural astronomy.

Where does the ray valley come from?

“In fact, like other research groups, we predicted, based on our calculations, even before this observation, that such a gap must exist,” explains co-author Christoph Mordasini, member of the National Pole of research competence (PRN). PLANETS. He heads the Division of Space Research and Planetary Sciences at the University of Bern. This prediction arose during his tenure as a scientist at the MPIA, which has been carrying out research in this area in collaboration with the University of Bern for many years.

The most commonly suggested mechanism to explain the emergence of such a radius valley is that planets might lose some of their original atmosphere due to irradiation from the central star, particularly from volatile gases like hydrogen and helium. “However, this explanation neglects the influence of planetary migration,” says Burn.

It has been established for about 40 years that, under certain conditions, planets can move in and out through planetary systems over time. The efficiency of this migration and the extent to which it influences the development of planetary systems impacts its contribution to the formation of the ray valley.

Enigmatic Sub-Neptunes

Two different types of exoplanets inhabit the size range surrounding space. On the one hand, there are rocky planets, which can be more massive than Earth and are therefore called super-Earths. On the other hand, astronomers are increasingly discovering so-called sub-Neptunes (also mini-Neptunes) in distant planetary systems, which are on average slightly larger than super-Earths.

“However, we do not have this class of exoplanets in the solar system,” Burn points out. “That is why, even today, we are not exactly sure of their structure and composition.”

However, astronomers largely agree that these planets have significantly larger atmospheres than rocky planets. Therefore, understanding how the characteristics of these sub-Neptunes contribute to the radius deviation is uncertain. Could this gap even suggest that these two types of worlds form differently?

Wandering ice planets

“Based on the simulations we have already published in 2020, the latest results indicate and confirm that the evolution of sub-Neptunes after their birth contributes significantly to the observed radius valley,” concludes Julia Venturini from the University from Geneva. She is a member of the PlanetS collaboration and led the 2020 study.

In the icy regions of their birthplaces, where the planets receive little heating radiation from the star, sub-Neptunes should indeed have sizes missing from the observed distribution. As these presumably icy planets approach the star, the ice melts, eventually forming a thick atmosphere of water vapor.

This process causes the radii of the planets to shift to larger values. After all, the observations used to measure planetary radii cannot differentiate whether the determined size is due only to the solid part of the planet or to an additional dense atmosphere.

At the same time, as already suggested in the previous photo, rocky planets “shrink” by losing their atmosphere. Overall, both mechanisms produce a lack of planets of a size around two Earth radii.

Physical computer models simulating planetary systems

“Theoretical research by the Bern-Heidelberg group has already significantly advanced our understanding of the formation and composition of planetary systems in the past,” explains Thomas Henning, director of the MPIA. “The current study is therefore the result of many years of joint preparatory work and constant improvements of the physical models.”

The latest results come from calculations of physical models that trace the formation of planets and their subsequent evolution. They encompass processes in the disks of gas and dust surrounding young stars that give rise to new planets. These models include the emergence of atmospheres, the mixing of different gases and radial migration.

“The properties of water at the pressures and temperatures found inside planets and their atmospheres were central to this study,” says Burn. Understanding how water behaves over a wide range of pressures and temperatures is crucial for simulations. This knowledge has only been of sufficient quality in recent years. It is this component that makes it possible to realistically calculate the behavior of sub-Neptunes, thus explaining the manifestation of extended atmospheres in warmer regions.

“It is remarkable how, as in this case, physical properties at the molecular level influence large-scale astronomical processes such as the formation of planetary atmospheres,” adds Henning.

“If we were to extend our results to colder regions, where the water is liquid, it could suggest the existence of aquatic worlds with deep oceans,” explains Mordasini. “Such planets could potentially support life and would make relatively simple targets for biomarker research, thanks to their size.”

Additional work to come

However, the ongoing work is only one important step. Although the simulated size distribution closely matches the observed one and the radius deviation is in the right place, the details still have some inconsistencies. For example, too many ice planets end up too close to the central star in the calculations. However, the researchers do not see this circumstance as a disadvantage but hope to learn more about planetary migration.

Observations with telescopes like the James Webb Space Telescope (JWST) or the under-construction Extremely Large Telescope (ELT) could also be useful. They would be able to determine the composition of planets based on their size, providing a test for the simulations described here.

The MPIA scientists involved in this study are Remo Burn and Thomas Henning.

Other researchers include Christoph Mordasini (University of Bern, Switzerland (Unibe)), Lokesh Mishra (University of Geneva, Switzerland (Unibe) and Unibe), Jonas Haldemann (Unibe), Julia Venturini (Unibe) and Alexandre Emsenhuber (Ludwig Maximilian University of Munich, Germany and Unibe).

NASA’s Kepler space telescope searched for planets around other stars between 2009 and 2018 and discovered thousands of new exoplanets during its operation. He used the transit method: when a planet’s orbit is tilted such that the plane is in the telescope’s line of sight, the planets periodically block some of the star’s light during their orbit. This periodic fluctuation in the star’s brightness allows indirect detection of the planet and determination of its radius.