Astronomers from Germany and Switzerland have uncovered evidence of how the enigmatic gap in the dimensions distribution of exoplanets at around two Earth radii emerges. Their computer simulations show that the migration of icy, so-called sub-Neptunes into the inner regions of their planetary systems could account for this phenomenon. As they draw closer to the central star, evaporating water ice forms an environment that makes the planets appear larger than of their frozen state. Concurrently, smaller rocky planets steadily lose a portion of their original gaseous envelope, causing their measured radius to shrink over time.
Ordinarily, planets in evolved planetary systems, reminiscent of the Solar System, follow stable orbits around their central star. Nonetheless, many indications suggest that some planets might depart from their birthplaces during their early evolution by migrating inward or outward. This planetary migration may also explain an commentary that has puzzled researchers for several years: the relatively low variety of exoplanets with sizes about twice as large as Earth, referred to as the radius valley or gap. Conversely, there are lots of exoplanets smaller and bigger than this size.
“Six years ago, a reanalysis of information from the Kepler space telescope revealed a shortage of exoplanets with sizes around two Earth radii,” Remo Burn explains, an exoplanet researcher on the Max Planck Institute for Astronomy (MPIA) in Heidelberg. He’s the lead writer of the article reporting the findings outlined in this text, now published in Nature Astronomy.
Where does the radius valley come from?
“In truth, we — like other research groups — predicted based on our calculations, even before this commentary, that such a spot must exist,” explains co-author Christoph Mordasini, a member of the National Centre of Competence in Research (NCCR) PlanetS. He heads the Division of Space Research and Planetary Sciences on the University of Bern. This prediction originated during his tenure as a scientist at MPIA, which has been jointly researching this field with the University of Bern for a few years.
Essentially the most commonly suggested mechanism to clarify the emergence of such a radius valley is that planets might lose a component of their original atmosphere on account of the irradiation from the central star — especially volatile gases like hydrogen and helium. “Nonetheless, this explanation neglects the influence of planetary migration,” Burn clarifies. It has been established for about 40 years that under certain conditions, planets can move inward and outward through planetary systems over time. How effective this migration is and to what extent it influences the event of planetary systems impacts its contribution to forming the radius valley.
Enigmatic sub-Neptunes
Two several types of exoplanets inhabit the dimensions range surrounding the gap. On one hand, there are rocky planets, which might be more massive than Earth and are hence called super-Earths. Then again, astronomers are increasingly discovering so-called sub-Neptunes (also mini-Neptunes) in distant planetary systems, that are, on average, barely larger than the super-Earths.
“Nonetheless, we shouldn’t have this class of exoplanets within the Solar System,” Burn points out. “That is why, even today, we’re not exactly sure about their structure and composition.”
Still, astronomers broadly agree that these planets possess significantly more prolonged atmospheres than rocky planets. Consequently, understanding how these sub-Neptunes’ characteristics contribute to the radius gap has been uncertain. Could the gap even suggest that these two varieties of worlds form otherwise?
Wandering ice planets
“Based on simulations we already published in 2020, the newest results indicate and make sure that as an alternative, the evolution of sub-Neptunes after their birth significantly contributes to the observed radius valley,” concludes Julia Venturini from Geneva University. She is a member of the PlanetS collaboration mentioned above and led the 2020 study.
Within the icy regions of their birthplaces, where planets receive little warming radiation from the star, the sub-Neptunes should indeed have sizes missing from the observed distribution. As these presumably icy planets migrate closer to the star, the ice thaws, eventually forming a thick water vapour atmosphere.
This process ends in a shift in planet radii to larger values. In any case, the observations employed to measure planetary radii cannot differentiate whether the determined size is on account of the solid a part of the planet alone or an extra dense atmosphere.
At the identical time, as already suggested within the previous picture, rocky planets ‘shrink’ by losing their atmosphere. Overall, each mechanisms produce an absence of planets with sizes around two Earth radii.
Physical computer models simulating planetary systems
“The theoretical research of the Bern-Heidelberg group has already significantly advanced our understanding of the formation and composition of planetary systems prior to now,” explains MPIA Director Thomas Henning. “The present study is, due to this fact, the results of a few years of joint preparatory work and constant improvements to the physical models.”
The most recent results stem from calculations of physical models that trace planet formation and subsequent evolution. They encompass processes within the gas and mud disks surrounding young stars that give rise to recent planets. These models include the emergence of atmospheres, the blending of various gases, and radial migration.
“Central to this study were the properties of water at pressures and temperatures occurring inside planets and their atmospheres,” explains Burn. Understanding how water behaves over a big selection of pressures and temperatures is crucial for simulations. This data has been of sufficient quality only lately. It is that this component which allows realistic calculation of the sub-Neptunes’ behaviour, hence explaining the manifestation of intensive atmospheres in warmer regions.
“It’s remarkable how, as on this case, physical properties on molecular levels influence large-scale astronomical processes reminiscent of the formation of planetary atmospheres,” Henning adds.
“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,” Mordasini says. “Such planets could potentially host life and could be relatively straightforward targets for looking for biomarkers due to their size.”
Further work ahead
Nonetheless, the present work is just a very important milestone. Although the simulated size distribution closely matches the observed one, and the radius gap is in the best place, the small print still have some inconsistencies. As an example, too many ice planets find yourself too near the central star within the calculations. Nonetheless, researchers don’t perceive this circumstance as a drawback but hope to learn more about planetary migration in this fashion.
Observations with telescopes just like the James Webb Space Telescope (JWST) or the under-construction Extremely Large Telescope (ELT) could also assist. They’d be able to determining the composition of planets depending on their size, thus providing a test for the simulations described here.
