Scientists have made a groundbreaking discovery that may explain the absence of super-Earths and mini-Neptunes in our universe. These planets, which are approximately twice the size of Earth but smaller than Neptune, have been classified as the “radius valley” or “radius gap” due to their scarcity. However, new research suggests that these missing planets may have taken different migration routes early on in their formation.
The study, led by Remo Burn from the Max Planck Institute for Astronomy, aimed to investigate the role of planetary migration in creating the radius valley. While scientists have long known that planets can move towards or away from their parent stars after formation, the effectiveness of this migration in shaping the radius valley was previously unknown.
The prevailing theory for the absence of super-Earths and mini-Neptunes revolves around stars irradiating the planets in close proximity, causing their atmospheres to shrink. However, Burn and his team believed that this explanation neglected the influence of planetary migration. To test their hypothesis, they conducted a re-analysis of a simulation they ran in 2020, considering processes in the gas and dust disks surrounding young stars, the emergence of atmospheres, and the migration of planets.
One crucial aspect of the simulation was understanding how water behaves under different pressures and temperatures. By factoring in these parameters, the team could calculate the behavior of mini-Neptunes more realistically. They found that mini-Neptunes are born in the outer regions of their systems and migrate towards their parent stars. This migration causes their icy material to thaw, resulting in a thick water atmosphere and increasing their radii beyond the planets in the radius gap.
On the other hand, super-Earths that migrate towards their host star or are born very close to it would have their atmospheres stripped by intense radiation. This would cause them to lose their atmosphere and become smaller, resulting in a peak in exoplanet sizes at only 1.4 times the width of Earth. Essentially, mini-Neptunes are moving out of the radius valley while super-Earths are evacuating it via the opposite exit, leading to a lack of planets around twice the size of Earth.
The implications of this research extend beyond the radius valley mystery. The simulations used in the study could have an impact on other areas of exoplanet science. For instance, if applied to cooler regions where water is liquid, the findings suggest the existence of water worlds with deep oceans. These planets could potentially host life and would be promising targets for searching for biomarkers.
The team’s research, published in the journal Nature Astronomy, sheds light on the migration of planets and its role in shaping the exoplanet population. By understanding these processes, scientists can gain valuable insights into the formation and evolution of planets in our universe. The discovery of different migration routes for super-Earths and mini-Neptunes provides a compelling explanation for the absence of planets in the radius valley, paving the way for further exploration and understanding of our cosmic neighborhood.