Scientists have made a groundbreaking discovery that may explain the absence of planets that are around twice the size of Earth. These planets, known as super-Earths or mini-Neptunes, have been dubbed the “missing” planets in space. However, new research suggests that these planets may have taken different routes out of what scientists call the “radius valley” or “radius gap.”
The radius valley refers to the scarcity of exoplanets that are between 1.6 and 2.2 times the size of Earth. Scientists have observed many exoplanets that are either smaller or larger than Earth, but those within this specific size range have been elusive. The most common explanation for this phenomenon is that stars irradiate the planets surrounding them, causing them to shrink. However, this explanation neglects the influence of planetary migration.
To investigate whether planetary migration could supplement the standard explanation, a team of researchers led by Remo Burn from the Max Planck Institute for Astronomy conducted a re-analysis of data from the Kepler space telescope. They wanted to understand if the migration of planets could explain why super-Earths and mini-Neptunes are rarely seen orbiting close to their stars.
Super-Earths are rocky or terrestrial planets, while mini-Neptunes have atmospheres that extend far beyond those of rocky planets. The team hypothesized that the existence of the radius valley could suggest different formations and evolutions for these two types of planets. They performed a simulation that factored in processes in the gas and dust disks surrounding young stars, the emergence of atmospheres, and the migration of planets.
The simulation revealed that moving toward a parent star had different effects on super-Earths and mini-Neptunes. Mini-Neptunes born in the icy outer regions of their systems would have their icy material thawed if they migrated inward toward their star. This would create a thick water atmosphere around these exoplanets, increasing their radii and shifting their widths beyond the planets in the radius gap. On the other hand, super-Earths that migrated toward their host star or were born very close would have their atmospheres stripped by intense radiation, causing them to become smaller.
In simple terms, mini-Neptunes are moving out of the radius valley, while super-Earths are evacuating it via the opposite exit. Both mechanisms result in a lack of planets around twice the width of Earth. This discovery not only solves the mystery of the missing super-Earths and mini-Neptunes but also has implications for other areas of exoplanet science.
The team’s simulations could potentially uncover the existence of water worlds with deep oceans in cooler regions. These planets could potentially host life and would be promising targets for searching for biomarkers. The research opens up new possibilities for understanding planetary formation and evolution, as well as the potential for habitable exoplanets.
The findings of this study were published in the journal Nature Astronomy on February 9th. The research provides valuable insights into the migration of planets and sheds light on the diversity of exoplanetary systems. With further exploration and analysis, scientists hope to uncover more secrets of the universe and potentially find habitable worlds beyond our own.