Two perplexing observations that have repeatedly surfaced among the more than 3,800 planetary systems cataloged to date can be explained by a new model that takes the interaction of forces acting on newborn planets into account. The rarity of exoplanets with a radius that is roughly 1.8 times that of Earth is one mystery referred to as the “radius valley.” In comparison to super-Earths with radii roughly 1.4 times that of Earth and mini-Neptunes with radii roughly 2.5 times that of Earth, NASA’s Kepler spacecraft observed planets of this size about 2-3 times less frequently. Peas in a Pod, or neighboring planets of a similar size that have been discovered in a large number of planetary systems, is the second mystery. These include TRAPPIST-1 and Kepler-223, both of which have nearly-harmonious planetary orbits.

According to André Izidoro of Rice University, the study’s co-corresponding author, “I think we are the first to explain the radius valley using a model of planet formation and dynamical evolution that self-consistently accounts for multiple constraints of observations.” We can also demonstrate that an exoplanet’s peas-in-a-pod feature is consistent with a planet-formation model that includes giant impacts. The NASA-funded CLEVER Planets project at Rice University, where Izidoro is a Welch Postdoctoral Fellow, used a supercomputer to simulate the first 50 million years of the evolution of planetary systems. In the model, young planets interact with the protoplanetary disks of gas and dust that give rise to them, drawing them nearer to their parent stars and trapping them in resonant orbital chains. Within a few million years, the chains are broken when orbital instability brought on by the disappearance of the protoplanetary disk results in collisions between two or more planets.

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Planetary systems that have kept their resonant orbital chains have been studied using planetary migration models. For instance, Izidoro and CLEVER Planets coworkers calculated the maximum amount of disruption TRAPPIST-1’s seven planet system could have endured during bombardment and still maintained its harmonious orbital structure in 2021 using a migration model. Izidoro collaborated in the new study with researchers from CLEVER Planets Rajdeep Dasgupta and Andrea Isella from Rice University, Hilke Schlichting from the University of California, Los Angeles, and Christian Zimmermann and Bertram Bitsch from the Heidelberg, Germany-based Max Planck Institute for Astronomy. Izidoro stated that the overcrowding caused by the migration of young planets towards their host stars frequently leads to catastrophic collisions that rob planets of their hydrogen-rich atmospheres. That implies that massive impacts, such as the one that created our moon, are probably a common result of planet formation.

According to the research, there are two different types of planets: mini-Neptunes, which are 2.5 times bigger than Earth and are rich in water ice, and super-Earths, which are dry, rocky, and 50% larger than Earth. New observations, according to Izidoro, appear to support the findings, which go against the conventional wisdom that both super-Earths and mini-Neptunes are only dry, rocky worlds. The scientists formulated hypotheses based on their findings that the James Webb Space Telescope of NASA can verify. For instance, they postulate that a portion of planets roughly twice the size of Earth will still have their original, hydrogen-rich atmosphere and be abundant in water. NASA (80NSSC18K0828), the Welch Foundation (C-2035-20200401), and the European Research Council all provided funding for the study (757448-PAMDORA).

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