Artist's impression of mini-Neptune TOI 560.01. Credit: Adam Makarenko / WM Keck Observatory

Planetary Migration and the Disappearance of Exoplanets in the Galaxy

A new model developed by astronomers my finally accounts for a scarcity of exoplanets with masses between super-Earths and mini-Neptunes.


tAs of November 7, 2022, astronomers have confirmed the existence of 5,197 distant alien worlds. The distant exoplanets are located in 3,888 planetary systems. But more importantly, there are 8,992 candidate planets that await confirmation. The galaxy we live in is a pretty massive place. Astronomers predict there to be hundreds of billions of planets in the Milky Way. And as our technology to explore space improves, so do these numbers. But the Galaxy we live in has many secrets and tricks up its sleeves. Scientists work hard to understand and uncover all of its mysteries. Now, astronomers have revealed that two puzzling observations have repeatedly been noted in over 3,800 planetary systems cataloged to date. However, both of these can be explained by a new model that explains how newborn planets are affected by forces.

Two mysteries

There is a puzzle known as the “radius valley.” It refers to the lack of exoplanets with a radius larger than Earth’s. Kepler’s observations of planets this size are 2-3 times less frequent than its observations of super-Earths 1.4 times greater than Earth’s radius and mini-Neptunes 2.5 times larger than Earth’s radius. The second mystery is the existence of neighboring planets of similar sizes found in hundreds of planetary systems and known as “peas in a pod.” Among these are TRAPPIST-1 and Kepler-223. Their orbits are also nearly in tune.
According to Rice University’s André Izidoro, co-author of a study published this week in Astrophysical Journal Letters, there’s an explanation. Izidoro and his team are the first to explain the radius valley by using a dynamical evolution model that satisfies multiple observational constraints.



Additionally, scientists demonstrated that a planet-formation model incorporating giant impacts fits the peas-in-a-pod nature of exoplanets. The Rice CLEVER Planets team, led by Izidoro, used a supercomputer to simulate the development of planetary systems for 50 million years using a planetary migration model. This model depicts protoplanetary disks of gas and dust that are involved in the formation of young planets, bringing them closer to their parent stars and locking them in resonant orbits. In a few million years, the chains are broken when the protoplanetary disk disappears. This causes orbital instability that leads to collisions between two or more planets. Astronomers have examined planetary systems with resonant orbital chains using planetary migration models. Using a migration model in 2021, Izidoro and colleagues calculated the maximum amount of disruption TRAPPIST -1’s seven-planet system could have withstood during bombardment and still maintain its harmonious orbits.

Unique characteristics

A planet’s hydrogen-rich atmosphere is stripped when it migrates towards its host star due to overcrowding and cataclysms from collisions,” Izidoro explained. Therefore, huge impacts, such as the one that formed our moon, are probably a generic result of planet formation.” Planets are classified into two flavors. We have super-Earths and mini-Neptunes. Each of these has its own characteristics. Super-Earths are dry, rocky, and 50% larger than Earth, while mini-Neptunes are bigger than Earth by about 2.5 times and are rich in water ice. In his explanation, Izidoro said new observations contradict the traditional notion that both mini-Neptunes and super-Earths are exclusively rocky and dry. The researchers made predictions using their findings, which NASA’s James Webb Space Telescope can verify. For example, they suggest that there will be several planets twice Earth’s size that retain their primordial hydrogen-rich atmospheres and have much water.


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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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