Missing planets twice Earth's size may point to migrating worlds

There is an explanation as to why so many planets are either super-Earths or mini-Neptunes.

There are different sizes and mass of planets. If you plotted the number of planets of each size on a graph, you would find two peaks, one at 1.4 times Earth's radius and the other at 2.4 times Earth's radius. The dip or valley between them is a sign of the scarcity of planets of that size.

Something is happening that means that planets that are two or three times the size of Earth are found less often. The first 50 million years of a typical planetary system's existence were modeled by the team led by Izidoro.

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One hypothesis is that the differences in composition between rocky super-Earths and hydrogen- and water-rich mini-Neptunes lead to the formation of planets of different sizes. The other hypothesis is that super-Earths lose their thick atmospheres as they migrate closer to their star.

The migration model is supported by the new simulations, as well as explaining why we frequently find chains of similarly sized exoplanets. When a planet's orbital periods fall into multiples of each other, resonance happens. The simulations show that the migration of planets within the disk of dust and gas causes a chain of worlds.

Astronomers know that the disk that allows this migration does not last indefinitely. As the young star begins to generate more energy, its radiation wind blows the disk away, causing the planets to be torn apart.

Izidoro said in a statement that the migration of young planets towards their host stars creates overcrowding and frequently results in cataclysmic collision that strip planets of their hydrogen-rich atmospheres. Giant impacts, like the one that formed our moon, are likely a generic outcome of planet formation.

The super-Earths that are rocky and dry, and the mini-Neptunes that did not migrate, were preferentially created by the migration of planets, the orbital destabilization and the loss of thick planetary atmospheres.

Izidoro believes that he is the first to explain the radius valley using a model of planet formation and evolution. The model incorporating giant impacts is consistent with the peas-in-a-pods feature of exoplanets.

This "peas-in-a-pods" characteristic is found in planetary systems such as TRAPPIST-1 which is home to seven rocky worlds of the same size. We should expect to find more multi-planet systems with similar sized planets in the future according to the new findings.

The results were published in a journal.

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