Early planetary migration explains gap in exoplanet radii

(ORDO NEWS) — Using supercomputer simulations, scientists were able to explain why we know so few exoplanets with a radius of about 1.8 Earth radii, and also why planets of similar size occur in many planetary systems, the orbits of which are in resonance.

It turned out that this is due to the early formation of systems and planetary migration.

Exoplanet scientists are concerned with two mysteries. One of them is that planets with a radius greater than that of the Earth, but less than that of Neptune (the so-called super-Earths and mini-Neptunes), the radius of the majority is about 1.4 or 2.4 times greater than at the Earth.

But at the same time, planets with a radius of 1.8 Earth radii are extremely rare.

The second mystery lies in the existence of planetary systems with a large number of similarly sized planets that are in orbital resonance. Such, in particular, are the stars TRAPPIST-1 and Kepler-223.

Scientists from Rice University (USA) explained this feature using modeling of the evolution of planetary systems. In addition, they showed that a model of planetary formation involving multiple collisions could result in multiple planets of a similar size.

The authors used a supercomputer, with which they were able to recreate the first 50 million years of the development of planetary systems.

According to the planetary migration model, the protoplanetary disk, consisting of gas and dust and giving rise to young planets, actively interacts with them and draws them closer to their parent stars.

In this case, orbital resonance often occurs – a situation in which the orbital periods of several planets correlate as natural numbers.

As a result, the planets periodically approach each other, being at certain points in their orbits.

However, within a few million years, the protoplanetary disk disappears, and orbital instability occurs, due to which the planets begin to collide with each other. These collisions deprive them of their hydrogen-rich atmospheres.

The simulation results showed that a small number of planets with a radius 1.8 times greater than that of the Earth is really a consequence of planetary migrations and collisions after the disappearance of the protoplanetary disk.

Systems that have managed to retain their original structure and orbital resonance are now being observed. For example, as many as seven planets revolve around a star in TRAPPIST-1 resonance.

Thus, collisions of young planets, like the one that formed the Moon, are probably a common result of planetary formation.

In their article, the scientists also made a number of predictions that can be tested using the James Webb Space Telescope.

For example, they suggested that parts of planets about twice the size of Earth still managed to retain their original hydrogen-rich atmosphere, and possibly water.

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