A new theory of rocky planet formation may shed light on the origin of “super-Earths”, which are several times the size of Earth and are the most common type of planet in the galaxy. Additionally, this theory may provide insight into why super-Earths within a single planetary system tend to exhibit similar dimensions, as if each system were limited to producing only one type of planet.

“As our observations of exoplanets have increased over the past decade, it has become clear that the standard theory of planet formation needs to be revised from the ground up. “We need a theory that can simultaneously explain the formation of terrestrial planets in our solar system and the origin of self-like systems of super-Earths, many of which appear to be rocky in composition,” says Konstantin Batygin. Planetary science at the California Institute of Technology (MS ’10, Ph.D. ’12) collaborated on the new theory with Alessandro Morbidelli of the Observatoire de la Côte d’Azur, France.

Planetary systems begin their life cycles as large rotating disks of gas and dust that coalesce in about a few million years. While most of the gas is accumulating in the star at the center of the system, the solid is slowly turning into asteroids, comets, planets and moons.

There are two different types of planets in our solar system: the smaller, rocky inner planets closest to the Sun, and the outer, larger, water- and hydrogen-rich gas giants further away from the Sun. In a previously published study Nature Astronomy In late 2021, this dilemma led Morbidelli, Batygin and their colleagues to suggest that planet formation in our solar system occurs in two distinct rings of the protoplanetary disk: the inner ring of small rocky planets. It also formed for the outer larger icy planets (two of them – Jupiter and Saturn – later turned into gas giants).

Super-Earths, as the name suggests, are more massive than Earth. Some even have hydrogen atmospheres, making them look almost like gas giants. Additionally, they often orbit close to their star, suggesting that they migrated to their current positions from more distant orbits.

“A few years ago, we created a model where super-Earths form in the icy part of the protoplanetary disk and migrate to the inner edge of the disk, closer to the star,” Morbidelli says. The model could explain the masses and orbits of super-Earths, but predicted that they were all water-rich. However, recent observations have shown that most super-Earths are rocky like Earth, even when surrounded by a hydrogen atmosphere. It was a death sentence for our old model.”

Over the past five years, scientists, including a team led by Andrew Howard, professor of astronomy at Caltech; Lauren Weiss, Associate Professor, University of Notre Dame; and Eric Petigura, Sagan’s former graduate student in astronomy at Caltech and now a professor at UCLA, studied these exoplanets and made an unusual discovery: Although there are many different types of super-Earths, all super-Earths are within a single planetary system. orbital range tends to be similar in size, mass, and other fundamental properties.

“Lauren discovered that, in a single planetary system, super-Earths are like ‘peas in a pod,'” says Howard, who was not directly involved in the Batygin-Morbidelli paper but reviewed it. “You actually have a planetary factory that only knows how to make one-mass planets and kick them out one by one.”

So what single process could have produced singular systems of rocky super-Earths as well as rocky planets in our solar system?

“The answer turned out to be related to something we found in 2020 but didn’t realize was related to planet formation more broadly,” Batygin says.

In 2020, Batygin and Morbidelli proposed a new theory for the formation of Jupiter’s four largest moons (Io, Europa, Ganymede and Callisto). Essentially, they showed that for a given dust particle size range, the force pulling the particles towards Jupiter and the force (or drift) carrying those particles in the external gas flow completely cancel each other out. This balance of power created a ring of material that formed strong building blocks for subsequent satellite formation. Additionally, the theory suggests that objects will grow in the ring until they are large enough to exit the ring via gas-driven migration. After that, they stop growing, which explains why the process produces stems of similar sizes.

In their new paper, Batygin and Morbidelli argue that the mechanism by which planets form around stars is basically the same. In the planetary state, large-scale concentration of solid rocky material occurs in a narrow band of the disk called the silicate sublimation line; this zone, silicate vapor condenses to form solid rocky pebbles.

“If you’re a speck of dust, you’re going to experience a significant upwind in the disk because the gas is spinning more slowly and you’re spinning towards the star; but if you are in vapor form, you simply spiral outward with the gas in the expanding disk. That’s why material builds up where you turn from steam to solid,” says Batygin.

The new theory identifies this band as a possible location for a “planet factory” that could eventually create multiple rocky planets of the same size. Also, when the planets are large enough, their interaction with the disk will tend to move these worlds inward, closer to the star.