Jan. 13 () –
A new theory on the formation of rocky planets offers an explanation for the origin of “super-Earths”, worlds several times more massive than ours and which are the most common in the galaxy.
In addition, it also explains why super-Earths in the same planetary system tend to be eerily similar in size, as if each system were only capable of producing a single type of planet.
“As our observations of exoplanets have been increasing over the past decade, it has become clear that the standard theory of planet formation needs to be revised, Starting with the fundamentals. We need a theory that can simultaneously explain the formation of the terrestrial planets in our solar system and the origins of self-similar super-Earth systems, many of which appear to be rocky in composition,” he says. it’s a statement Konstantin Batygin, Professor of Planetary Sciences at Caltech, who collaborated on the new theory with Alessandro Morbidelli of the Observatoire de la Côte d’Azur (France). nature astronomy published an article explaining their work.
Planetary systems begin their life cycles as large rotating disks of gas and dust that consolidate over the course of a few million years. Most of the gas accumulates in the star located in the center of the system, while solid material slowly coalesces into asteroids, comets, planets, and moons.
There are two different types of planets in our solar system: the smaller, rocky ones closest to the Sun, and the larger ones, gas giants rich in water and hydrogen, further from the Sun. In an earlier study published in Nature Astronomy at the end of 2021, this dichotomy led Morbidelli, Batygin, and their colleagues to suggest that the formation of planets in our solar system occurred in two distinct rings in the protoplanetary disk: an inner one where the small rocky planets formed and an outer one for the more massive icy planets (two of which -Jupiter and Saturn- later became gas giants).
Super-Earths, as their name suggests, are more massive than Earth. Some even have hydrogen atmospheres, giving them an almost gas giant appearance. Furthermore, they are often found orbiting close to their stars, suggesting that they migrated to their present location from more distant orbits.
“A few years ago we built a model according to which super-Earths formed in the icy part of the protoplanetary disk and migrated to the inner edge of the disk, close to the star,” explains Morbidelli. “The model could explain the masses and orbits of super-Earths, but it predicted that they were all rich in water. However, recent observations have shown that most super-Earths are rocky, like Earth, even though they are surrounded by an atmosphere. of hydrogen. That was the death knell for our old model.”
In the past five years, the story has gotten even stranger as scientists — including a team led by Andrew Howard, a professor of astronomy at Caltech; Lauren Weiss, an adjunct professor at the University of Notre Dame; and Erik Petigura, formerly a Sagan Postdoctoral Fellow in Astronomy at Caltech and now a professor at UCLA, have studied these exoplanets and made an unusual discovery: Although there are a wide variety of types of super-Earths, all super-Earths from the same planetary system they tend to be similar in terms of orbital spacing, size, mass, and other key features.
LIKE PEAS IN A PODS
“Lauren discovered that, within the same planetary system, super-Earths are like ‘peas in a pod’“, explains Howard, who was not directly involved in the Batygin-Morbidelli study, but has reviewed it. “Basically, we have a planet factory that only knows how to make planets of one mass, and it launches them one after another.”
So what single process could have given rise to the rocky planets in our solar system, but also to uniform rocky super-Earth systems?
“The answer turns out to be related to something we discovered in 2020 but didn’t realize applied to planet formation more broadly,” Batygin says.
In a 2020 paper published in The Astrophysical Journal, Batygin and Morbidelli proposed a new theory for the formation of Jupiter’s four largest moons (Io, Europa, Ganymede, and Callisto).
In essence, they showed that for a specific range of dust grain sizes, the force that drags the grains toward Jupiter and the force (or drag) that transports those grains in a flow of gas outward perfectly cancel each other out. That balance of forces created a ring of material that formed the solid building blocks for the later formation of the moons. Furthermore, the theory suggests that the bodies would grow in the ring until they were large enough to leave it due to gas-driven migration. They then stop growing, which explains why the process produces bodies of similar sizes.
In their new paper, Batygin and Morbidelli suggest that the mechanism for planet formation around stars is largely the same. In the planetary case, the large-scale concentration of solid rocky material occurs in a narrow strip of the disk called the silicate sublimation line, a region in which silicate vapors condense to form solid, rocky pebbles.
“If you’re a grain of dust, you feel a considerable headwind on the disk because the gas orbits a little slower, and you spiral in toward the star; but if you’re in vapor form, you just spiral out, along with with the gas in the expanding disk. So that place where you go from vapor to solid is where the material accumulates,” says Batygin.
The new theory identifies this band as the likely location for a “planet factory” that can, over time, produce several rocky planets of similar size. Furthermore, as the planets gain sufficient mass, their interactions with the disk will tend to pull these worlds inward, closer to the star.
