How are rocky planets really formed?

Planet composition

image: Artist’s illustration of a protoplanetary disk in which planets are forming.
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Credit: California Institute of Technology

A new theory of how rocky planets form could explain the origin of so-called “super-Earths” – a class of exoplanets several times the mass of Earth, and the most abundant type of planet in the galaxy.

Furthermore, it could explain why the disappearance of superplanets within a single planetary system appear so eerily similar in size, as if each system was only capable of producing one type of planet.

“As our observations of exoplanets have grown over the past decade, it has become clear that the standard theory of planet formation needs to be revised, starting from the basics. We need a theory that can simultaneously explain the formation of terrestrial planets in our solar system as well as the origins of autonomous systems. super-Earths, many of which appear rocky in composition,” says Caltech planetary science professor Constantin Batygin (MS ’10, PhD ’12), who collaborated with Alessandro Morbidelli of The Observatoire de la Côte d’Azur in France. about the new theory. A paper explaining their work has been published by natural astronomy it is january. 12.

Planetary systems begin their life cycle as large rotating disks of gas and dust that solidify over the course of a few million years or so. Most of the gas accumulates in the star at the center of the system, while solids slowly combine into asteroids, comets, planets and moons.

In our solar system, there are two distinct types of planets: the smaller, rocky inner planets that are closer to the sun, and the larger outer planets, hydrogen-rich gas giants that are farther from the sun. in a previous study Posted in natural astronomy At the end of 2021This dichotomy led Morbidelli, Batygin, and colleagues to suggest that the formation of planets in our solar system occurred in two different rings in the protoplanetary disk: an inner ring where the small rocky planets were formed, and an outer one for the more massive icy planets (two) of which – Jupiter and Saturn – later grew into giants. fizzy).

Super-Earth, as the name implies, is larger than Earth. Some of them even have hydrogen atmospheres, which makes them look almost like a gas giant. Moreover, they are often found orbiting close to their stars, which indicates that they have migrated to their current location from more distant orbits.

“A few years ago, we built a model in which super-Earths formed in the icy part of the protoplanetary disk and moved all the way to the inner edge of the disk, close to the star,” says Morbidelli. “The model could account for the masses and orbits of the superplanets but predicted that they were all rich in water. However, recent observations have shown that most superplanets are rocky, like Earth, even if they are surrounded by atmospheres of hydrogen. It was a death sentence for our old model.”

Over the past five years, the story has gotten stranger as scientists — including a team led by Andrew Howard, a Caltech professor of astronomy. Lauren Weiss, Assistant Professor at the University of Notre Dame; And Erik Petigura, formerly a Sagan postdoctoral researcher in astronomy at Caltech and now a professor at UCLA — studied these exoplanets and made an unusual discovery: While there are a variety of types of superplanets, all super-Earths within them are inclined. A single planetary system tends to be similar in terms of orbital spacing, size, mass, and other key features.

“Lauren discovered that super-Earths, within a single planetary system, are like peas in a pod,” says Howard, who was not directly related to Batygin-Morbidelli’s paper but did review it. “You basically have a planet factory that only knows how to make planets out of one mass, and it just puts them out one by one.”

So, what single process could lead to not only the rocky planets in our solar system but also the unified systems of rocky super-Earths?

“The answer turns out to be related to something we discovered in 2020 but didn’t realize applies to planet formation on a larger scale,” says Batygin.

In 2020, Batygin and Morbidelli propose a new theory to form the four largest moons of Jupiter (Io, Europa, Ganymede, and Callisto). In essence, they explained that for a specific size group of dust grains, the force that pulls the grains toward Jupiter and the force (or drift) that holds those grains in the outflow of gas cancel each other out completely. This balance of forces created a ring of material that formed the solid building blocks for the later formation of moons. Furthermore, the theory proposes that objects will grow in the ring until they are large enough to exit the ring due to gas-driven migration. After that, they stop growing, which explains why this process produces bodies of similar sizes.

Batygin and Morbidelli propose in their new paper that the mechanism for planet formation around stars is largely the same. In the case of planets, the widespread concentration of hard rocky material occurs in a narrow band in the disk called the silicate sublimation line—a region where silicate vapors condense to form hard rocky pebbles. “If you’re a grain of dust, you feel a big headwind in the disk because the gas is spinning a little more slowly, as you’re heading towards the star; but if you’re in vapor form, you’re simply rolling outward, along with gas in the expanding disk. So the place is Where you turn from vapor into solids is where materials accumulate,” says Batygin.

The new theory identifies this range as the likely location for a “planet factory” that could, over time, produce several similarly sized, rocky planets. Moreover, as planets grow massive enough, their interactions with the disk will tend to pull these worlds inward, closer to the star.

Batygin and Morbidelli’s theory was supported by extensive computer modeling but began with a simple question. “We looked at the current model of planet formation, knowing that it does not reproduce what we see, and asked, ‘What assertion do we take for granted? “The trick is to look at something that everyone considers to be true but for no good reason.”

In this case, the assumption was that the solid matter was dispersed throughout the protoplanetary disks. Batygin says that by abandoning this assumption and instead assuming that the first solid bodies formed in rings, the new theory could explain different types of planetary systems with a unified framework.

If the rocky ring contains a lot of mass, the planets grow until they migrate away from the ring, resulting in a system similar to a super-Earth. If the ring had a small mass, it would produce a system much like the terrestrial planets of our solar system.

“I’m an observer and an instrument maker,” Howard says, “but I pay very close attention to the literature.” “We get a regular shuffle of little but still important contributions. But every five years or so, someone comes out with something that creates a seismic shift in the field. This is one of those papers.”

The paper is titled “Formation of Rocky Superplanets from a Narrow Ring of Minor Planets”. Funding to support this research came from the California Institute of Technology, La Côte d’Azur Observatory, and the David and Lucille Packard Foundation, National Science Foundation, European Research Council.

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