‘Planet factories’ could explain the mysterious diversity of super-Earth’s alien worlds

Super-Earths are the most common type of planet in the galaxy, but much remains unknown about how these mysterious worlds form.

Now, in a new study, researchers suggest they can explain the origin of these mysterious worlds, as well as other rocky planets and moons, including Earth and its siblings.

In the 30 or so years since astronomers first began discovering exoplanets orbiting distant stars, researchers have discovered many strange new types of worlds unlike any known in our solar system. These include super-Earths, rocky worlds larger than our own and about 10 times heavier than Earth. Some super-Earths have atmospheres of hydrogen and helium, making them almost look like gas giants.

On the subject: These 10 super-extreme exoplanets are not from this world

Much remains unclear about the underlying reasons for the differences between the super-Earths and the terrestrial planets of the Solar System—Earth, Mercury, Venus, and Mars. For example, super-Earths are typically much larger than the solar system’s terrestrial planets, with diameters approaching that of ice giants such as Uranus and Neptune. However, “recent data have shown that extrasolar super-Earths tend to be rich in silicates, like the Earth itself,” study lead author Konstantin Batygin, a planetary astrophysicist at the California Institute of Technology (Caltech) in Pasadena, told .

In addition, stars often contain multiple super-Earths, and those orbiting the star together are often similar to each other in terms of size, mass, distance between their orbits, and other key characteristics. “Within a single planetary system, super-Earths are like ‘peas in a pod,'” Andrew Howard, a Caltech professor of astronomy who was not involved in the new study, said in a statement. “Basically, you have a planet factory that knows how to make planets of only one mass, and it just churns them out one by one.”

However, while super-Earths usually appear similar to each other when they orbit the same star, there is considerable variation in super-Earth types when comparing many different stars.

“The complexity of modeling the formation of a super-Earth is exacerbated by the fact that our own solar system does not contain any super-Earths, with the possible exception of a very distant one,” Batygin said.

Now, researchers may have come up with a scenario that could explain not only the origin of the super-Earth, but also the rocky planets and moons that populate our solar system. Batygin and his colleague Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in Nice, France, detailed their findings. (will open in a new tab) online Thursday (January 12) in Nature Astronomy.

Previous research has suggested that planetary systems begin as large spinning disks of gas and dust. These “protoplanetary disks” coalesce over a period of a few million years or so, with most of the gas concentrating on the star at the center of the system, while the solid material slowly coalesces into asteroids, comets, planets and moons.

To explain how smaller rocky planets near the Sun and larger gas giants farther afield appeared in our own solar system, a 2021 study (will open in a new tab)Batygin and his colleagues suggested that two separate rings participated in the formation of planets in the solar protoplanetary disk – an inner one for rocky planets and an outer one for more massive planets.

Now, in a new study, scientists are investigating whether this concept of multiple individual rings in protoplanetary disks can help explain the existence of a super-Earth.

“We looked at the existing model of planetary formation, knowing that it doesn’t reproduce what we see, and we asked, ‘Which statement are we taking for granted? Batygin said. “In this case, it turned out that the usual assumption is that the solid material inside the protoplanetary disks is evenly distributed. planetary systems with a single structure.

The researchers suggest that during planet formation, solid material is concentrated in a narrow band in the protoplanetary disk, where silicate vapor condenses to form rocky pebbles.

“If you’re a mote, you feel a lot of headwind in the disk because the gas orbits a little slower and you spiral towards the star, but if you’re in vapor form, you just spiral outward with the gas in the expanding disk.” , – Batygin said in a message. “So, where you go from vapor to solid, material accumulates.”

Related: Solar System Planets, Order and Formation: A Guide

Scientists speculate that this narrow band may act as a planetary factory, inside which rocky worlds form until they are large enough to leave the ring due to the forces created by the gas. Over time, this process can produce multiple rocky planets of the same size. This may help explain why all super-Earths around one star can look very similar to each other, but look completely different than super-Earths around another star, which can have a completely different planet factory.

If this ring contained a lot of mass, the planets would grow until they migrated away, resulting in a planetary system containing similar super-Earths. If the ring contained less mass, it would produce worlds that would look more like the terrestrial planets of our solar system.

Researchers first came up with this idea of ​​a narrow band giving rise to rocky bodies when they were studying how to explain the formation of Jupiter’s four largest moons—Io, Europa, Ganymede, and Callisto. Galileo first discovered these moons around the gas giant in the early 1600s.

“Over the years, it has been repeatedly pointed out that extrasolar super-Earths are very similar to enlarged versions of Galilean satellites,” Batygin said. “But the physical nature of this connection remained elusive. Among the most exciting aspects of our model is that it provides a theoretical framework that can explain the formation of the solar system’s terrestrial planets, extrasolar super-Earths, and the origin of the moons of Jupiter and Saturn in a unified way.”

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