Konstantin Batygin, professor of planetary sciences at the mythical Caltech, and Alessandro Morbidelli of the Observatoire de la Côte d’Azur recently published in Nature Astronomy a new theory on the formation of rocky planets, a theory that can explain a until now mysterious feature of the class of telluric planets called “Super-Earths”.
Batygin and Morbidelli, who have collaborated on several articles that can be found on Arxiv in recent years, are no strangers. In fact, Alessandro Morbidelli, along with his colleagues, is at the origin of the famous Nice model, proposed to explain the formation, structure and evolution of the solar system by including planetary migrations. He has also collaborated several times with Sean Raymond on these subjects, most notably with the model of the great tack, the grand tack. As for Konstantin Batygin, he is probably most famous for being at the origin of the hypothesis of the existence of a ninth planet in the solar system in January 2016 with his colleague at Caltech, astronomer Michael E. Brown.
The solar system is a laboratory for studying the formation of giant planets and the origin of life that can be used in conjunction with the rest of the observable universe for the same purpose. Mojo: Modeling the Origin of JOvian planets is a research project that has resulted in a series of videos presenting the theory of the origin of the solar system, and in particular the gas giants, by two renowned specialists, Alessandro Morbidelli and Sean Raymond. For a fairly accurate French translation, click the white rectangle at the bottom right. Then the English subtitles should appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © Laurence Honorat
The Properties of Super-Earths
In a statement from Caltech, Konstantin Batygin explains: “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, starting with the fundamentals. We need a theory that can simultaneously explain the formation of terrestrial planets in our solar system and the origins of self-similar super-Earth systems, many of which appear to have a rocky composition. »
We now know of thousands of exoplanets in the Milky Way, and oddly enough, when you find a cluster of super-Earths in a planetary system, they always seem to have very similar masses, although they vary from one system to another. Better still, all super-Earths within a single planetary system also tend to be similar in orbital spacing, size, and other key characteristics.
As a side note, let’s recall that such a body is generally defined by giving it a mass at least slightly greater than Earth’s, but no more than 10 Earth masses, that is, a rocky exoplanet distinct from the ice giants of the solar system and with a mass less than 69% of the mass of Uranus (the lightest giant planet in the solar system).
However, other numbers can also be found in the literature, so that there is still no real consensus on the mass limit.
The term “super-Earth” is also used by astronomers based on radius, so it refers to Earth-like exoplanets (from 0.8 to 1.2 Earth radius) but smaller than mini Neptunes with 2 to 4 Earth radii.
For a fairly accurate French translation, click the white rectangle at the bottom right. Then the English subtitles should appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © Laurence Honorat
The cosmochemistry of protoplanetary disks
Nevertheless, to explain these strange properties of super-Earths, Batygin and Morbidelli relied on part of the standard cosmogonic parameter for planet formation. It all starts with a disk of gas and dust of a certain mass and thickness. We get from dust to planets through a process of agglutination and accretion. A thermal and chemical gradient exists in the protoplanetary disc that causes the disc’s initially heated material to condense as it cools, yielding rocky planets near a host star because the high-temperature condensing minerals are formed there, while farther, beyond it , what is referred to as the ice or snow line, is dominated by silicate dust surrounded by an ice gangue. Instead, rock bodies are formed with a lot of ice, which can then attract more or less large amounts of gas.
The gas from the disk can exert some pressure on matter particles throughout the protoplanetary disk, and as long as the disk with gas persists between a few million and tens of millions of years, the disk can exert gravitational forces on the forming planets, causing planetary migrations.
Morbidelli explains: “A few years ago we built a model where super-Earths formed in the icy part of the protoplanetary disk and migrated to the inner edge of the disk near the star. The model could explain the masses and orbits of super-Earths, but predicts they are all water-rich. However, recent observations have shown that most super-Earths, like Earth, are rocky despite being surrounded by a hydrogen atmosphere. That was the death sentence for our former role model. »
The Explanations of Konstantin Batygin. For a fairly accurate French translation, click the white rectangle at the bottom right. Then the English subtitles should appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © Caltech
Rings in the protoplanetary disk
Inspired by another joint work on a theory of the formation of Jupiter’s moons, Batygin and Morbidelli therefore propose the following model.
The core idea is based on the existence of a ring in the protoplanetary disk that starts at a temperature below 1,400 Kelvin and allows silicates to condense. In this ring, the rocky planets of the solar system will emerge from planetary embryos over 1,000 kilometers in size.
But in the beginning it was only dust grains and small solid and rocky pebbles, subjected to the frictional force of the gas in the disk, that spiraled towards the central star. This frictional force disappeared below the ring as the dust and small pebbles there rapidly sublimated.
Calculations show that the inner edge of this ring is the boundary of a band in the ring in which planetary bodies accumulate until they reach a size where interactions with the protoplanetary disk (which can be complex) become dominant, forcing a planet to to migrate there its host star.
Because each protoplanetary disk has a different initial mass and thickness, the size and mass limit for a rocky planetary migration are not the same. Because of this, for each disk, each super-Earth formed migrates for a more or less constant size limit, thus ultimately yielding the series of similar super-Earths and in nearly equally spaced orbits that we have identified over the last few years.