Konstantin Batygin, professor of planetary sciences at the mythical Caltech, and Alessandro Morbidelli, from the Observatory of the Côte d’Azur published some time ago in Nature Astronomy a new theory of the formation of rocky planets, theory capable of explaining a mysterious characteristic until today of the class of telluric planets called “super-Earths”.
Batygin and Morbidelli, who have collaborated on several papers in recent years, which can be found at Arxiv, are not strangers. Alessandro Morbidelli is indeed, with his colleagues, at the origin of the famous Nice model proposed to explain the formation, structure and evolution of the Solar System by involving planetary migrations. He has also collaborated several times with Sean Raymond on these issues, notably with the model of the great tack, the Grand Tack. As for Konstantin Batygin, he is probably famous for having been at the origin, in January 2016, of the hypothesis of the existence of a ninth planet in the Solar System, with his colleague from Caltech, the 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, i.e. modeling the origin of the 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 of the gas giants by two specialists renowned, Alessandro Morbidelli and Sean Raymond. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Laurence Honnorat
The characteristics of super-Earths
In a press release 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 as well as the origins of self-similar super-Earth systems, many of which appear to be rocky in composition. »
We now know of thousands of exoplanets in the Milky Way and curiously, when you find a group of super-Earths in a planetary system, they seem to have very similar masses each time, although different 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.
Let us recall in passing that in general, such a body is defined by giving it a mass at least slightly greater than that of the Earth but not exceeding 10 terrestrial masses, that is to say that it is a rocky exoplanet different 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).
But other figures can be found in the literature, so that a consensus does not yet really exist about the mass limit.
The term “super-Earth” is also used by astronomers based on radius, so it refers to exoplanets similar to Earth (from 0.8 to 1.2 Earth radius), but more smaller than mini-Neptunes with 2 to 4 Earth radii.
To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Laurence Honnorat
The cosmochemistry of protoplanetary disks
To explain these curious characteristics of super-Earths, Batygin and Morbidelli nevertheless based themselves on part of the standard cosmogonic parameter for planetary formation. It all begins with a disc of gas and dust containing a given mass and of a given thickness. We pass from dust to planets by a process of agglutination and accretion. There exists in the protoplanetary disc a thermal and chemical gradient which causes the initially heated material of the disc to condense when it cools, giving near a central star rocky planets because the minerals condensing at high temperature are formed there, whereas further, beyond what is called the line of ice or snow, silicate dust surrounded by a gangue of ice is dominant. Instead, rocky bodies are formed with a lot of ice that can then attract gas in more or less large quantities.
The gas from the disk can exert some pressure on particles of matter throughout the protoplanetary disk and as long as the disk with gas persists, between a few million and ten million years, the disk can exert gravitational forces on the forming planets leading to planetary migrations.
Morbidelli also 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 that they are all water-rich. Recent observations, however, have shown that most super-Earths are rocky, like Earth, even though they are surrounded by a hydrogen atmosphere. This was the death sentence for our former role model. »
The explanations of Konstantin Batygin. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then 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 concerning a theory of the formation of the moons of Jupiter, Batygin and Morbidelli therefore propose the following model.
The central idea is based on the existence of a ring in the protoplanetary disk which begins when the temperature is below 1,400 kelvins, which allows silicates to condense. It is in this ring that the rocky planets of the Solar System will form from planetary embryos exceeding 1,000 kilometers in size.
But at the beginning, there were thus only grains of dust and small solid and rocky pebbles which underwent the force of friction of the gas in the disc and fell while spiraling in the direction of the central star. This frictional force disappeared below the ring since the dust and small pebbles quickly sublimated there.
Calculations show that the inner edge of this ring is the boundary of a band in the ring where planetary bodies will accumulate until they reach a size where interactions with the protoplanetary disk (which may be complex) will become dominant and force a planet to migrate towards its host star.
As each protoplanetary disk has a different initial mass and thickness, the size and mass limit for a rocky planet migration is not the same. This is why for each disc, each super-Earth formed then migrates for a more or less constant size limit, which therefore gives in the end the series of similar super-Earths and on almost equally spaced orbits that we have identified these last years.
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