For the first time, it seems that there is strong evidence that giant collisions, similar to the one that gave birth to the Earth-Moon system, occur well during the formation of exoplanets. The Kepler satellite has indeed flown around the star Kepler 107, an exoplanet abnormally dense compared to its three sisters known in the same system.
About 50 years ago, the arrival of Homo sapiens on the Moon would bring back many rock samples. They were very talkative about the composition of our natural satellite and its history. These rocks have not finished talking as shown by the recent example of a new analysis of a sample of the Apollo 14 mission. Thus, in the wake of the NASA lunar missions, two researchers, William K. Hartmann and Donald R. Davis, published, at that time, in the newspaper Icarus, a now historical article. The two men were inspired by the work on the formation of the planets of the Solar System, from the Soviet school led by Viktor Safronov. Based on the cosmochemical data provided by Apollo missions and the analysis of meteorites found on Earth, Hartmann and Davis had developed nothing less than a theory about the origin of the Moon. Alastair GW Cameron and William R. Ward also reached similar conclusions at the same time.
Theia and the origin of the Moon
According to these four researchers, some tens of millions of years after the beginning of the formation of the solar system, 4.56 billion years ago, a small planet the size of Mars and baptized Theia, in memory of the Greek deity, the mother of Helios (the Sun) and Selene (the Moon), collided with the proto-Earth. The debris of this collision would then have given birth to the Moon in the disc formed around the young Earth. Celestial mechanics tells us that a collision is far more likely than a capture of the moon by the Earth.
Above all, this explained why the average chemical composition of the Moon was surprisingly close to that of the Earth when it should not be: it should have been formed in another region of the Solar System and, from small celestial bodies, themselves also marked by differences (although they have points in common since they come from the same primitive nebula as the Sun). These common points can be seen by comparing the abundances of certain elements in meteorites, the Earth and the solar atmosphere .
This hypothesis did not really resonate in the scientific community until a conference was held in 1984 in Hawaii on the origin of the Moon. This gave rise to the publication of a book in 1986 which has since become a reference on this subject. From then on, the giant impact hypothesis became standard and many numerical simulations about it came into being, notably that of Robin Canup in the 1990s.
Beautiful images of synthesis, more artistic than scientific, illustrate, in this documentary, the collision between the Earth and Theia.
However, since 1995, in the world of exoplanets, there is now a back and forth between theories and observations concerning the formation of the Solar System and those of other planetary systems in the Milky Way. Progress in these two areas is therefore joint and we are also trying to determine how much our Solar System is typical or not in the Galaxy. This will help clarify the extent to which the appearance and evolution of life on Earth is a marginal phenomenon or not. Thus, we know that the existence of the Moon may have played a role in the appearance of life and especially the stabilization of the climate of the Earth.
One can, therefore, ask the question of whether collisions between the equivalent of proto-Earth and Theia are frequent or not. The publication of an article in Nature Astronomy (published on arXiv) is interesting on this subject. It comes from an international team of researchers, including members of the Department of Astrophysics-AIM Laboratory CEA but also many other prestigious institutes in Italy and Switzerland.
Kepler-107c, an exoplanet with a big metal core
Astronomers have combined several observations, from instruments such as NASA’s Kepler satellite and the Italian telescope Telescopio Nazionale Galileo, located at the Roque de los Muchachos Observatory (Canary Islands), to characterize more precisely the exoplanets discovered around the world.
Kepler spotted 4 exoplanets in orbit around Kepler 107 by the transits method. But they are in a particular configuration with orbital resonances so that the transit time variation method, aka TTV, does not make it possible to estimate their masses while their rays can be estimated. This is why the Italian telescope was used to practice the radial velocity method. Kepler’s data on asteroseismology have made it possible to specify our knowledge of the mass and the density of Kepler 107. Finally, masses and rays of the exoplanets could be determined with sufficient precision to draw astonishing conclusions concerning their densities.
Indeed, the two exoplanets closest to the star which are, in the order of remoteness, Kepler-107b and Kepler-107c, have slightly different radii (about 1.5 to 1.6 terrestrial radius), but Kepler-107c is more than twice as dense as Kepler-107b. For planetary scientists, this can only mean one thing: Kepler-107c has a very important metal core.
The existence of such dense planets could be explained by involving the X-ray and ultraviolet photoevaporation theory of a mini-Neptune having lost its atmosphere after migrating too close to its host star. Except that scenarios of this kind predict that Kepler-107b should be denser than Kepler-107c. According to the researchers, only the scenario of a similar giant collision between Earth and Theia would remain in the running.
Indeed, such a collision between two massive planets, and therefore differentiated, can lead to the melting of metal cores and the ejection of a large part of silicate coats. Numerical simulations naturally lead to the birth of Kepler-107 b and Kepler-107 c, according to this scenario and with their observed characteristics.
This is the first time that it has been confirmed that what happened to the Earth and the Moon in the Solar System can happen elsewhere.
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