Supernovae and water in rocky planets

An article published in the journal “Nature Astronomy” reports the results of a series of computer simulations conducted to better understand the mechanisms of rocky planets formation. A team of researchers concluded that there are probably two types of planetary systems: those similar to the solar system, with planets containing relatively little water, and those in which there are above all the so-called ocean planets or waterworlds. The difference may have been caused by the presence of a massive star nearby that ejected radioactive materials that have at least partially dried out the planets. This might have led to the emergence of a temperate climate on Earth.

According to current models, planets are formed inside a protoplanetary disk that orbits a star that’s also in formation or just formed thanks to an accretion that starts from gas and dust. An intermediate phase of this process consists of planetesimals, large asteroids composed of rocks and frozen water. The amount of water that ends up in a rocky planet depends on its proximity to its star as it depends on the fact that it’s at temperatures low enough to remain frozen. The problem is that a planet like the Earth should have accumulated a quantity of water much greater than that present today, therefore some process must have made at least a a part of it evaporate, if not sublimate and disperse in space.

Tim Lichtenberg, now at the University of Oxford and lead author of the article, began working on the problem to work on his doctoral thesis at the Institute of Astronomy and Geophysics at ETH Zurich, Switzerland. Together with other researchers from various universities, he developed models to simulate the formation of planets from planetesimals. According to their conclusions, if planetesimals are heated from the inside, part of their water content evaporates and in the case of our solar system this was caused by a supernova occurred in the cosmic neighborhood during its formation. Radioactive elements including aluminum-26 were injected into the solar system by stellar winds or materials ejected by the explosion and their decay heated up the planetesimals that were formed.

The simulations led the researchers to conclude that there are two types of star systems: those that had a supernova nearby during their formation with planets relatively poor in water and those far enough from massive stars to be born with a small amount of radioactive elements such as aluminum-26 but with ocean planets, which are covered with water. The image (Thibaut Roger, all rights reserved) shows an artistic representation of a planet like the Earth and therefore formed by planetesimals containing aluminum-26 that dried them at least partially (on the left) and an ocean planet formed by planetesimals that kept their water content (on the right).

Not everything was explained by these simulations. For example, they still need to understand the relationships between that type of dehydration caused by the warming of planetesimals and the formation of gas giant planets. The observations of the ones in the solar system and of exoplanets of that type showed that there’s also water inside them, how much does its amount matter?

Understanding how much water a rocky planet can contain is essential to understand its evolution. Water is essential for the development of life forms similar to those existing on Earth, but too much water would be a big problem. If the Earth’s mantle was covered by a great deep ocean, the climate would be very different and it would have thick layers of ice that would prevent a series of cycles such as the carbon one, which helps to have a stable and temperate climate. Basically, in the search for life forms similar to the Earth’s ones, understanding if a star system was born near a supernova might be another crucial factor.

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