A collision between two forming planets could unexpectedly help preserve an atmosphere. This result seems paradoxical: such an impact releases enough energy to heat rocks intensely and encourage gases to escape into space.
That is what a study published in Nature Astronomy by Darius Modirrousta-Galian of Shanghai Jiao Tong University and Jun Korenaga of Yale University shows.
Artist's impression of a collision between two rocky bodies, one the size of the Moon and the other the size of Mercury, in the young system of the star HD 172555.
Credit: NASA/JPL-Caltech.
Around a young star, dust and rocks gradually come together. Some of these bodies eventually collide and merge. During this period, a planet can also capture an envelope of gas, mainly composed of hydrogen and helium, from the disk that still surrounds its star. This is known as a primordial atmosphere, very different from the air we breathe on Earth.
Why doesn't an impact necessarily drive away the gases?
Two opposing effects occur during a major collision. On the one hand, the impact releases a considerable amount of energy. The planet's rocky layers heat up and can transfer their heat to the atmosphere. The gases then expand, making it easier for them to escape into space.
On the other hand, the planet gains material by absorbing the object that struck it. Its mass increases, making it more difficult for gases to escape its gravitational pull.
The researchers modeled the evolution of a rocky planet after a giant impact, taking both effects into account. Their finding: all the added mass helps retain the atmosphere more effectively, but not all the impact energy is used to heat the rocks beneath it.
Some of this energy is consumed in melting the mantle rocks, the layer between the surface and the core. This melting requires energy without entirely translating into a rise in temperature. In addition, the metallic core of the impactor can sink into the planet and remain extremely hot deep inside, without immediately transferring most of its heat to the mantle.
Under the conditions studied, the strengthening of gravity can therefore outweigh the warming that promotes gas loss.
A possible explanation for two major families of exoplanets
This theoretical discovery could explain a feature of planets observed around other stars. Astronomers detect many super-Earths, planets more massive than Earth, and sub-Neptunes, which are generally larger and surrounded by a thick gaseous envelope.
According to the proposed scenario, planets that grow as a result of major collisions could more easily retain a hydrogen-rich envelope and become sub-Neptunes. Conversely, smaller rocky planets could lose their primordial gases under the prolonged effects of radiation from their star, particularly its most energetic emissions.
The model therefore predicts that sub-Neptunes have, on average, more massive rocky interiors than super-Earths. This difference would thus not be related only to the presence or absence of gas, but also to the history of the collisions that shaped these worlds.
However, this is an explanation based on theoretical calculations. Measurements of exoplanet masses and radii could help test the structural differences predicted by the researchers.