2 Feb. () –
Astronomers have modeled a key phase in the formation of planets in our solar system: how centimeter ‘pebbles’ converge to form planetesimals ten to one hundred kilometers in size.
The simulation reproduces the original size distribution of planetesimals, which can be examined based on observations of modern asteroids. It also predicts the presence of near double planetesimals in our solar system, which should have formed very soon.
In the new study published in the journal Astrophysical JournalAstrophysicists Brooke Polak of the University of Heidelberg and Hubert Klahr of the Max Planck Institute for Astronomy (MPIA) used simulations to deduce key properties of so-called planetesimals, the intermediate-sized bodies from which planets formed. in our solar system about 4.5 billion years ago.
Using an innovative method to simulate the formation of planetesimals, the two researchers were able to predict the initial size distribution of planetesimals in our solar system: how many are likely to form in the different “size intervals” between about 10 km and 200 km. , reports the MPIA in a statement.
Several groups of objects in the current solar system, specifically the main belt asteroids and Kuiper belt objects, they are direct descendants of planetesimals that did not go on to form planets. Using existing reconstructions of the initial size distribution of main-belt asteroids, Polak and Klahr were able to confirm that their prediction fit the observations. Furthermore, their model successfully predicts the differences between planetesimals that form closest to the Sun and those that form farther away, as well as how many form as binary planetesimals.
The formation of planets around a star takes place in several stages. In the initial phase, cosmic dust particles from the protoplanetary disk swirling around a new star clump together, held together by electrostatic (van der Waals) forces, to form so-called pebbles a few centimeters in size. In the next phase, the pebbles come together to form planetesimals: space rocks between tens and hundreds of kilometers in diameter.
In the case of these larger objects, gravity is so strong that collisions between individual planetesimals form even larger, gravitationally bound, solid cosmic objects: planetary embryos. These embryos can continue to accumulate planetesimals and pebbles until they become terrestrial planets like Earth. Some may build up thick layers of hydrogen gas to become gas giants like Jupiter or ice giants like Uranus..
The simulations by Polak and Klahr go in a different direction, borrowing concepts from an apparently unrelated physical model: the kinetic description of a gas, in which myriads of molecules fly at high speed and collide with the sides of a container. exert a cumulative pressure on the walls of the container. When the temperature of the gas is low enough and the pressure high enough, the gas undergoes a phase transition and becomes a liquid. Under certain conditions, the phase transition can take a substance directly from the gaseous to the solid state.
Polak and Klahr’s simulation treated the small clumps of pebbles in a collapsing cloud into a protoplanetary disk in an analogous way to particles of this type of gas. Instead of explicitly modeling the collisions between the various groups of pebbles, they assigned a pressure to their “pebble gas.” For the so-called equation of state, which gives pressure as a function of density, they chose an adiabatic equation of state, the kind of equation that, in a situation of spherical symmetry, has a density structure similar to that of Earth.
With this choice, the gas in the pebbles can also undergo a phase change: At low density, there is a “gas phase” in which the separated pebbles frequently fly and collide. If the density is increased, one can move to a “solid phase”, in which the pebbles form solid planetesimals.. The key criteria for when the gas in the pebbles becomes a solid is whether or not the gravitational attraction of the pebbles is greater than the pressure sustained by the collisions.
Previous work by Hubert Klahr’s group had shown that planetesimal formation always begins with a compact cloud of pebbles within the protoplanetary disk collapsing in on itself, and they had also provided concrete values for the sizes of such separate collapsing regions. In this new work, Polak and Klahr study several versions of such a collapse region, each at a different distance from the Sun, starting with a distance as close as the orbit of Mercury and ending with a collapse region as far away as Neptune.
Because their simplified equations are far less complex than those of superparticle collision models, the researchers were able to use the computing power at their disposal to simulate finer details than ever before, down to the scales at which binary planetesimals can form as binaries. contact. Previous simulations, which lacked the ability to track such fine detail, simply assumed that two planetesimals coming close enough to form a close binary would have become a single structureless object, and thus they would miss those close binaries.
Their results paint an interesting picture of planetesimal formation as a whole. Distance from the Sun is key: a collapsing region very close to the Sun will produce a single planetesimal. At greater distances, each collapsing region will form more and more planetesimals at the same time. Also, the largest planetesimals form near the Sun. The largest planetesimals produced by a collapsing pebble cloud at the distance from Earth to the Sun they are around 30% more massive and 10% larger than those produced ten times farther. In general, the production of planetesimals turns out to be very efficient, since more than 90% of the available pebbles end up in the resulting planetesimals, regardless of its location in the solar system.
The simulation’s prediction of the size distribution of planetesimals is spot on. Of course, even in the case of the main belt asteroids, life continued for the past billion years, with numerous collisions breaking the largest planetesimals into smaller fragments. But analyzes that try to reconstruct the initial size distribution from what is seen today come up with results that are very similar to those of the new simulations.
