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Building planets as sandcastles

Planetary formation starts with small dust grains in disks surrounding young stars. But dust growth is hindered by collisional fragmentation and radial drift. So, how are the larger planetesimals, the building blocks of planets, formed? This is still an open question. Now, scientists of PlanetS have found a mechanism bridging the gap between the early and late stages of planetary formation. It takes place around the so called snow line where water condenses into solid ice.

By Wieńczysław Bykowski

Formation of planetary systems is extremely complex and many of its aspects are not well understood until now. The least certain gap in our knowledge concerns the connection between early and late stages of this process, or in other words, how micron-sized grains become kilometer- sized rocks. Scientists Joanna Drążkowska from the University of Zurich and Yann Alibert from the University of Bern, members of the NCCR PlanetS, performed numerical models that shed a new light onto this problem.
Key to their success was understanding how important is the role of water. “It is like the construction of sandcastles on the beach,” says Joanna Drążkowska, “you can build them if you add water to the sand. Water makes it sticky, otherwise it would fall apart.”

For building castles on the beach you have to add water to the sand. Water also plays an important role in the formation of planets. (Photo FSerega)

One of the major problems in our planet formation models is called “growth barriers”. The collisional velocities of dust aggregates are too high to allow growth past boulder sizes. What is more, already centimetre-sized pebbles decouple from the gas nebula and fall onto the star in a similar way the rain drops decouple from clouds producing rain. Until now, models dealing with the final stages of planetary formation would ignore these complications and assume an ad-hoc distribution of so called planetesimals, the kilometre-sized building blocks of planets, and just start from there.

The new work presents an efficient mechanism which makes it possible to finally close this gap. Models performed by Drążkowska and Alibert are based on the currently most widely accepted scenario of streaming instability. In this scenario, if there are sufficiently large dust aggregates concentrated in one location in the protoplanetary disc, the streaming instability generates overdense dusty filaments. Some of them are dense enough to collapse and form gravitationally bound kilometre-sized objects, the planetesimals, which are needed during the late phases of planet formation. In fact, the scientists think that some of the asteroids and comets in the solar system are the primordial planetesimals that survived from the planet formation era.

A question that remains is: how are these large dust aggregates formed and gathered together to initiate the streaming instability? Drążkowska and Alibert believe that they found an answer. A particular location in the protoplanetary disc known as the snow line, where the water condenses into solid ice, plays a significant role in their model. As the wet dust is more sticky, outside of the snow line the aggregates grow to larger sizes and thus drift more rapidly towards the star. Inside of the snow line the water ice evaporates, while the remaining dry dust slows down forming sort of a traffic jam. Some of the water particles are pushed out of the snow line by diffusion and re- condense, speeding the process of grain growth. As a result, there is an annulus of centimetre-sized icy pebbles just outside the snow line and those are large enough to trigger the streaming instability process and to initiate the next stages of planetary formation.

In the final result of the simulations, there is a massive annulus of planetesimals around the star, near the snow line. “The processes described likely sped-up the formation of Jupiter in our solar system,” claim the scientists, “in every model investigated by us there are more planetesimals than required by the standard minimum mass solar nebula”. It means, that there is enough material to quickly form a massive planetary core, so it can accrete its gaseous atmosphere before the protoplanetary nebula disperses. “Our results may be used by other scientists as an input to models dealing with later stages of planet accretion”, they add.

These findings open new research possibilities to the authors. Up to now, their models focussed on solar-type stars, but they plan to adapt them to protoplanetary disks around dwarf stars, like the famous Trappist-1. They will also test if similar mechanisms operate at condensation lines of another molecules, such as the carbon dioxide or ammonia observed in discs surrounding young stars.

Reference: “Planetesimal formation starts at the snow line” by Joanna Drążkowska and Yann Alibert,  Astronomy & Astrophysics, September 2017.

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