Just over 4.6 billion years ago, our planetary system was formed from a rotating disc of gas and dust at the edge of the Milky Way. The fact that both the Sun and our neighbouring planets are billions of years old makes it difficult to say with certainty exactly how this process happened. But by combining studies of our planetary system with analyses of the more than 5,500 exoplanets discovered in over 4,000 solar systems, the world's astronomers are painting an increasingly comprehensive picture of the intricate processes of planet formation every year.
The conventional wisdom has long been that the Earth and other planets formed over hundreds of millions of years thanks to multiple asteroid collisions. But in recent years, the rock theory has become increasingly common. It suggests that the Earth went from being a baby planet of rock and ice to its current size thanks to barely visible pebbles being sucked together into a celestial body over a much shorter period. And this is where ice and water vapour come in.
- Ice allows the particles to grow easier and faster. It also makes it easier for the clumped particles to cross the barrier required for them not to remain in the plant but to continue expanding into planets. "You can imagine hail in the Earth's atmosphere," says Katrin Ros, PhD in astrophysics at Lund University.
When she began her thesis, the role of ice in the planet formation process was a white spot on the astronomy map. Through computer simulations and laboratory experiments, Katrin Ros was able to show at several stages that ice is actually necessary for the millimetre-sized particles not to break up or just bounce off each other, but to actually clump together and form bodies the size of kilometres. These so-called planetesimals, such as comets and asteroids, are usually seen as the first building blocks of planets. But not everywhere are the conditions for this process favourable. Katrin Ros has turned the spotlight on the so-called frost line or ice line. This is the boundary, located at a certain distance from the star, where water in the form of vapour turns to ice. In the early solar system, the ice line was midway between Mars and Jupiter - which may explain why the smaller planets are inside this position while the larger ones are outside.
"At the ice line, it is easier for planetesimals to form, both because the proportion of solid material increases there, and because particles can grow through condensation and not just through collisions. This applies both to calm protoplanetary discs and to discs that have just undergone a stellar eruption where the temperature has suddenly increased," she says.
Katrin Ros' thesis has helped to open up new theoretical perspectives in planet formation. "The knowledge of how ice works may play an important role for astronomers searching for places where habitable, Earth-like planets may have formed.
- I hope that my results can serve as a small piece of the puzzle in the search for Earth number 2," says Katrin Ros.
The scientific papers are published in Katrin Ros' thesis: "The role of ice in planet formation".
Katrin Ros' profile in Lund University's research portal.