Giving planet formation another look
Composed of 66 high-precision antennas, the Atacama Large Millimeter/submillimeter Array (ALMA) observatory is the world’s largest radio telescope array. More than being a very big telescope, ALMA has also had a big impact on many different fields. But one of the fields it helped change the most is planet formation. “We know that young stars are often surrounded by a disc of gas and dust, out of which planets can form,” says Til Birnstiel, a professor of Theoretical Astrophysics at Ludwig Maximilian University of Munich. “With ALMA, we are able to see these planet-forming disks in high resolution for the very first time.” However, with these never-before seen images of dusty, planet building rings came new questions. “We wanted to know if planets are the cause or the result of these structures,” adds Birnstiel. So, armed with new models and with the support of the EU-funded DustPrints project, Birnstiel set out to dust the universe for the fingerprints of planet formation.
Planet formation happens faster than we thought
When the project first started, the general consensus was that planets were not the cause of these disc structures. Suffice to say this is no longer the case. “First, we showed that these structures are ubiquitous, that whenever one of these discs is observed in high enough resolution, one will also see some form of substructure,” explains Birnstiel. Using a new method to analyse these observations, researchers were able to show that the gas around those rings is perturbed just as it would be around a planet. This hypothesis was then proved via the first image of a planet inside a planet. “These findings really forced the field to accept that planets are the main cause of these structures, and that planet formation must work faster than we previously thought possible,” remarks Birnstiel.
What came first, the structure or the planet?
The project, which received support from the European Research Council, also discovered that the observed substructures all have the same amount of transparency. According to Birnstiel, this was odd as conditions between different young stars should vary. Turns out, the answer to this celestial conundrum was the active formation of asteroid-sized objects. “This really underlines the chicken-or-egg situation of planet formation,” notes Birnstiel. “Planets seem to be the cause of most observed structures, but the structures seem to be the best places to quickly build those planets in the first place.” Researchers are in the process of publishing simulations to demonstrate how planets can be formed sequentially, just like a chicken lays eggs from which new chickens hatch.
More work ahead
Not only did the DustPrints project show that planets form much faster than previously thought, it also developed the methods to show how this process works. Some of these tools have gone on to win awards, and many are already available and being used for further research. Furthermore, the project’s images of 20 young planet-forming discs with substructures have gained iconic status in the field and are now widely used in talks and presentations. That being said, there’s still more work to be done. “We need to understand how this planet forming gets started,” concludes Birnstiel. “This means studying the earliest stages of star formation, when the disc is just starting to form, to see if these initial stages affect – or even trigger – planet formation.” Birnstiel was just awarded an ERC Consolidator Grant for this very purpose – so be sure to watch this ‘space’!
Keywords
DustPrints, stars, planets, ALMA, planet formation, radio telescope, astrophysics, universe
