Astronomers from the University of Amsterdam have proposed a new method of planet formation for the mysterious TRAPPIST-1 system.

TRAPPIST-1 is fascinating, having an ultra-cool dwarf star orbited by seven known Earth-sized rocky planets. At least three of them may be habitable. All of these orbit within a tenth of the Earth-Sun distance (1 AU, astronomical unit), which is rather peculiar; none of our models of planet formation can really explain how they got here.

One theory we’ve got is the planets formed in the position they are now. However, that seems unlikely in this system because the initial disk from which they formed must have been very dense. If this were the case, the planets would be much larger.

Another theory suggested they migrated from further out in the system. But this theory does not explain why the planets are all roughly the same size as Earth.

So a new, third method may provide an answer. In a paper published in the journal Astronomy and Astrophysics, this team of Dutch researchers say it is actually the debris that formed the planets that migrated inwards, not the planets themselves.

"I hope that our model will help answer the question about how unique our own Solar System is compared to other planetary systems," Chris Ormel, the lead author on the study, said in a statement.

^{A previous video explaining the orbits of the TRAPPIST-1 planets}

This debris would have been in the form of pebbles, millimeter to centimeter-sized chunks of material swirling around a star after it forms. Some pebbles orbit in the outer reaches of the star’s disk, beyond the “ice line” where water turns to ice. In TRAPPIST-1, this ice line happens to be at 0.1 AU, within which all seven known planets orbit.

As the pebbles approach the ice line over 100,000 to 1 million years, they melt and lose their water ice. They can then clot together and form a proto-planet, which then moves inwards and picks up more material on the way. Once it reaches approximately the size of Earth, it ends up as close to the star as possible, and stays in orbit there. This then allows the next proto-planet to make its way inwards.

“Pebbles are mobile; they can easily drift towards the iceline,” Ormel told IFLScience.

This could help explain a number of things. For one, the disk around a star like this is thought to be tens to hundreds of AU in size. That the planets all formed in such a tight orbit suggests some form of migration took place. It could also explain the positions of planets in the TRAPPIST-1 system, and also the intriguing resonance between the planets.

“Chris’s work provides a fascinating narrative for the formation of planets orbiting stars with very low masses, like TRAPPIST-1,” Amaury Triaud from the University of Cambridge, who was not involved in this study but was a co-author in the initial discovery of TRAPPIST-1's seven planets, told IFLScience. “This study will likely create a lively debate, and it demonstrates how interesting these planetary systems like TRAPPIST-1 can be.”