Weird inside-out planet system may have formed one world at a time
Artist’s impression of the planetary system around the star LHS 1903
ESA
Astronomers have discovered a planetary system that appears to have formed in reverse. While most systems, like ours, have rocky planets closest to their star and gaseous planets farther away, the LHS 1903 system has a rocky world at its edges, challenging established models of planet formation.
The outermost of the system’s four planets was not immediately apparent in early observations from the Transiting Exoplanet Survey Satellite. These first measurements allowed researchers to identify a rocky planet slightly larger than Earth near the star, as well as two gaseous planets slightly smaller than Neptune beyond. But when Ryan Cloutier of McMaster University in Hamilton, Canada, and his colleagues tracked the system using eight other observatories, they spotted telltale signatures of a fourth world slightly larger than the system’s other rocky planet.
This rocky world, further from the star than its gaseous brothers, was unexpected. “These systems are not unknown, but they are rare – and systems that have this unique architecture and for which we can characterize them in detail are extraordinarily rare,” says Cloutier.
These details, including the size of the planets and the fact that they all orbit their star in periods of less than 30 Earth days, allowed researchers to test models of how these planets might have formed. “Producing a planet can be done with several mechanisms, but once you have to produce four different ones, you can start to distinguish between different models,” explains Solène Ulmer-Moll of Leiden University in the Netherlands. “You have to find a model that can explain them all.”
Most systems are thought to form all their planets around the same time from the same disk of dust and gas. The size and composition of planets depends on where they formed within this disk and what events happened to them subsequently, such as collisions with other worlds. However, for the LHS 1903 system, this model does not work.
If the LHS 1903 planets were born in the traditional way, the outermost one should have formed with a thick gaseous envelope like the middle two. This atmosphere could have been lost following a collision or bombardment of radiation, but the researchers’ simulations show that such a process would also have removed gas from one or both inner planets.
“It is very difficult to sculpt the most distant planet without affecting the gaseous planets closest to the star,” explains Cloutier. But the orbital dynamics of the system make it extremely unlikely that only one of the planets was born from the same disk.
Cloutier and his team found that the most likely way to create this system is through a process called “upside-down” planet formation. Here, a single planet forms and then migrates towards the star, making way for the next planet, and so on. This takes time, which is why planets are born in different environments as the protoplanetary disk evolves. “If it takes long enough, this last planet formed in an environment where there is no gas available,” says Cloutier. This system shows how diverse planetary formation processes in the universe could be, he says.
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