Weird planetary systems inside out may have formed a world at a time

Astronomers have found a planetary system that appears to have formed inside out. While most systems, like our own, have rocky planets closest to their stars and gaseous planets further out, the LHS 1903 system has a rocky world at its edge, challenging established models of planet formation.

The outermost of the system’s four planets was not immediately visible in the first observations from the Transiting Exoplanet Survey Satellite – the first measurements allowed scientists to identify a rocky planet slightly larger than Earth near the star, plus two gaseous planets slightly smaller than Neptune beyond. But when Ryan Cloutier of McMaster University in Hamilton, Canada, and his colleagues monitored the system using eight other observatories, they discovered the telltale signatures of a fourth world marginally larger than the other rocky planet in the system.

This rocky world, which is further away from the star than its gaseous siblings, was unexpected. “These systems are not unheard of, but they are rare—and the systems that have this unique architecture, and for which we can characterize them in detail, are exceptionally rare,” says Cloutier.

These details, including the size of the planets and the fact that they all orbit their star for periods of less than 30 Earth days, allowed the researchers to test models of how these planets may have formed. “Producing one planet can be done with several mechanisms, but when you need to produce four different ones, you can start distinguishing between different models,” says 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 at roughly the same time from the same disc of dust and gas. The sizes and composition of the planets depend on where they formed in that plate and what events, such as collisions with other worlds, happened to them afterwards. However, for the LHS 1903 system, that model does not work.

If the planets of LHS 1903 were born in the traditional way, the outermost one should have formed with a thick gaseous envelope like the two middle ones. That atmosphere could have been lost through a collision or bombardment with radiation, but the researchers’ simulations show that such a process would also have removed the gas from one or both of the inner planets.

“It’s very difficult for you to sculpt the outermost planet without affecting the gaseous planets that are closer to the star,” says Cloutier. But the orbital dynamics of the system make it extraordinarily unlikely that any of the planets were not born from the same disk.

Cloutier and his team found that the most likely way to create this system is through a process called “inside-out” planet formation. Here a single planet is formed which then migrates inwards towards the star, giving way to the next planet, and so on. This takes time, so the planets are born in different environments as the protoplanetary disc evolves. “The last planet, if it’s taken long enough, it’s formed in an environment where there’s no gas available,” says Cloutier. This system shows how different the planetary formation processes in the universe can be, he says.

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