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

The outermost of the system鈥檚 four planets wasn鈥檛 immediately apparent in initial observations from the Transiting Exoplanet Survey Satellite 鈥� those first measurements allowed researchers to identify one rocky planet a little bigger than Earth close to the star, plus two gaseous ones slightly smaller than Neptune beyond that. But when at McMaster University in Hamilton, Canada, and his colleagues followed up on the system using eight other observatories, they spotted the telltale signatures of a fourth world that is marginally bigger than the other rocky planet in the system.

This rocky world, which is further from the star than its gaseous siblings, was unexpected. 鈥淭hese systems are not unheard-of, but they鈥檙e rare 鈥� and the systems that have this unique architecture, and for which we can characterise them in detail, are extraordinarily rare,鈥� says Cloutier.

Those details, including the sizes of the planets and the fact that they all orbit their star in periods of less than 30 Earth days, made it possible for the researchers to test models of how these planets may have formed. 鈥淧roducing one planet can be done with several mechanisms, but once you need to produce four different ones, you can start to discriminate between different models,鈥� says at Leiden University in the Netherlands. 鈥淵ou have find a model that can explain all of them.鈥�

Most systems are thought to form all of their planets at about the same time from the same disc of dust and gas. The sizes and compositions of the planets are dependent on where they formed within that disc and what events, such as collisions with other worlds, happened to them afterwards. For the LHS 1903 system, though, that model doesn鈥檛 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 middle two. That atmosphere could have been lost through a collision or bombardment with radiation, but the researchers鈥� simulations show that such a process would have also stripped away the gas from one or both of the inner planets.

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鈥淚t鈥檚 really difficult for you to sculpt the outermost planet without affecting those gaseous planets that are closer to the star,鈥� says Cloutier. But the orbital dynamics of the system make it extraordinarily unlikely that any of the planets wasn鈥檛 born from the same disc.

Cloutier and his team found that the most likely way for this system to be created is through a process called 鈥渋nside-out鈥� planet formation. Here, a single planet forms and then migrates inwards towards the star, making room for the next planet, and so on. This takes time, so the planets are born in different environments as the protoplanetary disc evolves. 鈥淭hat final planet, if it鈥檚 taken long enough, it has formed in an environment where there鈥檚 no gas available,鈥� says Cloutier. This system goes to show how diverse the planetary formation processes in the universe might be, he says.

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