A strange rocky world lurks at the edge of an ‘inside-out’ planetary system discovered

Astronomers have discovered an unusual planetary system orbiting far beyond where rebellious rocky planets would be expected to form.

Rocky planets usually develop closer to their star, while gaseous planets form further out. This is the case in our solar system, where a vast asteroid belt draws a line between rocky Mars and gaseous Jupiter.

And in the newly discovered system of four planets, the first three follow this pattern. But the system also has a rocky planet orbiting its outer edge beyond the two gas giants. This suggests that the planets formed after the gas in the protoplanetary disk had already been used up, researchers reported in the February 12 issue of Science.

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“This strange obstacle makes this system a unique inside-out system,” study co-author Thomas Wilson, an exoplanet scientist at the University of Warwick in the UK, said in a statement. “Rocky planets don’t usually form outside of gaseous worlds far from their host stars.”

world of rebels

As the planets in our solar system formed and grew, dust, metals, and other solids were deposited on them. Farther away from the sun, substances such as water and methane remained solid because of the low temperatures. As the outer planet grew larger, it took in hydrogen and helium from its surroundings and grew into a gas giant.

“Historically, our theories of planet formation have been based on what we see and know about our solar system,” study co-author Isabelle Rebolido, a researcher who studies exoplanet systems at the European Space Agency (ESA), said in a statement. “As we observe more and more different exoplanet systems, we are starting to reconsider these theories.”

In a new study, Wilson and colleagues searched for exoplanets surrounding the red dwarf star LHS 1903, which was observed by the Transit Exoplanet Survey satellite from 2019 to 2023. When a planet passes between a star and a satellite, the starlight dims slightly. They then used the Exoplanet Characterization Satellite (CHEOPS) to study the properties of those planets.

Scientists have discovered four exoplanets orbiting this star, including a distant rocky planet. The researchers found that the planets likely did not form at the same time. Instead, they developed one by one, changing the environment in which each subsequent planet grew.

“By the time this last exoplanet forms, the planetary system may have already run out of gas, which is thought to be essential for planet formation,” Wilson said in a statement. “But here is a small, rocky world that defies expectations. It seems we have discovered it.” [the] This is the first evidence that planets formed in gas-depleted environments. ”

The researchers also ruled out the idea that distant rocky planets lost their atmospheres in collisions or swapped places with gaseous planets.

“Much remains a mystery about how planets form and evolve,” study co-author and ESA astrophysicist Maximilian Günther said in a statement. “Finding clues like this to solve this puzzle is exactly what CHEOPS is trying to do.”