Astronomers discover 'unique inside-out system' with a rocky planet far from where it belongs

Astronomers have spotted an unusual planetary system with a rebellious rocky planet orbiting well beyond where it would be expected to form.

Typically, rocky planets develop closer to a star, and gaseous planets form farther out — as is the case in our solar system, with a vast asteroid belt drawing the line between rocky Mars and gassy Jupiter.

And in the newly spotted system of four planets, the first three follow this pattern. But the system also has a rocky planet orbiting at its outer edge, beyond the two gas planets. That suggests the planet formed after the gas in the protoplanetary disk was already used up, researchers reported Feb. 12 in the journal Science.

"This strange disorder makes it a unique inside-out system," study co-author Thomas Wilson, an exoplanet scientist at the University of Warwick in the U.K., said in a statement. "Rocky planets don't usually form far away from their home star, on the outside of the gaseous worlds."

A rebel world

As the planets in our solar system formed and grew, they accreted dust, metals and other solids. Far from the sun, materials such as water and methane remained solid due to the cooler temperatures. As the outer planets got larger, they pulled in hydrogen and helium from their surroundings and grew into gas giants.

"Historically, our planet formation theories are based on what we see and know about our Solar System," study co-author Isabel Rebollido, a researcher who studies exoplanet systems at the European Space Agency (ESA), said in the statement. "As we are seeing more and more different exoplanet systems, we are starting to revisit these theories."

In the new study, Wilson and colleagues searched for exoplanets surrounding the red dwarf star LHS 1903, which had been observed by the Transiting Exoplanet Survey Satellite between 2019 and 2023. When a planet passes between the star and the satellite, it dims the star's light slightly. Then, they studied the properties of those planets, using the Characterizing Exoplanets Satellite (CHEOPS).

The scientists spotted four exoplanets orbiting the star, including the distant rocky one. The planets likely did not form at the same time, the team found; instead, they developed one by one and changed the environment in which each subsequent planet grew.

"By the time this final outer planet formed, the system may have already run out of gas, which is considered vital for planet formation," Wilson said in the statement. "Yet here is a small, rocky world, defying expectations. It seems that we have found [the] first evidence for a planet that formed in a gas-depleted environment."

The team further ruled out ideas that the distant rocky planet had lost its atmosphere in a collision or swapped places with a gas planet.

"Much about how planets form and evolve is still a mystery," study co-author Maximilian Günther, an astrophysicist at ESA, said in the statement. "Finding clues like this one for solving this puzzle is precisely what CHEOPS set out to do."