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

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Astronomers have spotted an unusual 'inside-out' planetary system where a rocky world seems to have formed far beyond the realm typically reserved for gas giants.

An illustration of a series of planets in a semicircle with a sun-like star of glowing gas toward the right of the image. The planets in the background are bluer while the one planet in the foreground is yellow and orange
A rocky planet orbits LHS 1903 farther out than the gas planets in the system, a new study finds (Image credit: ESA CC BY-SA 3.0 IGO)

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.

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."

Skyler Ware
Skyler Ware
Live Science Contributor

Skyler Ware is a freelance science journalist covering chemistry, biology, paleontology and Earth science. She was a 2023 AAAS Mass Media Science and Engineering Fellow at Science News. Her work has also appeared in Science News Explores, ZME Science and Chembites, among others. Skyler has a Ph.D. in chemistry from Caltech.

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