Astronomers have discovered a planetary system that appears to flip one of astronomy’s most reliable rules on its head; it orbits a small, faint red star known as LHS 1903.
LHS 1903 is a cool M-dwarf star, smaller and far less luminous than our sun. Using a combination of space- and ground-based telescopes, scientists identified four planets circling the star. The three innermost planets initially seemed to follow a familiar pattern seen across the Milky Way, with the closest planet being rocky and the next two being gas giants.
However, when researchers took a closer look at a fourth, more distant planet using the European Space Agency’s (ESA) Characterizing Exoplanet Satellite (CHEOPS), they found something surprising. Despite orbiting farther from the star than the gas giants, the outermost planet appears to be small and dense — likely rocky, similar in composition to Venus. That makes the system’s architecture rocky–gaseous–gaseous–rocky, a highly unusual arrangement that is rarely observed in planetary systems, according to the study.
In most systems, including our own solar system, rocky planets are found close to the star, while gas giants form farther out. Traditional models suggest intense stellar radiation near a star strips away light gases, leaving behind dense, rocky cores. Farther out, cooler temperatures allow planets to accumulate and retain thick atmospheres of hydrogen and helium, forming gas giants.
The newly characterized outer planet around LHS 1903 appears to defy that expectation.
After considering whether the planets might have swapped positions or whether the outer rocky planet lost its atmosphere in a collision, the team ruled out those explanations. Instead, they found evidence that the four worlds may have formed sequentially in a process known as inside-out planet formation, according to the statement.
In this scenario, planets form one by one, starting close to the star. Each newly formed planet sweeps up surrounding dust and gas, evolving in a different environment and potentially leaving the outer regions depleted of the material needed to form gas giants. By the time the fourth planet formed, the system may have already run out of gas, leaving behind a small, rocky world.
“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 first evidence for a planet that formed in a gas-depleted environment.”
The discovery suggests that planetary systems may evolve in more diverse ways than previously thought. As astronomers continue studying stars like LHS 1903, they may uncover more systems that challenge long-standing ideas about how planets form and where different types of worlds belong.
Their findings were published in the journal Science.