“Reverse” Solar System: Rocky Outer Planet Challenges Planet Formation Theories

A Planetary System That Defies Expectations

Our solar system is characterized by rocky planets close to the sun, like Earth and Mars, and gas giants further out, such as Jupiter and Neptune. Traditionally, astronomers believed many planetary systems might follow a similar pattern, though it was already known that significant diversity exists in exoplanet architectures.

Now, an international team of astronomers, led by the University of Warwick in collaboration with the National Institute for Astrophysics (INAF), the Italian Space Agency (ASI), and the Universities of Padua and Turin, has discovered a planetary system that partially challenges this pattern. The system, located in February 2026 observations of the Milky Way, presents an unusual configuration that is prompting a re-evaluation of planetary formation theories.

The “Inside-Out” System

The research, published in Science, focuses on LHS 1903, a small, faint, and cool red dwarf star approximately 116 light-years from Earth. Observations revealed a system resembling our own in its initial structure: a rocky planet orbiting close to the star, followed by two gas giants. However, using data from the European Space Agency’s (ESA) CHEOPS satellite, the team confirmed that the fourth planet, the furthest from the star, is also rocky. This configuration – rocky, gaseous, gaseous, rocky – is unusual and demonstrates that planetary systems can form in ways different from our solar system, though it’s not yet clear if this represents a universal pattern.

“This strange arrangement makes it a unique system,” said Thomas Wilson, the lead author of the study. “Rocky planets are not usually formed far from their host star, outside of the gas giants.”

Classical theories suggest that planets close to a star tend to be rocky because stellar radiation strips away their atmospheres, while gas giants form further out where lower temperatures allow them to retain gas. In this system, the outer rocky planet appears to have lost its atmosphere or never had one, and its existence indicates that planetary formation can occur even in gas-poor environments.

Sequential Formation and the Gas Supply

Initially, the researchers considered several hypotheses regarding the formation of this system, including the possibility that the planets had migrated from their original positions or that the outermost rocky planet had lost its atmosphere through a collision. However, they ruled out the latter when they found evidence that the four planets did not form simultaneously, but sequentially, from the inside out.

This sequential formation implies that each planet evolved in a different environment. By the time the outermost rocky planet formed, the system may have already run out of gas, making the formation of gas giants more difficult. “When this outermost planet formed, the solar system may have run out of gas, which is considered crucial for planet formation,” Wilson explained. “We have found the first evidence of a planet formed in a gas-poor environment.”

Implications for Planetary Formation Theories

The discovery not only identifies an unusual system but also offers clues about how other planetary systems different from our own might form. According to Isabel Rebollido, a researcher at ESA, “Historically, our theories about planet formation have been based on what we see and know about our own Solar System. As we observe more exoplanetary systems, we are beginning to revise our theories based solely on it.”

The findings suggest that the processes governing planet formation are more complex and varied than previously thought. The existence of a rocky planet so far from its star challenges the traditional understanding of where rocky planets can form and survive. It also raises questions about the role of gas availability in shaping planetary systems.

Further research will be needed to determine how common such “inside-out” systems are and to refine our models of planetary formation. The LHS 1903 system provides a valuable case study for understanding the diversity of planetary architectures in the galaxy and the conditions under which planets can form and evolve.

The discovery also highlights the importance of continued exoplanet research and the development of new observational techniques. The CHEOPS satellite, with its ability to precisely measure the sizes of exoplanets, played a crucial role in confirming the rocky nature of the outermost planet in the LHS 1903 system. Future missions, such as the James Webb Space Telescope, will provide even more detailed observations of exoplanets, allowing astronomers to probe their atmospheres and compositions and further unravel the mysteries of planetary formation.

Recent research also suggests that gravitational mechanics can play a role in how interstellar objects, like 3I/ATLAS, maneuver within our solar system. A hidden solar passage could facilitate a “reverse Solar Oberth maneuver,” allowing an object to decelerate and potentially redirect toward Earth. This type of gravitational interaction would be optimal for a spacecraft attempting to enter our solar system undetected, as noted by NASA in response to claims that 3I/ATLAS could be an advanced alien spacecraft. While unrelated to the LHS 1903 system, this illustrates the complex interplay of gravitational forces in our solar neighborhood.