Astronomers have discovered a surprising planetary system that does not follow the usual rules of how planets form.
The finding, led by researchers from the University of Warwick using a European Space Agency telescope, reveals a distant rocky planet orbiting far from its star—something scientists rarely see.
In our solar system, the pattern is clear. The inner planets, such as Mercury, Venus, Earth, and Mars, are small and rocky. Farther out, the planets are much larger and made mostly of gas, like Jupiter and Saturn.
This arrangement has been considered typical across the galaxy. But observations of a faint red dwarf star called LHS 1903 have uncovered a system that breaks this pattern.
The system contains four planets. Closest to the star is a rocky planet, followed by two gas-rich worlds, which fits expectations.
However, the outermost planet is also rocky instead of gaseous. This unexpected arrangement has been described as an “inside-out” system because it reverses the usual order of planet types.
Scientists used the European Space Agency’s CHEOPS space telescope to study the planets in detail.
The discovery puzzled researchers because rocky planets are not thought to form far from their stars. Typically, strong radiation from a star strips gas from nearby planets, leaving behind solid rocky cores. Farther away, where temperatures are cooler, planets can hold onto gas and grow into giants.
The team considered several explanations for the unusual system. They wondered if the planets had swapped positions over time or if the outer rocky planet had lost its atmosphere in a violent collision. However, evidence did not support those ideas. Instead, the researchers proposed that the planets may have formed one after another, starting close to the star and moving outward in a process known as inside-out planet formation.
In this scenario, each new planet forms from the leftover material after earlier planets have already taken what they need. By the time the outermost planet formed, the system may have run out of gas, leaving only rocky material behind. This could explain why the distant planet never became a gas giant.
The discovery is important because it suggests that planetary systems can evolve in more diverse ways than scientists once believed. For decades, theories about planet formation were based largely on our own solar system. Now, as astronomers find more unusual exoplanets, those ideas are being reconsidered.
Researchers say this rocky outer world could be a rare exception or the first example of a new type of planetary system. Either way, it highlights how much remains unknown about how planets form and evolve.
The CHEOPS mission was designed to study exoplanets and uncover clues about their origins. Discoveries like this one are helping scientists piece together the complex story of how planetary systems develop across the universe, showing that nature often has surprises that challenge long-held assumptions.
