Astronomers have now found a nearby planetary system that breaks that familiar pattern.

An international team led by Thomas Wilson of the University of Warwick used ESA's Characterising Exoplanet Satellite, Cheops, along with other space and ground based observatories, to study the red dwarf star LHS 1903 and its planets.

LHS 1903 is a small, cool M dwarf star that shines less brightly than the Sun.

Earlier work had identified three planets orbiting this star.

Measurements showed that the innermost of these worlds is rocky, while the two outer ones are gaseous planets with thick atmospheres.

Cheops observations revealed something unexpected: a fourth planet orbiting even farther from LHS 1903.

Follow up analysis indicates that this outermost planet is small and rocky, even though it resides where a gas rich planet would normally be expected.

The resulting architecture is rocky, gaseous, gaseous and then rocky again, an inside out order that defies standard planet formation models.

Current theories predict that close to a young star, intense radiation strips away surrounding gas, leaving behind compact rocky planets.

Farther out in the cooler regions of a protoplanetary disc, gas can accumulate around solid cores to form gas giants or gas rich worlds.

A rocky planet at large distance from its star therefore poses a puzzle.

The team examined whether violent events could explain the observations, such as a collision with a large asteroid, comet or planetary body that might have blown away a gaseous envelope from the outer planet.

They also considered whether the planets could have swapped orbits over time, reshuffling the system.

Simulations and orbital calculations did not support these scenarios, leading the researchers to favour a different explanation.

Their preferred picture is that the planets around LHS 1903 did not all form at once, but instead emerged one after another.

In the conventional view, planets grow from a disc of gas and dust as many planetary embryos form roughly simultaneously and then evolve over millions of years into planets of various sizes and compositions.

In contrast, LHS 1903 appears to host a system where the star may have given rise to planetary siblings sequentially, a process known as inside out planet formation.

That idea was proposed about a decade ago, but until now evidence for it has been limited.

In this case, the outer rocky planet seems to be a latecomer that formed in an environment very different from that of its inner siblings.

By the time this world took shape, the system may have been largely depleted of gas that is normally considered crucial for building planets with thick atmospheres.

The discovery suggests that planet formation can proceed in a gas depleted disc, producing a small rocky planet where a gas rich world would normally be expected.

This makes the outer planet either a rare outlier or an early example of a wider class of planets that current models do not yet fully capture.

Scientists involved in the work argue that such systems push them to revisit assumptions built from the Solar System alone.

Traditional planet formation theories were largely developed from what is known about the eight planets orbiting the Sun.

As new instruments find more diverse exoplanet systems, including ones that look nothing like our own, theorists are adjusting their models to accommodate this growing variety.

Cheops project scientist Maximilian Gunther notes that many aspects of planet formation and evolution remain poorly understood, and that systems such as LHS 1903 provide crucial clues.

The mission is designed to measure exoplanet sizes and orbits with high precision, helping researchers link planetary properties to their formation histories.

The LHS 1903 system adds to a list of so called weird planetary systems that challenge expectations.

Each of these discoveries forces astronomers to question how universal the Solar System's layout really is.

In turn, they help place the Sun and its planets in context within the wider population of planetary systems in the Milky Way.

The work also raises questions about how often inside out formation might occur and whether other systems host late forming rocky planets in gas poor regions.

As surveys continue and instruments improve, more examples may come to light.

That would allow astronomers to test whether LHS 1903 is unique or part of a broader trend that demands a substantial revision of planet formation theory.

Research Report: Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903

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