A Planetary System That Formed in the Wrong Order Challenges Standard Models

The story of how planets form has been told with some confidence for decades. Rocky planets assemble close to their star, where intense radiation strips away gas and leaves only solid material. Gas giants form further out, where conditions are cool enough for thick atmospheres to accumulate. Our solar system fits this pattern reasonably well, and observations of thousands of other planetary systems have broadly confirmed it.

The four-planet system around LHS 1903, a small red M-dwarf star in our cosmic neighborhood, does not fit this pattern. And the anomaly at its outer edge, identified using ESA's CHaracterising ExOPlanet Satellite (Cheops), is generating serious attention among researchers who study how planets are born.

The unexpected fourth planet

Thomas Wilson at the University of Warwick led an international team that had classified three of LHS 1903's planets using data from ground-based and space-based telescopes. The innermost planet appeared rocky, consistent with expectations. The next two were gaseous. Normal enough.

It was only when the team analyzed Cheops observations that a fourth planet appeared - the furthest from the star. When they characterized it, the result was surprising: the planet appeared to be rocky.

"That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-and then rocky again," Wilson said. "Rocky planets don't usually form so far away from their home star."

The anomaly sits directly where current formation theory predicts a gas-rich environment should produce, at minimum, a gas-rich planet. Instead, there is a small rocky world.

Ruling out simpler explanations

Before concluding that a new formation mechanism is needed, Wilson and his colleagues tested more mundane explanations. Could a giant impact - an asteroid or comet collision - have stripped the outer planet's original atmosphere? Could the planets have migrated, swapping positions at some point in the system's history? The team ran simulations and examined the orbital timing of the four planets. Neither scenario survived scrutiny.

The ruling-out process is significant. It means the rocky outer planet is probably not a normal gas planet that underwent some secondary modification. It appears to have formed as a rocky planet - and to have done so in conditions that should not have permitted it.

Inside-out formation: a decade-old idea gets its strongest evidence

The explanation that best fits the data draws on a theoretical model proposed roughly a decade ago called inside-out planet formation. In the standard view, all of a star's planets form simultaneously from a disc of gas and dust - a swirling protoplanetary disc that dissipates over a few million years. In inside-out formation, planets form sequentially, one after another, from the inside of the disc outward. By the time the outermost planet begins to assemble, the inner planets are already mature.

For LHS 1903, this sequence implies the four planets did not form at the same time. The inner rocky planet formed first, followed by the two gas giants. The outer rocky planet formed last - but by then, the disc's gas had been largely consumed or dissipated by the already-formed gas giants and by stellar radiation. The small outer world assembled from whatever solid material remained in what Wilson calls a "gas-depleted environment."

"By the time this outer planet formed, the system may have already run out of gas," Wilson said. "Yet here is a small, rocky world, defying expectations. It seems that we have found first evidence for a planet which formed in what we call a gas-depleted environment."

Cheops's role and the limits of one observation

ESA's Cheops is designed specifically to characterize known exoplanets rather than discover new ones, using ultra-precise photometry to measure planet sizes during transits. Its sensitivity to small planets makes it well suited to detecting a faint outer world that other surveys might overlook. The satellite's science team has been clear that a single anomalous system is not sufficient to revise formation theory - but it is sufficient to demand an explanation.

"Much about how planets form and evolve is still a mystery," said Maximilian Gunther, ESA's Cheops project scientist. "Finding clues like this one for solving this puzzle is precisely what Cheops set out to do."

The broader context matters here. As exoplanet surveys accumulate more systems, the solar system's ordered sequence of small-rocky-then-large-gaseous planets increasingly looks like one outcome among many. LHS 1903 adds a category that may turn out to be more common than its novelty currently suggests.

The paper describing the findings, "Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903" by T.G. Wilson et al., was published in Science on February 12, 2026.