An international scientific team, led by the University of Warwick and involving the Canary Islands Institute of Astrophysics, has used the European Space Agency's Cheops satellite to discover that the planetary system around the star LHS 1903 challenges current planet formation theories with the unusual order of its planets. Surprisingly, the most distant outer planet might be rocky and seems to have formed later – in a different environment than the other planets around the star. The study is published in the prestigious journal Science.

In our Solar System, the inner planets (Mercury to Mars) are rocky, and the outer planets (Jupiter to Neptune) are gaseous. This planetary pattern – rock then gas - is consistently observed across the Milky Way. That was, until an international team of scientists, led by Dr. Thomas Wilson Assistant Professor in the Department of Physics, University of Warwick, took a closer look at a star called LHS 1903. Their observations reveal a system of four planets that breaks this convention.

Based on observations from ESA's Cheops (CHaracterising ExOPlanet Satellite) satellite, the team has found that the planetary system of the star LHS 1903 does not fit current models of planetary formation. The reason for this is the atypical arrangement of its worlds: unexpectedly, the outermost planet could be rocky, and everything points to it having originated at a later stage and under conditions different from those of the other planets in the system.

The four planets of LHS 1903

The planets around LHS 1903 (a cool faint red dwarf star) begin with a rocky planet orbiting close by and then two gas worlds– the expected planetary pattern. However, using ESA’s CHaracterising ExOPlanet Satellite (CHEOPS), the astronomers saw a surprising fourth planet at the system’s outer edge. And this outermost planet was rocky, not gaseous. "This strange disorder makes it a unique inside-out system” says first author Dr. Thomas Wilson. “Rocky planets don’t usually form far away from their home star, on the outside of the gaseous worlds.”

Traditional models suggest that the closest planets to stars are rocky because stellar radiation sweeps away their gaseous atmospheres, leaving dense, solid cores behind. Gas giants form farther out in cooler regions where gas can accumulate, and planets can hold onto it. Yet the distant rocky world orbiting LHS 1903 appeared to have either lost its gaseous atmosphere or never formed one.

Taking it one planet at a time

Thomas and his colleagues set out to explore various explanations for this rogue rocky planet. Could the rocky and gaseous planets have swapped places? Or had the rocky planet lost its atmosphere in a collision? They ruled out those theories. Instead, they found evidence that the four planets did not form at the same time but instead formed one after another— a process called inside-out planet formation - which could explain the rocky planet.

According to our current understanding, planets form from discs of gas and dust around stars (called protoplanetary disc). The gas and dust clumps together into planetary embryos all at roughly the same time. These clumps then evolve into planets of different sizes and compositions over millions of years, but all the planets began forming in the same environment at the same time.

In contrast, the most compelling theory to explain the rocky planet says that LHS 1903 gave birth to its four planets one after another from the inner to outermost planet, instead of bearing quadruplets at once. This means that each planet evolved separately, sweeping up nearby dust and gas, with further out worlds waiting their turn for the next planet to form.

Dr. Thomas Wilson explains what this means for the rocky planet: “By the time this final outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. 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.”

A late bloomer from another era

Much like how younger siblings grow up in a world that is different from their elders, this small rocky planet seems to have evolved and formed in a very different environment than its older sibling-planets - in an outer area of the disc where gas had already run out.

Isabel Rebollido, Research Fellow at ESA points out: “Historically, our planet formation theories are based on what we see and know about our Solar System. As we are seeing more and more different exoplanet systems, we are starting to revisit these theories.”

Maximilian Günther, Cheops project scientist at ESA adds: “Much about how planets form and evolve is still a mystery. Finding clues like this one for solving this puzzle is precisely what CHEOPS set out to do.”

"This small rocky outer planet might still be an odd exception, or it might be the first clue to a new pattern in how planetary systems evolve. Either way, it is a discovery begging for an explanation that lies beyond our typical understanding of how planets form.

For Enric Pallé, researcher at the IAC and co-author of the study, “this small rocky outer planet could remain a strange exception, or it could be the first clue to a new pattern in the evolution of planetary systems. In any case, it is a discovery that demands an explanation that goes beyond our usual understanding of how planets are formed.”

In addition to Pallé, Roi Alonso, Felipe Murgas, Hannu Parviainen, and Emma Esparza-Borges from the IAC also collaborated on the study.

Article: Thomas Wilson et al. “Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903”. Science, 2026. DOI: 10.1126/science.adl2348

Contact at the IAC:
Enric Pallé, epalle [at] iac.es (epalle[at]iac[dot]es)