A global team of astronomers, led by the University of Warwick, have used a European Space Agency (ESA) telescope to discover a planetary system that turns our understanding of planet formation upside down, with a distant rocky world.
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 from the University of Warwick, took a closer look at a star called LHS 1903. Their observations, published in Science , reveal a system of four planets that breaks this convention.
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 the outermost planet was rocky, not gaseous.
“This strange disorder makes it a unique inside-out system” says first author Dr. Thomas Wilson, Assistant Professor in the Department of Physics, University of Warwick. “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.
A late bloomer from another era
Dr. Wilson and his colleagues set out to explore various explanations for how this rogue rocky planet was formed. 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, as would be expected, but formed one after another in a process called inside-out planet formation. If LHS 1903 gave birth to its four planets one after another, from the inner to outermost planet, then each planet evolves in turn, sweeping up nearby dust and gas and leaving further out planets to wait and potentially evolve in a different environment.
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.”
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.
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.”
ENDS
Notes to Editors
The paper ‘Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903’ is published in Science . DOI: 10.1126/science.adl2348
More information, including a copy of the paper, can be found online at the Science press package at https://www.eurekalert.org/press/scipak/ or contact scipak@aaas.org.
For more information please contact:
University of Warwick: Matt Higgs | Media & Communications Officer | Email: Matt.Higgs@warwick.ac.uk | Phone: +44(0)7880 175403
European Space Agency (ESA): Media Relations Team | media@esa.int
Additional Quotes
Dr. Ancy Anna John is a member of the University of Birmingham research team which made a key contribution towards confirming the existence and properties of the planets. Reflecting on the discovery,
Dr. Ancy Anna John said: “Confirming the uniqueness of this remarkable system using my specialised analysis pipeline was incredibly exciting. It truly felt like standing at the forefront of scientific discovery. I’m grateful to be part of an international team of outstanding astronomers working together on a discovery that challenges convention.”
Image
Title: Artist impression of the planetary system around the star LHS 1903
Caption: LHS 1903 is a small red M-dwarf star that is cooler and shines less brightly than our Sun. Scientists used telescopes in space and on Earth to discover four planets orbiting LHS 1903. With those telescopes, they classified the three closest planets to the star as the innermost being rocky, and the two that follow it gas giants.
In a recent study, scientists used ESA’s Cheops satellite to investigate the fourth planet in this system. They noted that it is much smaller and further away from its star than the other planets and discovered that it might be rocky, like Venus. This is surprising because a planet order of rocky-gaseous-gaseous-rocky in a planetary system is unusual and rarely observed in the Universe.
Note that the distances and sizes of the planets are not to scale – the outer fourth planet is much smaller than the other three planets in the system.
Alt Text: artist impression of a planetary system with four planets, around a small red star, called LHS1903.
Image description: This image shows a planetary system with four planets circling around a star. In the foreground, a large reddish-brown rocky planet with craters is visible. Behind it, there are three smaller planets: one with blue and white colours, another with blue and brown bands, and the third with a rocky surface. In the top right corner of the image, a small but bright red star illuminates the scene, with space dust and distant stars visible in the background.
Credit: ESA
Licence: CC BY-SA 3.0 IGO or ESA Standard Licence
About the University of Warwick
Founded in 1965, the University of Warwick is a world-leading institution known for its commitment to era-defining innovation across research and education. A connected ecosystem of staff, students and alumni, the University fosters transformative learning, interdisciplinary collaboration, and bold industry partnerships across state-of-the-art facilities in the UK and global satellite hubs. Here, spirited thinkers push boundaries, experiment, and challenge convention to create a better world.
Science
Experimental study
Not applicable
Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903
12-Feb-2026
The authors declare that they have no competing financial interests