Science & Technology (Commonwealth Union) – Astronomers from the University of Birmingham who joined hands with researchers from around the world to find a distant planetary system that goes against long-held ideas about how planets form.
The planets closest to the Sun in our Solar System, from Mercury to Mars, are rocky. The planets farther out, from Jupiter to Neptune, are mostly made of gas. This pattern of moving from solid inner worlds to gaseous outer giants has been seen all over the Milky Way—until now.
Using a telescope operated by the European Space Agency, the team focused on a star known as LHS 1903. Writing in the journal Science, they describe a four-planet system that defies this traditional structure. Located around 116 lightyears from Earth, it is the most distant planetary system of its kind currently observable within our galaxy.
The planets orbiting LHS 1903 — a dim, cool red dwarf — initially follow the expected layout, with a rocky planet nearest the star followed by two gaseous planets. However, observations from ESA’s CHEOPS (CHaracterising ExOPlanet Satellite) uncovered an unexpected fourth planet on the outer edge of the system that is rocky instead of gaseous.
Dr Ancy Anna John, a co-author of the study and member of the Birmingham research team, played a significant role in verifying the planets’ existence and determining their characteristics.
Reflecting on the find, Dr John said confirming the system’s uniqueness through his specialist analysis pipeline was a thrilling moment. He described it as feeling like being on the cutting edge of scientific discovery and said he was proud to collaborate with an international group of leading astronomers on research that challenges established ideas.
Conventional theories indicate that the planets closest to a star are typically rocky. Intense stellar radiation is thought to strip away lighter gases, leaving behind compact, solid cores. In contrast, gas giants are believed to form farther from their stars, in cooler regions where gas can gather and be retained. However, the rocky planet orbiting LHS 1903 appears either to have lost its gaseous envelope or to have formed without one altogether.
The international research team was led by first author Dr Thomas Wilson of the University of Warwick. He noted that the unusual arrangement makes the system effectively “inside-out”, as rocky planets are not normally expected to form beyond gas giants on the outer edges of a system.
To understand the anomaly, the scientists investigated several possible scenarios. They thought about whether the rocky and gaseous planets might have switched places or whether a collision could have taken the atmosphere away from the rocky planet. After ruling out those options, they found proof that the four planets did not form at the same time. Instead, they probably formed one after the other in a process called “inside-out planet formation,” which could explain why the rocky world is there.
Current models suggest that planets form within rotating discs of gas and dust that surround young stars. These discs are called protoplanetary discs. In these discs, material comes together to form planetary embryos at about the same time. Over millions of years, these embryos grow into planets of different sizes and compositions.
The co-author Dr Annelies Mortier, from the University of Birmingham, explained “Each planet evolved separately, sweeping up nearby dust and gas, with further out worlds waiting their turn for the next planet to form.”
The fourth, smaller rocky planet appears to have developed under markedly different conditions from the others, forming in the outer region of the protoplanetary disc after the surrounding gas had already been depleted.
Isabel Rebollido, who is a Research Fellow at ESA, says “Historically, our planet formation theories are based on what we see and know about our Solar System. As we are seeing more different exoplanet systems, we are starting to revisit these theories.”
