Astrophysics turned upside down by unique ‘inside out’ planetary system

University of Birmingham astronomers joined a global team of scientists to discover a distant planetary system that turns scientific understanding of planet formation upside down.

In the Solar System, the inner planets (Mercury to Mars) are rocky, and the outer planets (Jupiter to Neptune) are gaseous. This pattern – rock to gas – has been consistently observed across the Milky Way until now.

Using a European Space Agency (ESA) telescope, the international research team took a closer look at a star called LHS 1903. Publishing their observations in Science, the researchers reveal a system of four planets that breaks this convention. This is the furthest observable planetary system still in the Milky Way – some 116 lightyears away from Earth.

The planets around LHS 1903, a cool faint red dwarf star, begin as expected with a rocky planet orbiting close by and then two gas worlds. ESA’s CHaracterising ExOPlanet Satellite (CHEOPS) then reveals a surprising fourth planet at the system’s outer edge which is rocky, rather than gaseous.

Dr Ancy Anna John is a co-author of the paper, and member of the Birmingham research team which made a key contribution towards confirming the existence and properties of the planets.

Reflecting on the discovery, Dr. 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.”

Traditional models suggest 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 rocky world orbiting LHS 1903 appears to have either lost its gaseous atmosphere or never formed one.

The global team of astronomers was led by first author Dr Thomas Wilson from the University of Warwick, who said: “This strange disorder makes it a unique inside-out system. Rocky planets don’t usually form far away from their home star, on the outside of the gaseous worlds.”

The research team set out to explore various explanations for the rogue rocky planet. Could the rocky and gaseous planets have swapped places? Had the rocky planet lost its atmosphere in a collision? They ruled out those theories, but 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.

Current understanding says that planets form from discs of gas and dust around stars – a phenomenon called protoplanetary disc. Gas and dust clumps together into planetary embryos at roughly the same time. These clumps evolve into planets of different sizes and compositions over millions of years.

Co-author Dr Annelies Mortier, from the University of Birmingham, said: “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 small rocky planet seems to have evolved in a very different environment than the other planets in an outer area of the protoplanetary disc where gas had already run out.

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.”

Isabel Rebollido, Research Fellow at ESA, said: “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.”

Scientists are unsure whether this small rocky outer planet might be an odd exception or the first clue to a new pattern in how planetary systems evolve. The discovery hints at an explanation that lies beyond our typical understanding of how planets form.