An international team of astronomers has discovered a distant planetary system that challenges long-standing theories of how planets form.
Across our galaxy, astronomers routinely observe a characteristic pattern in planetary systems: rocky planets orbiting close to their host star with gas giants farther away. Our own Solar System follows this rule, with the inner planets — Mercury, Venus, Earth and Mars — composed of rock and iron, and the outer planets — Jupiter, Saturn, Uranus and Neptune — being predominantly gaseous.
This pattern stems from a well-established theory of planet formation: Intense radiation from the host star strips away gas accumulated by close-in planets, leaving behind bare rocky bodies. Further from the star, cooler conditions allow thick atmospheres to build, forming gaseous planets.
But a newly discovered planetary system orbiting the star LHS 1903 breaks this rule. The findings were published this week in Science.
LHS 1903 is a red dwarf star that is smaller and fainter than our Sun. The team, co-led by researchers Ryan Cloutier from McMaster and Thomas Wilson from the University of Warwick, used observations from ground and space-based telescopes to detect and classify three planets around LHS 1903.
They found one rocky inner planet, followed by two gaseous planets akin to miniature Neptunes — exactly what astronomers would expect.
After years of effort characterizing the three known planets around LHS 1903, new data from the European Space Agency’s CHEOPS satellite revealed something unexpected: a fourth planet known as LHS 1903 e, farthest from the star, that appears to be rocky.
“We’ve seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems. But now, the discovery of a rocky planet in the outer part of a system forces us to rethink the timing and conditions under which rocky planets can form,” says Cloutier, an assistant professor in the Department of Physics and Astronomy.
The researchers tested multiple explanations: Was the planet struck by a massive object that blew away an atmosphere? Did the planets shift their ordering over time? Numerical simulations and orbital analyses ruled out these possibilities.
The team’s analysis points to a more surprising possibility: The planets in the LHS 1903 system may not have formed all at once. Instead, they may have emerged sequentially, each under slightly different conditions as the system evolved.
Current models suggest that planets form within a protoplanetary disc — a mixture of gas and dust, where material can clump together into several planetary embryos at roughly the same time. Over millions of years, these bodies gradually develop into fully-formed planets of different sizes and compositions.
But the unusual architecture of LHS 1903 hints at a different process — one known as inside-out planet formation — where planets form sequentially, one after the other, in ever-changing environments. Their final compositions reflect the local conditions at the time of final assembly, which dictates whether they form gas-rich or rocky.
This theory can explain why LHS 1903 e appears so different from its neighbours. By the time it began forming, the disc surrounding the star may have already been depleted of the gas typically needed to build a thick atmosphere.
“It’s remarkable to see a rocky world forming in an environment that shouldn’t favour that outcome. It challenges the assumptions built into our current models,” says Cloutier.
The discovery raises broader questions about whether LHS 1903 is an anomaly or an early example of a pattern scientists have yet to recognize, he says.
“As telescopes and detection methods become more precise, we are strengthening our ability to find planetary systems that don’t resemble our own and that don’t conform to longstanding theories,” Cloutier says.
“Each new system adds another data point to a growing picture of planetary diversity — one that forces scientists to rethink the processes that shape worlds across the galaxy.”