Children often learn the order of the planets in the solar system using quirky mnemonic phrases like “Educated mom just served me nachos.” These proverbs will help you remember Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
In our solar system, planets are divided into two main groups. From Mercury to Mars, the inner planets closest to the Sun are rocky worlds. Further outward are the gas giants from Jupiter to Neptune.
Astronomers have long believed that this arrangement reflects a common pattern throughout the universe. Influential theories of planet formation suggest that rocky planets form close to their stars, while gas giants develop further away, where cooler conditions allow thicker atmospheres to accumulate. Observations of many planetary systems generally support this idea.
But a newly studied star system called LHS 1903 may cast doubt on that understanding.
Strange planetary system around LHS 1903
LHS 1903 is a small, cool red dwarf star known as an M dwarf. It is darker and cooler than our sun. Researchers led by Thomas Wilson at the University of Warwick in the UK combined observations from several space and ground-based telescopes to study planets orbiting stars.
The research team initially identified three planets. The closest planet looked rocky, while the next two appeared gaseous. The arrangement matched what scientists expected.
The researchers then analyzed data from the European Space Agency’s signature ExOPlanet satellite (Cheops) and discovered something unexpected. A fourth planet, orbiting further away from the star, appeared to be rocky as well.
“So this is an inside-out system, and the order of the planets is rocky – gas – gas – then rocky again. Rocky planets don’t usually form very far from their host star,” Thomas said.
Current models suggest that rocky planets form near stars as intense radiation strips away the gas around the developing planet’s core. In distant locations, cold temperatures allow gas to accumulate in thick atmospheres, forming gas giants.
ESA Cheops project scientist Maximilian Günther said the discovery highlights how little scientists still understand about the origins of planets.
“There’s still a lot of mystery about how planets form and evolve. Finding clues like this to solve this puzzle is exactly what Khufus is trying to do.”
Scientists test possible explanations
The researchers did not immediately conclude that existing theories of planet formation were wrong. Instead, they investigated whether other events could explain this unusual system.
One possibility is that the outer rocky planets once had thick atmospheres that were later blown away by collisions with giant asteroids, comets, or other large objects. Another idea was that the planet may have changed position over time.
The research team used simulations and calculations of the planet’s orbital behavior to rule out those explanations.
Their research instead pointed to a more unusual possibility. The planets around LHS 1903 may not have formed at the same time. In fact, it may have evolved one after another.
The standard theory is that planets form within giant disks of gas and dust called protoplanetary disks. Multiple planetary embryos emerge at about the same time and gradually evolve into full-fledged planets over millions of years.
However, in this system, researchers believe that the stars may have formed the planets one after another, rather than all at once. Scientists proposed this idea, known as inside-out planet formation, about a decade ago, but there has been limited evidence to support it until now.
A rocky planet born late
The discovery is even more interesting because the outer rocky planets may have formed under very different conditions than their neighboring worlds.
“By the time this exoplanet formed, the planetary system may have already been depleted of the gases thought to be essential for planet formation, but here we have a small, rocky world that defies expectations. We seem to have found the first evidence that planets formed in what are called gas-starved environments,” Thomas said.
This unusual rocky planet may represent a rare cosmic oddity or reveal a broader trend that astronomers are not yet aware of. In any case, scientists say the system cannot be easily explained using current theories alone.
Rethinking planet formation
“Historically, our theories of planet formation have been based on what we see and know about the solar system,” points out Isabelle Rebolido, now a researcher at ESA. “As we observe more and more different exoplanet systems, we are starting to reconsider these theories.”
As telescopes become more powerful, astronomers continue to discover increasingly rare planetary systems across the galaxy. These strange worlds are forcing researchers to rethink their assumptions about how planets form and evolve.
The discovery could also change the way scientists view our solar system. Rather than representing a universal blueprint, its ordered structure may be just one example of many vastly different planetary configurations in the universe.
