What happens when a planetary system refuses to follow the usual script? Astronomers say they have found exactly that around LHS 1903, a small red dwarf about 116 light-years from Earth, where four planets circle in an order that current formation models did not expect.
Instead of a neat progression from rocky worlds near the star to gas-rich worlds farther out, this system seems to go rocky, gas, gas, and rocky again, and the team says that odd layout could be one of the strongest signs yet that some planets form one by one in a gas-depleted disk.
That final planet is the one pulling most of the attention. Called LHS 1903 e, it completes an orbit in 29.3 days, has a mass of 5.79 Earths, a radius 1.732 times Earth’s, and appears to be a rocky super-Earth with no thick gaseous envelope, even though it sits farther from the star than two puffier neighbors.
In a field where astronomers usually sort small worlds into rocky planets and atmosphere-rich ones, this one looks like the piece that landed in the wrong box.
A planetary lineup nobody expected
For the most part, astronomers see a familiar pattern when they study planetary systems. Closer to a star, intense radiation tends to strip away gas and leave denser rocky planets behind, while farther out, cooler conditions make it easier for planets to keep thick atmospheres.
That is the broad logic behind our own Solar System, and it is also the pattern many exoplanet systems follow.
LHS 1903 seemed to fit that picture at first. Earlier observations had already revealed three planets, with an inner rocky world and two farther-out planets with extended atmospheres, but CHEOPS later exposed a fourth outer planet that appears rocky too, and the full system spans orbital periods from 2.2 to 29.3 days.
Textbook diagrams usually look tidy, but this one looks more like a shuffled deck.
Why the outer planet matters so much
The outer world, LHS 1903 e, is not just another exoplanet added to a crowded catalog. Its measured density points to a compact, rocky planet, and as Thomas Wilson, the study’s lead author, put it, “Rocky planets don’t usually form so far away from their home star.”
That simple line gets to the heart of the discovery, because standard models say planets forming farther out should find it easier to gather and keep gas.
So what happened here? The researchers tested two obvious ideas: that the planets may have traded places over time or that the outer planet might have lost its atmosphere after a giant collision, and they found those explanations did not fit the system well enough. That leaves a more intriguing possibility, and that is where this story really gets interesting.
A possible case of planets forming one at a time
The team’s favored explanation is called inside-out planet formation. In that scenario, planets do not all grow together inside the same gas-rich environment. Instead, they form sequentially, with inner planets taking shape first and sweeping up nearby material while later worlds are left to form under different conditions.
In practical terms, that means LHS 1903 e may have arrived late, after much of the gas in the protoplanetary disk had already disappeared. That would help explain why a planet with roughly 5.79 Earth masses ended up rocky instead of gas-wrapped, and why the authors describe it as possible evidence for formation in a gas-depleted environment.
It may sound like a small technical detail, but it cuts right into one of the biggest questions in exoplanet science, namely how common planetary blueprints are actually built.
Why this faraway system matters back on Earth
No, LHS 1903 is not offering a second Earth. The two rocky planets are both larger than Earth and circle their dim red dwarf in just 2.2 and 29.3 days.
What matters here is the challenge they pose to theories largely built from our own Solar System, and as ESA researcher Isabel Rebollido put it, “we are starting to revisit these theories.”
That is why LHS 1903 could become a valuable benchmark for future work. Astronomers now have a compact system that contains both rocky planets and atmosphere-rich worlds, along with a concrete case that may show what happens when a later planet forms after the gas is mostly gone.
The study was published in Science.
