LHS 1903: New Planet System Challenges Planetary Formation Theories

For decades, astronomy has built its models of planetary formation with a clear premise: systems follow a relatively predictable order – small, rocky worlds orbit close to their star. Further out reside gas giants, grown large by accumulating thick atmospheres in the cool regions of the protoplanetary disk. This logic fits our Solar System and thousands of exoplanets detected in recent years. However, a recent discovery has forced a re-evaluation of that schema.

An international team of over 150 astronomers has described a system around the red dwarf star LHS 1903 that challenges prevailing physical models. The research combined data from NASA’s TESS satellite, ground-based observatories in Mexico, the Canary Islands, and Hawaii, and the European Space Agency’s CHEOPS satellite. The result was the identification of four planets with an arrangement unlike anything previously known.

The star LHS 1903 is located approximately 120 light-years from Earth. It’s a type M red dwarf, cooler and less luminous than our Sun. These stars dominate the universe in number, though they are not visible to the naked eye from Earth. Orbiting it are four worlds with distinctly different characteristics.

The first planet, closest to the star, is rocky and shares similarities with Earth in terms of density. Then come two intermediate planets with significant gaseous envelopes, comparable in composition to Neptune, though somewhat smaller. Up to this point, the structure aligns with the classic pattern: an inner rocky world followed by gas-rich bodies.

The surprise came with the outermost planet. In that distant region, where models anticipate the presence of gas giants or bodies with thick atmospheres, lies a fourth, small, rocky planet, with a radius 1.7 times that of Earth and a mass approximately six times greater. Its density is similar to Earth’s, indicating a composition dominated by rock and metal.

“This system doesn’t fit into anything we knew before; it’s messy,” says astronomer Ignasi Ribas, a co-author of the study, published in the journal Science.

The planetary architecture is thus rocky, gaseous, gaseous, rocky – a sequence that deviates sharply from the established norm. Thomas Wilson, from the University of Warwick, put it directly: “That makes this system an inverted one – or ‘inside-out’ – with a rocky-gaseous-gaseous and then rocky again order. Rocky planets aren’t usually formed so far from their host star.”

Traditional theory holds that planets arise from a disk of gas and dust surrounding a young star. In the inner regions, intense radiation strips away much of the gas, leaving behind compact rocky cores. In the outer regions, the cold allows for the accumulation of thick atmospheres, giving rise to gas giants. It’s also assumed that all planets in a system form within a similar timeframe, within the first few million years.

The LHS 1903 system calls that simultaneity into question. Researchers analyzed various scenarios to explain the fourth rocky planet. They evaluated the possibility of a giant impact stripping away its atmosphere, and whether the planets exchanged positions over their orbital evolution. After dynamic simulations and precise calculations of orbital periods, those hypotheses were discarded.

The alternative that gained traction is more disruptive. The data suggests the planets didn’t form at the same time. Instead, they formed one after another. The first three planets arose in an early stage, when the disk still contained abundant gas. The fourth rocky planet appeared millions of years later, in an environment depleted of gas.

“By the time this outer planet formed, it’s possible the system had already run out of gas, which is considered vital for planet formation. However, here’s a small, rocky world that defies expectations. It seems we’ve found the first evidence of a planet that formed in what we call a gas-poor environment,” Wilson explained.

Ribas agrees with this temporal interpretation: “The three inner planets have formed earlier and have been enriched in gas when it was possible, and the external would have appeared some million years later.”

If this interpretation is correct, the LHS 1903 system experienced two episodes of planet genesis. The first produced a rocky world and two planets with gaseous envelopes. The second yielded a late-forming rocky planet, in a scenario lacking sufficient gas to form a giant. This sequence breaks with the idea that the protoplanetary disk definitively determines the system’s structure.

The conceptual impact isn’t limited to chronology. It also affects the understanding of the transition between gas-rich and gas-poor planets. Ribas summarizes: “There is a particularly great interest in understanding the architecture of planetary systems.” He adds, “This proves this transition between planets rich in gas and planets poor in gas. This represents what separates the planets that have large gas envelopes [the vast majority discovered so far] from those that have no gas envelope or a very small one, like our own Earth, because its atmosphere is really very small compared to that of the gas giants. This system is especially good for understanding where that limit, that transition, is.”

The finding gains greater relevance considering the statistical context. To date, more than 6,000 exoplanets and nearly 4,500 planetary systems have been identified. None have shown clear evidence of a planet forming after its companions.