This "reversed" planetary system that defies all our theories about the formation of worlds

An exception that challenges the standard model

For a long time, we believed we understood the universal blueprint for planetary formation. By observing our own solar system, a pattern seemed to emerge: rocky planets, whose icy and gaseous atmospheres have been blown away, orbit closest to their star, while gas giants form in distant outer regions. Of course, we now know that our system is not the only model for the entire cosmos.

Previous discoveries have already shown the existence of gas giants orbiting closer to their star than Mercury does to the Sun, as well as rocky worlds far more massive than Earth. However, the prevailing hypothesis remained that, within multiplanetary systems, the segregation between inner rocky planets and outer gas giants was the norm. Now, scientists have identified a system that defies this fundamental expectation, calling our certainties into question.

LHS 1903: A Puzzling Configuration

At the heart of this discovery lies LHS 1903, a star classified as a red dwarf—cold and low-luminosity. This system hosts four planets. So far, the observations seemed to follow a familiar pattern with the first planet, LHS 1903 b. This is the innermost world, a rocky super-Earth whose radius is 40% larger than that of our planet, and where temperatures are extremely high.

Things get more complicated with the next two planets, LHS 1903 c and LHS 1903 d. These planets are much larger and are classified as sub-Neptunian worlds. Their estimated density is consistent with that of gas giants. This is where the classical model begins to break down, but the real problem for theorists lies in the nature of the outermost planet, LHS 1903 e.

The Mystery of the “Sandwich” Planet

The planet LHS 1903 e is impressively large, with a radius about 73% larger than Earth’s. However, unlike its immediate neighbors, estimates of its density suggest that it is a rocky world. This is where the mystery lies: this system appears to consist of two gas giants sandwiched between two rocky worlds—a configuration we did not think possible.

Dr. Thomas Wilson, the study’s lead author and an assistant professor at the University of Warwick, highlighted the uniqueness of this discovery in a statement reviewed by IFLScience. He explains: “This strange arrangement makes it a unique, upside-down system. Rocky planets don’t usually form close to their star, on the outside of the gas giants.”

CHEOPS: The Tool That Revealed the Anomaly

This unique system was studied in detail using the CHEOPS (CHaracterising ExOPlanet Satellite) satellite. This is a European Space Agency (ESA) mission whose goal is to determine the size and properties of planets orbiting nearby stars. In this specific case, the instrument stumbled upon an anomaly that could upend our models of planetary formation.

Maximilian Günther, a CHEOPS project scientist at ESA, commented on this breakthrough: “Much of how planets form and evolve remains a mystery. Finding clues like this to solve that puzzle is precisely what CHEOPS set out to do.”

Isabel Rebollido, a research scientist at ESA, adds some historical context on the need for this reevaluation: “Historically, our theories about planet formation have been based on what we see and know about our solar system. As we observe more and more diverse exoplanetary systems, we are beginning to reevaluate these theories.”

Delayed Formation

How could such a system have come into being? Researchers are putting forward a bold hypothesis: the planets did not form simultaneously, but one after another, starting with the one closest to the star and moving outward. Each planet would have accumulated the available material around itself, but by the time LHS 1903 e formed, the building blocks were beginning to run out.

Dr. Wilson summarizes this scenario of material scarcity as follows: “By the time this outermost planet formed, the system may have already exhausted its gas reserves, which are considered vital for planet formation. Yet here is a small rocky world that defies expectations. It seems we have found the first evidence of a planet formed in a gas-depleted environment.”

Conclusion: The hunt is on

This study, which puts our astronomical knowledge into perspective, was published in the prestigious journal Science. The discovery of such a system suggests a high statistical probability: if one system of this type exists, others must necessarily exist elsewhere in the galaxy.

The hunt to track down these “inverted” systems is now underway. Astronomers will scan the sky with renewed attention, armed with the certainty that the universe still has many exceptions to reveal to us.

Source: iflscience.com

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This "reversed" planetary system that defies all our theories about the formation of worlds