Faraway solar system challenges theories of how planets form

(CN) — For decades, astronomers thought they had planet formation figured out. Rocky worlds huddle close to their stars. Gas giants circle farther out. It's a pattern repeated across our galaxy, a cosmic rule modeled on our own solar system.

Until now.

An international team of astronomers led by the University of Warwick has discovered a planetary system that flips the script. Orbiting a faint red dwarf star called LHS 1903, four planets arrange themselves in an unexpected order: rock, gas, gas and then, impossibly, rock again.

"This strange disorder makes it a unique inside-out system," said Dr. Thomas Wilson, assistant professor in the Department of Physics at the University of Warwick and first author of the study. "Rocky planets don't usually form far away from their home star, on the outside of the gaseous worlds."

The discovery, published Thursday in the journal Science, came through observations from the European Space Agency's CHaracterising ExOPlanet Satellite, known as CHEOPS. The telescope revealed the fourth planet at the system's outer edge, a rocky world sitting where conventional wisdom says only gas giants should exist.

In our solar system, the inner quartet — Mercury, Venus, Earth and Mars — are all rocky. The outer planets, Jupiter, Saturn, Uranus and Neptune, are gaseous. This arrangement makes physical sense. Close to a star, intense radiation blasts away lighter gases, leaving only dense cores behind. Farther out, in cooler regions, planets can accumulate and retain thick gaseous envelopes.

The rocky outer planet circling LHS 1903 appears to violate these principles. It either lost a gaseous atmosphere it once had or never formed one at all.

Wilson and his colleagues explored potential explanations. The planets may have migrated, swapping positions after formation. The outer planet may have lost its atmosphere in a violent collision. They ruled out both scenarios.

Instead, the evidence points toward something more fundamental. The planets did not form simultaneously. Instead, they assembled sequentially through a process called inside-out planet formation.

In this model, LHS 1903 gave birth to its planets one after another, starting from the innermost and working outward. Each planet swept up nearby dust and gas as it formed, altering the environment for the next planet waiting in line.

"By the time this final outer planet formed, the system may have already run out of gas, which is considered vital for planet formation," Wilson said, "Yet here is a small, rocky world defying expectations. It seems that we have found first evidence for a planet that formed in a gas-depleted environment."

This rocky outer planet might be a fluke, or it could represent a previously unknown pathway in planetary evolution. The discovery forces astronomers to confront the limitations of theories built mostly on observations of our own solar system.

"Historically, our planet formation theories are based on what we see and know about our Solar System," said Isabel Rebollido, research fellow at ESA. "As we are seeing more and more different exoplanet systems, we are starting to revisit these theories."

Since the first exoplanet discovery in 1995, thousands of worlds have been cataloged, each offering new insights and leading to new questions.

"Much about how planets form and evolve is still a mystery," said Maximilian Günther, CHEOPS project scientist at ESA. "Finding clues like this one for solving this puzzle is precisely what CHEOPS set out to do."

Follow @JeremyYurow