This Bizarre Star System Is Inside Out, and Astronomers Aren’t Sure How

Astronomers have discovered a star system that’s out of order, with a late bloomer planet that may have sprung up at a different time than its neighbors. The discovery defies the norms of planet formation across the cosmos, raising questions over how this weird rocky world broke a familiar pattern.

The inner planets of our solar system, Mercury, Venus, Earth, and Mars, are rocky worlds, while the outer solar system is reserved for gas giants like Jupiter, Saturn, Uranus, and Neptune. Other star systems in the Milky Way have followed the same pattern, with rocky planets forming closer to their host star and gas planets ending up further away.

That said, a closer look at a star called LHS 1903 revealed something out of the ordinary. The closest planet to the star is rocky, followed by two gas worlds. Everything seemed well and good, until the outermost planet of the system turned out to be a rocky world.

“This strange disorder makes it a unique inside-out system†Thomas Wilson, a physics professor at the University of Warwick and lead author of a new paper detailing the discovery, said in a statement. “Rocky planets don’t usually form far away from their home star, on the outside of the gaseous worlds.â€

Out of order

LHS 1903 is a small red M-dwarf star, which is cooler and less bright than our Sun. The team behind the recent study combined observations by different space telescopes to initially identify three planets orbiting around the star. It wasn’t until the scientists were analyzing observations made by the European Space Agency’s CHEOPS space telescope that they discovered a small, fourth planet.

As the team looked closer at the outermost planet of this star system, they were shocked to discover that it was in fact a rocky world. This order of rocky, gaseous, gaseous, rocky defies our current understanding of how planets form around their host star.

According to planet formation theories, the radiation from a star strips the closer planets of most of their gas, resulting in rocky worlds. Meanwhile, planets that form further out away from the star do so in conditions that are cool enough for gas to accumulate, resulting in a gaseous world.

The newly discovered rocky planet, however, either lost its gaseous atmosphere or may have never formed one to begin with.

Odd one out

The scientists behind the discovery set out to uncover the mystery behind the disorganized star system. They tested different scenarios through simulations, questioning whether the planets may have swapped places at some point or if the rocky planet could have been slammed by an asteroid so hard that it blew away its atmosphere.

Those theories did not pan out. Instead, the team found evidence that the planets may not have formed at the same time, but were instead birthed one after the other. Planets form from protoplanetary discs of gas and dust, the leftover material around a star. They typically arrive into the cosmos at the same time.

This star, however, may have delivered one planet after another instead of all of them clumping into different worlds at the same time. “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.â€

The runt of the litter may either be an odd exception to planet formation, or the first indication of a different pattern across the cosmos.

“Historically, our planet formation theories are based on what we see and know about our solar system,†Isabel Rebollido, a research fellow at ESA, said in a statement. “As we are seeing more and more different exoplanet systems, we are starting to revisit these theories.â€