Inside-Out Planetary System Challenges Planet Formation Theories
Scientists have discovered a rare “inside-out” planetary system around the red dwarf star LHS 1903, where a rocky planet orbits farther from the star than two gas giants. This surprising arrangement challenges traditional models of planet formation and suggests planets may develop one by one in changing environments.
The Standard Model of Planet Formation
Typically, when observing planetary systems, a pattern emerges: rocky planets orbit close to their star, while gas giants reside farther out. Our solar system exemplifies this, with Mercury, Venus, Earth, and Mars being rocky, and Jupiter, Saturn, Uranus, and Neptune being gas-rich. This layout is explained by the prevailing theory of planet formation.
Young stars emit radiation that strips lightweight gases from planets forming nearby, resulting in dense, rocky cores. At greater distances, cooler temperatures allow planets to retain atmospheres and grow into gas giants. European Space Agency’s Cheops satellite observations have widely confirmed this pattern.
LHS 1903: A Rule-Breaking System
The planetary system orbiting LHS 1903, a red dwarf smaller and dimmer than our Sun, defies this expected structure. Research published in Science details the findings. Researchers, led by Prof. Ryan Cloutier of McMaster University and Prof. Thomas Wilson of the University of Warwick, combined observations from Earth-based and space telescopes to analyze the system.
They initially identified three planets: a rocky world closest to the star, followed by two gas-rich planets similar to scaled-down Neptunes. This arrangement aligned with standard theories. However, further measurements, including modern observations from Cheops, revealed a fourth planet, LHS 1903 e, orbiting farthest from the star and appearing to be rocky.
“We’ve seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems. But now, the discovery of a rocky planet in the outer part of a system forces us to rethink the timing and conditions under which rocky planets can form,” says Cloutier, an assistant professor in the Department of Physics and Astronomy.
Exploring Alternative Explanations
The researchers investigated whether a massive impact could have stripped away a thick atmosphere from LHS 1903 e, or if the planets had migrated and swapped positions over time. Detailed computer simulations and orbital dynamics studies ruled out these possibilities.
Inside-Out Planet Formation
The team concluded that the planets around LHS 1903 may not have formed simultaneously, but sequentially, as the environment around the star changed. This “inside-out” planet formation scenario suggests that each planet’s composition depends on the local environment at the time of its formation.
By the time LHS 1903 e began to form, much of the surrounding gas in the protoplanetary disc may have dissipated, leaving insufficient material to build a thick atmosphere. EarthSky explains this process further.
“It’s remarkable to spot a rocky world forming in an environment that shouldn’t favour that outcome. It challenges the assumptions built into our current models,” Cloutier adds. He suggests that LHS 1903 may be an early example of a pattern scientists have yet to fully recognize.
“As telescopes and detection methods turn into more precise, we are strengthening our ability to uncover planetary systems that don’t resemble our own and that don’t conform to longstanding theories,” he says. “Each new system adds another data point to a growing picture of planetary diversity – one that forces scientists to rethink the processes that shape worlds across the galaxy.”
About LHS 1903
LHS 1903 is located approximately 116 light-years from Earth in the constellation Lynx. According to Wikipedia, it is a red dwarf star with a mass about 54% that of our Sun. The star hosts four known exoplanets, designated b, c, d, and e.