Astronomers have discovered a planetary system that defies conventional understanding of how planets form. Orbiting the red dwarf star LHS 1903, located roughly 116 light-years away, are four planets arranged in an unexpected order: rocky, gaseous, gaseous, and then rocky, moving outward from the star. This configuration challenges existing models that predict rocky planets closer to the star and gas giants further out.
The discovery, reported on February 12 in the journal Science, suggests a history of significant upheaval within the system. “Bad stuff does happen in young planetary systems,” explains Andrew Cameron, an astronomer at the University of St. Andrews in Scotland. “This one has the look of something that’s been turned inside out.”
LHS 1903 is a relatively small star, about half the mass of our sun, and significantly cooler and dimmer. The four planets orbiting it are all relatively close in, completing orbits in less than 30 days. Their sizes range from approximately 1.4 to 2.5 times the radius of Earth, placing them in the category of super-Earths and mini-Neptunes.
The initial discovery was made by NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2019. Subsequent observations from both ground-based and space-based telescopes allowed scientists to precisely determine the planets’ masses and densities, providing clues about their composition.
The prevailing theory of planet formation posits that rocky planets develop closer to a star, where the intense heat prevents the condensation of gases. Further out, where temperatures are cooler, gas giants are expected to form, accumulating large atmospheres. The LHS 1903 system initially appeared to follow this pattern for its first three planets. However, the fourth planet, the outermost one, is rocky, a surprising finding.
“It would be like finding a world resembling Venus out past the orbit of Neptune,” researchers noted. This unexpected arrangement suggests that the outer planets of LHS 1903 underwent significant migration early in the system’s history.
Planetary migration is a well-established concept in our own solar system’s formation. It’s believed that gravitational interactions between Jupiter and Saturn caused them to shift their orbits inward, disrupting the arrangement of other planets and scattering asteroids. A similar process may have occurred at LHS 1903.
Several scenarios could explain the unusual configuration. A large object may have collided with the fourth planet, stripping away its atmosphere. Alternatively, gravitational scattering could have rearranged the planets, pushing the rocky planet outward and the gas giants inward. The fourth planet may have simply formed late in the system’s evolution, “just as the system ran out of gas,” according to Cameron.
The discovery of LHS 1903 provides a valuable opportunity to refine our understanding of planetary formation and the dynamic processes that shape planetary systems. It highlights the diversity of planetary arrangements that can exist and underscores the need for continued research to unravel the complexities of exoplanetary systems. Further observations and modeling will be crucial to determine the precise mechanisms that led to this “inside-out” configuration and to assess how common such systems might be in the galaxy.
The star LHS 1903 is a red dwarf, a type of star smaller and cooler than our sun. These stars are the most common type in the Milky Way galaxy, and they often host planetary systems. However, the conditions around red dwarfs are different from those around sun-like stars, and the formation and evolution of planets in these systems are still not fully understood.
The planets in the LHS 1903 system orbit their star very closely, much closer than the planets in our solar system orbit the sun. This is typical of planets orbiting red dwarfs, as the habitable zone – the region around a star where liquid water could exist on a planet’s surface – is much closer to the star for these cooler stars.
The research team continues to analyze data from the LHS 1903 system, hoping to gain further insights into its unique characteristics and the processes that shaped its planetary arrangement. This discovery serves as a reminder that our solar system may not be representative of all planetary systems in the universe and that there is still much to learn about the diversity of worlds beyond our own.
