Astronomers have identified a planetary system that challenges conventional understanding of how planets form. Orbiting the red dwarf star LHS 1903, this system features an unexpected arrangement: rocky planets both close to the star and in the outer reaches, separated by what would typically be gas giant territory. The discovery, made using data from space-based and ground-based telescopes, suggests that planetary systems can be far more diverse – and dynamically unstable – than previously thought.
LHS 1903, located roughly 116 light-years away, is a relatively cool and dim star, significantly smaller than our Sun. Initial observations revealed four planets orbiting the star. The three innermost planets were initially classified as rocky, while the two outermost were thought to be gas giants, mirroring the pattern seen in our own solar system and many others. However, further analysis revealed a surprising twist: the outermost planet appears to be rocky, not gaseous.
This “inside-out” configuration – rocky-gaseous-gaseous-rocky – is particularly puzzling. The prevailing theory of planet formation posits that intense radiation from young stars strips away gases from planets forming close in, leaving behind rocky worlds. Further out, cooler temperatures allow planets to retain their atmospheres, leading to the formation of gas giants. This explains the arrangement in our solar system, with Mercury, Venus, Earth, and Mars being rocky, and Jupiter, Saturn, Uranus, and Neptune being gas giants.
The discovery of a rocky planet in the outer orbit of LHS 1903 suggests that this process isn’t always straightforward. “Bad stuff does happen in young planetary systems,” says Andrew Cameron, an astronomer at the University of St. Andrews in Scotland, hinting at a history of gravitational upheaval within the system. The current arrangement suggests the system may have been “turned inside out” through some disruptive event.
The planets in the LHS 1903 system are relatively close to their star, all orbiting within less than 30 days. They range in size from approximately 1.4 to 2.5 times the radius of Earth, placing them in the range between super-Earths and mini-Neptunes. Precise measurements of their masses and densities, enabled by observations from instruments like NASA’s Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Cheops satellite, have been crucial in determining their compositions.
Cheops, specifically, played a key role in refining the understanding of the system. Observations from Cheops helped scientists classify the planets and identify the unusual order. The satellite’s ability to precisely measure the sizes of exoplanets was instrumental in this discovery.
The implications of this finding are significant. It suggests that planetary systems can form in a wider variety of configurations than previously assumed, and that gravitational interactions between planets can play a more significant role in shaping their final arrangement. The discovery challenges the long-held belief that rocky planets are always found closer to their stars and gas giants further out.
While the exact mechanism that led to this unusual configuration remains unclear, astronomers theorize that gravitational interactions between the planets could have caused them to migrate from their original positions. This migration could have resulted in the outer planet being pushed further out, where it was able to retain its rocky composition. Alternatively, the system may have experienced a period of instability, with planets colliding or being ejected, leading to the current arrangement.
The discovery highlights the complexity of planet formation and the need for more research to understand the diverse range of planetary systems that exist in the Milky Way. Further observations of LHS 1903 and other similar systems will be crucial in refining our models of planet formation and gaining a more complete understanding of how planets come to be.
The unusual nature of the LHS 1903 system also raises questions about the habitability of planets in such configurations. The presence of rocky planets in the outer reaches of the system could potentially provide habitable environments, but the specific conditions would depend on a variety of factors, including the planet’s atmosphere and its distance from the star. Further research will be needed to assess the potential for life on these worlds.
The research team, led by Thomas Wilson from the University of Warwick, combined data from multiple telescopes to reach their conclusions. This collaborative effort underscores the importance of combining observations from different instruments to gain a more comprehensive understanding of exoplanetary systems. The findings were reported on in Science and further detailed in reports released on by McMaster University.
