A new discovery of an exoplanetary system, located about 116 light-years from Earth, is challenging our understanding of planet formation. Researchers from NASA and the European Space Agency (ESA) found this system, which includes four planets orbiting a red dwarf star named LHS 1903.
What stands out is the unique arrangement of these planets. The closest one is rocky, followed by two gaseous planets, and surprisingly, the outermost planet is also rocky. This setup goes against what we typically see in our solar system and others, where rocky planets are usually closer to the star, and gas giants are found farther out.
Astronomers often believe that planets form within a disk of gas and dust surrounding a star. In this warmer inner region, lighter compounds like water vaporize, allowing only denser materials like rock and iron to form solid planets. That’s why rocky planets are generally found near their stars.
Farther out, beyond the “snow line,” temperatures are low enough for gases to freeze into ice, aiding the formation of gas giants like Jupiter and Saturn. This process means that larger planets form quickly, pulling in vast amounts of gas as they grow.
Thomas Wilson, a physics professor at the University of Warwick, noted, “Typically, we see rocky planets close to the star, but here, we have a rocky planet far away from its host star.” This is a pivotal finding that suggests a different formation process could be at play.
The unusual rocky planet, named LHS 1903 e, is about 1.7 times the size of Earth, qualifying it as a “super-Earth.” This raises questions about how it ended up so far from the star when similar planets in other systems typically follow the more expected patterns.
Wilson suggests that this system’s planets formed in distinct environments. He believes that LHS 1903 e likely formed later than the inner planets, when there was less gas and dust remaining for the formation of new planets.
The planetary system was identified using NASA’s Transiting Exoplanet Survey Satellite (TESS), which was launched in 2018. Follow-up data was gathered using ESA’s CHaracterising ExOPlanet Satellite (Cheops), launched in 2019. This collaboration highlights the importance of international efforts in astrophysics.
After confirming the strange arrangement, scientists tested various theories to explain LHS 1903 e. They considered whether it might have formed from a collision with another planet or emerged from a gas-rich planet that lost its shell. However, these ideas were not supported by their data.
Instead, the researchers propose a “gas-depleted” formation mechanism. This means that planets formed one after another, starting with the innermost planet, moving outward. Because LHS 1903 e formed later, there was not much material left for its construction.
In our solar system, the gas giants formed first, followed by rocky planets. LHS 1903 e adds a new twist to this familiar story, suggesting that its formation might require a new perspective on how planets develop around red dwarf stars.
Sara Seager, a planetary science professor at MIT, calls this finding “some of the first evidence for flipping the script” on planetary formation. Yet, she acknowledges that this interpretation is complex and further research is necessary to fully understand it.
The unique nature of this system makes it a valuable case for studying planetary evolution. Heather Knutson, a professor at Caltech, believes LHS 1903 e could host various atmospheres and be capable of supporting water, making it an exciting candidate for observation by the James Webb Space Telescope.
Ana Glidden, a postdoctoral researcher at MIT, emphasizes that this system can serve as a natural laboratory to study planet formation around stars different from our sun. Understanding these mechanisms can illuminate the diverse pathways of planetary evolution.
Néstor Espinoza, an astronomer at the Space Telescope Science Institute, adds that this discovery will keep scientists busy debating and analyzing for years. It may not only challenge existing models but also enrich our understanding of the universe.
