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The researchers have identified four celestial bodies around this star. The three innermost planets follow an expected pattern: the first one is rocky, like Earth, and the next two are gas giants, similar to Jupiter.
LHS 1903 is a small red dwarf star that is cooler and less luminous than our Sun. Scientists used space and ground-based telescopes to discover four planets orbiting LHS 1903. With these telescopes, they classified the three planets closest to the star: the innermost is rocky, and the next two are gas giants.
Note that the distances and sizes of the planets are not to scale — the fourth outer planet is much smaller than the other three planets in the system.
Credit: ESA
The surprise comes from the fourth planet, located farther from the star. Unlike what is usually observed, this distant world is not a gas giant, but appears small and dense, likely rocky. This gives the system an unusual sequence: rocky, gaseous, gaseous, rocky.
This result questions established models. Indeed, in the majority of systems, like our own, rocky planets are found close to the star, because the intense heat drives away light gases. Farther out, lower temperatures allow gas giants to form by accumulating hydrogen and helium. The LHS 1903 system does not follow this rule.
Several hypotheses have been examined to explain this configuration. The researchers have ruled out the idea that the planets changed places or that the outer rocky planet lost its atmosphere in a collision. They suggest instead that the planets formed one after the other, from the inside out. Each new planet would then have absorbed the dust and gas around it, changing the environment for the next ones.
Thus, when the fourth planet formed, the system may have already exhausted its gas, an element necessary for creating gas giants. This observation indicates that planetary systems can evolve in more diverse ways than previously imagined. Studying other similar stars could therefore reveal new planetary architectures.
Methods for detecting exoplanets
The search for exoplanets, these planets orbiting other stars, relies on several techniques. One of the most common is the transit method, which observes dips in a star's brightness when a planet passes in front of it. This approach allows determining the planet's size and its distance from the star, providing clues about its composition.
Another important method is radial velocity, which measures the slight wobbles of the star caused by the gravitational pull of the planets. By analyzing these movements, scientists can estimate the planets' masses. Combined, these techniques offer a more complete picture of planetary systems.
To study LHS 1903, astronomers used both space telescopes, like the European Space Agency's CHEOPS, and ground-based instruments. This combination allows for gathering precise data on the position and characteristics of planets, even around low-luminosity stars like red dwarfs.
These technological advances make the discovery of atypical systems possible. By refining detection methods, researchers hope to find more planets in unexpected configurations.