Helium escape from exoplanet LHS 1140b reveals atmospheric evolution
New data published in Nature indicates a helium loss rate of 100,000 kilograms per second, helping define the 'cosmic shoreline' in the LHS 1140 system.
Astronomers have detected helium escaping from the atmosphere of the rocky exoplanet LHS 1140b, located approximately 50 light-years from Earth. The findings, published in Nature, indicate an escape rate of approximately 100,000 kilograms of helium per second. This observation, derived from near-infrared imaging at the Las Campanas Observatory, suggests the planet has lost its original hydrogen envelope, leaving behind a helium-rich upper atmosphere.
The study provides critical insight into the 'cosmic shoreline' within the LHS 1140 system, a boundary distinguishing between planets that retain atmospheres and those that do not. The data places this shoreline between the inner planet, LHS 1140c, which receives significantly more radiation, and the outer planet, LHS 1140b. While helium was not detected during the transit of LHS 1140c, it was observed extending well beyond the radius of LHS 1140b, forming both leading and trailing tails.
Researchers used imaging hardware at the Las Campanas Observatory in the Atacama Desert to capture signals before, during, and after the transit of LHS 1140b. The presence of helium tails indicates that high-energy radiation from the host star, LHS 1140a, is driving the atmospheric escape. These optical observations were corroborated by X-ray imaging from the XMM-Newton satellite, which confirmed the energetic environment capable of stripping material from the planet.
The rate of helium loss offers clues about the planet's remaining composition. If substantial hydrogen were present, it would likely shield the helium from stellar radiation. The current escape rate suggests that any unreacted hydrogen was lost long ago, while heavier elements such as oxygen and nitrogen, along with molecules like water and methane, remain retained in the atmosphere. The study notes that the planet’s atmosphere may have been significantly larger in the past, given the higher activity levels typical of red dwarf stars in their early years.
Observations conducted one year after the initial detection failed to identify helium, indicating variability in the atmospheric loss. This fluctuation helps explain why the signal remains detectable despite the high escape rate. The research, led by a team of US-based scientists, underscores the complex dynamics of planetary evolution and the factors that determine whether a world retains an atmosphere or becomes an airless rock over billions of years.
