(Symbolbild). Around 6,300 confirmed exoplanets are now known to astronomy, yet the critical measurement for rocky worlds in habitable orbits has eluded researchers until now. The exoplanet LHS 1140 b, located 48 light-years away, orbits a cool red dwarf star in the constellation Cetus. A ground-based telescope in Chile has now provided the signal that researchers have awaited for two decades.
(Foto: © Forschung und Wissen)
A rocky planet located 48 light-years away delivers the first solid signal of an atmosphere within its star’s habitable zone. This detection was made not with a space telescope, but with a spectrograph on a mountain in Chile. A model had predicted this finding, and the measurement statistically confirmed it. In contrast, no comparable atmosphere was found on the sibling planet in the same system.
Detecting planets in distant solar systems is challenging, as they do not emit their own light and are often overshadowed by the brightness of their considerably larger star. However, astronomers have confirmed over 6,319 exoplanets through various methods, determining their size, mass, or orbital periods. When a planet passes in front of its star, some starlight filters through its gaseous envelope, causing specific wavelengths to diminish. From this subtle change in color composition, researchers can infer the atmospheric constituents of the planet. This technique works best for dense gas envelopes, while thin atmospheres provoke greater scientific interest due to their closer resemblance to Earth-like conditions. The complexity of such measurements is evident from past milestones, such as the first detection of an atmosphere around a rocky planet that was far from any habitable zone.
The habitable zone refers to the range around a star where pressure and temperature allow liquid water to exist on a planet’s surface. For a cool red dwarf star, this zone is very close to the star because its luminosity is only a fraction of solar output. This proximity makes these planets prime candidates for exoplanet research, as a solid surface and moderate temperatures are essential prerequisites for life as we know it. However, a third condition remained unfulfilled until now, as the absence of an atmosphere would preclude pressure regulation and heat transport, making it impossible for liquid water to exist on the surface. To date, all known rocky planets within habitable zones have failed to display a gas envelope, raising the question of whether the intense radiation from red dwarf stars could obliterate such atmospheres. A team from the Center for Astrophysics Harvard & Smithsonian, led by Collin Cherubim, has now filled this gap.
A Signal from 48 Light-Years Away
The exoplanet LHS 1140 b was discovered in 2017 in the constellation Cetus. It has about 5.6 times the mass of Earth and a radius 1.73 times that of Earth, classifying it as a super-Earth. Its central star is a red dwarf that it orbits every 24.7 days, remaining within the habitable zone despite the close proximity. Cherubim’s model predicted that this particular planet should possess a helium-rich gas envelope. Rather than waiting for observing time on the James Webb Space Telescope, the researchers turned to the WINERED spectrograph at the Magellan Observatory in Chile. The analysis, published in the journal Science, showed statistically significant results according to co-author David Charbonneau. The detection came from a ground-based instrument, significantly enhancing the methodology for future campaigns.
Why Helium Matters
The researchers measured helium, which escapes from the outer layer of the atmosphere into space, leaving a distinct infrared signature. Paradoxically, escaping gas indicates the existence of an atmosphere, as only a present atmosphere can lose material. Furthermore, this finding aligns with previous observations that disputed a hydrogen-dominated gas envelope and instead indicated a gas mixture with high average molecular weight. What is particularly insightful is the direct comparison within the system itself. The researchers found no atmosphere in the also-observed sibling planet LHS 1140 c, suggesting that neither a measurement artifact nor a star-specific characteristic created the signal. According to Robin Wordsworth, this discovery answers a question that has puzzled researchers for two decades: while Earth-like planets have been identified and located in habitable zones, it remained uncertain whether any of them could retain their gas envelope.
Outstanding Questions About Composition
What the atmosphere actually contains cannot yet be determined with the current data. Helium confirms the existence of the envelope but reveals little about its primary components; whether water vapor is present will require further observations. The researchers estimate that the planet is several billion years old, similar to Earth, implying that the gas envelope has withstood stellar winds over prolonged periods. This realization is significant as red dwarf stars are the most common type in the Milky Way, and their radiation has long been considered a probable atmospheric killer. This finding is part of a series of discoveries, including the search for potentially habitable exoplanets using spectral methods. While this does not directly indicate the presence of life, it does provide compelling evidence that the third condition for habitability may indeed be achievable.
Science, Study on the Helium Atmosphere of Exoplanet LHS 1140 b; doi:10.1126/science.aea9708
