Proxima Centauri b is the closest known exoplanet to our solar system, orbiting Proxima Centauri just over four light-years away. This planet has captured scientific interest because it circles its parent star within the region where liquid water could potentially exist on its surface. The question of whether this rocky world is potentially habitable hinges on the presence of a stable atmosphere. Scientists are focused on determining if that atmosphere contains oxygen, the gas considered most indicative of life as we know it.
What We Know About Proxima Centauri b
Proxima Centauri b is estimated to have a minimum mass of approximately 1.07 times that of Earth, suggesting it is a rocky or terrestrial world. Its orbital period is incredibly short, completing a full revolution around its star in only 11.2 Earth days. This close orbit places the planet squarely within the habitable zone of its dim, red dwarf star.
The planet’s distance from Proxima Centauri is about 0.05 Astronomical Units, roughly one-twentieth the distance between Earth and the Sun. Despite this proximity, the star is so small and faint that the planet receives about 65% to 70% of the energy Earth receives from the Sun. This energy input is theoretically sufficient to maintain liquid water if other conditions, such as a stable atmosphere, are met.
The Search for Atmospheric Oxygen
Oxygen is considered a primary biosignature because, on Earth, its high abundance is overwhelmingly a byproduct of life, specifically photosynthesis. While oxygen can be produced abiotically through processes like the breakdown of water molecules by ultraviolet light, the quantities found in Earth’s atmosphere are sustained by a global biological engine. A significant detection of oxygen, especially alongside other gases like methane, would therefore suggest a robust biological process at work.
To search for atmospheric gases, scientists use a method called transit spectroscopy, which requires the planet to pass directly in front of its star from our perspective. When this transit occurs, starlight travels through the thin ring of the planet’s atmosphere before reaching our telescopes. Specific molecules in the atmosphere absorb light at characteristic wavelengths, allowing researchers to build a chemical fingerprint of the atmosphere.
Unfortunately, Proxima Centauri b is not known to transit its star in a way that is easily observable, making the conventional use of transit spectroscopy extremely challenging for this world. Consequently, there is no confirmed detection of a significant atmosphere, let alone one rich in oxygen, around Proxima Centauri b.
Environmental Factors Affecting Atmospheric Retention
The nature of the host star, Proxima Centauri, presents a major obstacle to the existence of a stable, oxygen-rich atmosphere. Proxima Centauri is an M-dwarf, a type of star known for its magnetic activity and frequent, powerful stellar flares. These flares are intermittent bursts of high-energy radiation that can be hundreds of times more powerful than flares from our own Sun.
The planet is constantly bombarded by intense high-energy radiation, receiving an estimated 30 times more extreme-UV radiation and 250 times more X-rays than Earth. Over billions of years, this steady stream of energetic particles and radiation works to strip away lighter atmospheric gases. Any initial atmosphere, including water vapor that could be broken down to form oxygen, faces a constant threat of being eroded and lost to space.
Proxima Centauri b is likely tidally locked, meaning one side perpetually faces the star while the other is in eternal darkness. This condition, which results from the planet’s close orbit, complicates atmospheric stability. While an atmosphere would be necessary to redistribute heat from the scorching dayside to the freezing nightside, the planet’s magnetic field is likely compromised by this close proximity and rotation.
How Scientists Will Confirm the Planet’s Composition
Despite the current observational difficulties, the definitive answer regarding Proxima Centauri b’s atmosphere is expected to come from next-generation instruments. The James Webb Space Telescope (JWST) offers the sensitivity required to probe the planet’s conditions using methods other than traditional transit spectroscopy. JWST’s Mid-Infrared Instrument (MIRI) can measure the planet’s thermal emission, which is the heat radiating from its surface and atmosphere.
By observing the planet throughout its orbit, scientists can measure how heat is distributed between the dayside and nightside. If a substantial atmosphere is present, it will efficiently transport heat to the far side, leading to a smaller temperature difference. JWST may also be able to detect ozone, which serves as a proxy for a free-oxygen atmosphere.
Future extremely large ground-based telescopes, such as the Extremely Large Telescope (ELT), will also play a role. These telescopes will use sophisticated instruments to directly image the planet and analyze the light reflected from its surface or atmosphere. Scientists anticipate being able to confirm or deny the presence of specific atmospheric gases, including oxygen and water, within the next decade.