Proxima Centauri b sits in the habitable zone of our nearest stellar neighbor, but whether it can actually support life depends on a tangle of unresolved questions about its atmosphere, magnetic field, water history, and the punishing radiation environment created by its host star. Climate models show plausible scenarios with liquid water on the surface, yet the same models rely on assumptions about conditions we have not yet measured directly. The planet is less a confirmed habitable world and more a high-priority test case for understanding whether rocky planets around red dwarf stars can be livable at all.
What We Know About the Planet Itself
Proxima Centauri b orbits its star every 11.2 days at a distance of roughly 0.05 astronomical units, receiving about 70% of the energy Earth gets from the Sun. Climate simulations have modeled it with an assumed mass of 1.4 times Earth’s, a radius about 10% larger, and surface gravity slightly stronger than ours. Those simulations tested different rotation states, including synchronous rotation (one face permanently toward the star) and spin-orbit resonances where the planet rotates faster than it orbits.1Astronomy & Astrophysics. The habitability of Proxima Centauri b. II. Possible climates and observability The orbital eccentricity is constrained to be less than 0.35, which leaves open whether the planet is tidally locked in synchronous rotation or caught in a 3:2 resonance like Mercury, where it rotates three times for every two orbits.2PubMed Central. The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants That distinction matters enormously for climate. A synchronously rotating planet has a permanent dayside and nightside, creating extreme temperature contrasts that an atmosphere would need to redistribute. A 3:2 resonance spreads heating more evenly.
The Flare Problem
Proxima Centauri is an M5.5 red dwarf, a small, cool star prone to violent magnetic outbursts. Observations from the TESS satellite over about 50 days detected 72 flare events, which works out to roughly 1.5 flares per day. The strongest flares in that dataset had energies around 1030 to 1032 erg in the TESS band, with bolometric energies roughly five times higher. The flare frequency distribution suggests superflares with energies of 1033 erg hit about three times a year, and truly extreme events at 1034 erg could occur every other year.3The Astrophysical Journal. Flaring Activity of Proxima Centauri from TESS Observations: Quasiperiodic Oscillations during Flare Decay and Inferences on the Habitability of Proxima b Those are staggering rates compared to what Earth experiences from the Sun.
Millimeter-wavelength observations from the ALMA radio telescope add another layer of concern. Over roughly 50 hours of observation, researchers detected 463 flares at millimeter wavelengths, with the brightest one boosting the signal by a factor of a thousand above the star’s quiet output. The power-law slope of the flare frequency distribution at millimeter wavelengths was much steeper than what optical and X-ray observations show, meaning small flares are far more common than previously expected. If millimeter flare rates predict extreme-UV flare rates, the cumulative radiation hitting Proxima b from frequent small flares could be substantially worse than estimates based on optical data alone.4The Astrophysical Journal. The Proxima Centauri Campaign—First Constraints on Millimeter Flare Rates from ALMA
This is not just a surface-sterilization concern. Extreme-UV and energetic-particle bombardment can strip away an atmosphere over time, which leads to the next big question.
Can the Planet Hold Onto an Atmosphere?
An atmosphere is the minimum requirement for surface habitability, providing pressure to keep water liquid, warmth through greenhouse effects, and shielding from radiation. The problem for Proxima b is that its close orbit puts it squarely in the path of a hostile stellar wind. Modeling using magnetohydrodynamic simulations adapted from Venus and Mars found that ion escape rates from an unmagnetized Proxima b would be about a hundred times higher than those for Earth, Venus, or Mars. Even with a planetary magnetic field, escape rates remain elevated for certain stellar wind pressures and are still higher than what we observe in our own solar system’s rocky planets.5The Astrophysical Journal Letters. Is Proxima Centauri b Habitable? A Study of Atmospheric Loss
A magnetic field helps, but how much it helps depends on factors that interact in complicated ways. A stronger stellar wind or a weaker planetary magnetic field increases the area over which ions can escape into space along open field lines at the poles. The geometry of the stellar magnetic field relative to the planet’s own field orientation matters too, affecting the size of the polar cap region where atmospheric material leaks out.6The Astrophysical Journal Letters. On the Magnetic Protection of the Atmosphere of Proxima Centauri b In short, a magnetic field is necessary but may not be sufficient if the stellar wind is strong enough to compress it and open up large escape channels.
Does Proxima b Even Have a Magnetic Field?
We do not know, and we cannot measure it directly with current technology. Interior modeling based on the planet’s estimated mass and composition suggests it could generate a magnetic field in the range of 0.06 to 0.23 Gauss, compared to Earth’s field of roughly 0.25 to 0.65 Gauss. However, if the planet is tidally locked and rotating slowly, any field it generates would likely be multipolar rather than a clean dipole like Earth’s. A multipolar field offers less coherent shielding and creates more complex magnetic topology with additional escape routes for atmospheric particles. Worse, if Proxima b has more than about 10% of its mass in volatile materials (water and ices), the models predict magnetic fields would be either nonexistent or very weak.7Monthly Notices of the Royal Astronomical Society. Characterizing the possible interior structures of the nearby Exoplanets Proxima Centauri b and Ross-128 b
There is an uncomfortable irony here: a planet rich in water, which you might think is good for habitability, could end up with a weaker magnetic field, making it harder to protect whatever atmosphere and water it has. The relationship between interior composition, magnetic protection, and atmospheric retention creates a web of interdependencies that makes simple habitability claims unreliable.
Water History and the Early Runaway Phase
Red darf stars like Proxima Centauri are much more luminous in their youth than in their main-sequence lifetimes. Before Proxima dimmed to its current output, the planet would have been blasted with enough energy to trigger a runaway greenhouse state for up to roughly 200 million years. During that phase, surface water would have vaporized and migrated to the upper atmosphere, where ultraviolet radiation could split water molecules and allow hydrogen to escape to space. Modeling of this process found that total water loss during the runaway phase was below about 0.4 Earth ocean equivalents at one estimate of the habitable zone boundary, and below roughly one Earth ocean equivalent at another.8Astronomy & Astrophysics. The habitability of Proxima Centauri b
Those numbers leave room for the planet to have retained water, especially if it started with more than Earth did. Planets that form farther from their star and migrate inward, or that receive water through impacts and outgassing, could begin with several ocean-equivalents. Whether Proxima b had that much to start with is unknown, but the calculations suggest the early hot phase was not necessarily a death sentence for its water supply.
What the Climate Might Look Like
If Proxima b has an atmosphere and water, what happens on the surface? The answer depends heavily on ocean dynamics. Simulations using coupled ocean-atmosphere models found that an ocean-covered Proxima b could maintain surface liquid water over a much broader area than simpler models predicted, though at much colder temperatures. Ocean currents transport heat from the warm substellar region outward, preventing the dayside from becoming too hot while spreading warmth toward the nightside. Interestingly, adding more greenhouse gases does not always produce more open ocean, because the atmospheric circulation can shift between different dynamic regimes.9PubMed. Habitable Climate Scenarios for Proxima Centauri b with a Dynamic Ocean
The presence or absence of continents also matters. Simulations testing different land-mass sizes found that dayside ice-free ocean and nutrient upwelling were maintained regardless of continent size. But when the substellar land mass exceeded about 20% of the planet’s surface area, the climate transitioned from a state where ocean currents spread heat outward (sometimes called a “lobster” pattern) to a more symmetric “eyeball” pattern where temperatures drop off evenly from the substellar point and heat transport is restricted. The study also found that differences between atmospheric models had a bigger effect on the climate results than whether ocean dynamics were included, which is a humbling reminder of how uncertain these predictions remain.10The Astrophysical Journal Letters. The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b
Even Without a Surface, Subsurface Oceans Are Possible
If the surface proves too hostile, there is a backup scenario. Even a planet with a frozen surface could harbor a liquid ocean beneath an ice shell, sustained by internal heat from tidal forces and radioactive decay. Proxima b is actually one of the most favorable candidates for this kind of subsurface ocean. Modeling of internal heating rates suggests the planet’s ice shell could be relatively thin, and its geological activity rate likely exceeds that of Jupiter’s moon Europa. If cryovolcanic eruptions occur, they could vent water directly from the subsurface ocean into space, making such an ocean potentially detectable with future telescopes.11The Astrophysical Journal. Prospects for Cryovolcanic Activity on Cold Ocean Planets
This idea mirrors our growing understanding of habitability in our own solar system, where moons like Europa and Enceladus are considered among the most promising places to look for life despite being far from the traditional habitable zone. A frozen Proxima b with a warm subsurface ocean would be a very different kind of habitable world from Earth, but not necessarily a lifeless one.
Could Life Survive the UV Radiation?
Surface radiation on a planet orbiting an active M dwarf would be intense during flares, but this does not necessarily rule out biology. On Earth, organisms use multiple strategies to cope with ultraviolet damage, including protective pigments, DNA repair enzymes, and biofluorescence, which converts harmful UV photons to longer, safer wavelengths. Organisms can also simply avoid direct exposure by living beneath soil, in rock crevices, or underwater. The UV surface environment on planets around active M dwarfs, while harsh by Earth standards, does not clearly exceed what robust Earth life can tolerate using known biological defenses.12Monthly Notices of the Royal Astronomical Society. Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems
Early Earth itself endured periods of elevated UV flux before the ozone layer was fully established, and life not only survived but diversified during that time. The question is not really whether organisms could theoretically tolerate Proxima b’s surface conditions, but whether life would have enough time and stability to arise in the first place, and that depends on the atmosphere and water questions discussed above.
Reading the Atmosphere from Here
The most immediate way to constrain Proxima b’s habitability is to detect or rule out an atmosphere. One proposed approach involves targeting the 15-micrometer absorption band of carbon dioxide using the James Webb Space Telescope’s mid-infrared spectrograph. If the planet has an Earth-like atmosphere, carbon dioxide could potentially be detected within a few days of observations, but only if both the instrument’s spectral response and the stellar spectrum can be determined to a precision of better than one part in ten thousand between adjacent spectral channels.13The Astronomical Journal. Detecting Proxima b’s Atmosphere with JWST Targeting CO2 at 15 μm Using a High-pass Spectral Filtering Technique That precision requirement is formidable and has not yet been demonstrated for this target.
Even if atmospheric gases are detected, interpreting them correctly will be tricky. Modeling of Proxima b’s atmospheric chemistry shows that cosmic-ray-induced reactions create spectral features that could be confused with biosignatures. For example, a feature near 5.3 micrometers that on Earth correlates with nitrogen oxide production from solar energetic particles turned out in models of Proxima b to be a weak carbon dioxide absorption band. That same spectral feature appeared at identical strength for both flaring and quiescent stellar conditions, meaning it could easily be misinterpreted as a biological or flare-driven signal when it is actually just CO2.14The Astrophysical Journal. Proxima Centauri b: A Strong Case for Including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies Future observations will need careful photochemical modeling alongside spectral data to avoid false positives.
Confirming the Planet Is Real
It is worth noting that detecting Earth-mass planets through radial velocity measurements is among the hardest tasks in exoplanet science. Proxima b’s signal is on the order of one meter per second in radial velocity wobble, buried in stellar noise from the very magnetic activity that makes the star interesting. Independent confirmation efforts have tested different methods for suppressing non-planetary signals in the radial velocity data, and the planet’s existence has held up, but the original detection sparked enough concern about stellar contamination that verification was treated as essential.15Astronomy & Astrophysics. Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities The planet is now widely accepted as real, but the episode illustrates how close-in planets around active stars sit right at the edge of our detection capabilities.
Proxima b in a Crowded System
Proxima b is not alone. The system includes at least one other confirmed planet, Proxima d, orbiting even closer to the star, and a more distant candidate, Proxima c. The two innermost planets are separated by just 0.02 astronomical units, which creates strong gravitational interactions. Stability analysis shows the system is very likely stable as long as the initial eccentricities of planets b and d stay below about 0.2, though mutual orbital inclinations between 95° and 142° often lead to instability.16The Astrophysical Journal. Orbital Stability and Secular Dynamics of the Proxima Centauri Planetary System Separate dynamical mapping work found that Proxima b’s orbit is stable with eccentricities below 0.4, and the best-fit orbital parameters fall well within the strongly stable region.17Monthly Notices of the Royal Astronomical Society. Dynamical evolution and stability maps of the Proxima Centauri system
Orbital stability matters for habitability because a planet that gets periodically kicked into a highly eccentric orbit would experience wild temperature swings and potentially lose its atmosphere more quickly. The current evidence suggests Proxima b’s orbit is well-behaved, but companions in the system could also influence its rotation state, potentially pushing it out of tidal lock and into a spin-orbit resonance that would alter the climate picture.
The Broader M-Dwarf Question
Proxima b is not just one planet’s story. Roughly a third of rocky planets around M dwarfs orbit within the habitable zone, making them the most common category of potentially habitable world in the galaxy.18ScienceDirect (Elsevier / Physics Reports). The habitability of planets orbiting M-dwarf stars If planets in tight orbits around flare-active stars turn out to be categorically uninhabitable, a huge fraction of candidate worlds drops off the list. If they can retain atmospheres and sustain liquid water despite the flares, the number of potentially habitable planets in the Milky Way rises enormously. Proxima b, being the closest and one of the most studied, serves as the bellwether for this entire class of worlds.
Getting There Someday
Remote observation has limits. The Breakthrough Starshot initiative envisions sending gram-scale probes to the Alpha Centauri system (which includes Proxima Centauri) at 20% the speed of light, reaching it in about 20 years of travel. A system model for this mission estimates capital costs around $8 billion for the ground-based laser array, assuming projected costs of a penny per watt for lasers and $500 per square meter for optics.19Acta Astronautica. The Breakthrough Starshot system model One proposed approach would launch hundreds of these tiny probes over a year-long campaign, gradually forming a mesh network roughly 100,000 kilometers across centered on Proxima b at flyby time, allowing them to combine their signals for optical communication back to Earth.20arXiv. Swarming Proxima Centauri: Optical Communication Over Interstellar Distances
These concepts remain firmly in the technology-development phase, and the engineering challenges are immense. But the fact that serious system-level engineering studies exist for missions to Proxima b reflects how central this particular planet has become to questions about habitability beyond our solar system. Whether the answer turns out to be a thriving ocean world, a frozen shell hiding life underneath, or a barren rock stripped of its atmosphere billions of years ago, finding out would reshape how we think about the prevalence of life in the galaxy.