Imagine looking up at the night sky and knowing that the closest planet beyond our solar system is sitting right there, just over four light-years away. Not in some distant galaxy. Not a blurry dot in a telescope image from the edge of the observable universe. It is our next-door neighbor, orbiting the nearest star to the Sun, and it is roughly the same size as Earth.
That planet is Proxima Centauri b, and ever since its discovery was announced in 2016, it has been the exoplanet people ask about more than any other. It is close, it is rocky, and it sits in the habitable zone of its star — the region where liquid water could exist on the surface. On paper, it sounds like the perfect candidate for a second Earth. The reality, as you are about to find out, is far more complicated, and honestly, far more interesting.
The short answer: Proxima Centauri b is the closest known exoplanet to Earth, about 4.24 light-years away. It is a rocky world at least 1.17 times the mass of Earth, orbiting its red dwarf star every 11.2 days inside the habitable zone. We do not yet know whether it has an atmosphere or any water, and its star’s violent flares may have stripped its air away long ago — so whether anything could live there remains an open question.
What Is Proxima Centauri b?
Proxima Centauri b, often shortened to Proxima b, is an exoplanet — a planet that orbits a star other than the Sun. It circles Proxima Centauri, a small red dwarf star that is the closest star to our solar system. The planet lies in the southern constellation of Centaurus, which means it is best seen from the Southern Hemisphere, though you cannot actually see the planet itself with any telescope available to the public. What astronomers detected was not the planet’s light, but the tiny gravitational tug it exerts on its star.
The “b” in its name follows the standard naming rule for exoplanets: the first planet found around a star gets the letter b, the next one gets c, and so on. Proxima Centauri already had its star, so the planet became Proxima Centauri b. It is also sometimes called Alpha Centauri Cb, because Proxima Centauri is the third member of the Alpha Centauri star system.
Here are the key facts at a glance:
| Fact | Detail |
|---|---|
| Distance from Earth | About 4.24 light-years |
| Host star | Proxima Centauri, a red dwarf |
| Discovered | 24 August 2016 |
| Discovery method | Radial velocity (Doppler spectroscopy) |
| Minimum mass | About 1.17 Earth masses |
| Orbital period | About 11.2 Earth days |
| Distance from its star | About 0.0485 AU |
| Location | Habitable zone of Proxima Centauri |
What makes this planet special is not just its distance. Thousands of exoplanets have been found since the first ones were confirmed in the 1990s, and most of them are gas giants or scorched worlds orbiting far away. Proxima Centauri b is different because it combines three things at once: it is the nearest, it is roughly Earth-sized, and it sits in the temperate zone of its star. No other known planet ticks all three boxes.
How Far Away Is Proxima Centauri b?
Proxima Centauri b sits about 4.24 light-years from Earth, which works out to roughly 40 trillion kilometers, or about 25 trillion miles. That is the kind of number that sounds small in astronomy and impossible in everyday life at the same time.
To put it in perspective, light itself — the fastest thing in the universe — takes more than four years to make the trip. The Sun’s light reaches us in about eight minutes. The nearest planet beyond our solar system is over 270,000 times farther away than the Sun.
Why “close” still means impossibly far
Our fastest spacecraft would need tens of thousands of years to get there. Voyager 1, the most distant human-made object, is travelling at about 17 kilometers per second, and even at that speed the journey to Proxima Centauri b would take well over 70,000 years. The Parker Solar Probe, the fastest spacecraft ever built, would still need thousands of years. Chemical rockets simply cannot close a gap measured in light-years within a human lifetime.
That is why astronomers call Proxima Centauri b “on our doorstep” while engineers call it unreachable. Both are right. In a universe where most interesting planets sit hundreds or thousands of light-years away, four light-years is practically next door. For a species that has never sent a probe beyond its own solar system, it might as well be on the other side of forever.
How Was Proxima Centauri b Discovered?
The discovery of Proxima Centauri b was announced on 24 August 2016 by the European Southern Observatory, and it was the result of a dedicated observing campaign with a name straight out of a science fiction novel: the Pale Red Dot campaign.
The Pale Red Dot campaign
The campaign was led by astronomer Guillem Anglada-Escudé, then at Queen Mary University of London. His team used the HARPS spectrograph at the European Southern Observatory’s La Silla Observatory in Chile to watch Proxima Centauri night after night, looking for the tiniest wobble in the star’s motion. The name was a deliberate nod to Carl Sagan’s famous “Pale Blue Dot” description of Earth.
Astronomers had suspected for years that Proxima Centauri might host a planet. Earlier hints had come and gone without confirmation, and the star’s own restless activity made the signal hard to trust. The Pale Red Dot campaign was designed to settle the question once and for all, with intensive observations over several months in early 2016.
The radial velocity method
The team found Proxima Centauri b using the radial velocity method, also called Doppler spectroscopy. Here is how it works in plain language:
- A planet’s gravity pulls on its star, just as the star’s gravity pulls on the planet.
- That pull makes the star wobble slightly toward us and away from us as the planet orbits.
- When the star moves toward us, its light shifts slightly toward the blue end of the spectrum. When it moves away, the light shifts toward the red.
- By measuring these tiny shifts over time, astronomers can work out the planet’s orbital period and its minimum mass.
For Proxima Centauri b, the wobble was almost absurdly small. The planet tugs its star back and forth at a speed of only about 1.4 meters per second — roughly walking pace. Picking that signal out of a star that is constantly churning with magnetic activity was a remarkable achievement, and the results were published in the journal Nature on 25 August 2016.
Later observations with the ESPRESSO spectrograph on the Very Large Telescope confirmed the planet in 2020, and newer measurements from the NIRPS spectrograph in 2025 refined the minimum mass estimate to around 1.06 Earth masses. The exact value depends on the method and the data set, but every estimate lands in the same neighborhood: this is an Earth-mass world.
How Big Is Proxima Centauri b?
Proxima Centauri b has a minimum mass of about 1.17 Earth masses, based on the original discovery measurements. That “minimum” part matters, and it is worth understanding, because it is the reason scientists are careful about how they describe this planet.
The radial velocity method can only measure the component of the star’s wobble along our line of sight. If we are viewing the planet’s orbit edge-on, the measured mass is the true mass. If the orbit is tilted, the true mass is higher. Because the tilt of Proxima Centauri b’s orbit is still unknown, 1.17 Earth masses is a lower limit, not a final answer. Statistically, the true mass is probably less than about two to three Earth masses, which still keeps it firmly in rocky-planet territory.
What we know — and what we are guessing
- Mass: At least about 1.17 times Earth’s mass, and very likely not more than a few Earth masses.
- Radius: Unknown. The planet does not pass in front of its star from our point of view, so we cannot measure its size directly. If it is rocky like Earth, its radius is probably around 1.1 to 1.3 times Earth’s.
- Composition: Almost certainly rocky. A planet this small cannot hold on to a thick hydrogen atmosphere the way a gas giant does, so scientists expect a solid surface, possibly with an iron core like Earth’s.
- Surface gravity: Probably similar to Earth’s, perhaps slightly stronger, given the mass estimate.
The frustrating part is that the most basic questions — how wide is it, what is it made of, does it have an atmosphere — are still unanswered. We know its mass and its orbit, and almost everything else is educated inference.
What Is Proxima Centauri b’s Orbit Like?
Proxima Centauri b completes one orbit around its star in just 11.2 Earth days. Its year is shorter than two of our weeks. It circles its star at a distance of about 0.0485 AU, which is roughly 7.25 million kilometers — only about one-twentieth of the distance between Earth and the Sun.
That sounds impossibly close, and it is, by solar system standards. Mercury, the closest planet to the Sun, orbits at 0.39 AU, about eight times farther from its star than Proxima Centauri b is from its own. But Proxima Centauri is a very different kind of star from the Sun, and that changes everything about what such a tight orbit means.
A year in 11 days
Because the planet is so close to its star, it is almost certainly tidally locked, the same way the Moon is tidally locked to Earth. One side of Proxima Centauri b probably faces its star permanently, baked in endless daylight, while the other side sits in permanent darkness. Whether the boundary between day and night — the terminator line — could be temperate enough for liquid water is one of the big open questions.
A tidally locked world is not automatically dead. A thick atmosphere could carry heat from the dayside to the nightside, evening out temperatures across the planet. An ocean could do the same. But without an atmosphere, the dayside would be scorching and the nightside frozen, with only a narrow band in between where temperatures might be tolerable.
Is Proxima Centauri b in the Habitable Zone?
Yes — Proxima Centauri b orbits inside the habitable zone of its star, the region sometimes called the Goldilocks zone, where temperatures are just right for liquid water to exist on the surface of a rocky planet, assuming it has the right kind of atmosphere.
The reason a planet can sit twenty times closer to its star than Earth sits to the Sun and still be temperate is that Proxima Centauri is a feeble star. It is a red dwarf with only about 12 percent of the Sun’s mass, a surface temperature of around 3,050 Kelvin (compared with the Sun’s 5,778 Kelvin), and a total luminosity roughly one-thousandth of the Sun’s. It is so dim that you cannot see it with the naked eye, even though it is the closest star to us.
Because the star is so faint, its habitable zone huddles close in. Proxima Centauri b receives about 65 percent of the sunlight Earth receives from the Sun — less than Earth, but far more than Mars gets. Its estimated equilibrium temperature, before accounting for any atmosphere, is around 234 Kelvin, or about minus 39 degrees Celsius. That is cold, but a greenhouse atmosphere like Earth’s could warm the surface well above freezing.
The habitable zone is a starting point, not a guarantee
It is important to be clear about what “habitable zone” actually means. It is a statement about temperature and distance, not a promise of life. Venus sits near the inner edge of the Sun’s habitable zone and is a 465-degree-Celsius hellscape. Mars sits near the outer edge and is a frozen desert. Being in the zone is necessary for surface water, but it is nowhere near enough on its own. A planet also needs an atmosphere, the right chemistry, and a star that does not strip everything away — and that last point is where Proxima Centauri b runs into serious trouble.
Could There Be Life on Proxima Centauri b?
Nobody knows. That is the honest answer, and any article that tells you otherwise is overselling what we have actually measured. No telescope has detected an atmosphere on Proxima Centauri b, let alone signs of life, and as of 2026, none has directly measured its surface temperature either.
The case for optimism is straightforward. Proxima Centauri b is rocky, roughly Earth-sized, and sits in the habitable zone of its star. If it formed with water and held on to a reasonably thick atmosphere, liquid water could exist on its surface, and where liquid water exists, life as we know it has a chance.
The case against is just as straightforward, and it comes down to the star. Proxima Centauri is not a calm, steady sun like ours. It is a flare star, and its tantrums are violent enough to matter for anything trying to live on a planet sitting right next to it.
What life would need to survive there
For life to exist on Proxima Centauri b, the planet would probably need several things working in its favor:
- A magnetic field strong enough to deflect the star’s charged particles, the way Earth’s magnetic field protects us.
- A thick atmosphere that can absorb ultraviolet radiation and redistribute heat from the dayside to the nightside.
- Liquid water on the surface or beneath it, stable over billions of years.
- A way to recover from flares, whether through deep oceans, underground habitats, or an atmosphere that rebuilds itself.
Some scientists think a strong magnetic field and a thick atmosphere could be enough. Others think the star’s onslaught would strip any Earth-like atmosphere away in a cosmologically short time. Both camps are working from the same data, and the data is not yet good enough to settle the argument.
The Problem With Its Star: Flares and Stellar Wind
If Proxima Centauri b has an enemy, it is the star it orbits. Proxima Centauri is a red dwarf, and red dwarfs are famously unruly. They are small, cool, and magnetically hyperactive, and Proxima Centauri is one of the more dramatic examples.
Superflares that would dwarf anything on Earth
In March 2016, astronomers observed a superflare from Proxima Centauri that made the star about 68 times brighter in visible light. Researchers estimated that flares of that size might happen at least five times a year. Then, on 1 May 2019, a coordinated set of telescopes caught something even more extreme: the star brightened by a factor of more than 14,000 in far ultraviolet light, as measured by the Hubble Space Telescope, with the most violent burst lasting less than ten seconds. It was one of the most extreme stellar flares ever recorded from any star.
A flare like that would flood a nearby planet with ultraviolet and X-ray radiation. On Earth, a flare a tiny fraction of that size can disrupt radio communications and damage satellites. On Proxima Centauri b, repeated superflares could destroy an ozone layer, sterilize the surface, and gradually strip away the atmosphere altogether.
A stellar wind thousands of times stronger than the Sun’s
The flares are only half the problem. Proxima Centauri b is also exposed to stellar wind pressures estimated at more than 2,000 times what Earth experiences from the solar wind. That is a constant, grinding pressure that can erode an atmosphere particle by particle, year after year, for billions of years.
Earth survives its own solar wind because of its magnetic field, which deflects charged particles around the planet. Whether Proxima Centauri b has a magnetic field is unknown, and if it is tidally locked, its slow rotation — one turn every 11.2 days — might not generate a strong dynamo in its core. Without that shield, the wind would have a clear run at whatever air the planet started with.
Proxima Centauri b vs Earth: How Do They Compare?
Side-by-side comparisons are the fastest way to see why Proxima Centauri b is both tantalizingly familiar and deeply alien. Here is how the two worlds stack up on the things we actually know.
| Feature | Earth | Proxima Centauri b |
|---|---|---|
| Distance from its star | 1 AU (150 million km) | 0.0485 AU (7.25 million km) |
| Orbital period | 365.25 days | 11.2 days |
| Mass | 1 Earth mass | At least ~1.17 Earth masses |
| Host star | G-type yellow dwarf (the Sun) | M-type red dwarf |
| Star’s mass | 1 solar mass | About 0.12 solar masses |
| Starlight received | 100% | About 65% |
| In the habitable zone? | Yes | Yes |
| Known atmosphere? | Yes | Unknown |
| Tidally locked? | No | Very likely yes |
| Flares from its star | Occasional, mild | Frequent superflares |
The table makes the paradox obvious. Proxima Centauri b is the most Earth-like planet we know of in terms of mass and temperature, orbiting the closest star we know of — and yet it may be one of the least Earth-like places in terms of what it is actually like to stand on the surface. Same size, same temperature range, completely different sky: a dim red sun that never sets on one side of the planet and never rises on the other, flaring without warning.
What Do We Still Not Know About Proxima Centauri b?
For all the attention this planet gets, the list of things we do not know about it is long, and it is worth being honest about that. Science journalism often smooths over the gaps. Here, the gaps are the story.
- Does it have an atmosphere? This is the single most important unanswered question. Without an atmosphere, there is no surface water, no weather, and no protection from radiation. Proxima Centauri b does not transit its star from our point of view, which rules out the usual method of studying exoplanet atmospheres, where starlight filters through the air during a transit. Astronomers will need direct imaging or new techniques to answer this one.
- Does it have water? If the planet formed with water and kept its atmosphere, oceans are possible. If the star stripped the atmosphere early, any water may have been lost to space billions of years ago.
- What is its true mass and radius? Until we know the tilt of its orbit, the mass is a minimum. Until we can measure its size, its density — and therefore whether it is truly rocky — stays an estimate.
- Is it tidally locked? Almost certainly, given how close it is to its star, but this has not been directly confirmed.
- Are there other planets in the system? Yes — Proxima Centauri also hosts Proxima Centauri c, a more distant super-Earth announced in 2020, and Proxima Centauri d, a small sub-Earth-mass planet announced in 2022 with a 5.1-day orbit. Proxima Centauri b is the one in the habitable zone, though, which is why it gets the attention.
Why telescopes like JWST cannot just look at it
You might wonder why the James Webb Space Telescope has not simply pointed at Proxima Centauri b and answered all of this. The problem is geometry. Most exoplanet atmospheres studied so far were observed during transits, when the planet crosses in front of its star. Proxima Centauri b does not transit from Earth’s point of view, so that door is closed. Directly imaging a small, dim planet sitting a hair’s breadth from its star, even a nearby star, pushes current telescopes to their limits. Future instruments — extremely large ground-based telescopes and dedicated space missions — may finally crack it, but for now, the planet keeps its secrets.
Breakthrough Starshot: Could We Ever Visit Proxima Centauri b?
In 2016, the same year Proxima Centauri b was discovered, a project called Breakthrough Starshot was announced with an audacious goal: send a fleet of tiny spacecraft to the Alpha Centauri system, powered not by rockets but by giant ground-based lasers pushing feather-light sails to a fifth of the speed of light.
The idea, backed by investor Yuri Milner and supported by figures including Stephen Hawking and Mark Zuckerberg, is to build gram-scale probes — essentially a computer chip attached to a reflective sail a few meters across — and accelerate them with a powerful laser array. At 20 percent of light speed, the journey to Proxima Centauri would take about 20 years, and the probes would fly past the system snapping photographs and sending data back to Earth.
What Starshot could — and could not — do
- It could give us our first close-up images of Proxima Centauri b and its neighboring planets, something no telescope on Earth can do.
- It could measure the planet’s atmosphere and surface during a high-speed flyby, if the instruments survive the encounter.
- It could not slow down, land, or orbit. The probes would streak through the system in a matter of hours.
- It could not carry humans. A gram-scale chip is not a starship, and the laser array needed to push it would be one of the largest engineering projects ever attempted.
Starshot remains a concept, not a funded mission with a launch date. Enormous technical challenges stand in the way, from building a laser array powerful enough to surviving collisions with interstellar dust at relativistic speeds. But it matters because it is the only plan on the table that could reach Proxima Centauri b within a human lifetime. Every other approach takes tens of thousands of years. If humanity ever sees this planet up close, it will probably be through something like Starshot — or through telescopes we have not built yet.
Frequently Asked Questions
How far is Proxima Centauri b from Earth?
Proxima Centauri b is about 4.24 light-years from Earth, or roughly 40 trillion kilometers. It is the closest known exoplanet to our solar system, orbiting the nearest star to the Sun.
When was Proxima Centauri b discovered?
Proxima Centauri b was discovered in 2016, with the announcement made on 24 August 2016 by the European Southern Observatory. It was found by the Pale Red Dot campaign using the radial velocity method at La Silla Observatory in Chile.
Is Proxima Centauri b habitable?
It is in the habitable zone of its star, which means liquid water could exist there if the planet has the right atmosphere — but habitability has not been established. The star’s violent flares and intense stellar wind may have stripped away any atmosphere, so scientists are split on whether life could survive there.
How long is a year on Proxima Centauri b?
A year on Proxima Centauri b lasts just 11.2 Earth days. The planet orbits its star at a distance of about 0.0485 AU, roughly one-twentieth of the Earth-Sun distance.
Can we see Proxima Centauri b with a telescope?
No. The planet itself cannot be seen directly with any amateur telescope, and even professional telescopes cannot yet image it directly. Astronomers detected it indirectly, by measuring the tiny wobble it causes in its star’s motion.
Could humans ever travel to Proxima Centauri b?
Not with current technology. Even our fastest spacecraft would need tens of thousands of years to make the trip. The Breakthrough Starshot concept aims to send gram-scale laser-propelled probes there in about 20 years, but it is still an unbuilt concept, not an active mission.
Is Proxima Centauri b the closest exoplanet to Earth?
Yes. At about 4.24 light-years away, Proxima Centauri b is the closest known exoplanet to Earth. Its star, Proxima Centauri, is also the closest star to the Sun.
Final Thoughts
Proxima Centauri b is the best and worst of exoplanet science rolled into one small world. It is the closest exoplanet we know, roughly the size of Earth, sitting in the habitable zone of our nearest stellar neighbor — and it may be a barren rock blasted by flares, stripped of its air, with one side locked in permanent day and the other in permanent night. Or it may be a world with oceans, weather, and a thick atmosphere that shrugs off its star’s tantrums. Right now, both stories fit the data.
That uncertainty is exactly why this planet matters so much. Proxima Centauri b is a test case for one of the biggest questions in astronomy: can rocky planets around red dwarfs — the most common stars in the galaxy — actually hold on to their atmospheres and stay habitable? Most stars in the Milky Way are red dwarfs, and most of them have planets in their habitable zones. If worlds like Proxima Centauri b turn out to be habitable, then potentially habitable planets are everywhere. If they turn out to be airless and scorched, our search for life narrows dramatically.
So keep an eye on this little red neighbor. The next generation of telescopes may finally tell us whether it has an atmosphere, and one day, a tiny sail pushed by a laser might send back the first photograph of an alien world just four light-years from home. Until then, Proxima Centauri b remains what it has been since 2016: the most fascinating maybe in all of astronomy.
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