Kepler-22 b

A sub-Neptune

2.10 Earth radii, likely a rocky core under a thick gas or steam envelope rather than a walkable surface. With 9.1 Earth masses packed into that size, its density points to rock and iron rather than gas.

The orbit

Its year takes 290 Earth days (0.8 Earth years). The orbit is strongly stretched (eccentricity 0.72), so its distance from the star, and with it the climate, swings dramatically.

Temperature and starlight

Its equilibrium temperature (the airless-world estimate) is about 6 °C. In the range where, with the right atmosphere, liquid water is conceivable; a real atmosphere could shift the true surface figure a lot (Earth's equilibrium value is −18 °C, yet we average +15 °C). It receives 1.01 times Earth's starlight, the same order as our own world.

Its sun

Its star is Sun-like (5,596 K, G5 V; the Sun is 5,772 K).

How far is that, really?

At 635 light-years, tonight's light from this system left it around the year 1391. A Voyager-speed probe would need roughly 11,231,917 years to get there.

How we found it

It was found by the transit method: the planet crosses its star's face on schedule, dimming it by a tiny, repeating fraction. The dip's depth gives the planet's size. The discovery was announced in 2011 from Kepler.

Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.

01 The Discovery That Changed the Search for Life

Kepler-22b earned its place in history on 5 December 2011, when NASA announced it as the first known transiting planet to orbit within the habitable zone of a Sun-like star. The word "transiting" is crucial: it means the planet was caught passing directly across the face of its host star as seen from Earth, an alignment that allowed Kepler's photometer to measure its size with precision. The story actually began much earlier, though. The planet's very first transit was recorded on 12 May 2009, only three days into the Kepler space telescope's scientific operations — an almost immediate payoff for the mission. A second transit followed, and then a third was detected on 15 December 2010. Because the orbital period is about 290 days, patient waiting was required between each crossing. Additional confirmation came from the Spitzer Space Telescope and from ground-based observatories, ensuring that the signal was not a false positive before the landmark announcement was made.

02 A Star Almost Identical to Our Own Sun

Kepler-22, the star at the center of this system, is a G-type star — the same broad category as our Sun. The resemblance is close but not perfect. Kepler-22 is 3% less massive than the Sun and 2% smaller in volume, making it a slightly more compact, slightly cooler object. Its surface temperature is 5,518 K compared with the Sun's 5,778 K, a difference of 260 degrees that shifts the star's color very slightly toward orange. At roughly 4 billion years old, Kepler-22 is also about 600 million years younger than the Sun, which has had 4.6 billion years to evolve. One practical consequence of the star's dimmer nature is that its apparent magnitude as seen from Earth is 11.5, well below the naked-eye limit of about 6 — you would need a telescope to spot it at all in the constellation Cygnus. The star's lower luminosity, about 25% less than the Sun's total light output, is a key factor in evaluating whether Kepler-22b could be warm enough for liquid water.

03 Why Orbit Distance Alone Does Not Tell the Whole Story

Kepler-22b orbits its star at an average distance about 15% less than the Earth-Sun distance of roughly 150 million kilometers. If Kepler-22 shone as brightly as the Sun, that closer orbit would make the planet significantly hotter than Earth. But Kepler-22 puts out about 25% less light than the Sun, and those two factors — shorter distance and dimmer star — largely cancel each other out. The result is a location where, under the right conditions, liquid water could theoretically exist on the surface. Scientists call this band around a star the habitable zone, and Kepler-22b sits inside it. However, recent modeling suggests the situation is more nuanced: estimates give Kepler-22b more than a 95% probability of falling within the broader "empirical" habitable zone defined by the recent Venus and early Mars limits, but less than a 5% chance of lying in the more strictly defined conservative habitable zone derived from cloud-free climate models. Its exact position within or outside that narrower band remains uncertain.

04 A Radius Twice Earth's — and Not Much Else Known

The single most reliable piece of physical information about Kepler-22b is its radius. Transit observations allow astronomers to measure how much starlight the planet blocks, which directly reveals its size relative to the star. The radius was originally estimated at 2.4 times Earth's, but improved data revised that figure downward to 2.1 Earth radii as of 2023. A planet of that size sits in an awkward gap in our classification system: too large to be confidently called a rocky "super-Earth," yet potentially too small to be a fully gas-dominated planet like Neptune. Its mass, by contrast, remains deeply uncertain. The radial velocity method — which detects a planet's gravitational tug on its star — has been applied to this system but has not yet yielded a precise mass measurement. As of 2023, astronomers can only say with confidence that the mass is at most 9.1 Earth masses, a significant tightening from the early upper limit of 124 Earth masses at 3-sigma confidence announced at discovery.

05 Ocean World, Gas Ball, or Something Else? Deeper

Without a measured mass, planetary scientists cannot calculate Kepler-22b's density, and without density they cannot determine whether the planet is rocky, watery, or wrapped in thick gas. What radial velocity data has managed to rule out is an Earth-like rocky composition — that scenario is excluded to at least 1-sigma uncertainty. What remains are more exotic possibilities. One leading idea is that Kepler-22b is an "ocean planet," a world covered by a deep global ocean with little or no exposed land. Scientist Natalie Batalha, a member of the Kepler project team, noted that if the planet is mostly ocean with a small rocky core, life in such an ocean is not beyond possibility. Another scenario is a volatile-rich composition with a liquid or gaseous outer shell, which would make Kepler-22b similar to Kepler-11f, one of the smallest known gas planets. The planet has also been compared to Enaiposha, a water-rich world, though unlike Enaiposha, Kepler-22b orbits within the habitable zone.

06 Three Climate Futures Depending on One Unknown Deeper

The surface temperature of Kepler-22b depends almost entirely on a factor that cannot yet be measured: its atmosphere. Scientists use three benchmark scenarios to bracket the possibilities. If the planet has no atmosphere at all, its equilibrium temperature — calculated assuming an Earth-like albedo, meaning the same fraction of light is reflected — would be approximately 279 K, or 6 °C. That is comfortably above freezing and actually a bit warmer than Earth's own bare-rock equilibrium temperature of 255 K, or minus 18 °C. If its atmosphere provides a greenhouse effect comparable in magnitude to Earth's, the average surface temperature would rise to about 295 K, or 22 °C — a pleasant, shirtsleeve climate. But if the greenhouse effect resembles Venus's dense, CO₂-rich blanket, the average surface temperature would reach a crushing 733 K, or 460 °C, hot enough to melt lead. The actual answer depends entirely on the atmospheric composition and thickness, both of which remain completely unknown.

07 What a Stretched Orbit Would Mean for the Surface Deeper

Kepler-22b's orbital inclination is known to be approximately 90 degrees — nearly edge-on from Earth's perspective, which is why transits are visible at all. However, the eccentricity of the orbit, which describes how stretched or circular the path is, has not been measured with any precision. As of 2023, only an upper limit on eccentricity exists; the radial velocity data have not pinned down a specific value. This matters enormously for habitability. If Kepler-22b travels a nearly circular orbit, the amount of stellar energy it receives stays relatively constant throughout the year, supporting stable surface conditions. But if the orbit is highly elliptical, the planet would swing considerably closer to and farther from Kepler-22 during each 290-day year, creating dramatic swings in surface temperature. Scientists have noted that in such a scenario, the surface temperature variance would be very high, potentially cycling between frozen and scorched conditions within a single orbit — an environment that would challenge any form of life as we understand it.

08 Ruling Out Large Hidden Moons Deeper

One intriguing question about any planet in a habitable zone is whether it hosts moons — bodies that could themselves provide stable environments, or that could influence the planet's axial tilt and climate stability in the way Earth's Moon influences ours. For Kepler-22b, a dedicated effort has been made to address this. The Hunt for Exomoons with Kepler project, known by its acronym HEK, combed through the Kepler photometry of the planet looking for transit timing variations and transit duration variations. Both of these subtle effects can be caused by the gravitational presence of an orbiting satellite tugging the planet slightly forward or backward in its orbit. No such variations were detected. The absence of a signal allowed scientists to set a firm upper limit: Kepler-22b does not have any moon with a mass greater than 0.54 Earth masses. A moon of that size would itself be a substantial world — larger than Mars — so the constraint is meaningful, though smaller moons remain entirely possible and undetectable with current data.

09 How Far Away 640 Light-Years Really Is

Kepler-22b sits approximately 640 light-years from Earth, equivalent to about 200 parsecs, in the constellation Cygnus the Swan. To put that distance in perspective: light, traveling at nearly 300,000 kilometers per second, takes 640 years to make the crossing. The light arriving at Kepler's detectors when it first spotted the planet's transit in May 2009 had left the Kepler-22 system sometime around the year 1369. Even the fastest spacecraft humanity has ever launched, Voyager 1, travels at roughly 17 kilometers per second and would take more than 11 million years to cover such a distance. The system is invisible to the unaided eye not only because of this enormous distance but also because Kepler-22 itself has an apparent magnitude of only 11.5, far too faint to see without optical aid. The region of Cygnus that Kepler monitored was chosen partly because it sits away from the densest part of the Milky Way, giving the telescope cleaner lines of sight to hundreds of thousands of stars simultaneously.

10 SETI's Interest and the Speculation About Life

The combination of habitable zone location, ocean planet speculation, and Sun-like host star made Kepler-22b one of the most discussed candidates for extraterrestrial life shortly after its announcement. The SETI Institute, which searches for signals that might indicate technological civilizations, has specifically included Kepler-22b among its top candidates for research following the 2011 announcement. The scientific case was articulated most directly by Natalie Batalha of the Kepler team, who said that if the planet is mostly ocean with a small rocky core, the existence of life in that ocean is not beyond the realm of possibility. It is important to note that this remains pure speculation — there is no evidence of life, no measurement of atmospheric composition, and no confirmed understanding of whether liquid water actually exists at the surface. The planet's mass, density, atmospheric properties, and actual surface conditions are all still unknown. What has been established is simply that the orbit falls in a zone where conditions could, in principle, allow liquid water to exist.

11 Kepler-22b in Fiction and Popular Culture

Few exoplanets have captured the popular imagination as quickly as Kepler-22b. Within a year of the 2011 announcement, it appeared as a central plot device in a teen science fiction novel titled Kepler 22b, written by Bangladeshi author Muhammad Zafar Iqbal. The planet also became a playable map in Łukasz Jakowski's strategy video games Age of Civilizations, released in 2014, and Age of History II in 2017, where players must control 404 provinces to win. Sri Lankan author Binendra used it as the setting for a children's science fiction book series called Kepler, beginning in 2019. The 2020 American television series Raised by Wolves, filmed in South Africa, used Kepler-22b as its setting. The planet also lent its name to a track on the album Omnium Gatherum, the twentieth studio album by King Gizzard and the Lizard Wizard. It appears as the source of a mysterious radio signal in the project Pluribus and as the destination of the starship Stalwart II in Scott Sigler's short story collected in The Apocalypse Triptych, edited by John Joseph Adams and Hugh Howey.

12 What Remains Unknown and Why It Matters Deeper

Despite more than a decade of follow-up observations, the fundamental nature of Kepler-22b remains unsolved. The mass is still not directly measured — only an upper limit of 9.1 Earth masses exists as of 2023, improved from earlier cruder estimates but still not a detection. Without mass, there is no density, and without density there is no way to distinguish between a rocky world, a water world, and a sub-Neptune gas planet. The orbital eccentricity is also unconstrained beyond an upper limit, leaving climate stability an open question. No atmospheric spectrum has been obtained, so the greenhouse effect, if any, is entirely unknown. The albedo — how reflective the surface is — was simply assumed to be Earth-like for the purpose of equilibrium temperature calculations, not measured. The HEK project ruled out massive moons but cannot exclude smaller ones. In short, the planet is defined mainly by what it is not: it is not Earth-sized, it is probably not rocky like Earth, it is not accompanied by a large moon, and it is not orbiting outside the habitable zone. What it actually is — its interior, its surface, its air — remains one of the most tantalizing open questions in exoplanet science.

About this record

The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.