A tiny dip in a distant star’s light can reveal an entire planet. In 2013, observations from the Kepler space telescope introduced three worlds that helped bring the search for potentially habitable planets into sharper focus: Kepler-62e, Kepler-62f and Kepler-69c.
The announcement was exciting because their sizes and orbits made them worth investigating. Understanding what was measured—and what remained a possibility—makes the discovery even more interesting.
The voices and modelling behind the 2013 excitement
The 2013 discovery was part of a broader effort to find an Earth analogue. Kepler principal investigator Bill Borucki described that ambition in remarks reported by Space.com: “We’re moving very rapidly towards finding an Earth analogue around a star like the sun.” Borucki described Kepler-62e and Kepler-62f as looking “very good as possibilities for looking for life.” He also said: “I think we’re making excellent progress in that direction. We have a number of candidates that look good.” The optimism concerned prospects for further investigation, not a detection of life. In the same period, Lisa Kaltenegger and colleagues explored possible climates for Kepler-62e and Kepler-62f. Their water-world scenarios came from modelling, not from observations of oceans on the planets.
Kaltenegger’s description of the modelled worlds made the hypothetical setting explicit:
“There may be life there, but could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy.”
The announcement covered seven planets: five orbiting Kepler-62 and two orbiting Kepler-69. The two systems therefore offered more than isolated candidates; they placed the potentially temperate worlds among planetary neighbours. JPL’s April 2013 release located Kepler-62 roughly 1,200 light-years away in Lyra and Kepler-69 about 2,700 light-years away in Cygnus. It described Kepler-62 as about one-fifth as luminous as the Sun. Three inner planets in that system complete their orbits in roughly six, twelve and eighteen days, in much hotter conditions than the two outer worlds. Borucki and colleagues’ discovery paper in Science reports radii of 1.31, 0.54, 1.95, 1.61 and 1.41 times Earth’s for Kepler-62b through Kepler-62f, respectively. The range includes a planet smaller than Earth as well as the larger outer pair. Kepler-62 is smaller and cooler than the Sun.
The climate models distinguished the closer, warmer Kepler-62e from the more distant Kepler-62f. In Harvard’s April 2013 account, the outer planet, Kepler-62f, would need substantial carbon-dioxide greenhouse warming to maintain a liquid ocean rather than freeze.
Co-author Dimitar Sasselov described the modelled contrast: “Kepler-62e probably has a very cloudy sky and is warm and humid all the way to the polar regions. Kepler-62f would be cooler, but still potentially life-friendly.” These descriptions express possible climates under model assumptions, not a weather report from another world.
Which three planets did Kepler discover?
The April 2013 announcement described two planetary systems containing three super-Earth-size planets considered candidates for habitable-zone conditions. Kepler-62e and Kepler-62f orbit the same star; Kepler-69c belongs to a different system.
The table below uses rounded radii and orbital periods from the linked NASA Science catalogue entries. Radius describes size, not mass.
| Planet | Radius relative to Earth | One orbit |
|---|---|---|
| Kepler-62e | 1.61 times | 122.4 Earth days |
| Kepler-62f | 1.41 times | 267.3 Earth days |
| Kepler-69c | 1.71 times | 242.5 Earth days |
For example, a radius 1.41 times Earth’s means roughly 41 percent larger in radius. It does not mean 41 percent more massive, and it does not tell us what someone would find on the surface.
How did Kepler find worlds it could not photograph?
Launched in March 2009, Kepler was a dedicated search for planets around other stars. Contemporary RT reporting described it as a $600 million observatory. Its central technique was to monitor stars for repeated decreases in brightness. When a planet crosses in front of its star from our viewpoint, it blocks a small portion of the light. These events are called transits. In its April 2013 release, JPL distinguished 2,740 candidates detected by Kepler from 122 confirmed planets. Keeping those categories separate is essential: a candidate signal still needs analysis to establish what caused it. Those numbers describe the mission’s progress in 2013, not its final total.
The timing of repeated transits helps establish an orbital period. The amount of light blocked, together with information about the star, helps estimate the planet’s size. That is a remarkable amount to learn from a faint signal, but it does not produce a photograph of continents, oceans or clouds. NASA Science’s guide to finding and characterising exoplanets explains how transits fit alongside other detection methods.
Does a “super-Earth” have to be like Earth?
In this discovery announcement, “super-Earth-size” describes planets larger than Earth. It is not a promise of familiar weather, breathable air or a solid surface. A name that sounds reassuring can cover worlds with very different properties.
The attractive planet pictures accompanying the discovery are artists’ interpretations. They help communicate possibilities and relative sizes, but their colours and surface details should not be read as observations.
What does the habitable zone tell us?
The habitable zone is the range of distances from a star where liquid water could exist on a planet’s surface under suitable conditions. Its location depends on the star. A cooler, dimmer star has a different zone from a hotter, brighter one.
That makes orbital location a starting point. It does not demonstrate that a planet has an ocean, a suitable atmosphere or living organisms. The wording “could support liquid surface water” carries assumptions that a headline can easily lose.
Why Kepler-69c needs extra care
Already in May 2013, Stephen Kane, Thomas Barclay and Dawn Gelino examined Kepler-69c’s likely conditions in A Potential Super-Venus in the Kepler-69 System, accepted for publication in The Astrophysical Journal Letters. They concluded it was unlikely to lie within the star’s habitable zone, even under optimistic assumptions.
Their analysis estimated that the planet received about 1.91 times the sunlight energy per unit area received by Earth. They suggested that a Venus-like interpretation could be more appropriate for its atmosphere and habitability.
This was an interpretation based on the planet’s properties and modelling, not a direct measurement of a Venus-like atmosphere. The distinction matters when comparing the three worlds. Similarities in size do not make them equally promising Earth analogues, and receiving too much stellar energy can be as consequential as receiving too little. Kepler-69c illustrates why the interpretation of a discovery may change when researchers examine its likely energy balance more closely.
What can these discoveries teach us?
Astronomy advances by turning broad possibilities into more precise questions. These worlds helped researchers investigate the relationship between planet size, starlight and potential habitability. Follow-up analysis then refined that picture.
For another perspective, explore the ocean beneath Saturn’s moon Enceladus. Its water is hidden under ice, illustrating why the search for habitable environments also reaches beyond the familiar picture of an Earth-like surface warmed by a nearby star.
Another way astronomy reads changes in light is gravitational lensing, illustrated by Hubble’s repeated images of a quasar. Kepler’s transit method measures a passing planet’s dimming of its star; lensing reveals what intervening gravity does to light from a background source.
The search continued beyond Kepler. The seven roughly Earth-sized planets of TRAPPIST-1 now allow researchers to ask a more demanding question: which worlds have atmospheres, and what do the observations actually rule out?
Kepler’s discoveries also belonged to a wider change in how astronomers understood the abundance of small planets. In a separate 2012 study, an ESO HARPS team estimated that roughly 40 percent of red dwarfs might host a super-Earth in the habitable zone. The estimate came from a survey of 102 stars and had a wide uncertainty range, reported as 28–95 percent around a central value of 41 percent. It was a population estimate, not a census of inhabited worlds. The result suggested that such planets could number in the tens of billions across the Milky Way.
Placing that estimate beside Kepler’s individual discoveries shows two complementary questions: what are particular planets like, and how common might comparable worlds be?
Kepler planet illustrations
“alien earth”Discovery coverage from 2013: RT.
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