“Six billion Earth-like planets” is a number large enough to stop a conversation. Its meaning, however, rests on a much more careful question: how common might small, potentially rocky planets be around stars similar to our Sun?
The headline grew from a 2020 analysis by University of British Columbia astronomers Michelle Kunimoto and Jaymie Matthews, using data from NASA’s Kepler mission. It described an estimate of a population, not a catalogue containing billions of newly discovered worlds.
The researchers described what the estimate meant
In the original 2020 account, Michelle Kunimoto’s result was quoted as an upper limit:
"place an upper limit of 0.18 Earth-like planets per G-type star."
This corresponds to fewer than one such planet per five G-type stars on average, not five planets for each star. Co-author Jaymie Matthews explained the Galactic extrapolation:
"Our Milky Way has as many as 400 billion stars, with seven per cent of them being G-type. That means less than six billion stars may have Earth-like planets in our Galaxy."
Kunimoto also described the forward-modelling procedure in her own words:
"I started by simulating the full population of exoplanets around the stars Kepler searched. I marked each planet as 'detected' or 'missed' depending on how likely it was my planet search algorithm would have found them. Then, I compared the detected planets to my actual catalogue of planets. If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars."
What counted as Earth-like?
In UBC’s explanation of the study, the relevant planets were roughly Earth-sized, potentially rocky and in the habitable zones of Sun-like G-type stars. A habitable zone is a range of conditions in which surface liquid water could be possible, given a suitable atmosphere.
The analysis placed an upper limit of 0.18 such planets per G-type star. Extrapolation to the galaxy produced the widely shared “as many as six billion” framing.
The definition does not establish blue oceans, breathable air, continents or living things. Those are additional properties that cannot be read directly from a size estimate and an orbit.
Why the planets we detect are not the whole population
Kepler searched for small dips in a star’s brightness when a planet crossed in front of it from our viewpoint. A planet can be present without producing a detectable signal: the orbit may be poorly aligned for a transit, the dip may be too faint or the observing period may be too short to capture enough events.
Kunimoto used forward modelling to investigate that missing population. Simulated planets were passed through a representation of the detection process, then the predicted detectable sample was compared with the real catalogue.
It is rather like estimating the fish in a lake after understanding which sizes your net tends to catch. The analogy is limited, but the essential point holds: the observed sample and the full population are different things.
Why another study produced a different headline
You may also have encountered the estimate of at least 300 million potentially habitable worlds. Different star samples, planet-size ranges, detection corrections and habitable-zone assumptions can produce different summaries.
Before deciding that two numbers contradict each other, compare what is being counted and whether the result is an upper bound, a lower bound or a central estimate. Those details are more informative than the largest number in the title.
The exciting result is the question becoming measurable
Astronomers cannot visit these hypothetical populations one world at a time. They can, however, use a carefully characterised survey to learn how common different types of planet may be.
That information helps frame future searches: which stars to observe, which signals may be rare and how much observing time a programme might need. Our guide to Kepler’s planet discoveries provides background on the method.
The number invites wonder. Understanding the estimate lets that wonder rest on something sturdier than the assumption that “Earth-like” means another Earth has already been found.
Original references and creator profiles: Kepler planet-hunting spacecraft; study; said that their calculations.
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