Composite of spacecraft work and an illustrated planetary scene, retained as context for exoplanet research. The planets are not a census photograph. Source.
In its 2020 analysis, NASA reported that there are 300 million planets in the Milky Way alone that could harbour life. The estimate concerns rocky planets in the habitable zones of Sun-like stars, with substantial uncertainty about occurrence rates and actual surface conditions. NASA described at least 300 million potentially habitable worlds; this does not establish that any of them supports life.
While that may seem like a lot of planets in our local area of space, the study estimated that several such planets could occur within about 30 light-years, including one potentially within about 20 light-years; these were statistical expectations, not four confirmed nearby habitats. This means that light from a hypothetical planet 20 light-years away takes 20 years to reach Earth, quite a distance. If we had a telescope strong enough to examine the planet closely, we would see it as it was 20 years ago and the same is true vice-versa, a hypothetical observer looking from that distance in 2020 would receive light that left Earth around 2000.
The data was gathered by the NASA deep-space telescope 'Kepler' which explored and mapped the skies, thereby giving us a good idea of how many far-off stars actually have planets surrounding them. Indeed, in Kepler's time in operation, over 9 years, it was able to identify 2,600 planets beyond our solar system.
The work of Kepler and its relationship with the recent research on the possibility of life on other planets was published in the The Astronomical Journal. Steve Bryson of NASA, who led the work, stated:
"Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable. Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision."
We may not though have to wait to explore distant planets before we can ascertain whether we are alone in the universe. Perseverance subsequently landed in February 2021 in Jezero Crater on Mars, where its mission included searching rocks for possible signs of ancient microbial life.
Some scientists believe that parts of ancient Mars had rivers, lakes and other liquid-water environments and therefore, could be thought to have had life upon it. If this is the case, it may well be that fossils of previously living creatures are still accessible with the right equipment. In 2020, a reported phosphine signal in Venus’s atmosphere prompted discussion of possible explanations. The detection and interpretation were disputed, and neither established organisms in the clouds nor showed that the gas could only come from life.
What the researchers counted
The team, led by Steve Bryson, focused on planets with radii between half and one-and-a-half times Earth’s radius, orbiting stars broadly similar to the Sun. That size range was chosen to concentrate on worlds more likely to be rocky.
They then investigated which planets could receive an amount of stellar energy compatible with surface liquid water under the study’s assumptions. The calculation is an occurrence-rate estimate: it concerns how common a class of planet may be.
It is not a list of 300 million measured atmospheres. Nor does it mean that astronomers verified oceans, living organisms or conditions comfortable for humans on those worlds.
Why Gaia made the calculation better
Distance from a star is only part of the story. Stars differ in temperature and energy output, so the same orbital distance can mean very different conditions around different stars.
Gaia supplied information that helped the researchers characterise the host stars more accurately. Combined with Kepler’s planetary signals, this allowed a more informative calculation of the energy reaching a possible planet.
A simple comparison is standing near different heaters: the distance matters, but so does the heater. For a planet, atmosphere and reflectivity add further complications. A habitable-zone label captures one useful part of a much larger environmental picture.
A population estimate is not a promise about a neighbour
Large numbers can make a discovery feel close at hand. But an estimate that a class of object is common does not specify which nearby target will turn out to belong to that class.
The practical value is in planning. If potentially rocky, temperate worlds are relatively common, that affects expectations for future surveys and the instruments needed to investigate them. It turns a broad hope into a question that can be tackled statistically.
How does this compare with the six-billion headline?
The separate UBC analysis behind the “six billion Earth-like planets” headline used its own definitions and modelling approach. In comparing studies, look first at the selected stars, the planet-size range, the treatment of missed detections and how uncertainty is expressed.
A lower, conservative galactic estimate and an upper-limit estimate are not interchangeable. Placing their headlines side by side without those details can create a disagreement that is partly about definitions.
For an example involving actual named planets rather than a galactic population, explore the TRAPPIST-1 system. Together, the two perspectives show what makes exoplanet science so rewarding: learning about individual worlds while gradually discovering how they fit into a much larger family.
Additional source and image references: NASA’s announcement · Technician working on spacecraft equipment beside an exoplanet illustration
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