For most of the time astronomers have thought seriously about other worlds, the basic numbers were missing. Nobody knew whether planets like Earth were ordinary or freak accidents, because we had a sample of one solar system and no way to count the rest. That has changed.
Two NASA telescopes, Kepler and the Transiting Exoplanet Survey Satellite, have turned the question from whether planets exist elsewhere into how the ones that do are distributed. The short version is that small planets are common, and rocky worlds in the right orbits may be common as well. What that leaves behind is a harder question, and it is not about the planets.
What Kepler actually counted
Kepler, which operated from 2009 until it ran out of fuel in 2018, did not photograph planets. It stared at a single patch of sky, about a quarter of one per cent of it, watching for the tiny, repeated dimming that happens when a planet passes in front of its star. From that narrow window it built a statistical census of the wider galaxy.
The central finding is that planets outnumber stars. Most stars have at least one, and the most common kind are not giants like Jupiter but small worlds between the size of Earth and Neptune, the super-Earths and sub-Neptunes that have no equivalent in our own system. TESS, launched in 2018, now scans almost the whole sky for the same dimming around brighter, nearer stars, marking out the ones worth a closer look.
How common is “Earth-like”?
The harder number is the fraction of stars with a roughly Earth-sized planet in the habitable zone, the band of orbits where a planet could, in principle, hold liquid water on its surface. Pinning it down means extrapolating from that small Kepler patch, and the answer carries wide error bars.
An early estimate from Erik Petigura and colleagues in 2013 put it at about 22 per cent of Sun-like stars, give or take eight. A later analysis led by Steve Bryson at NASA’s Ames Research Center, combining Kepler’s final data with star measurements from Europe’s Gaia mission, landed higher, at roughly half, while spelling out a range from a conservative 7 per cent to an optimistic 75 per cent. On even the most cautious reading, NASA put the number of potentially habitable worlds in the galaxy at 300 million or more.
The figures are soft, and the researchers who produced them say so. What they support is a modest claim, which is that such worlds are probably not rare.
What the habitable zone does not tell you
The phrase carries more weight than it should. A planet in the habitable zone is one at the right distance for liquid water, given a suitable atmosphere. It is not a planet known to have water, an atmosphere, or anything at all living on it.
Bryson’s team was careful on this point, noting that surface water is only one of many conditions life would require. As his colleague Ravi Kopparapu put it, habitability is not simply a matter of distance from a star, because not every star is alike and neither is every planet. A rocky world in the habitable zone is a candidate for further study, not a finding.
The question that is left
This is where the mystery moves. If small rocky planets in temperate orbits are common, then the galaxy has had a great many places, and a great deal of time, in which something could have arisen and made itself known.
So far, nothing has.
The physicist Enrico Fermi is said to have framed it over lunch in 1950 as a plain question: where is everybody? The radio searches that began with Frank Drake’s work in the 1960s, and continue today, have returned silence.
It is worth being careful about what that silence means. The searches so far have covered only a tiny fraction of the stars, the radio frequencies and the spans of time over which a signal might arrive, and a distant civilisation need not be broadcasting at all. The steps between a habitable-zone planet and a detectable neighbour, life beginning, growing complex, lasting, and choosing to signal, are entirely unmeasured. Absence of evidence, here, is not evidence of absence. It is mostly a measure of how little has been examined.
What to watch
Counting planets cannot settle this.
The next step is to look at them directly. The James Webb Space Telescope is already taking the measure of some nearby small planets’ atmospheres, though drawing a faint signal of life out of that data sits at the very edge of what it can do. The longer-term hope is NASA’s planned Habitable Worlds Observatory, a flagship designed to image a couple of dozen potentially Earth-like planets and read their atmospheres for gases that might, handled carefully, point to life. Whether the silence is telling us something, or only that we have barely begun to look, is a question for those instruments rather than for the census that raised it.
