Posted by Astronomers have confirmed thousands of worlds around other stars, and the number of planets that can support life, confirmed, is still zero. That is not pessimism, it is a statement about how hard the measurement is. What the field has instead is a shortlist: half a dozen planets that keep reappearing in telescope proposals, funding pitches and conference talks, each one interesting for its own reason and each one carrying a specific, named problem that stops anybody calling it habitable.
Knowing why each of them made the list tells you more about the search than any tidy definition of habitability ever will.
The usual starting point is the habitable zone, the band of orbits where a planet could hold liquid water on its surface given a reasonable atmosphere. Useful, and crude. Venus sits close enough to the inner edge that some calculations put it inside the zone, and its surface is hot enough to melt lead under ninety atmospheres of carbon dioxide. Orbital distance sets the energy budget. The atmosphere decides how it gets spent, and the atmosphere is almost always the thing we cannot see.
The One Next Door
Proxima Centauri b was announced in August 2016 by a team led by Guillem Anglada-Escude, off the back of a campaign called Pale Red Dot that pointed the HARPS spectrograph at La Silla at a single star, night after night, hunting a stellar wobble of barely a metre per second. The wobble was there. The planet has a minimum mass around 1.3 times Earth’s, an orbit of 11.2 days, and one advantage nothing else on the list can ever match: it is 4.24 light-years away, around the nearest star to the Sun.
Then the problems start.
Proxima Centauri is a red dwarf with a temper. In March 2016 the Evryscope array in Chile caught a superflare that brightened the star dozens of times over in visible light for a few minutes, and ALMA has watched others since. Proxima b orbits at roughly a twentieth of the Earth-Sun distance, so it takes that radiation full in the face, repeatedly, over billions of years. Whether it kept an atmosphere through all of that is unknown. Nobody has detected one. From our line of sight the planet does not transit its star, which removes the cheapest way of looking, so the answer will probably wait for telescopes that can image the planet directly.
Seven Planets Around a Star Barely Bigger Than Jupiter
TRAPPIST-1 is the system that reorganised everyone’s priorities. In February 2017 a team led by Michael Gillon announced seven roughly Earth-sized planets around an ultracool dwarf about forty light-years away in Aquarius, a star with something like nine percent of the Sun’s mass. All seven orbits would fit comfortably inside Mercury’s. TRAPPIST-1e is the one that holds a place on the shortlist: close to 0.92 Earth radii, a density consistent with rock, and receiving roughly the stellar energy Earth gets from the Sun.
It is also the best-instrumented case in existence, because all seven planets transit. Starlight filters through whatever atmosphere is there on its way to us, and JWST can read which molecules ate which wavelengths.
The early returns were sobering. JWST measured the dayside of the innermost planet, TRAPPIST-1b, at roughly 500 kelvin, about what bare rock with no air to move heat around would give you. TRAPPIST-1c came back looking similarly stripped. Those two sit much closer to the star than e does, so this is not a verdict on the whole system, but it sharpens the question the system was meant to answer: can a small planet hold onto its air next to a star like this one? Observations of e itself are still being gathered and argued over. Ask again in a couple of years.
A second complication applies to almost every red dwarf world here. A planet orbiting that close is very likely tidally locked, one hemisphere in permanent daylight, the other in permanent night. Climate models disagree about whether that is fatal. A thick atmosphere or a deep ocean could ferry heat to the dark side; a thin one would let the night side freeze out into a growing cap of ice that eventually swallows the air entirely. Both outcomes come from respectable physics.
Two Names Left Over from the Kepler Years
Kepler-186f, announced in April 2014, was the first Earth-sized planet found in the habitable zone of another star, and it earned its headlines. Around 1.1 Earth radii, a 130-day orbit around a red dwarf roughly half the Sun’s size, some five hundred light-years away in Cygnus. It receives about a third of the light Earth does, near the cold edge, where the difference between temperate and frozen solid comes down entirely to greenhouse gases nobody can measure at that range around a star that faint. Kepler-186f is a milestone. It is not really a target.
Kepler-452b, announced in July 2015, was presented as an older cousin of Earth: a 385-day orbit around a Sun-like star roughly 1,400 light-years off, the star itself well over a billion years older than ours. The complications piled up afterwards. At about 1.6 Earth radii it sits above the size where planets tend to stop being rock and start being small gas-wrapped worlds. And a 2018 reanalysis of Kepler’s statistics concluded that the detection could not be validated with confidence, meaning the planet may not be there at all. Worth remembering whenever a list of planets that can support life gets passed around: confirmed comes in grades.
K2-18b and the Argument That Will Not Settle
K2-18b is not Earth-like under any reading. Roughly 8.6 Earth masses and 2.6 Earth radii, orbiting a red dwarf about 124 light-years away in Leo, it is a sub-Neptune, the most abundant class of planet in the galaxy and the one class our own system somehow lacks.
It made the list on the strength of its air. Hubble found water vapour in its atmosphere in 2019. JWST followed in 2023 with methane and carbon dioxide and a conspicuous absence of ammonia, a combination consistent with the hycean scenario proposed by Nikku Madhusudhan’s group at Cambridge: a deep liquid-water ocean under a hydrogen envelope. The same spectrum carried a faint suggestion of dimethyl sulphide, a molecule that on Earth is produced almost entirely by marine plankton.
That faint suggestion has since consumed an extraordinary amount of the field’s attention. A 2025 follow-up claimed a stronger detection; independent reanalyses of the same data reported that the signal weakens or vanishes depending on how the atmosphere is modelled, and that dimethyl sulphide’s fingerprint overlaps awkwardly with other molecules. Nothing has been retracted. Nothing has been confirmed. Anyone who wants to know what a real biosignature dispute looks like should watch this one: slow, statistical, and unresolved for years at a stretch.
The Quiet Star
LHS 1140b gets less press and probably deserves more. Found in 2017 by the MEarth-South telescopes in a search led by Jason Dittmann, it circles a red dwarf about fifty light-years away in Cetus on a 25-day orbit inside the habitable zone. It is roughly 1.7 Earth radii and several times Earth’s mass, dense enough that most of it has to be rock and iron rather than gas.
The attraction is the star. LHS 1140 is old, slow-rotating and magnetically calm by red dwarf standards, nothing like the fireworks Proxima puts on. A planet there stood a decent chance of keeping whatever atmosphere it was born with. JWST observations reported in 2024 turned up tentative evidence of nitrogen, which would be the first hint of a secondary atmosphere on a temperate, mostly rocky world anywhere, if it survives scrutiny. The same data leave open a second reading, in which the planet is a water world, its ocean frozen over except for a warm patch facing the star. Both interpretations are still live.
What Would Actually Settle Any of This
Transit spectroscopy is a starved technique. You are chasing a change of a few tens of parts per million in the light of a distant star, stacking dozens of transits before the signal outruns the noise, and starspots on an active dwarf can imprint features that mimic planetary molecules. Two competent teams can pull different conclusions from one dataset, which is precisely what keeps happening.
The next steps are already being built. The Extremely Large Telescope in Chile, with its 39-metre mirror, should be able to attack the atmospheres of nearby rocky planets from the ground. Beyond it sits the concept of a large space observatory designed to blot out starlight and image Earth analogues around Sun-like stars directly, a project measured in decades.
The standard of proof will be high, and it should be. Oxygen alone means nothing; it can build up photochemically on a planet losing its water. Methane alone means nothing. What would move the needle is a set of molecules that should not coexist in chemical equilibrium, seen repeatedly, on a planet whose geology has been ruled out as the source. Phosphine on Venus in 2020 is the cautionary tale everyone cites now, a detection that arrived loudly and spent the following years shrinking under reanalysis.
So the shortlist of planets that can support life will keep changing, and some of the names above will drop off it. That is the process working, not failing. If you would rather follow these arguments while they are still open than read the tidy version a decade later, the exoplanet and astrobiology coverage at SETIworld is written for exactly that, and there is room for another curious reader in the conversation.
