Exoplanet Colonization

This brief owns the destination question. Not how to get there — that belongs to Interstellar Probes, Fusion Spacecraft and Beam-Powered Propulsion — but what is actually in the catalogue, what has been measured about it, and what would have to be true of a world before it could be called a target. The framing under test is that habitable exoplanets are settlement targets.

The catalogue is genuinely spectacular and it is the established core of this subject. More than six thousand planets are confirmed, the count is rising monthly, occurrence rates for rocky habitable-zone worlds around Sun-like stars are measured to within about a factor of ten, and a habitable-zone planet was confirmed eleven and a half light-years away in August 2026. None of that is in dispute and this brief carries it with the strongest flag.

The trouble starts one step later. The settlement-relevant subset is smaller than the headline by more than two orders of magnitude, the criterion that defines it is a cut on radius and insolation and nothing else, and the number of terrestrial habitable-zone exoplanets confirmed to hold an atmosphere is zero. That last figure is not a gap in a survey. It is the result of the survey: JWST went looking at the nearest and best-characterised system in the sky and found bare rock on one planet, no thick carbon dioxide on a second, and an unresolvable answer on the third.

So the page is organised around measurements rather than around scenarios. Sections 2 and 3 carry the catalogue, the occurrence rates, the JWST results, the flare record of the nearest star, and the arithmetic of a transit. The speculative material — that any of these places could be settled — is flagged as such throughout, and the one biosignature claim that reached the public was reversed three times in seven months by three independent groups. The most interesting thing in this subject is the measurement, not the inference.

Established The catalogue, current rather than remembered. NASA's public exoplanet page, updated 30 August 2026, states “more than 6,000 exoplanets” and cites 6,300+ on the same page, with “thousands more candidate planets await confirmation.” The NASA Exoplanet Archive crossed 6,000 on 17 September 2025, thirty years after the first detection, with over 8,000 candidates pending. Any brief still saying “thousands are confirmed” is a year and a half out of date.

Established And the archive is live, which matters for a subject where the numbers move. Releases logged through August 2026 include Gl 725 B c, a habitable-zone planet 11.4 light-years away (20 August 2026), the first microlensing planet confirmed by TESS (30 July 2026), and 55 new atmospheric spectra covering 34 planets. The discovery machine is working. The characterisation machine is the bottleneck, and the rest of this section is about what it has returned.

Established The settlement-relevant number is roughly two orders of magnitude smaller than the headline, and it is worth doing the arithmetic in public. The Planetary Habitability Laboratory's Habitable Worlds Catalog lists up to 70 potentially habitable worlds, split into a 29-planet conservative sample (“more likely to have a rocky composition and support surface liquid water”) and a 41-planet optimistic sample (“might include water worlds or mini-Neptunes, with a lower likelihood of habitable conditions”), against “over five thousand known exoplanets” at its last update on 21 March 2024. 29 out of ~6,000 is under 0.5%.

Established The criteria matter more than the count, and they are published verbatim. The conservative sample requires 0.5 < R_p ≤ 1.6 Earth radii, or 0.1 < minimum mass ≤ 3 Earth masses, in the conservative habitable zone. That is a statement about a planet's size and about how much light falls on it. It says nothing about atmosphere, magnetic field, surface water, plate tectonics, stellar activity, or tidal locking — and every one of those is unmeasured for every one of the twenty-nine. The catalogue's maintainers are a habitability laboratory and therefore an interested party; the criteria are stated openly, which is the appropriate response.

Established Occurrence rates are the genuine achievement of this field and should be reported with their error bars. Bryson and colleagues (arXiv:2010.14812, 2021), from Kepler DR25 with Gaia astrometry, for planets of 0.5–1.5 Earth radii around stars of 4800–6300 K: 0.37 (+0.48 / −0.21) to 0.60 (+0.90 / −0.36) planets per star in the conservative habitable zone, and 0.58 (+0.73 / −0.33) to 0.88 (+1.28 / −0.51) in the optimistic zone, with “95% confidence that, on average, the nearest HZ planet around G and K dwarfs is about 6 pc away” and roughly four within 10 parsecs. The lower bound of the conservative range is 0.16 and the upper is 1.5: the rate is known to within about a factor of ten, which is a real result and is not a targeting tool.

Established Now TRAPPIST-1, the system that was going to be the answer. The first planet measured has no atmosphere, and the measurement is unusually clean. Greene, Bell, Ducrot, Dyrek, Lagage and Fortney, Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST (Nature 618:39–42, 2023): secondary eclipse depth at 15 micrometres of 861 ± 99 ppm, detected at 8.7 sigma combined, giving a dayside brightness temperature of 503 (+26 / −27) K. The zero-albedo, no-redistribution bare-rock prediction is 508 K. The measurement lands on it.

Established The paper states what that rules out, and the exclusion is broad. “Atmospheres dominated by CO₂ or O₂ with some CO₂ (outgassed or desiccated) with surface pressures as low as 10 bar should have significant absorptions in the F1500W filter bandpass, reducing the observed brightness temperature to 300 K or less”, and “Our data are clearly incompatible with the T_eq = 400 K, CO₂, or O₂ + CO₂ models.” The conclusion: the planet “absorbs nearly all of the incident stellar flux and does not have a high-pressure or optically-thick atmosphere” — “little or no atmosphere of any kind.”

Established The second planet measured has no thick carbon dioxide, and the paper adds a system-wide inference that is more damaging than the planet-specific one. Zieba, Kreidberg and colleagues, No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c (Nature, 19 June 2023): planet-to-star flux ratio at 15 micrometres of 421 ± 94 ppm, dayside brightness temperature about 380 K, ruling out cloud-free oxygen/carbon-dioxide mixtures from 10 bar with 10 ppm CO₂ down to 0.1 bar of pure CO₂, and disfavouring a Venus-like sulfuric-acid-cloud atmosphere at 2.6 sigma. The formation inference: fewer than 9.5 Earth oceans of water in the system — a volatile-poor history that bears on all seven planets, not just this one.

Frontier The third and most important planet is undetermined, and the reason is the single most useful methodological fact in this brief. Glidden and colleagues, JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e (ApJL 990:L53, 2025): “we do not obtain strong evidence for or against an atmosphere.” Hydrogen-rich atmospheres with CO₂ and CH₄ are excluded. CO₂-rich atmospheres at Venus or Mars surface pressures are weakly disfavoured at 2 sigma. Still allowed: a nitrogen-rich atmosphere with trace CO₂ and CH₄, and bare rock — both fit adequately and neither fits fully.

Established And the limiting factor is not the telescope. Four JWST visits showed significant stellar contamination which varied between visits: the star's own spots and faculae imprint features on the transmission spectrum that mimic or mask planetary ones. The published strategy for getting past it is consecutive transits paired with TRAPPIST-1 b as a bare-rock reference, so the star's contribution can be subtracted. The wall in front of habitability measurements at M dwarfs is the star, and no amount of exposure time removes it.

Established Proxima b is the nearest candidate anywhere, and its star is the problem. Anglada-Escude and colleagues (Nature 536:437, 2016) report a period near 11.2 days, a minimum mass near 1.3 Earth masses, and a semi-major axis of about 0.05 AU — within the range where liquid surface water is possible around a star with roughly 0.0017 of the Sun's luminosity. The star is moderately active with an X-ray luminosity comparable to the Sun's, which around a star that faint is an enormous ratio.

Established The flare record is the number to carry. MacGregor and colleagues (arXiv:2104.09519, 2021), from a multiwavelength campaign in 2019, caught an event on 1 May 2019 in which Proxima Centauri brightened by a factor of more than 1,000 at millimetre wavelengths and more than 14,000 in the far ultraviolet, with a rise time under 5 seconds and a second event 510 seconds later. From TESS and Evryscope frequency distributions, “flares with energies and amplitudes larger than the May 1 event occur once per day in the optical.” The discovery team closed with a question rather than an answer: “Can a planet truly be habitable in this environment?” — noting that “necessary pieces are missing from our current understanding of M dwarf flares in order to answer that question.”

Frontier The modelling that connects flares to atmospheric loss points the wrong way for habitability, and specifically at the habitable zone. A 2025 study of the AU Mic system finds that for close-in planets quiescent extreme-ultraviolet output dominates, but at habitable-zone separations the flare contribution “is necessary to deplete primordial atmospheres fully” and can accelerate total atmospheric loss “by a few billion years.” The erosion mechanism is strongest exactly where the water would be. This is modelling rather than measurement and carries the middle flag accordingly.

Established Tidal locking is not in dispute for a planet at 0.05 AU; it is a geometric consequence. Frontier The habitability consequences of tidal locking are modelled and not observed, and the literature contains climate simulations reaching both optimistic and pessimistic conclusions depending on atmospheric mass and circulation assumptions — assumptions that, for every real candidate, are unconstrained because no atmosphere has been detected.

Frontier K2-18 b is the worked example of how not to read a spectrum, and the record is now three independent reversals deep. Madhusudhan and colleagues reported dimethyl sulfide and dimethyl disulfide in a JWST MIRI spectrum at roughly 3 sigma in 2025. Taylor (arXiv:2504.15916, 22 April 2025) found a flat baseline preferred in five of six tests, only about 2 sigma support for Gaussian features at the claimed peaks, and reduced chi-squared near unity: “no strong statistical evidence for spectral features.”

Established Two more reversals followed within seven months, by different teams using different methods. Luque, Piaulet-Ghorayeb, Radica, Xue, Zhang, Bean, Samra and Steinrueck (A&A 700, August 2025) ran the first joint 0.6–12 micrometre analysis and found that adding the species “causes ln Z to decrease by 0.3, indicating no evidence for these species”, attributing marginal preferences to “limiting the number of molecules considered... and oversensitivity to small changes between data reductions” — and estimating that about 25 more MIRI transits would be needed even for a 3-sigma rejection. Stevenson and colleagues (AJ 170(5), 3 November 2025) found that 87.5% of retrievals under their preferred binning do not support the detection, that “none are significantly detected at >3 sigma”, that ethylene and chloroethane fit at least as well, and that “red noise — rather than an astrophysical signal — plagues the mid-IR data.”

Established Three teams, three methods, one answer. A brief that says the claim is “disputed by independent reanalyses” without naming them is under-reporting the strongest thing in the record. The original paper was not re-obtained during this research pass and no figure from it is printed here except as characterised by its three rebuttals, which is the correct discipline when the claim under discussion is a claim about statistics.

Established Finally the transit arithmetic, done with real numbers rather than assumed drives. Voyager 1 cruises at 17.0 km/s relative to the Sun and was at 164.7 AU as of 21 August 2024, having launched in September 1977. Proxima Centauri is about 4.25 light-years — 4.02 × 10¹³ km, roughly 268,000 AU, about 1,600 times further than Voyager has travelled in forty-nine years. The division: 4.02 × 10¹³ km divided by 17 km/s is 2.36 × 10¹² seconds, or about 75,000 years. The figure that circulates is correct and it survives checking.

Established The optimistic substitution does not rescue it, and it is worth stating why. Parker Solar Probe reached 430,000 mph, about 692,000 km/h or 192 km/s, on 24 December 2024 — the fastest human-made object. That is a perihelion speed deep inside the Sun's gravity well, not a departure speed, so it bounds “how fast have we made a thing go” and not “how fast can we send a thing away.” Even granting it as a cruise speed, which physics does not, Proxima is about 6,600 years away. Anything faster belongs to Interstellar Probes, Fusion Spacecraft and Beam-Powered Propulsion; this brief carries two agency measurements and two divisions and hands over.

Established Position one is a measurement and the strongest thing on this page: rocky habitable-zone planets around Sun-like stars are common. 0.37 to 0.60 per star in the conservative zone for G and K dwarfs, with the nearest about 6 parsecs away at 95% confidence. Position two is also established: they are common around M dwarfs too, at roughly 15–45% per star on the post-2000 literature. These two positions are the reason the subject exists and they are the only ones in this list carrying the strongest flag.

Frontier Position three, and it is the live one: can a planet at an M dwarf keep an atmosphere against extreme-ultraviolet output and flares? No single holder; genuinely contested. The evidence so far: TRAPPIST-1 b has none, TRAPPIST-1 c has no thick carbon dioxide, TRAPPIST-1 e is undetermined, and AU Mic modelling says the flare contribution dominates atmospheric stripping precisely at habitable-zone separations. Three data points from one system, all pointing the same way, is suggestive and is not a population result.

Frontier Position four: TRAPPIST-1 e may still hold a secondary nitrogen atmosphere. Glidden and colleagues, with the crucial rider that bare rock fits about as well and that the limiting term is stellar contamination varying between visits. This is the single most consequential open question in the subject, and it is open in the strict sense: the data do not favour either answer.

Speculative Position five: Proxima b is habitable. Proposed at discovery and widely repeated. The orbit is real and the minimum mass is real; the star produces an optical flare larger than a millimetre-1,000-fold, far-ultraviolet-14,000-fold event roughly once a day, and the team that measured that declined to say whether a planet can be habitable there. Declining to answer, in print, in the discovery paper, is a stronger statement than most negative results.

Speculative Position six: K2-18 b shows a biosignature. Held by the original team at roughly 3 sigma; reversed independently by three groups within seven months, one of which attributes the feature to red noise and two of which name ordinary molecules that fit at least as well. Position seven: a world can be confirmed habitable before a mission is committed. Implicit in every settlement proposal and unsupported: no instrument has confirmed an atmosphere on any terrestrial habitable-zone exoplanet.

Frontier Position eight is the requirement ladder, and it is the most useful single object in this brief. Confirmed to exist: established, for about 6,000. Terrestrial radius and conservative habitable zone: established, for 29. Confirmed to retain an atmosphere: established for zero. Atmosphere characterised for composition: frontier, and the one biosignature attempt has been reversed three times. Stellar environment survivable: frontier, and the nearest case flares daily. Surface conditions known: none. Reachable: 75,000 years at demonstrated cruise speed. Settleable — the Aurora term: unmeasured for every candidate.

Handwave Position nine: generation ships or suspended animation close the transit gap. Not this brief's to assess — Interstellar Probes owns the vehicle and Multi-Planetary Civilization owns whether a closed human population can be sustained at all — but the 75,000-year figure is this brief's, and it is the number those proposals exist to defeat. Position ten: terraforming an exoplanet. It compounds every unsolved problem in the ladder above with everything in Terraforming, and no candidate world is characterised well enough for the question to be posed.

Established The first bottleneck is stellar contamination, and it is the one the field named itself. An M dwarf's spots and faculae imprint wavelength-dependent structure on a transmission spectrum that can mimic or cancel a planetary signal, and on TRAPPIST-1 e it varied between visits. This is not photon noise and more integration time does not fix it. The published route past it — consecutive transits with a known bare-rock planet in the same system as a reference — is a scheduling and modelling solution, not an instrument one.

Established The second is that emission and transmission spectroscopy at 15 micrometres is near the edge of what JWST can do for Earth-sized planets, and the eclipse depths say so. 861 ± 99 ppm at 8.7 sigma is a strong detection; 421 ± 94 ppm is a weaker one; and the answer for the habitable-zone planet is a non-answer. The instrument was good enough to exclude thick atmospheres and is not good enough to confirm thin ones, which is precisely the regime a habitable world would occupy. Collecting area is Mega-Telescopes' subject and it is the binding term here.

Frontier The third is statistical rather than physical, and the K2-18 b episode is its case study. Marginal spectral features at 2–3 sigma are sensitive to binning choices, to the number of molecules included in a retrieval, and to correlated noise. Luque and colleagues estimate roughly 25 additional MIRI transits would be needed for a 3-sigma rejection of the original claim. The cost of disproving a marginal detection exceeds the cost of making one, and that asymmetry is a structural hazard for the whole biosignature programme.

Frontier The fourth is the host star, and it is unfixable. The planets that are easiest to characterise orbit the stars that are worst for habitability: M dwarfs give deep transits, frequent repeats and favourable contrast, and they also flare daily, emit hard ultraviolet, and tidally lock their habitable zones. The selection effect that made these worlds observable is the same effect that makes them hostile, and no instrument removes it — only a different, harder target class does.

Established The fifth is distance, and it is arithmetic. 75,000 years at 17 km/s; about 6,600 years even at a perihelion speed that cannot be used as a cruise speed. No measurement in this brief changes if the trip is shortened, and no shortening changes the fact that zero candidate atmospheres have been confirmed. The two problems are independent, which is why the destination question is worth separating from the propulsion question at all.

Handwave And a sixth that is a hole rather than an obstacle: settleability. Habitability as measured is a cut on radius and insolation. Settleability — whether a formally habitable world could actually support a landed population — has no measurement, no proxy and no proposed instrument. Interstellar Civilization Models needs this term as a model parameter and it does not exist.

Established Three adjudicated dependencies, and all three are the same seam at different depths: this brief owns the destination, they own the journey. Interstellar Probes owns mission architecture, sail and beam engineering, and every claim about shortening a transit. Fusion Spacecraft and Beam-Powered Propulsion own the drive physics one level down. This brief uses exactly two propulsion numbers, both agency-sourced, performs two divisions with them, and stops.

Established The reverse direction of that seam is the part worth stating explicitly, because it is where the collision would otherwise happen. A propulsion brief should not adjudicate habitability or cite a habitability catalogue as justification for a mission. The destination question has an answer independent of drive performance, and the answer is currently that no candidate world is known to have air.

Frontier The fourth relationship is not a dependency but a supply: Interstellar Civilization Models needs the Aurora term from this brief. That brief owns the settleable fraction as a model parameter; this one owns what is known about it, which is almost nothing. Stating that the settleable fraction is unmeasured for all twenty-nine conservative-sample worlds is the substantive contribution, and it is more useful to the models than a fabricated value. The reverse holds too: II-13 does not re-list exoplanet counts.

Established Fifth, Mega-Telescopes, which is not a typed edge here but is the physical constraint behind every negative result above. Whether a thin secondary atmosphere on a habitable-zone terrestrial planet can be detected at all is a question about collecting area, spectral stability and the ability to model a variable star — not about exoplanet science. The wall in this brief is an instrument wall wearing an astrophysical disguise.

Speculative And one dependency this brief cannot discharge: planetary protection. Forward contamination of a possibly-inhabited world is a live constraint rather than a rhetorical one. The governing framework was not obtained during this research and no category or numerical requirement from it is printed here. What can be said without it: the same measurements that failed to find atmospheres are the only tool that could establish a world is uninhabited before a mission is committed, and they cannot currently do so for any of the twenty-nine.

Established Experiment one is already designed and is the highest-value observation in the subject: consecutive transits of TRAPPIST-1 e paired with TRAPPIST-1 b as a bare-rock reference. The point is to subtract the star. Because b is now measured as having little or no atmosphere, it functions as a contamination standard within the same system observed at the same epoch. If this works it converts an undetermined result into a determination in either direction, which is exactly what a well-posed experiment does.

Frontier Experiment two: enough MIRI time on K2-18 b to reject rather than to claim. Luque and colleagues price it at roughly 25 more transits for a 3-sigma rejection. The scientific value of spending that time is high and the institutional incentive to spend it is low, because a rejection generates no headline. Naming the number is the useful thing this brief can do about it.

Established Experiment three: emission photometry on the remaining TRAPPIST-1 planets, particularly d, f and g. The b and c results were obtained the same way and cost modest amounts of time. A system-wide census of which planets retain atmospheres would turn one system into a population statistic about M-dwarf atmospheric retention — the question section 3 flags as the live one — and the volatile-poor inference from c's water budget is already a system-wide claim that such a census would test.

Frontier Experiment four: long-baseline flare monitoring of the nearest M dwarfs across the millimetre-to-ultraviolet range simultaneously. The 1 May 2019 Proxima event was caught only because nine instruments were watching at once, and it lasted under five seconds at rise. The flare energy distribution, not the mean activity level, is what determines atmospheric loss, and it is measured for essentially one star.

Speculative Experiment five: an agreed statistical protocol for biosignature claims, pre-registered. Binning choices, molecule lists and noise models decided before the data are examined, with a stated threshold. Three independent reanalyses of one claim in seven months is a healthy field responding correctly to a weak result, and it is also an enormous waste of the field's scarcest resource.

Frontier Experiment six, which nobody can run yet: a direct measurement of surface conditions on any of the twenty-nine. No instrument now operating or funded resolves surface pressure, surface temperature or the presence of liquid water on a terrestrial exoplanet. Every claim about a candidate world's surface is an inference from a radius and an insolation, and saying so is the honest position rather than a pessimistic one.

Established The engineering requirement in this subject is measurement stability, not spacecraft. Every result in section 2 came from photometry at the parts-per-million level in the mid-infrared, and the differences between a detection, a non-detection and an artefact are of the order of a hundred parts per million. An 861 ppm eclipse depth with a 99 ppm uncertainty is what “this planet has no atmosphere” looks like in practice.

Frontier The second requirement is a stellar model good enough to subtract. The TRAPPIST-1 e result is limited by contamination that varies between visits, which means the required object is a time-dependent model of a spotted M dwarf's photosphere, validated against a planet in the same system known to be bare. That is a stellar-astrophysics deliverable standing in the way of an exoplanet result, and it is the clearest example on this page of a bottleneck sitting outside the field that owns the question.

Established Third, retrieval software and its statistics. The K2-18 b episode turned on binning, on the molecule list included in the retrieval, and on whether correlated noise was modelled — three software choices, each defensible, together spanning the difference between a biosignature announcement and a flat line. The engineering requirement is that these choices be declared and their sensitivity published alongside the result, which the three rebuttal papers did and which is why the reversal was possible at all.

Established Fourth, catalogue engineering, which is unglamorous and load-bearing. The confirmed count, the candidate count and the habitable-subset count come from three different maintained products with different update cadences — and one of them, the habitability catalogue, was last updated in March 2024. A brief quoting 29 conservative-sample worlds against a 6,300-planet total is combining a 2024 numerator with a 2026 denominator, and should say so. This one does.

Speculative And fifth, the requirement this brief explicitly does not own: everything that would make 4.25 light-years a shorter number. Propulsion, power, shielding, autonomy and closure are Interstellar Probes', Fusion Spacecraft', Beam-Powered Propulsion' and Multi-Planetary Civilization's. What this brief supplies to them is the target list, and the target list currently contains no world confirmed to have an atmosphere.

Established The primary seam is with propulsion, and it is drawn on purpose. Interstellar Probes owns how to get there; this brief owns whether there is a there. Two agency speed figures cross the seam — Voyager's 17 km/s and Parker's 192 km/s — and nothing else. Fusion Spacecraft and Beam-Powered Propulsion hold the drive physics whose absence makes 75,000 years the operative number.

Frontier Second, Interstellar Civilization Models, which needs two things from here and gets one. It gets the occurrence rate, which is measured. It does not get the settleable fraction, which is not. The models run on a free parameter and this brief is the reason it is free.

Established Third, Multi-Planetary Civilization, which asks whether a second world secures the species. The interstellar arm of that argument inherits this brief's findings directly: the nearest terrestrial candidates have no measured atmosphere and sit 75,000 years away at demonstrated cruise speed. That brief owns the closure and population argument; this one supplies the destination facts it reasons from.

Speculative Fourth, Terraforming, adjacent and downstream of everything unsolved here. Modifying a world's atmosphere presupposes knowing what atmosphere it has, and for every candidate in the conservative sample that is unknown. Fifth, Mega-Telescopes: the instrument programme that would move any of this, and the reason the current answers are bounded the way they are.

Established And one boundary drawn to prevent a specific error: Civilization Timelines owns the reading of “becoming interstellar” as a developmental stage a civilisation passes through, and flags that reading speculative. This brief does not frame settlement as a stage. It treats a destination as an empirical object with measurable properties, most of which have not been measured. Biosignature and technosignature spectroscopy share instruments but not questions: any claim about industrial markers in an exoplanet atmosphere belongs to Interstellar Archaeology.

Established The first institutional fact is a good one and deserves saying: the exoplanet catalogue is a functioning public scientific infrastructure. A confirmed-planet archive with a public news log, dated releases, candidate counts and reproducible criteria is the reason this brief can quote a number from August 2026 rather than a recollection. Most of the subjects on this map have no equivalent.

Frontier The second is an update-cadence problem with a real consequence. The habitability catalogue that supplies the twenty-nine conservative-sample worlds was last updated in March 2024, against a confirmed count that has risen by more than a thousand since. The ratio quoted everywhere — twenty-nine out of six thousand — is therefore a lower bound on the numerator and a current denominator, and the honest version says so rather than presenting a clean fraction.

Established Third, an announcement-norm problem that this field has just worked through in public and largely got right. A roughly 3-sigma biosignature claim reached global coverage; three independent groups reanalysed it within seven months; all three found the evidence insufficient; two named ordinary molecules that fit at least as well; one attributed the feature to red noise. The self-correction worked. What it cost was three teams' time and a durable public impression that a biosignature was found.

Frontier Fourth, an allocation question that is genuinely hard. Confirming that a planet has no atmosphere is cheap and decisive; confirming that it has a thin one is expensive and may be impossible with current apertures. A time-allocation committee optimising for decisive results will systematically fund the observations that produce negative habitability answers, which is scientifically sound and produces a literature that looks more pessimistic than the underlying reality warrants. This brief carries the negatives and flags the asymmetry.

Speculative Fifth, planetary protection, named and not quantified. The international framework governing forward contamination of possibly-inhabited bodies was not obtained during this research and no category or requirement from it is printed here. The institutional gap that can be stated without it is that no body has authority over, or a procedure for, a destination outside the solar system, and Space Law and Governance owns what exists in the near case.

Frontier The first ethical question is contamination, and it is not hypothetical in structure even though it is remote in time. Committing a mission to a world that might host life requires knowing whether it does, and the instruments that would tell us are the same ones that have so far failed to detect an atmosphere on any terrestrial habitable-zone planet. The epistemic gap and the ethical gap are the same gap: we cannot currently establish that a target is uninhabited.

Established The second is the public-communication ethics of marginal claims, and this subject has just run the experiment. A 3-sigma feature became a worldwide story about possible life; three reanalyses reversed it; the reversals reached a fraction of the original audience. The harm is not to the field's internal process, which worked, but to the currency of the word “biosignature” — and the next genuine detection will be received in a landscape this episode helped shape.

Frontier Third, the ethics of “potentially habitable” as a public label. In the catalogue it denotes a cut on radius and insolation. In circulation it denotes a place one might live. The gap between those two meanings is where nearly all the popular misunderstanding of this subject lives, and the catalogue's own criteria are published precisely so that the gap can be closed by anyone who looks.

Speculative Fourth, the escape framing, which this brief hands over rather than argues. Exoplanet settlement is sometimes offered as an answer to terrestrial risk. Multi-Planetary Civilization owns that argument and finds it conditional on a self-sufficiency that has never been demonstrated anywhere. What this brief adds is the destination side of the same audit: at 75,000 years to the nearest candidate, with no candidate confirmed to have air, exoplanet settlement is not a risk-reduction measure on any timescale a risk argument can use.

Established And fifth, a smaller point about attribution. The habitability catalogue is maintained by a habitability laboratory, and several of the most-cited optimistic framings in this subject come from institutions whose funding depends on the subject being promising. That is normal and it should be marked; equally, the JWST results that went against habitability were produced by teams who proposed those observations hoping for the opposite answer, and that is the strongest form of evidence available anywhere in this cluster.

Established The civilizational content of this brief is a single arithmetic pair. Six thousand planets confirmed; twenty-nine that pass a two-parameter cut; zero with a confirmed atmosphere. Thirty years of the most successful observational programme in modern astronomy has produced a catalogue of extraordinary richness and not one world anyone can say is habitable.

Frontier The second implication is about where the next thirty years go. The characterisation results so far say that the easy targets — close-in planets at small, bright-in-contrast stars — are the hostile ones, and that the star itself, not the telescope, is the wall. Progress on the destination question therefore requires either much larger apertures or a fundamentally better model of a spotted M dwarf, and the second is cheaper.

Speculative Third, the effect on the settlement argument, which is severe and often unstated. Every interstellar settlement proposal assumes a destination worth the journey. The measured record supplies candidates selected on radius and insolation, no confirmed atmospheres, an active three-way reversal of the one biosignature claim, and a transit time of seventy-five millennia at demonstrated speed. The proposal is not refuted; it is unsupported at the destination end, which is the end nobody audits.

Frontier Fourth, and it cuts the other way, because a brief that only carries the negatives is misreporting too. Occurrence rates say rocky habitable-zone worlds are common and the nearest is a few parsecs away. A habitable-zone planet was confirmed at 11.4 light-years in August 2026. The population is there. What is missing is any means of knowing whether any member of it has air, and that is a solvable instrumental problem rather than a fact about the universe.

Established And fifth, the reframing this brief actually recommends. Exoplanet science is not a settlement programme with a slow start. It is the most productive observational field of the last three decades, whose findings happen to be discouraging for settlement and encouraging for almost everything else. Reading it primarily as a search for a second home understates it.

These horizons track what can be measured about candidate worlds, not when anyone might go — nothing in this brief supports forecasting the second.

• 10 yr: Frontier TRAPPIST-1 e is either determined or not, and the deciding factor is whether stellar contamination can be subtracted using b as a bare-rock reference across consecutive transits. Established The confirmed-planet count continues rising and the habitable-zone subset continues to be dominated by two-parameter cuts, because the characterising observations are far more expensive than the discovering ones. Frontier The K2-18 b claim is either rejected at 3 sigma or left unresolved; the observing cost of a rejection is published and the institutional incentive to pay it is weak.
• 25 yr: Frontier If a substantially larger aperture flies, thin secondary atmospheres on terrestrial habitable-zone planets move from undetectable to marginal, and the number of worlds with a confirmed atmosphere becomes non-zero or is shown to be hard to make non-zero. Speculative A population statistic on M-dwarf atmospheric retention becomes possible with a few tens of systems characterised, and it is the result that would settle position three. Speculative No surface measurement of any terrestrial exoplanet is in prospect on this horizon.
• 50 yr: Speculative Direct characterisation of a habitable-zone terrestrial planet around a Sun-like star — the target class that avoids the flare problem entirely — is the plausible outcome of a very large space telescope programme, and it is the first observation that could plausibly identify a world anyone would call a target. Handwave Any claim about a launched interstellar mission at this horizon is Interstellar Probes' to make or decline, and the 75,000-year figure remains the number to beat until a drive exists.
• 100 / 250+ yr: Handwave Beyond forecasting. Established The structural statement that survives any horizon is the arithmetic: 4.02 × 10¹³ km at 17 km/s is 75,000 years, and even the fastest object humanity has accelerated — at a perihelion speed that cannot be used for departure — gives about 6,600 years. Speculative Every scenario shorter than that is a claim about a propulsion technology, and every scenario at all is a claim about a destination whose atmosphere is currently unmeasured.

• Depends on Three edges, all the same seam at different depths, and all of them about the journey rather than the destination. Interstellar Probes owns mission architecture and everything that would shorten a transit; Fusion Spacecraft and Beam-Powered Propulsion own the drive physics one level below it. This brief depends on them in the strict sense: its central arithmetic — 75,000 years to Proxima at Voyager's demonstrated 17 km/s, about 6,600 years even at Parker's unusable perihelion speed — is a statement about their subject matter, and the destination question only becomes actionable if one of them produces a result. The reverse does not hold, and that asymmetry is the point: the destination question has an answer today, and the answer is that no candidate world is confirmed to have an atmosphere.
• Requires (not on this map) Two constraints that are not briefs on this map. The first is a scientific result nobody has produced: an atmosphere confirmed on a terrestrial planet in a conservative habitable zone. Until one exists, every target in the catalogue is selected on radius and insolation alone, and the ladder in section 3 stops at its third rung. The second is institutional: no body has authority over, or a procedure for, forward contamination of a destination outside the solar system, and the framework governing the near case was not obtained during this research, so no category or requirement from it is printed here.
• Enables No typed enabling edge is claimed. A catalogue of candidate worlds selected on radius and insolation, with zero confirmed atmospheres among them, does not enable a downstream capability. What this brief supplies instead is a parameter that Interstellar Civilization Models needs and cannot get — the settleable fraction, the Aurora term — and the substantive finding is that it is unmeasured for every world in the conservative sample. A supplied absence is not an enabling edge and is not recorded as one.
• Adjacent Interstellar Civilization Models owns population and distribution modelling and takes the occurrence rate from here. Multi-Planetary Civilization owns whether a second world secures the species and inherits this brief's destination findings for its interstellar arm. Mega-Telescopes owns the aperture and stability that bound every negative result on this page. Terraforming is downstream of an atmospheric characterisation nobody has achieved. Interstellar Archaeology owns industrial markers in exoplanet atmospheres, which share instruments with biosignature work and are a different question. Civilization Timelines owns the reading of interstellar settlement as a developmental stage, which this brief does not adopt.

Established “Thousands of exoplanets have been confirmed.” The count passed 6,000 on 17 September 2025 and NASA cites 6,300+ as of August 2026, with over 8,000 candidates pending. Established The correction matters because the interesting ratio is between that number and the settlement-relevant subset — 29 in the conservative habitable-zone sample, under 0.5% — and quoting a stale total makes the ratio look better than it is.

Established “There are twenty-nine potentially habitable planets.” There are twenty-nine planets that pass a published cut: radius between 0.5 and 1.6 Earth radii, or minimum mass between 0.1 and 3 Earth masses, inside the conservative habitable zone. Established That is a statement about size and about incident starlight. Atmosphere, magnetic field, surface water, plate tectonics, stellar activity and tidal locking are unmeasured for all twenty-nine. The catalogue publishes its criteria; the misconception is created downstream of it.

Established “TRAPPIST-1 is our best bet for a second Earth.” Two of its planets have now been measured. Established TRAPPIST-1 b's dayside is 503 K against a 508 K bare-rock prediction — the planet does nothing an atmosphere would do — and TRAPPIST-1 c rules out carbon-dioxide atmospheres from 10 bar down to 0.1 bar, with a formation inference of fewer than 9.5 Earth oceans of water for the whole system. Frontier TRAPPIST-1 e remains open, with bare rock and a thin nitrogen atmosphere fitting about equally well.

Established “JWST just needs more time on TRAPPIST-1 e.” Time is not the limit. Established Four visits showed significant stellar contamination that varied between visits — the star's own surface changes the spectrum — and the published route past it is consecutive transits with a bare-rock planet in the same system as a reference, which is a modelling and scheduling strategy rather than an integration-time one. Reporting the result as “not enough signal” misstates what the paper says.

Established “K2-18 b shows signs of life.” Three independent groups reanalysed the claim within seven months and all three found the evidence insufficient. Established One found a flat baseline preferred in five of six tests with only ~2 sigma support for the features; one found that adding the molecules decreased the Bayesian evidence and priced a 3-sigma rejection at about 25 more transits; one found 87.5% of retrievals unsupportive, named ethylene and chloroethane as fitting at least as well, and attributed the feature to red noise. Speculative The original paper was not re-obtained for this brief and no figure from it is printed here except as its rebuttals characterise it.

Speculative “Proxima b is an Earth-like planet next door.” It has a real orbit, a real minimum mass near 1.3 Earth masses, and a semi-major axis of 0.05 AU around a star with 0.0017 of the Sun's luminosity. Established That star brightened by more than 1,000-fold in the millimetre and more than 14,000-fold in the far ultraviolet in a single event with a rise time under five seconds, and events larger than that occur roughly once a day in the optical. Frontier The team that measured it asked in print whether a planet can be habitable in that environment and declined to answer. Nothing is known about whether the planet has an atmosphere.

Frontier “Tidal locking is not really a problem — models show habitable climates.” Some models do, and others do not, and every one of them depends on an assumed atmospheric mass and circulation. Established For every real candidate the atmosphere is unmeasured, so the input that decides the model's output is exactly the thing nobody has. The locking itself is a geometric consequence of a close orbit and is not in dispute.

Established “The 75,000-year figure is a pessimistic estimate.” It is a division. Established 4.02 × 10¹³ km to Proxima divided by Voyager 1's measured 17.0 km/s is 2.36 × 10¹² seconds. Established The optimistic substitution — Parker Solar Probe's 192 km/s — is a perihelion speed inside the Sun's gravity well rather than a departure speed, and even granting it gives about 6,600 years, longer than recorded history. Anything faster is a claim about a drive that does not exist, and it belongs to Interstellar Probes.

Handwave “Generation ships or suspended animation make the distance irrelevant.” They make the transit survivable in principle, which is a different claim, and they are assessed elsewhere on this map. Speculative What this brief can say is that the destination end of those proposals is unaudited: they are journeys to worlds selected on radius and insolation, none of which is confirmed to have air. Multi-Planetary Civilization carries what is known about sustaining a closed human population, and the answer there is also negative.

Established And the framing itself. “Habitable exoplanets are settlement targets” fails on the first word rather than the last. Established No exoplanet is known to be habitable, because none is known to have an atmosphere; the label “potentially habitable” is a two-parameter cut; the two nearest and best-studied systems have returned bare rock, no thick carbon dioxide, an undetermined third case limited by starlight rather than sensitivity, and a nearest star that flares daily. Frontier The catalogue is a triumph and the destination question is unanswered, and those two statements are both true and rarely printed together.