{"id":"a6cc043b-1212-4927-b50b-7aab137bd58d","arxiv_id":"2502.09702","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Earth-mass planets in the habitable zones of stars below 0.4 solar masses are unlikely to retain CO2- or N2-dominated atmospheres, and current JWST rocky-planet targets all sit inside the atmospheric-loss region.","lead":"This paper calculates the distance from a star at which an Earth-sized planet can keep a CO2 or N2 atmosphere, and compares it with the habitable zone. It finds that planets around the smallest stars (under about 0.4 solar masses) are unlikely to keep any atmosphere, and that all JWST-observed rocky planets are outside this retention distance.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The solar-scaled EUV spectrum assumption in Sect. 2.3 is load-bearing: it is least valid for the fully convective M dwarfs that drive the <0.4 Msun conclusion.","rationale":"The central claim requires reliable exobase temperatures across stellar mass, and the weakest step is the assumption that one solar EUV spectrum, uniformly scaled, represents all hosts. This assumption is acknowledged in Sect. 2.3, and the paper's own reference to VL24 Fig. 7 shows that spectral shape can affect the pressure-temperature profile. The concern is amplified because the strongest conclusion applies to fully convective M dwarfs, for which the solar analog argument is least applicable. The qualitative trend of decreasing ARD with increasing stellar mass and rotation is likely robust, so I do not oppose the overall verdict, but the quantitative boundary at 0.4 Msun is not settled by the presented models. The reader's weakest assumption identifies the same issue, and the conditional verdict remains appropriate.","tokens_in":17095,"tokens_out":11518,"duration_ms":124055,"concrete_test":"Rerun the Kompot models for a 0.3 Msun and a 0.5 Msun host at F_EUV = 6-14 F_EUV,sun using a reconstructed M-dwarf XUV spectrum (e.g., Fontenla et al. 2016 or the TRAPPIST-1 proxy used in VL24) normalized to the same 10-92 nm integrated flux. Recompute the critical catastrophic irradiance and ARD for the CO2- and N2-dominated compositions. If the 0.4 Msun threshold shifts outside the 0.35-0.45 Msun range, the headline conclusion requires rephrasing; if it stays within that range, the spectral-shape concern is not load-bearing.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 2.3 assumes that one solar spectrum, uniformly scaled in flux, represents the XUV irradiation of all hosts from 0.1 to 1.2 Msun. The exobase temperature enters Jeans escape exponentially and is sensitive to spectral shape, as shown in VL24 Fig. 7. The paper justifies the assumption by noting that most known planets orbit relatively evolved main-sequence stars, but the headline claim concerns fully convective M dwarfs, where the EUV spectrum is line-dominated and the X-ray-to-EUV ratio differs from the Sun. The ARD and the 0.4 Msun threshold are therefore least secure exactly where the conclusion is strongest. The paper acknowledges this uncertainty and the lower-limit nature of Jeans escape, but the caveat does not remove the risk: if a realistic M-dwarf spectrum yields a cooler exobase at fixed 10-92 nm flux, Jeans loss decreases and the threshold could move below 0.4 Msun; if it yields a hotter exobase, the claim is strengthened. Given the exponential sensitivity of escape to exobase temperature, the central mass boundary should be treated as provisional until this spectral-shape sensitivity is quantified.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper introduces the 'atmospheric retention distance' (ARD): the minimum orbital distance at which an Earth-mass planet with a CO2- or N2-dominated secondary atmosphere would lose the equivalent of one Earth atmosphere in 10 Myr by Jeans escape. The ARD is computed by combining thermochemical upper-atmosphere models (Kompot; Van Looveren et al. 2024) with stellar evolution and rotation models (Johnstone et al. 2021a) for stellar masses 0.1-1.2 Msun, ages 1-12000 Myr, and slow/medium/fast initial rotators, and is compared to the conservative habitable zone of Kopparapu et al. (2013, 2014). The authors find that the HZ-ARD overlap appears earlier around slowly rotating stars; that HZ planets around fully convective stars with masses below about 0.4 Msun are unlikely to retain atmospheres; that initial rotation affects retention probability; and that scheduled JWST cycle 1/2 Earth-sized rocky targets, including HZ planets, lie closer than the ARD. They validate the model against Archean Earth pressure constraints and Venus's resurfacing history, and stress that Jeans escape is a lower limit to total loss.","tokens_in":17365,"tokens_out":9515,"duration_ms":96924,"significance":"If the results are robust, the ARD provides a simple, physically motivated target-selection criterion for JWST and Ariel and a falsifiable prediction that HZ rocky planets around very-low-mass M dwarfs should lack thick CO2/N2 atmospheres. The paper builds on published, externally benchmarked models rather than fitting to the targets; the Earth-Archean and Venus comparisons are concrete consistency checks. Its main strengths are the explicit treatment of the calculation as a lower limit on loss, the systematic coverage of stellar mass, age, rotation, and atmospheric composition, and the clear presentation of a new diagnostic quantity. The principal uncertainties are the scaled-solar EUV spectral assumption for M dwarfs, the adopted 1 Gyr 'grace period', and the exclusion of hydrodynamic and non-thermal loss; these make the quantitative mass threshold provisional pending sensitivity tests.","major_comments":[{"comment":"The assumption that one solar EUV spectrum, uniformly scaled in flux, represents the XUV irradiation of all hosts from 0.1 to 1.2 Msun is load-bearing for the central <0.4 Msun threshold. As the paper recognizes, VL24 Fig. 7 shows that the exobase thermal structure is sensitive to spectral shape, and Jeans escape depends exponentially on exobase temperature. The stated justification (most known planets orbit relatively evolved main-sequence stars) does not apply to the fully convective M dwarfs on which the headline conclusion rests. Please add a quantitative sensitivity analysis using reconstructed M-dwarf EUV spectra (e.g., Fontenla et al. 2016; Namekata et al. 2023) and report how the ARD and the 0.4 Msun boundary change.","section":"Sect. 2.3"},{"comment":"The ARD is based on Jeans escape only, which the authors correctly call a lower limit, yet the captions state 'above these lines, atmospheres can be retained' and the conclusions state that HZ planets around stars above 0.4 Msun 'are most likely to retain' atmospheres. Because hydrodynamic and non-thermal losses are excluded, an orbit outside the Jeans-based ARD does not guarantee retention; it only means that Jeans escape is below the adopted threshold. I recommend reframing the ARD as a necessary-condition boundary (inside which loss is guaranteed) and qualifying all retention-zone language accordingly.","section":"Sect. 2.2 and Figs. 2-3"},{"comment":"The adopted 1000 Myr 'grace period' is an input assumption rather than a model output, and it directly sets the age at which the 0.4 Msun threshold is evaluated in Fig. 3. A different grace period would shift the mass boundary; the paper does not derive this timescale from the outgassing and thermal-evolution literature it cites or test its sensitivity. Please show how the ARD-HZ overlap and the threshold mass change for grace periods of, e.g., 500 and 2000 Myr, or provide a quantitative justification for 1000 Myr from the magma-ocean and outgassing models.","section":"Sect. 3.1"},{"comment":"The ARD threshold itself (loss of 1 Earth atmosphere in 10 Myr, approximately 1.6e4 kg/s) is an order-of-magnitude choice. Because the comparison to outgassing rates governs whether a planet can replenish what it loses, the threshold should be presented explicitly as a diagnostic and the paper should show how the ARD curves change if the threshold is varied by, say, a factor of a few. This is especially relevant for the claim that planets inside the ARD are 'unlikely to retain any atmosphere', which depends on the threshold exceeding plausible sustained outgassing.","section":"Sect. 2.2"}],"minor_comments":[{"comment":"The phrase 'fall outside the ARD' is ambiguous; since Fig. 4 shows targets orbiting closer than the ARD, I recommend using 'inside the ARD' or 'closer than the ARD' consistently.","section":"Abstract and Sect. 3.2"},{"comment":"The quantity FEUV,⊕ is introduced with units of erg/cm2, but a flux should carry erg cm^-2 s^-1; please correct the units or define the quantity as a time-integrated fluence.","section":"Sect. 2.3"},{"comment":"The phrase 'keep in might' should read 'keep in mind'.","section":"Sect. 3.2"},{"comment":"The conclusion refers to fully convective stars as 'M < 0.35 Msun', while the abstract and Sect. 3.2 use 'masses under 0.4 Msun'; please reconcile these numbers and state the adopted boundary criterion.","section":"Sect. 4"},{"comment":"Please state that the ARD curves are for a slowly rotating star at 5000 Myr and that younger or faster-rotating hosts would move the curves outward, as noted for TOI-700 d in the text.","section":"Fig. 4 caption"},{"comment":"The modelled irradiance grid is 6-14 FEUV,⊕, but the Venus discussion in Sect. 3.1 cites values up to 24 FEUV,⊕; clarify whether the latter are linear scalings outside the simulated grid or extrapolations, since the text elsewhere says results are not extrapolated beyond the simulated models.","section":"Sect. 2.2 and Table 1"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within A&A scope and the ARD concept is a useful addition. The main risk to the headline result is the combination of the scaled-solar EUV spectrum and the 1 Gyr grace period; I would consider accepting after the authors supply the requested sensitivity tests and tighten the retention language. I do not see a circularity problem: the model is validated against independent geological constraints and uses published benchmark models."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague — this is worth reading if you work on rocky exoplanet observations. The paper extends the ARD idea from TRAPPIST-1 to a grid of stellar masses, ages, and initial rotation rates, and the bottom line is: HZ planets around fully convective stars below about 0.4 M_sun are unlikely to keep a CO2 or N2 atmosphere, and every JWST Cycle 1/2 Earth-size target falls outside the retention distance. The qualitative ordering by stellar mass and rotation is robust and clearly presented.\n\nWhat I like: the authors are upfront about what their model does and doesn't do. They compute only Jeans escape and explicitly call it a lower limit; they exclude water; they use one solar EUV spectrum scaled in flux and say why. They also test the machinery on the Solar System — the Archean Earth and Venus — and get plausible consistency with isotope records. That kind of grounding matters.\n\nThe soft spots are real but not fatal. The solar-scaled EUV spectrum is the biggest one. Exobase temperature enters Jeans escape exponentially, and VL24's own Fig. 7 shows spectral shape changes the pressure-temperature profile. The stress-test note is right: the 0.4 M_sun boundary is least secure exactly where the claim is strongest, for fully convective M dwarfs whose EUV spectra are line-dominated and different from the Sun. The authors acknowledge this and frame the work as general trends, but they don't quantify the sensitivity. That should be in the paper, ideally before the JWST target plot. A second-order concern is the loss threshold (1 Earth atmosphere in 10 Myr) and the ad hoc 1000 Myr grace period for early evolution; both are defensible but arbitrary.\n\nI don't think the central argument collapses. The omitted processes (hydrodynamic escape, non-thermal loss) only add loss, so the ARD is a lower limit and the 'unlikely to retain' conclusion is conservative in that direction. If a realistic M-dwarf spectrum cools the exobase, Jeans loss drops and the threshold might move lower; if it heats, it moves higher. Either way the trend with mass and rotation survives. So I'd treat the exact 0.4 M_sun as provisional, not the qualitative result.\n\nWho benefits: observers planning JWST/Ariel target lists, and anyone building escape models for secondary atmospheres. The paper deserves a serious referee; I'd send it out. I'd also bring it to a reading group, partly because the spectral-shape question is a nice discussion point.","headline":"A useful, honest synthesis that gives observers a clear reason to expect airless rocky planets around small M dwarfs, with the main caveat being the solar-scaled EUV spectra.","tokens_in":17898,"tokens_out":3104,"would_cite":true,"duration_ms":30680,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Earth-mass planets around the smallest stars probably cannot keep CO2 or N2 atmospheres.","keywords":["atmospheric retention distance","secondary atmospheres","Jeans escape","habitable zone","M dwarf stars","stellar XUV evolution","rocky exoplanet atmospheres","JWST target selection"],"falsifier":"A JWST or Ariel detection of a thick CO2- or N2-dominated atmosphere on an Earth-mass rocky planet in the habitable zone of a fully convective star below about 0.4 solar masses and older than 1 Gyr would contradict the central claim that such planets are unlikely to retain any atmosphere.","tokens_in":16892,"feed_emoji":"🪐","tokens_out":10465,"duration_ms":88611,"temperature":0.7,"pith_summary":"This paper tries to establish when an Earth-mass rocky planet can hold on to a CO2- or N2-dominated secondary atmosphere, and whether that overlaps with the liquid-water habitable zone. It combines thermochemical upper-atmosphere escape models with stellar rotation and X-ray/ultraviolet (XUV) evolution to define an atmospheric retention distance, the closest orbit at which thermal Jeans escape — molecules at the top of the atmosphere moving faster than escape velocity — stays below volcanic outgassing replenishment. The central result is that around fully convective stars below about 0.4 solar masses, the habitable zone lies inside the retention distance for gigayear timescales, so Earth-mass HZ planets there are unlikely to retain any atmosphere. The paper also finds that slower initial stellar rotation lets the habitable zone and retention distance overlap earlier, and that all Earth-like rocky planets targeted by JWST in cycles 1 and 2 fall outside the retention distance. If right, many prime targets in the search for habitable rocky exoplanets are predicted to be airless, and future target selection should favour stars above 0.4 solar masses.","feed_headline":"Habitable zones of the smallest stars likely strip rocky atmospheres","feed_subtitle":"New models put every JWST Earth-sized target outside the atmosphere retention distance.","key_machinery":"The central object is the atmospheric retention distance (ARD): the closest orbital distance at which a 1 Earth-mass planet retains a given CO2/N2 atmosphere, defined by comparing Jeans escape rates from the Kompot 1D thermochemical upper-atmosphere model with a threshold loss rate of about 16,000 kg/s, equivalent to losing Earth's entire atmosphere in 10 Myr and comparable to sustained volcanic outgassing. The critical XUV irradiance at which this loss rate is reached is converted into a distance using stellar luminosity evolution tracks for slow, medium, and fast initial rotators across stellar masses from 0.1 to 1.2 solar masses and ages from 1 Myr to 12 Gyr. The habitable zone is computed separately from climate-model limits, and the comparison of these two distances carries the argument.","core_discovery":"The paper's central claim is that habitability requires not just liquid-water insolation but atmospheric retention, and these two conditions are often mutually exclusive for low-mass stars. Concretely, it claims that for a 1 Earth-mass planet with a CO2- or N2-dominated atmosphere, there is a minimum orbital distance, the atmospheric retention distance, inside which Jeans escape removes the entire atmosphere faster than plausible outgassing can replace it. Combining these retention distances with stellar evolution models, the paper finds that the ARD lies outside the HZ for stars below roughly 0.4 solar masses at ages of a gigayear or more, because fully convective stars spin down slowly and stay X-ray and ultraviolet active; HZ planets around such stars are therefore unlikely to retain atmospheres unless outgassing is extreme. It further claims that initial rotation rate matters: a fast-rotating star keeps high XUV irradiance longer, delaying HZ-ARD overlap. Finally, it claims that all Earth-like rocky exoplanets observed by JWST in cycles 1 and 2, including HZ planets, currently orbit inside the ARD and are not expected to retain atmospheres.","pith_inferences":["If the EUV spectral shape of M dwarfs differs strongly from the scaled solar spectrum used here, exobase temperatures and therefore retention distances could shift; this is testable by recomputing escape rates with observed M-dwarf EUV spectra.","Because only Jeans escape is modelled and hydrodynamic and non-thermal losses are ignored, the ARDs are optimistic upper limits; including those channels would push retention distances outward and strengthen the airless-planet prediction.","A natural extension is to vary planet mass and water content: water vapour enhances loss, so wetter planets would have larger ARDs, while more massive rocky planets up to about 2 Earth masses would have smaller ones, changing the ranking of JWST and Ariel targets.","If the claim holds, future target lists should prioritise planets around older, slowly rotating stars above 0.4 solar masses rather than M-dwarf HZ planets."],"forward_implications":["Around stars below about 0.4 solar masses, the habitable zone lies inside the atmospheric retention distance at ages of 1000 Myr and beyond, so Earth-mass HZ planets there are unlikely to retain any CO2- or N2-dominated atmosphere.","For stars above about 0.6 solar masses, the entire HZ can lie outside the ARD by 2000 Myr, allowing retention and constraining possible atmospheric compositions such as N2 near the outer HZ and CO2 near the inner edge.","A faster initial stellar rotation keeps XUV output high for longer and delays HZ-ARD overlap, so identical planets around identical-mass stars can have different atmospheric fates depending on the star's birth rotation.","None of the Earth-like rocky planets observed by JWST in cycles 1 and 2 are expected to retain an atmosphere, and a detection around such a target would require rapid, sustained volcanic replenishment.","The Solar System validation implies that the early Earth needed a CO2-dominated atmosphere and that Venus could have retained a thick atmosphere until roughly 3.5 Gyr ago."],"supporting_citations":[{"why":"Supplies the Kompot escape-rate simulations and the TRAPPIST-1 atmospheric retention distance method that this paper extends to other stellar masses.","marker":"Van Looveren et al. (2024)"},{"why":"Provides the stellar rotation evolution tracks and XUV luminosity histories used to turn critical irradiances into retention distances over time.","marker":"Johnstone et al. (2021a)"},{"why":"Describes the Kompot thermochemical upper-atmosphere model that produces the thermal profiles and Jeans escape rates.","marker":"Johnstone et al. (2018)"},{"why":"Supplies the climate-model habitable zone boundaries used to define the HZ.","marker":"Kopparapu et al. (2013)"},{"why":"Provides the planet-mass-dependent HZ update used for the 1 Earth-mass case.","marker":"Kopparapu et al. (2014)"},{"why":"Supplies the empirical solar spectrum that is scaled uniformly to represent the XUV irradiation of all modelled stars.","marker":"Claire et al. (2012)"},{"why":"Gives the outgassing rates and the 10-40 percent Jeans escape share used to set the replenishment-versus-loss comparison.","marker":"Catling & Kasting (2017)"},{"why":"Defines the rocky-planet radius boundary used to select Earth-like JWST targets for comparison with the ARD.","marker":"Rogers (2015)"}],"fun_headline_variants":["Smallest stars' habitable zones likely lack atmospheres","Under 0.4 solar masses, HZ planets lose their atmospheres","JWST's rocky targets orbit inside atmosphere-loss zone","X-ray-UV activity dooms atmospheres on HZ planets of small stars","Fast-rotating stars prolong atmosphere loss for HZ planets"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The calculations assume that one solar EUV spectrum, scaled uniformly in brightness, represents the XUV output of every star from 0.1 to 1.2 solar masses; if low-mass stars emit a genuinely different spectral shape, the predicted escape rates and retention distances could shift.","fun_headline_variants_meta":{"raw":{"variants":["Smallest stars' habitable zones likely lack atmospheres","Under 0.4 solar masses, HZ planets lose their atmospheres","JWST's rocky targets orbit inside atmosphere-loss zone","X-ray-UV activity dooms atmospheres on HZ planets of small stars","Fast-rotating stars prolong atmosphere loss for HZ planets"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001644,"raw_usage":{"total_tokens":6600,"prompt_tokens":1082,"completion_tokens":5518,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":698,"completion_tokens_details":{"reasoning_tokens":5429}},"tokens_in":698,"tokens_out":5518,"duration_ms":44208,"temperature":1.0,"reasoning_tokens":5429,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T20:45:53.545328+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A JWST or Ariel detection of a thick CO2- or N2-dominated atmosphere on an Earth-mass rocky planet in the habitable zone of a fully convective star below about 0.4 solar masses and older than 1 Gyr would contradict the central claim that such planets are unlikely to retain any atmosphere.","supporting_citations":[{"cited_title":"P., Güdel, M., Lammer, H., & Kislyakova, K","cited_arxiv_id":null,"evidence_quote":"Describes the Kompot thermochemical upper-atmosphere model that produces the thermal profiles and Jeans escape rates."},{"cited_title":"M., Schottelkotte, J., et al","cited_arxiv_id":null,"evidence_quote":"Provides the planet-mass-dependent HZ update used for the 1 Earth-mass case."},{"cited_title":"W., Sheets, J., Cohen, M., et al","cited_arxiv_id":null,"evidence_quote":"Supplies the empirical solar spectrum that is scaled uniformly to represent the XUV irradiation of all modelled stars."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the outgassing rates and the 10-40 percent Jeans escape share used to set the replenishment-versus-loss comparison."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Defines the rocky-planet radius boundary used to select Earth-like JWST targets for comparison with the ARD."}],"review_version":1}