Claims of detections of gases in exoplanet atmospheres often rely on comparisons between models including and excluding specific chemical species. However, the space of molecular combinations available for model construction is vast and highly degenerate. Only a limited subset of these combinations is typically explored for any given detection. As a result, apparent detections of trace gases risk being artefacts of incomplete modelling rather than robust identification of atmospheric constituents, especially in the low-signal-to-noise regime. Here, using the sub-Neptune K2-18 b as a case study, we show that recent biosignature claims vanish when the model space is expanded, with numerous alternatives providing equally good or better fits. We demonstrate that the significance of a claimed detection relies on the choice of models being compared, and that model preference does not in itself imply the presence of a specific gas. We recommend treating model comparisons instead as relative adequacy tests, which should be supported by theoretical predictions and complementary metrics of statistical significance to attribute a signal to a particular gas. Reported detections of gases in exoplanet atmospheres, including claims of biosignatures on K2-18 b, disappear when broader models are tested, revealing that such detections often reflect modelling limits rather than real signals.

Abstract Outflows play a key role in the star and planet formation processes. Some outflows show discrete clumps of cold molecular gas moving at extremely high velocities (EHVs) of ~100 km s−1, known as ‘molecular bullets’, that are likely closely associated with their primary driving agent. Here we present ALMA CO (J = 3–2) observations of a bright EHV molecular bullet that reveal its morphology in detail down to scales of 30 au and its kinematic structure across the entire intermediate velocity range (~30–100 km s−1). These provide important insights into how outflows transfer mass and momentum to the surrounding medium. The observed channel maps display several sequences of ring-like features whose velocity increases and size decreases with projected distance from the driving source, each sequence tracing a thin, bow-shaped shell culminating on axis in a bright EHV head. The shape, kinematics and mass of each shell all agree remarkably well with the simplest textbook models of momentum-conserving bowshocks produced by a time-variable EHV jet. The dynamical timescale between consecutive shells is of a few decades, with the latest ejection event coinciding with the protostar optical/infrared outburst observed in ~1990. The very strong evidence for bowshock-driven entrainment induced by jet variability revealed by this work suggests that accretion bursts, and therefore variations in the disk snowlines, should occur on decade timescales, which could substantially impact grain growth and planet formation. Access options Get Nature+, our best-value online-access subscription Prices may be subject to local taxes which are calculated during checkout The ALMA raw data are available from the ALMA Science Archive (https://almascience.eso.org/aq/) using the project identifiers 2015.1.01229.S (for Cycle 3 data) and 2016.1.101305.S (for Cycle 4 data). 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Tabone for his useful comments and suggestions on this work. G.A., G.B.-C., I.d.G.-M., A.K.D.-R., G.A.F., J.F.G. and M.O. acknowledge financial support from grant nos. PID2020-114461GB-I00, PID2023-146295NB-I00 and CEX2021-001131-S, funded by MCIN/AEI/10.13039/501100011033. G.B.-C., G.A.F. and M.O. acknowledge financial support from Junta de Andalucia (Spain) grant no. P20-00880 (FEDER, EU). G.B-C acknowledges support from grant no. PRE2018-086111, funded by MCIN/AEI/ 10.13039/501100011033 and by ‘ESF Investing in your future’ and thanks ESO Science Support Discretionary Fund for their financial support under the 2024 SSDF 06 project. S.C. acknowledges support from Conseil Scientifique of Observatoire de Paris and from the Programme National de Physique et Chimie du Milieu Interstellaire (PCMI) of CNRS/INSU co-funded by CEA and CNES. A.-K.D.R acknowledges support from STFC grant no. ST/T001488/1. G.A.F. also acknowledges support from the Collaborative Research Centre 956, funded by the Deutsche Forschungsgemeinschaft (DFG) project ID 184018867, the DFG for funding through SFB 1601 ‘Habitats of massive stars across cosmic time’ (sub-project B1) and the University of Cologne and its Global Faculty programme. R.E. acknowledges partial financial support from grant nos. PID2020-117710GB-I00 and CEX2019-000918-M funded by MCIN/ AEI /10.13039/501100011033. J.M.T. acknowledges support from grant no. PID2023-146675NB funded by MCIN/AEI/10.13039/501100011033 and by the programme Unidad de Excelencia María de Maeztu CEX2020-001058-M. L.F.R. acknowledges support from grant no. CBF-2025-I-2471 of SECIHTI, Mexico. L.A.Z. acknowledges financial support from grant nos. CONACyT-280775, UNAM-PAPIIT IN110618 and IN112323, México. This work makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01229.S, ADS/JAO.ALMA#2016.1.01305.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This publication use images based on observations made with the NASA/ESA Hubble Space Telescope and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). G.B.-C. led the data reduction, analysis, modelling, interpretation and writing of the paper. G.A., S.C., M.O., A.C.R., G.A.F. and R.E. contributed important inputs to the methodology, modelling and interpretation of the results and to the writing of paper sections. G.A., J.F.G. and A.K.D.-R. contributed to the data reduction. G.A. led the ALMA observation proposal, conceived and prepared with contributions from M.O., J.F.G., A.K.D.-R., J.M.T., L.F.R., E.M., I.d.G.-M. and P.T.P.H. All authors participated in discussions of the results and contributed to the paper preparation and revision. Peer review information Nature Astronomy thanks Somnath Dutta, Klaus Hodapp and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Extended data Observed outflow features surrounding SVS 13 at different scales (see their observational properties in Supplementary Table 1). Large boxes indicate zoomed-in regions. The synthesized beams are shown in the bottom right corner of the panels. Velocities are relative to VLA 4B (vLSR = + 9.3 km/s)58. Top left: CARMA CO(J=1-0) map of the large-scale SHV molecular outflow63,105. The emission has been integrated from − 11.6 to − 3.1 km/s (blue lobe) and from 0.7 to 8 km/s (red lobe). The location of the EHV molecular bullets51,52 (triangles), Herbig-Haro objects (squares), and radio sources proposed as YSOs58,106 (plus signs, where SVS 13 is the largest) is indicated. Top right: HST optical image (F606W filter) of the region, showing the outflow cavity, with the SMA map of the CO(2-1) emission of the EHV bullets52 (with velocities from − 161 to +129 km/s) overlapped in blue contours. Bottom left: Our ALMA CO(3-2) map (not corrected for the primary beam response) of the central region of the molecular outflow (beam=0.527″ × 0.333″, PA=2.7 deg) is shown in contours. The emission has been integrated from − 126.4 to − 6.5 km/s (blue lobe) and from 6.5 to 60.8 km/s (red lobe). Contours are 3, 6, 10, 15, 21, 28, and 37 times 0.41 Jy/beam km/s (blue lobe) and 0.32 Jy/beam km/s (red lobe).The H2 arcuate features60 are plotted as arcs. Bottom right: Close-up toward Bullet 1. Our ALMA CO(3-2) map of the blueshifted emission (beam=0.537″ × 0.333″, PA=2.7 deg), integrated from − 9.3 to − 126.4 km/s (thus including the SHV, IHV, and EHV components), is shown in contours. Contours are 3, 6, 10, 16, and 25 times 0.61 Jy/beam km/s. The colored filled areas represent the heads of the detected families of rings (see Fig. 3). The green squares represent the location of two CO clumps (where c2 coincides in position and velocity with the SiO knot reported by ref. 53). Image of the velocity-integrated intensity of the SO(88 − 77) line observed with the ALMA 12-m array using a low spectral resolution spectral window dedicated to continuum observation, with a channel spacing of 13.6 km/s. The emission has been integrated in the LOS velocity range from -9.8 to -118.6 km/s relative to the velocity of VLA 4B (+9.3 km/s)58. The positions of the two protostars of the SVS 13 binary56 are indicated by plus signs. The synthesized beam, shown in the bottom left corner, is 0.54″ × 0.36″ (PA=8.37 deg). Contours are -3, 3, 5, 8, 13, 20, 30, 50, 80, 140, 260 times 0.04 Jy/beam km/s. The image has not been corrected by the primary beam response. A sample of spectral channel images of the CO(J=3-2) emission observed by ALMA with a synthesized beam of 0.554″ × 0.352″ (PA=3.1 deg), where natural weighting has been used. The obtained data cover a range of velocities wider than the high angular resolution data shown in Fig. 2. The positions of the two protostars of the SVS 13 binary56 are indicated by plus signs. The LOS velocity, relative to the velocity of VLA 4B (vLSR = + 9.3 km/s)58, is shown in the top left corner of each image. The width of each of the spectral channels shown in the figure is 2.12 k/ms, which corresponds to the average of 10 native channels. The rms of the images is 8 mJy/beam. Images have not been corrected by the primary beam response. The synthesized beam is plotted as an ellipse in the bottom left corner of the first image. Position angle as a function of the dynamical time for the heads of the families of rings (plotted as triangles and labeled F I to F VI; see Methods), as well as for other features associated with the SVS 13 outflow (we plot atomic features as circles, and molecular as rectangles), such as HH objects61,107 (plotted as squares and labeled HH7, HH11), [FeII] jet (triangle labeled as Microjet), and H2 arcs60 (squares labeled as HC1, HC2, HC3). Colors indicate features with position angles around 160 deg (in blue) and in the range of 120-140 deg (in gray). The data for the families of rings (F I-VI) are presented as mean values, and error bars represent the standard deviation (see ‘Dynamical times of the heads of the families of rings’ in Methods). For the rest of the objects we represent the values and uncertainties as reported in the literature52,60,61,107. We assumed inclination angles from 22 to 25 deg. See Methods and Supplementary Table 1 for details on the calculations. Epoch is given in Julian years, with epoch 2000 corresponding to the standard definition of Julian epoch J2000.0. Dynamical times correspond to the date of our high-angular resolution ALMA observations (epoch 2016.69). The bowshock is seen in a reference system moving at the velocity vjet of the working surface. The cylindrical jet beam has a diameter 2rjet, and the impinging ambient gas moves to the left at a velocity vjet − vamb in this reference system. We show a cylindrical coordinate system (x*,r), where r is the cylindrical radius and x* the distance measured from the head of the working surface towards the outflow source. The working surface ejects material sideways at a velocity v0 (which is approximately equal to the post-cooling region sound speed of ~ 10 km/s). This sideways ejection interacts with the impinging ambient gas, forming a thin shell bowshock that has a well defined locus rb(x*), and locally has a slope \(\tan (\alpha )={\rm{dr}}_{\rm{b}}/{{\rm{dx}}}^{*}\). Top left: Geometry of the bowshock shell, where black vectors show velocities of the shell, blue vectors their LOS projections, and the shaded band outlines the spatial extent of the emitting region in the velocity range of a given channel map. Top center: Sketch of the corresponding channel image. Due to projection effects, the observed ring emission looks wider on the side closest to the star, while the far side appears narrower. Top right: Observed channel image (rotated so that the top faces SVS 13) illustrating the asymmetry of the rings, with the side closest in projection to the star appearing brighter and wider than the opposite side. Middle row: At low optical depths, in the side closest to the star, the emission appears spread over a wider range of radii and with lower intensity than on the far side, where it appears narrower but with higher intensity (left). When convolved with an elongated beam (center), the intensity becomes almost uniform throughout the ring, except near the positions where the beam axis is tangent to the ring (a well-known beam-filling-factor effect95). When noise is added (right), no substantial asymmetries are detected. Bottom row: When the optical depth is high enough (left), the side closer to the star becomes almost as bright, but more extended, than the far side, where any increase in intensity is hindered by opacity saturation. When convolved with the beam (center), the intensity drops in the far (narrower) side of the ring, because of the smaller beam-filling factor, producing an asymmetry similar to that observed. A model image with noise (right panel) shows that a qualitative agreement with observations (top panel) can be achieved. In the modeling, the assumed velocity dispersion is 2 km/s, the inclination angle is 20 deg, and the channel width is 0.53 km/s. Decomposition of the observed CO (J = 3–2) channel map emission of Bullet 1 in SVS 13 into elliptical rings. The LOS velocity, relative to VLA 4B (vLSR = +9.3 km s−1 (ref. 58)), ranges from −0.9 km s−1 to −102.5 km s−1. Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

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