J/A+A/705/A260ivo://CDS.VizieR/J/A+A/705/A260CDSLacedelli G.Palle E.Luque R.Ikuta K.Tabernero H.M.,Zapatero Osorio M.R.Almenara J.M.Pozuelos F.J.Jankowski D.Narita N.,Fukui A.Nowak G.Hirano T.Ishikawa H.T.Kimura T.Hori Y.,Collins K.A.Howell S.B.Jiang C.Murgas F.Osborn H.P.,Astudillo-Defru N.Bonfils X.Charbonneau D.Fausnaugh M.,Geraldia-Gonzalez S.Gozdziewski K.Guerra P.Hayashi Y.Hodapp K.,Horne K.Isogai K.Jafariyazani M.Kagetani T.Kawai Y.Kawauchi K.,Krishnamurthy V.Kotani T.Kudo T.Kurokawa T.Kuzuhara M.Mori M.,Nishikawa J.Nugroho S.K.Omiya M.Schwarz R.P.Sefako R.Shporer A.,Srdoc G.Teng H.Watanabe N.2026-03-02T06:25:50Z2026-01-27T08:46:21ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frmultiple-starsexoplanetsradial-velocityvisible-astronomySub-Neptunes are absent in the Solar System, yet they are the most common category of planets found in our Galaxy. This kind of planet challenges the internal structure models and prompts investigation on their formation and evolution, as well as pushing towards atmospheric characterisation studies to break the degeneracy in their inner composition. We report here the discovery and characterisation of new sub-Neptunes orbiting two similar M dwarfs, TOI-521 (Teff=3544+/-100K, V=14.7mag), and TOI-912 (Teff=3572+/-100K, V=12.7mag). Both stars host a transiting planetary candidate identified by TESS and are part of the THIRSTEE follow-up program, which aims at understanding the sub-Neptune population through in-depth and precise characterisation studies on a population level. We analysed TESS light curves, ground-based photometry and high-precision ESPRESSO, HARPS and IRD radial velocities to confirm the planetary nature of both candidates, infer precise orbital and physical parameters of the planets and investigate the presence of additional planets in the systems. The two stars host nearly identical planets in terms of mass and radius. TOI-521 hosts a transiting sub-Neptune in a 1.5-days orbit with radius and mass of R_b_=1.98+/-0.14R_{Earth}_ and M_b_=5.3+/-1.0M_{Earth}_, respectively. Moreover, we identified an additional candidate at 20.3-days, with a minimum mass of Mpsin(i)=10.7M_{Earth}_, currently not detected to transit in our photometric dataset. Similarly, the planet orbiting TOI-912 is a 4.7-d sub-Neptune with R_b_=1.93+/-0.13R_{Earth}_ and M_b_=5.1+/-0.5M_{Earth}_. Interestingly, TOI-912 b likely possesses an unusually high eccentricity (e=0.58+/-0.02), and it is probably undergoing strong tidal dissipation. If such eccentricity is confirmed, it would make it one of the most eccentric sub-Neptunes known to date. TOI-521 b and TOI-912 b have very similar densities (4g/cm^3^), and they lie in the degenerate region of the mass-radius diagram where different compositions are plausible, including a volatile-rich composition, or a rocky core surrounded by a H-He envelope. When compared to the other THIRSTEE M-dwarf targets, our sample supports the division of sub-Neptunes into two distinct populations divided by a density gap. Both planets are interesting targets for atmospheric follow-up in the context of understanding the temperature-atmospheric feature trend that starts to emerge thanks to JWST observations.2026A&A...705A.260Lhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/705/A260CatalogResearchIsServedByTAP VizieR generic servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/705/A260https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/705/A260http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/705/A260https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/705/A260https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/705/A260http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/705/A260GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapOpticaldefaultJ/A+A/705/A260/toi912eESPRESSO RVs and activity indicators of TOI-912recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintBJDBarycentric Julian Date (TDB)dtime.epochdoubleRVRadial velocitym/sphys.veloc;pos.heliocentricfloate_RVRadial velocity uncertaintym/sstat.errorfloatdLWDifferential line widthm**2.s**-2spect.line.widthfloate_dLWDifferential line width uncertaintym**2.s**-2stat.errorfloatCRXChromatic indexm/sspect.indexfloate_CRXChromatic index uncertaintym/sstat.errorfloatNaISodium line index (doublet I)spect.index;spect.linefloate_NaISodium line index (doublet I) uncertaintystat.errorfloatNaIISodium line index (doublet II)spect.indexfloate_NaIISodium line index (doublet II) uncertaintystat.errorfloatHalphaH-alpha line indexspect.index;spect.linefloate_HalphaH-alpha line index uncertaintystat.error;spect.linefloatFWHMFull width half maximumm/sspect.line.widthfloate_FWHMFull width half maximum uncertaintym/sstat.error;phys.angSizefloatBISBisector spankm/sphys.velocdoublee_BISBisector span uncertaintykm/sstat.errordoubleSMWS-indexphot.flux;arith.ratiofloate_SMWS-index uncertaintystat.errorfloatJ/A+A/705/A260/toi912hHARPS RVs and activity indicators of TOI-912recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintBJDBarycentric Julian Date (TDB)dtime.epochdoubleRVRadial velocitym/sphys.veloc;pos.heliocentricdoublee_RVRadial velocity uncertaintym/sstat.errorfloatFWHMFull width half maximumkm/sspect.line.widthfloatContrContrast of the CCFphot.flux;arith.ratiodoubleBISBisector spankm/sphys.velocfloatJ/A+A/705/A260/toi521eESPRESSO RVs and activity indicators of TOI-521recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintBJDBarycentric Julian Date (TDB)dtime.epochdoubleRVRadial velocitym/sphys.veloc;pos.heliocentricdoublee_RVRadial velocity uncertaintym/sstat.errorfloatFWHMFull width half maximumm/sspect.line.widthfloate_FWHMFull width half maximum uncertaintym/sstat.error;phys.angSizefloatContrContrast of the CCFphot.flux;arith.ratiofloate_ContrContrast of the CCF uncertaintystat.errorfloatBISBisector spankm/sphys.velocdoublee_BISBisector span uncertaintykm/sstat.errordoubleJ/A+A/705/A260/toi521iIRD RVs and activity indicators of TOI-521recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintBJDBarycentric Julian Date (TDB)dtime.epochdoubleRVRadial velocitym/sphys.veloc;pos.heliocentricfloate_RVRadial velocity uncertaintym/sstat.errorfloatFWHMFull width half maximumkm/sspect.line.widthfloate_FWHMFull width half maximum uncertaintykm/sstat.error;phys.angSizefloatdVLine asymmetrym/sspect.line.asymmetry;spect.linefloate_dVLine asymmetry uncertaintym/sstat.errorfloatdLWDifferential line widthm**2.s**-2spect.line.widthfloate_dLWDifferential line width uncertaintym**2.s**-2stat.errorfloatCRXChromatic indexm/sspect.indexfloate_CRXChromatic index uncertaintym/sstat.errorfloat