Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
M. G. Soto, G. Anglada-Escudé, S. Dreizler, K. Molaverdikhani, J. Kemmer, C. Rodríguez-López, J. Lillo-Box, E. Pallé, N. Espinoza, J. A. Caballero, A. Quirrenbach, I. Ribas, A. Reiners, N. Narita, T. Hirano, P. J. Amado, V. J. S. Béjar, P. Bluhm, C. J. Burke, D. A. Caldwell, D. Charbonneau, R. Cloutier, K. A. Collins, M. Cortés-Contreras, E. Girardin, P. Guerra, H. Harakawa, A. P. Hatzes, J. Irwin, J. M. Jenkins, E. Jensen, K. Kawauchi, T. Kotani, T. Kudo, M. Kunimoto, M. Kuzuhara, D. W. Latham, D. Montes, J. C. Morales, M. Mori, R. P. Nelson, M. Omiya, S. Pedraz, V. M. Passegger, B. V. Rackham, A. Rudat, J. E. Schlieder, P. Schöfer, A. Schweitzer, A. Selezneva, C. Stockdale, M. Tamura, T. Trifonov, R. Vanderspek, D. Watanabe
AAstronomy & Astrophysics manuscript no. toi1640 © ESO 2021February 24, 2021
Mass and density of the transiting hot and rocky super-EarthLHS 1478 b (TOI-1640 b)
M. G. Soto (cid:63) , G. Anglada-Escudé , , , S. Dreizler , K. Molaverdikhani , , J. Kemmer , C. Rodríguez-López ,J. Lillo-Box , E. Pallé , , N. Espinoza , J. A. Caballero , A. Quirrenbach , I. Ribas , , A. Reiners , N. Narita , , , ,T. Hirano , , , P. J. Amado , V. J. S. Béjar , , P. Bluhm , C. J. Burke , D. A. Caldwell , , D. Charbonneau ,R. Cloutier , K. A. Collins , M. Cortés-Contreras , E. Girardin , P. Guerra , H. Harakawa , A. P. Hatzes ,J. Irwin , J. M. Jenkins , E. Jensen , K. Kawauchi , T. Koyati , , , T. Kudo , M. Kunimoto , M. Kuzuhara , ,D. W. Latham , D. Montes , J. C. Morales , , M. Mori , R. P. Nelson , M. Omiya , , S. Pedraz ,V. M. Passegger , , B. Rackham , A. Rudat , J. E. Schlieder , P. Schöfer , A. Schweitzer , A. Selezneva ,C. Stockdale , M. Tamura , , , T. Trifonov , R. Vanderspek , and D. Watanabe (A ffi liations can be found after the references) Received dd February 2021 / Accepted dd Month 2021
ABSTRACT
One of the main objectives of the
Transiting Exoplanet Survey Satellite (TESS) mission is the discovery of small rocky planets around bright,nearby stars. Here, we report the confirmation and characterization of the transiting super-Earth planet orbiting LHS 1478 (TOI-1640). The star isan inactive red dwarf ( J = . . ± . M (cid:12) and 0 . ± . R (cid:12) , respectively,and an e ff ective temperature of 3381 ±
54 K. It was observed by TESS in four sectors. These data revealed a transit-like feature with a period of1.949 days. We combined the TESS data with three ground-based transit measurements, 57 radial-velocity (RV) measurements from CARMENES,and 13 RV measurements from IRD to determine that the signal is produced by a planet with a mass of 2 . + . − . M ⊕ and radius 1 . + . − . R ⊕ . Theresulting bulk density of this planet is 6.66 g cm − , consistent with a rocky planet with a Fe- and MgSiO -dominated composition. Although theplanet would be too hot to sustain liquid water on its surface (its equilibrium temperature is of about ∼
595 K), spectroscopic metrics based on thecapabilities of the forthcoming
James Webb Space Telescope , and the fact that the host star is rather inactive indicate that this is one of the mostfavorable known rocky exoplanets for atmospheric characterization.
Key words. planets and satellites: detection - planets and satellites: fundamental parameters - techniques: photometric - techniques: radialvelocities
1. Introduction
Stars smaller than the Sun o ff er a number of advantages for thedetection of exoplanets with both the Doppler (e.g. Proxima b,Anglada-Escudé et al. 2016) and the photometric transit methods(e.g. the TRAPPIST-1 system, Gillon et al. 2017). Both are indi-rect methods that rely on the planet imprinting a signal onto thestar-light. The Doppler technique measures the radial velocityof the star by measuring to high accuracy the wavelength shift ofnumerous absorption features in the stellar spectrum. The ampli-tude of the signal is larger for a larger planet-star mass ratio andfor short orbital periods. The transit method relies on measuringthe light blocked by the planet as it crosses the stellar disk. Inthis case the signal is proportional to the planet-star surface arearatio, and it repeats once per orbital period. In both methods,short-period planets are therefore more easily detectable.Exoplanet surveys with the Doppler technique are typi-cally conducted with ground-based high-resolution spectrome-ters ( R = λ/δλ >
50 000) that are kept in a very stable envi-ronment and are calibrated against spectral features of a refer-ence source measured in the laboratory. This is the case for theCARMENES (Quirrenbach et al. 2016) spectrometer, which wasspecifically designed to obtain maximal precision on red-dwarfstars. The CARMENES survey has led to numerous new exo- (cid:63)
E-mail: [email protected] planet discoveries in the super-Earth down to Earth mass regime,in hot to warm temperate orbits (e.g. Luque et al. 2018; Zech-meister et al. 2019; Stock et al. 2020). Several of these planetshave been detected both in transit and by radial velocities (Luqueet al. 2019; Dreizler et al. 2020; Kemmer et al. 2020; Nowaket al. 2020; Bluhm et al. 2020). On the other side, the NASA
Transiting Exoplanet Survey Satellite mission (TESS, Rickeret al. 2015) has been surveying most of the sky for signals oftransiting planets, with the goal of detecting those that shouldenable a more straightforward atmospheric characterization. Todate, TESS has revealed numerous exoplanet candidates transit-ing nearby M dwarfs (Astudillo-Defru et al. 2020; Dreizler et al.2020; Gan et al. 2020; Kanodia et al. 2020, to give a few).One key element in the characterization and study of tran-siting planets is their validation using Doppler spectroscopy,which in turn produces a measurement of their masses, allow-ing us to derive their mean bulk densities and put constrains ontheir composition. Finally, the atmospheres of transiting plan-ets can be studied by performing spectroscopic measurementsduring transits (the planet blocks more light at certain wave-lengths where the molecules in its atmosphere deter more lightproducing deeper transits), and secondary eclipses (the spectrumof thermal emission of the planet is also a ff ected by the pres-ence of absorbing molecules, producing a shallower dip in thelight curve when the planet goes behind the star). Among these, Article number, page 1 of 13 a r X i v : . [ a s t r o - ph . E P ] F e b & A proofs: manuscript no. toi1640 and thanks to the more favorable radius ratio between the planetand the star, exoplanets transiting red-dwarfs are the best onesfor atmospheric characterization. The measured spectrum of anexoplanet can be a ff ected by the presence of spots and activeregions in the visible part of the star during transits (Rackhamet al. 2018): the less active the star is, the cleaner and easier tointerpret is the measured spectrum of a planet.Due to its properties (transiting, Doppler signal in the fewm / s regime), and host star (small, relatively nearby and low stel-lar activity), LHS 1478 satisfies all the favorable conditions tobecome a prime target for characterization of rocky terrestrialplanets.In this paper, we validate the transiting exoplanetLHS 1478 b (TOI-1640 b). We provide an overview of the mea-surements with photometry (initial detection, stellar activity, or-bital period and planet size), imaging (validation against falsepositives), and radial velocity (confirmation, stellar activity, andmeasurement of the planet mass) in Sec. 2. Stellar parameters arerevised in 3.1, and a joint analysis to constrain the planet proper-ties is given in Sec. 3.2. We discuss the results in the context ofterrestrial planet candidates and further follow-up in Sec. 4, andsummarize our work in Sec. 5.
2. Data
TOI-1640 was observed by TESS during sectors 18, 19, 25, and26 of its primary mission. The data was processed through theScience Processing Operations Center (SPOC; Jenkins et al.2016), and transiting planet search algorithms (Jenkins 2002;Jenkins et al. 2010) detected a signal with a period of 1.9495 ± ±
111 parts per million(ppm) in December 2019 based on the data from sector 18. Af-ter reviewing the results of the data validation reports (Twickenet al. 2018; Li et al. 2019), the TESS Science O ffi ce alerted TOI-1640 to the community on 14 January 2020 .We obtained the photometric light curve, corrected for sys-tematics (PDC; Smith et al. 2012), from the Mikulski Archivefor Space Telescopes (MAST), using the lightkurve pack-age (Lightkurve Collaboration et al. 2018). The data are shownin Fig. 1. We performed a period search using the Box-FittingLeast Squares (BLS; Kovács et al. 2002) and the Transit LeastSquares (TLS; Hippke & Heller 2019) algorithms (Fig. A.1),and detected with both a signal with a period of 1.949 days and asignal detection e ffi ciency (SDE) > tpfplotter (Aller et al. 2020). No sources contaminating theTESS aperture are found in the Gaia
DR2 catalog (Gaia Collab-oration et al. 2018) down to a magnitude contrast limit of 6 magcompared to our target. Also, the
Gaia
EDR3 renormalized unitweight error (
RUWE ) value for this target is 1.26, below the criti-cal value of 1.40 that is an indicator that a source is non-single orhas a problematic astrometric solution (Lindegren et al. 2020). https://tess.mit.edu/toi-releases/ https://archive.stsci.edu https://github.com/KeplerGO/Lightkurve https://github.com/jlillo/tpfplotter In order to exclude the presence of close-by contaminants toour target, we observed LHS 1478 with the AstraLux highspatial resolution camera (Hormuth et al. 2008), located atthe 2.2 m telescope of the Calar Alto Observatory (Almería,Spain). This instrument uses the lucky-imaging technique to ob-tain di ff raction-limited images by obtaining thousands of short-exposure frames (below the atmospheric coherence time) to sub-sequently select the ones with the highest Strehl ratio (Strehl1902) and combine them into a final high spatial resolution im-age. We observed this target on the night of 25 February 2020under good weather conditions with a mean seeing of 1.0 arcsec.We obtained 41 710 frames with 20 ms exposure time for a to-tal exposure of 83.4 s in the Sloan Digital Sky Survey z (cid:48) filter(“SDSSz”), with a field-of-view windowed to 6 × astrasens package with theprocedure described in Lillo-Box et al. (2012, 2014). Both the5 σ sensitivity curve and the image are shown in Fig. 2. We canexclude sources down to 0.2 arcsec with magnitude contrast of ∆ z < python implementa-tion of this approach ( bsc ) , which uses the TRILEGAL galac-tic model (v1.6, Girardi et al. 2012) to retrieve a simulated sourcepopulation of the region around the corresponding target . Thissimulated population is used to compute the density of starsaround the target position (radius r = ∆ m b , max = . z (cid:48) passband, corresponding to the maximum contrast of a blendedeclipsing binary that could mimic the observed transit depth.Thanks to our high-resolution image, we estimate the probabil-ity of an undetected blended source to be 0.2 %. The probabilityof such undetected source to be an appropriate eclipsing binaryis even lower, and thus we conclude that the transit signal is notdue to a blended eclipsing binary. Ground-based transit photometry was obtained to confirm theTESS transit event and to refine the ephemeris of the planet. Weused the TESS Transit Finder, a customized version of the Tapirsoftware package (Jensen 2013), to schedule our observationsbased on the preliminary ephemeris from the SPOC light curve.We obtained a total of three transit detections, which we includein our joint fit. The individual observations are described below.
TOI-1640 was first observed by the MEarth-North telescope ar-ray on the 14 January 2020. MEarth-North is located at the Fred https://github.com/jlillo/astrasens http://stev.oapd.inaf.it/cgi-bin/trilegal This is done in python by using the astrobase implementation byBhatti et al. (2020).Article number, page 2 of 13. G. Soto et al.: Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
790 800 810 820 830 840
Time (BJD - 2458000) R e l a ti v e f l ux Sector 18 Sector 19 a)
990 1000 1010 1020 1030Sector 25 Sector 26880 900 9201050510 R a d i a l v e l o c it y ( m / s ) b) Time (BJD - 2458000)
Phase R e l a ti v e f l ux c) Phase R a d i a l v e l o c it y ( m / s ) d) Fig. 1: Data and joint fit results with juliet (Sect. 3.2). a) TESS photometry for TOI-1640, with the black line representing thebest transit + GP fit. b) Radial velocity data from CARMENES and IRD, with the jitter term added to the uncertainties. The blackline is the best fit Keplerian model. c) Phased-folded TESS data, with the GP component removed. The black line is the best transitfit. d) Phased-folded RV data. The black line is the best fit Keplerian model, and the red-shaded area represents the 68% confidenceinterval. . . . . . . C on t r a s t ( m ag )
10% cont.5% cont.2% cont. ∆ m(TOI-1640 b) AstraLux/SDSSz − X (arcsec) − − Y ( a r cs e c ) Fig. 2: AstraLux 5 σ sensitivity curve (main panel) of the high-spatial resolution image obtained for LHS 1478 (inset) in theSDSS z (cid:48) photometric band.Lawrence Whipple Observatory on Mt. Hopkins, Arizona, andconsists of eight 40 cm telescopes equipped with Apogee U42 cameras and custom RG715 passbands. We reduced the MEarthphotometry following the standard procedures outlined in Irwinet al. (2007) and Berta et al. (2012), using a 6 arcsec aperture.TOI-1640 was observed continuously with all eight tele-scopes from an airmass of 1.4 to 2.0 with 14-second exposures.The observations were taken through intermittent clouds andproduced a tentative transit detection amid large residual sys-tematics with a photometric dispersion of 4.2 parts-per-thousand(ppt). Although the combined light curve of the primary tar-get TOI-1640 from all eight telescopes did not yield a reliabletransit detection, these data were still used to rule out nearbyeclipsing binaries in 101 of 103 sources within 2.5 arcmin anddown to a di ff erential magnitude of 8.6 mag. The two unclearedsources are faint ( ∆ = The first reliable ground-based detection of a transit of TOI-1640was obtained on 31 January 2020 using the RCO 40 cm telescopelocated at the Grand-Pra Observatory near Valais Sion, Switzer-land. We observed a full transit in the Sloan i (cid:48) passband with anexposure time of 45 seconds. We used the AstroImageJ soft-
Article number, page 3 of 13 & A proofs: manuscript no. toi1640
Phase R e l a ti v e f l ux RCO
Binned data 0.04 0.02 0.00 0.02 0.04
Phase
LCO
Binned data
Fig. 3: Phased folded RCO ( left ) and LCOGT ( right ) data. The black line represents the best transit model from juliet . The smallpoints show the raw data, while the large points show the data binned into 10 minute bins.ware package (Collins et al. 2017) to perform di ff erential pho-tometry using 5.1 arcsec apertures, and to detrend against air-mass to produce the light curve of TOI-1640 depicted in the leftpanel of Fig. 3. With our reduction, we achieved a photometricprecision of 1.1 ppt in 3-minute bins. We detected the transit ofTOI-1640 with a mid-transit time of ≈ We observed two additional full transits of TOI-1640 on 27 Au-gust 2020 and 7 October 2020 from the Las Cumbres Obser-vatory Global Telescope (LCOGT) (Brown et al. 2013) 1.0 mnetwork node at McDonald Observatory. The images were cali-brated by the standard LCOGT
BANZAI pipeline (McCully et al.2018).Both light curves were obtained in the Pan-STARSS z -short passband with exposure times of 40 and 50 sec-onds in the August and October sequences, respectively. Weused AstroImageJ to perform di ff erential photometry using5.8 arcsec apertures and to detrend against airmass. The result-ing combined and phase folded light curves are included in theright panel of Fig. 3. With our reduction, we achieve photometricprecisions of 0.7 ppt and 0.5 ppt in 3-minute bins, which resultedin a high S / N transit detection in each LCOGT light curve. BothLCOGT transit detections were at the time predicted by the re-fined ephemeris of TOI-1640 from the RCO detection.
We obtained a total of 57 spectra with CARMENES from 28January 2020 to 16 October 2020, all taken with an exposuretime of 1800 s. The visual channel (VIS) spectra were reducedwith
CARACAL (Zechmeister et al. 2014; Caballero et al. 2016b),and the radial velocities (RVs) were extracted using
SERVAL (Zechmeister et al. 2018), along with selected activity indica-tors (Sect. 3.1). The RVs were corrected for barycentric motion, instrumental drift, secular acceleration, and nightly zero-points(Tal-Or et al. 2019; Trifonov et al. 2020). The CARMENES RVsare shown in Fig. 1. A generalized Lomb-Scargle periodogram(GLS; Zechmeister & Kürster 2009) of the RV data shows a pe-riod at 1.949 days, consistent with the transit signal, with a the-oretically computed False Alarm Probability (FAP) of less than1% (Fig. 4). The CARMENES RVs are listed in Table A.1.
We observed LHS 1478 using the InfraRed Doppler (IRD) in-strument at the Subaru 8.2 m telescope (Tamura et al. 2012;Kotani et al. 2018) between September and December 2020. Atotal of 13 spectra were obtained with an integration time of900 s. IRD is a fiber-fed spectrograph and we injected light fromthe laser-frequency comb into the second fiber for the simulta-neous wavelength calibration. The raw data were reduced intoone-dimensional spectra using the IRAF software as well as acustom code (Kuzuhara et al. 2018; Hirano et al. 2020) to sup-press the bias and correlated noise of the detectors. The typicalsignal-to-noise (S / N) ratio of the reduced spectra ranged from 60to 95 per pixel at 1000 nm.Following Hirano et al. (2020), we analyzed the observedspectra to extract precise RVs. In doing so, we combined mul-tiple frames to obtain a high S / N template for the RV analysis,after removing the telluric lines as well as the instrumental pro-file of the spectrograph. The relative RV was then measured withrespect to this template for each spectrum. The resulting relativeRVs after the correction for the barycentric motion of Earth arelisted in Table A.2 and shown in Fig. 1; the internal RV error wastypically 3–6 m s − for each frame.
3. Analysis
The stellar parameters for this target were estimated using theCARMENES VIS stacked stellar template produced by
SERVAL . T e ff , log g , and iron abundance [Fe / H] were determined throughspectral fitting with a grid of PHOENIX-SESAM models, fol-
Article number, page 4 of 13. G. Soto et al.: Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
Table 1: Stellar parameters of LHS 1478Parameter Value ReferenceName LHS 1478 Luy79Karmn J02573 + α (J2000) 02 57 17.51 Gaia
EDR3 δ (J2000) +
76 33 13.0
Gaia
EDR3SpT M3.5 V Cab16 J [mag] 9 . ± .
026 Skr06 G [mag] 12 . ± . Gaia
EDR3 T [mag] 11 . ± . (cid:36) [mas] 54 . ± . Gaia
EDR3 d [pc] 18 . ± . Gaia
EDR3 T e ff [K] 3381 ±
54 This worklog g [cgs] 4 . ± .
06 This work[Fe / H] [dex] − . ± .
19 This work M (cid:63) [ M (cid:12) ] 0 . ± .
012 This work R (cid:63) [ R (cid:12) ] 0 . ± .
008 This work ρ (cid:63) [g cm − ] a . ± . L (cid:63) [10 − L (cid:12) ] 71 . ± . H α [Å] + . ± .
026 This work v sin i [km s − ] < P rot [d] [6.4] b New16
References.
Luy79: Luyten (1979); Sta18: Stassun et al. (2018); Cab16:Caballero et al. (2016a); Skr06: Skrutskie et al. (2006);
Gaia
EDR3:Gaia Collaboration et al. (2020); Sta19: Stassun et al. (2019); Cif20:Cifuentes et al. (2020); Mar21: Marfil et al., in prep; New16: Newtonet al. (2016).
Notes. ( a ) Derived from M (cid:63) and R (cid:63) . ( b ) Flagged as a non-detection or undetermined detection by Newtonet al. (2016). lowing Passegger et al. (2019), using the upper limit v sin i = . − of Reiners et al. (2018) and Marfil et al (in prep.), whodid not detect any rotational velocity. The luminosity L (cid:63) was es-timated by integrating the spectral energy distribution, with thephotometric data used listed by Cifuentes et al. (2020). The stel-lar radius R (cid:63) was determined through the Stefan–Boltzmann law,and the stellar mass M (cid:63) through the mass-radius relation fromSchweitzer et al. (2019). We estimated the overall activity levelof the star from the pseudo-equivalent width of the H α line aftersubtraction of an inactive stellar template (pEW’H α ) followingSchöfer et al. (2019). We obtained a value of + . ± .
026 Å,which indicates that LHS 1478 is a fairly inactive star (Je ff erset al. 2018; Schöfer et al. 2019).The star has a rotational period of 6.4 days, as listed by New-ton et al. (2016), but the detection is deemed inconclusive. Welooked at the SAP (Simple Aperture Photometry, Morris et al.2020) and PDC data from TESS and, after masking the transitsand performing a Lomb-Scargle periodogram, we could not de-tect any signal with a period of 6.4 days. We therefore agree with Table 2: Posterior distributions from the joint fit. The uncertain-ties represent the 68% CI of the obtained distributions.Parameter Value Stellar density ρ (cid:63) [g cm − ] a . + . − . Orbital parametersP [d] 1 . + . − . T [BJD] 2458786 . + . − . r b . + . − . r b . + . − . p = R p / R (cid:63) . + . − . b = ( a / R (cid:63) ) cos i . + . − . i [deg] 87 . + . − . a / R (cid:63) . + . − . K [m s − ] 3 . + . − . t T [h] 0 . + . − . Derived planetary parametersM p [ M ⊕ ] 2 . + . − . R p [ R ⊕ ] 1 . + . − . ρ p [g cm − ] 6 . + . − . a [AU] 0 . + . − . T eq [K] c + − Notes. ( a ) Derived from lightcurve fitting, using the relations from Sea-ger & Mallén-Ornelas (2003) ( b ) Parameterization from Espinoza (2018) for p and b . ( c ) Assuming zero Bond albedo.
Newton et al. (2016) in that the signal is a non-detection and maynot correspond to the real rotational period for this star.Time series of the activity indicators CRX, dLW, and H α (Zechmeister et al. 2018) were extracted from the spectra with SERVAL . We find no signs of periodic variations similar to theplanet orbital period or the putative stellar rotational period of6.4 days (Fig. 4). The only exception is for the H α data, wherethere is a signal at ≈ = ffi cient | r | < . Gaia
EDR3 is shown in Table 1.
We used juliet (Espinoza et al. 2019) to perform a joint fitof the TESS, RCO, LCOGT, CARMENES, and IRD data. Weused the e ffi cient, uninformative sampling scheme of Kipping(2013) to parameterize the limb-darkening in the TESS data, to-gether with a quadratic law. For the RCO and LCOGT data weused a linear limb-darkening law. We also followed the parame-terization presented in Espinoza (2018), with parameters r and r , to fit for p and b , the planet-to-star radius ratio and impactparameter, respectively. The TESS data were fitted separately Article number, page 5 of 13 & A proofs: manuscript no. toi1640 P o w e r Period (days) P o w e r Fig. 4: GLS periodograms for the CARMENES RV and RVresiduals from the fit (top panel), and activity indices plus win-dow function (bottom panel). The window function was scaleddown to have power comparable to the GLS periodograms. Thedotted horizontal lines represent the 1%, 5%, and 10% FAP. Thered arrow at the top is the position of the orbital period of theplanet, and the red dotted vertical lines are aliases of that period.The black dotted vertical line represents the 6.4 days period fromNewton et al. (2016).by sector, each with its own relative flux o ff set and jitter, butwe imposed identical limb-darkening coe ffi cients for the transitcurves from all sectors. We fitted the LCOGT data separately foreach observing night, but as with the TESS data we imposed anidentical limb-darkening coe ffi cient for the two nights. The de-trending of the TESS data was done by incorporating a GaussianProcess (GP) model; we used the celerite Matern kernel withhyperparameters ( σ GP , ρ GP ), the amplitude and time scale of theGP, respectively (Foreman-Mackey et al. 2017). The same GPmodel was used for each TESS sector. An initial search for theoptimal parameters for this system was done with exostriker (Trifonov 2019), using all datasets, and the results were used tobuild the priors for juliet , which are shown in Table A.3. Theobtained posterior probabilities are listed in Tables 2 and A.4.We construct our full model, shown in Figs. 1 and 3, fromthe 68% confidence interval (CI) of the posterior distributionfor each parameter. We find that the signal observed both in thephotometric and RV data is consistent with a 2 . + . − . M ⊕ and1 . + . − . R ⊕ planet, with a density of 6 . + . − . g cm − , orbitingthe star with a period of 1 . + . − . days.Since there is a brighter star ( G = .
30 mag) about120 arcsec north of LHS 1478, we checked for possible contami-nation in the TESS light curve. We analyzed the transit data fromTESS and LCOGT, using the GP-detrended data, leaving the di-lution factor as a free parameter for the TESS data, and fixingthe limb darkening coe ffi cients to the values interpolated fromlimb darkening tables (Claret et al. 2013; Claret 2017). The pa-rameters from Table 2 were very well recovered, and the best fitdilution factor is below 1%, and it is below 13% at 95% confi-dence. We conclude that the TESS light curve is therefore notcontaminated. Mass ( M ) R a d i u s ( R ) OEarth Like100% MgSiO
50% H O - 50% EarthTransiting PlanetsTransiting Planets(CARMENES)
Fig. 5: Mass-radius diagram for LHS 1478 b (red star). The col-ored lines represent the composition models from Zeng et al.(2016, 2019). Transiting planets around M-dwarfs with massand radius measurements are shown in black (detected withCARMENES) and gray circles.A TLS search in the light curve residuals from the TESSdata showed two significant periods at 13.9 and 27.8 days, butthese are due to the sampling of the data (Fig. A.3). A GLS pe-riodogram of the residuals of the RV data does not show anysignificant signals, either (Fig. 4).
4. Discussion and characterization prospects
Given the measured properties, we made a first exploratory guessof the planet composition. We compared its mass and radius withthe models from Zeng et al. (2016, 2019), shown in Fig. 5, plusother transiting planets around M-dwarfs from the literature .We find that LHS 1478 b is compatible with a bulk composi-tion of ∼
30 % Fe +
70 % MgSiO , which makes it comparableto Earth’s, thus strongly supporting the notion that it is a rockyworld indeed.Atmospheric characterization of rocky planets with (some)properties similar to Earth is one of the pivotal developmentsexpected in the forthcoming years, thanks to the deployment ofnew ground and space-based facilities. The potential of a tar-get for detailed characterization is a complicated function of theplanet and host star properties, and the instrument to be used. Inthis sense, not all transiting planets that have interesting proper-ties are equally suitable for actual characterization.As a first approximation of the suitability of LHS 1478 b, wecalculate the spectroscopic metrics from Kempton et al. (2018),which were developed to rank the best TESS targets for the in-strumentation aboard the JWST. We estimate the emission spec-troscopy metric (ESM) and transmission spectroscopy metric(TSM) to be 7.28 and 19.35, respectively. The upper panel ofFigure 6 shows the ESMs of rocky exoplanets with measuredmasses, either through RVs or TTVs, and puts LHS 1478 b inthat context. The ESM of 7.28 is slightly lower than the 7.5threshold set by Kempton et al. (2018), but it is very close to thatof Gl 1132 b, which is considered in that work as a benchmarkrocky planet for emission spectroscopy. The lower panel illus-trates TSM values. An acceptable TSM value for this class ofplanet is around 12 or higher (Table 1 in Kempton et al. 2018), Data on transiting planets from https://carmenes.caha.es/ext/tmp/ .Article number, page 6 of 13. G. Soto et al.: Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
15 30 45 60 75 90
Distance from Earth (pc) E m i ss i o n Sp ec t r o s c o p y M e t r i c ( E S M ) LTT1445Ab GJ357b L168-9bGJ9827cGJ1252b K2-233bLTT3780bGJ1132b LHS1140cT-1b K2-137bL98-59b GJ3473b
TOI 1640 b
Radius (R )0.71.3 400450500550600650700750800850 E qu ili b r i u m t e m p er a t u re ( K )
10 15 20 25 30 35 40
Distance from Earth (pc) T r a n s m i ss i o n Sp ec t r o s c o p y M e t r i c ( T S M ) LTT1445AbGJ357b L168-9b GJ9827cGJ1252bLTT3780bGJ1132bLHS1140c K-138bT-1dT-1e T-1fT-1bT-1g T-1c GJ3473b
TOI 1640 b
Radius (R )0.71.3 400450500550600650700750800850 E qu ili b r i u m t e m p er a t u re ( K ) Fig. 6: Upper) The emission spectroscopy metric (ESM), andLower) transmission spectroscopy metric (TSM) for exoplanetswith a radius less than 1.5 R ⊕ and a mass determination by RVsor TTVs. LHS 1478 b (TOI-1640 b) is labeled and marked withthicker black borderline in both panels.which implies that LHS 1478 b is likely an appropriate candi-date for atmospheric characterization through transmission spec-troscopy as well.As a second refinement to its suitability for characterization,we assess the potential chemical species that could be detectedin its atmosphere using near future instrumentation. Molecularfeatures such as water, carbon dioxide, or methane should betypically observable on these kind of planets if they maintain asubstantial atmosphere (Molaverdikhani et al. 2019a,b), but thedetectability of such features can also be obscured by the pres-ence of clouds (Molaverdikhani et al. 2020). In order to quantita-tively assess the potential of atmospheric characterization obser-vations of LHS 1478 b with the JWST, we calculate a few atmo-spheric models and their composition using the photo-chemicalcode ChemKM (Molaverdikhani et al. 2019a) and their corre-sponding spectra using petitRADTRANS (Mollière et al. 2019).We assumed a stellar radiation environment similar to that mea-sured for the well-studied star GJ 667C (T e ff ≈ feature at around 4.5 µ m. The amplitudesof these features is between 50 to 100 ppm, which should bemeasurable given that the star is rather bright. This was verifiedusing P and E xo (Batalha et al. 2017) simulations of the NIRISS-SOSS, NIRSpec-G395M, and MIRI-LRS instruments on boardof JWST.Hydrogen-dominated atmospheres with temperatures be-low 900 K are expected to have significant photochemically-produced hazes (e.g. He et al. 2018; Gao et al. 2020). If sucha scenario applies to LHS 1478 b, it would result in the ob-scuration of spectral features, particularly in the case of anenhanced metallicity (see bottom panel in Figure 7). A rela-tively flat transmission spectrum might thus be indicative of ametallicity-enhanced atmosphere covered by haze, which mightbe indistinguishable from the planet having no atmosphere at all.If (a close to) flat spectrum was observed, ground-based high-resolution spectroscopy could then be used to detect unobscuredmolecular features and break the associated degeneracies.In summary, the simulations in this section indicate thatLHS 1478 b belongs to the small family of planets, along withGJ 357 b (Luque et al. 2019) and GJ 1132 b (Berta-Thompsonet al. 2015), where we can realistically expect to obtain mean-ingful measurements and constraints with next-generation spacetelescopes such as JWST.
5. Conclusions
In this paper we have presented TESS and ground-based pho-tometric observations, together with CARMENES and IRDDoppler spectroscopy, of the star LHS 1478. We determineLHS 1478 b to have a mass of 2 . + . − . M ⊕ , a radius of1 . + . − . R ⊕ and a bulk density of 6 . + . − . g cm − , which is con-sistent with an Earth-like composition. The star is remarkablyinactive, and we see no signs of additional planetary signals inthe photometry or the radial velocities, thus greatly simplifyingthe analysis.The equilibrium temperature of this planet places it in arecently found group of warm rocky planets, together withGJ 357 b and GJ 1132 b, which are ideal for atmospheric studies.The fact that the planetary signal is very isolated in both pho-tometry and radial velocity, with no stellar activity or additionalcompanions contaminating it, together with the proximity of thesystem to the Sun and the brightness of the star, make this anideal target for near future transit observations with JWST andground-based high-resolution spectrometers. Acknowledgements.
CARMENES is an instrument at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointlyby the Junta de Andalucía and the Instituto de Astrofísica de Andalucía(CSIC). CARMENES was funded by the Max-Planck-Gesellschaft (MPG),the Consejo Superior de Investigaciones Científicas (CSIC), the Ministerio deEconomía y Competitividad (MINECO) and the European Regional Develop-ment Fund (ERDF) through projects FICTS-2011-02, ICTS-2017-07-CAHA-4, and CAHA16-CE-3978, and the members of the CARMENES Consortium(Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Lan-dessternwarte Königstuhl, Institut de Ciències de l’Espai, Institut für Astro-physik Göttingen, Universidad Complutense de Madrid, Thüringer Landesstern-warte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte,Centro de Astrobiología and Centro Astronómico Hispano-Alemán), with ad-ditional contributions by the MINECO, the Deutsche Forschungsgemeinschaftthrough the Major Research Instrumentation Programme and Research UnitFOR2544 “Blue Planets around Red Stars”, the Klaus Tschira Stiftung, thestates of Baden-Württemberg and Niedersachsen, and by the Junta de Andalucía.This work is partly financed by the Spanish Ministry of Economics and Com-petitiveness through project PGC2018-098153-B-C31. This paper includes datacollected by the TESS mission. Funding for the TESS mission is providedby the NASA’s Science Mission Directorate. Resources supporting this work
Article number, page 7 of 13 & A proofs: manuscript no. toi1640 CH CO H O Fig. 7: Synthetic transmission atmospheric spectra of LHS 1478 b including haze opacity (solid blue lines) and without haze opacity(solid red lines). Simulated observations and estimated uncertainties are shown for the JWST NIRISS-SOSS, NIRSpec-G395M, andMIRI-LRS configurations, assuming two transits and binned for R =
50 (filled circles with error bars). Top : Fiducial models withsolar abundance. Bottom : Metallicity enhanced by a factor of 100. were provided by the NASA High-End Computing (HEC) Program throughthe NASA Advanced Supercomputing (NAS) Division at Ames Research Cen-ter for the production of the SPOC data products. This work makes use ofobservations from the LCOGT network. LCOGT telescope time was grantedby NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP isfunded by NSF. This work is partly supported by Grant-in-Aid for JSPS FellowsGrant Number JP20J21872, by JSPS KAKENHI Grant Numbers 22000005,JP15H02063,JP18H01265, JP18H05439, JP18H05442, JP19K14783, by JSTPRESTO Grant Number JPMJPR1775, and by a University Research SupportGrant from the National Astronomical Observatory of Japan (NAOJ).
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Aohoku Place, Hilo, HI 96720, USA Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Taut-enburg, Germany Deptartment of Physics & Astronomy, Swarthmore College, Swarth-more PA 19081, USA Department of Astronomy, School of Science, The Graduate Uni-versity for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka,Tokyo, Japan MIT Kavli Institute for Astrophysics and Space Research, Mas-sachusetts Institute of Technology, Cambridge, MA 02139, USA Departamento de Física de la Tierra y Astrofísica and IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM),Facultad de Ciencias Físicas, Universidad Complutense de Madrid,28040, Madrid, Spain Department of Astronomy, The University of Tokyo, 7-3-1 Hongo,Bunkyo-ku, Tokyo 113-0033, Japan Centro Astronómico Hispano-Alemán, Observatorio de Calar Alto,04550 Gérgal, Almería, Spain Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112,21029 Hamburg, Germany Homer L. Dodge Department of Physics and Astronomy, Universityof Oklahoma, 440 West Brooks Street, Norman, OK 73019, USA Department of Earth, Atmospheric and Planetary Sciences, Mas-sachusetts Institute of Technology, Cambridge, MA 02139, USA NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt,MD 20771, USA Hazelwood Observatory, Australia Planetary Discoveries, Fredericksburg, VA 22405, USAArticle number, page 9 of 13 & A proofs: manuscript no. toi1640 B L S S D E P MAX = 1.950 d1.0 10.0Period (days)505101520 TL S S D E P MAX = 1.949 d
Fig. A.1: BLS (top panel) and TLS (bottom panel) signal de-tection e ffi ciency (SDE) for the TESS light curve. The verticalblue lines correspond to aliases of the maximum period, high-lighted by the blue-shaded region. The dashed horizontal line inboth panels corresponds to the the threshold for transit detection(SDE >
8) from Aigrain et al. (2016).
Appendix A: Additional figures and tables
Article number, page 10 of 13. G. Soto et al.: Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
Table A.1: Radial velocity data and activity indicators from CARMENES VIS.BJD RV RV error CRX dLW H α -2450000 m s − m s − m s − Np − m s − ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Article number, page 11 of 13 & A proofs: manuscript no. toi1640
Pixel Column Number P i x e l R o w N u m b e r E N
Sector 18
123 45 678 9101112
Flux ×10 (e ) Pixel Column Number
Sector 19 m = -2 m = 0 m = 2 m = 3 m = 5 Flux ×10 (e ) Pixel Column Number
Sector 25
Flux ×10 (e ) Pixel Column Number
Sector 26
Flux ×10 (e ) Fig. A.2: TESS target pixel files for all four sectors that observed the target TOI-1640. The red-shaded area corresponds to theapertures used to extract the photometry. The nearest sources to the target with up to 6 magnitude di ff erence from Gaia are markedwith red circles. TESS pixel scale is ∼
21 arcsec.Table A.2: Radial velocity data from IRD.BJD RV RV error-2450000 m s − m s − Period (days)5.02.50.02.55.07.510.012.5 S D E P orb = 1.949 days 0.040.020.000.020.040.060.080.10 W F P o w e r Fig. A.3: TLS of the TESS light curve residuals, after subtract-ing the GP + transit model. The dashed green lines are the aliasesof the 13.9 days signal. The black line represents the windowfunction of the data. Table A.3: Priors used for the juliet run.Parameter Prior a M lc b [ppm] N (0 , . σ lc b [ppm] LU (0 . , q , TESS U (0 , q , TESS U (0 , q , LCO,RCO U (0 , µ RV [m s − ] c N (0 , . σ RV [m s − ] c LU (10 − , P [d] N (1 . , . T [BJD-2458000] N (786 , . r U (0 , r U (0 , a / R (cid:63) N (16 . , . K [m s − ] U (0 . , e Fixed(0) σ GP, TESS [ppm] LU (10 − , ) ρ GP, TESS [d] LU (10 − , ) Notes. ( a ) N : Normal distribution, LU : Log-Uniform distribution, U :Uniform distribution. ( b ) The same prior distributions were assumed for the photometric data,where lc stands for TESS sectors 18, 19, 25, 26, the two nights of pho-tometry from LCOGT, and RCO. ( c ) The same prior distributions were assumed for the radial velocitydata, where RV stands for CARMENES and IRD.Article number, page 12 of 13. G. Soto et al.: Mass and density of the transiting hot and rocky super-Earth LHS 1478 b (TOI-1640 b)
Table A.4: Posterior distributions for the instrumental parame-ters. The uncertainties represent the 68 % CI of the obtained dis-tributions. Parameter Value
Photometric parametersM
S18 [10 − ppm] 3 . + . − . σ S18 [ppm] 1 . + . − . M S19 [10 − ppm] − . + . − . σ S19 [ppm] 1 . + . − . M S25 [10 − ppm] − . + . − . σ S25 [ppm] 2 . + . − . M S26 [10 − ppm] − . + . − . σ S26 [ppm] 1 . + . − . M LCO1 [10 − ppm] − . + . − . σ LCO1 [ppm] 977 . + . − . M LCO2 [10 − ppm] − . + . − . σ LCO2 [ppm] 856 . + . − . M RCO [10 − ppm] 10 . + . − . σ RCO [ppm] 651 . + . − . q , TESS a . + . − . q , TESS a . + . − . q , LCO . + . − . q , RCO . + . − . RV parameters µ CARMENES [m s − ] 0 . + . − . σ CARMENES [m s − ] 1 . + . − . µ IRD [m s − ] − . + . − . σ IRD [m s − ] 2 . + . − . GP parameters GP σ, TESS [10 − ppm] 339 . + . − . GP ρ, TESS [d] 0 . + . − . Notes. ( a ))