LHS 475 b: A Potential Venus Analog Orbiting a Nearby M Dwarf

Based on photometric observations by TESS, we present the discovery of a potential Venus analog transiting LHS 475, an M3 dwarf located 12.5 pc from the Sun. The mass of the star is 0.274 ± 0.015 M ☉. The planet, originally reported as TOI 910.01, has an orbital period of 2.0291010 ± 0.0000017 days and an estimated radius of 0.975 ± 0.058 R ⊕. We confirm the validity and source of the transit signal with MEarth and Las Cumbres Observatory Global Telescope ground-based follow-up photometry. We present radial velocity data from CHIRON that rule out massive companions. In accordance with the observed mass–radius distribution of exoplanets as well as planet formation theory, we expect this planetary companion to be terrestrial, with an estimated radial velocity semiamplitude of 1.1 m s−1. LHS 475 b is likely too hot to be habitable but is a suitable candidate for emission and transmission spectroscopy.


Introduction
The Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) was launched in 2018 and has already yielded a plethora of planet discoveries.At the time of this writing, there are 326 confirmed TESS planets as well as thousands of TESS Objects of Interest (TOIs) still awaiting validation.Of particular interest are planets transiting the nearest red dwarfs.Such planets have larger relative sizes compared to their host star.This leads to a larger transit depth, facilitating planet detection.It also increases the feasibility for detailed atmospheric characterization of the planet via transit spectroscopy, which is among the goals of the recently launched James Webb Space Telescope (JWST).Stars with masses between 0.1 and 0.3 M ☉ have been studied extensively by the MEarth team through photometric monitoring combined with high-resolution spectroscopy as part of a study that is volume-complete within 15 pc (Nutzman & Charbonneau 2008;Berta et al. 2012;Irwin et al. 2015;Winters et al. 2021).As a result, these stars have well defined physical parameters (including masses) and multiplicity information.However, only a handful of stars with masses below 0.3 M ☉ located within 15 pc of the Sun are known to host transiting planets: GJ 1132 (Berta-Thompson et al. 2015), GJ 1214 (Charbonneau et al. 2009), LHS 1140 (Dittmann et al. 2017;Ment et al. 2019), LHS 3844 (Vanderspek et al. 2019), LTT 1445A (Winters et al. 2019(Winters et al. , 2022)), TOI 540 (Ment et al. 2021), and TRAPPIST-1 (Gillon et al. 2016(Gillon et al. , 2017)).
Follow-up observations of TOIs, typically by ground-based observatories, are essential for multiple reasons.First, they provide an independent validation of the transiting planet, ruling out instrument systematics as the cause of the transit-like signal.Second, the sky-projected size of each individual TESS pixel is large (21″ × 21″) and typically contains many background light sources in addition to the intended target.Subsequently, follow-up observations are crucial to exclude background stars (including unresolved eclipsing binaries) as potential sources of the transit signal.Third, ground-based photometric observations are a relatively cost-effective way to increase the total number of observed transits, leading to more precise estimates for modeled system parameters as well as refined ephemerides, which are often essential for planning additional follow-up observations.Finally, spectroscopic follow-up observations can yield estimates for the planet's mass, a crucial parameter that cannot be determined from transit photometry.
The vast majority of confirmed transiting planets have estimated radii larger than that of the Earth.Unlike planets substantially larger than Earth, Earth-sized and smaller planets are likely too small to hold on to a substantial volatile envelope.
Therefore, they are highly likely to be terrestrial in nature with a mostly rocky interior composition.The existence of a population of terrestrial planets distinct from larger sub-Neptunes and water worlds is evidenced by the bimodal radius distribution of planets (Owen & Wu 2013;Fulton et al. 2017).The gap between the two populations occurs at planet radii between 1.5 and 2.0 R ⊕ and has been found to exist around Sun-like stars (Fulton et al. 2017;Fulton & Petigura 2018) as well as K and M dwarfs (Cloutier & Menou 2020), with planets below these radii mainly expected to have terrestrial bulk compositions (Weiss & Marcy 2014;Dressing et al. 2015;Rogers 2015).This interpretation is also consistent with simulations based on planet growth models (e.g., Zeng et al. 2019).Therefore, we expect an Earth-sized or smaller planet to be terrestrial with a high degree of confidence even in the absence of a mass estimate from radial velocity or transit timing variation data.
Here we present the discovery and subsequent ground-based validation observations of LHS 475 b, an Earth-sized planet in the Venus zone (as defined by Ostberg & Kane 2019 based on incident stellar flux) orbiting a nearby M dwarf.Section 2 describes the properties of the host star.Section 3 summarizes various data sets used in this study.We describe our modeling procedure in Section 4 and present results in Section 5. Finally, Section 6 summarizes the article and offers a brief discussion.
There are no additional light sources listed within 10″ in either the Two Micron All-Sky Survey (2MASS) catalog or the TESS Input Catalog (TIC).Leveraging the high proper motion of the target, we used archival images from the 2MASS (Skrutskie et al. 2006) as well as the Digitized Sky Survery (DSS) to rule out the presence of any distant background stars at the location of LHS 475 at the time of the observations presented in this paper (years 2019-2021).The 2MASS images were taken in year 2000 while the DSS images are from years 1976-1999.While there are no visible sources in the 2MASS J, H, and K images, the bluer DSS images do reveal an additional light source at a sky-projected distance of 7″ from LHS 475 at the time of the observations.We later identified this background source within the Gaia DR3 catalog with an ID of 6347643496607834880.It has a B P − R P color of 1.05 mag, in contrast to the B P − R P = 2.73 mag estimate for LHS 475.It is 6.7 mag fainter than LHS 475 in the Gaia bandpass, and based on the color difference, we estimate a difference close to 8 mag in the TESS and MEarth bandpasses.As a result, this object would be too faint to produce the planetary transits described within this work.We further investigate the presence of other nearby bright objects in Section 3.5.
We adopt a K-band apparent brightness of K = 7.686 ± 0.042 mag from 2MASS, yielding an absolute brightness of M K = 7.205 ± 0.042 mag.We also adopt J-and H-band fluxes from 2MASS and VRI photometry from Jao et al. (2011).Using the relation between stellar mass and K-band luminosity for main-sequence M dwarfs in Benedict et al. (2016), we estimate a stellar mass of M = 0.274 ± 0.015 M ☉ .We use two independent radius-mass relations to estimate the stellar radius: we obtain R = 0.281 ± 0.013 R ☉ from optical interferometry of single stars (Boyajian et al. 2012, Equation (10)) and R = 0.291 ± 0.014 R ☉ from eclipsing binary measurements (Bayless & Orosz 2006).We adopt a weighted average of R = 0.286 ± 0.013 R ☉ as the final radius (we adopt the smaller of the two uncertainties for the final value since the individual errors are mostly systematic in nature).We check for consistency with the mass-radius relation for M dwarfs by Mann et al. (2015) which yields R = 0.287 ± 0.026 R ☉ .Using the mass and radius, we estimate a stellar surface gravity of g log 4.964 0.046 =  .We employ bolometric corrections (BCs) to determine the luminosity of LHS 475.We use Table 5 of Pecaut & Mamajek (2013) to interpolate between the V − K color and BC V , obtaining a V-band correction value of BC V = −2.348± 0.050 mag and a bolometric luminosity of L = 0.00907 ± 0.00049 L ☉ .Next, we apply the third-order polynomial fit between V − J and BC V in Mann et al. (2015, and subsequent erratum) to derive BC V = − 2.249 ± 0.041 and L = 0.00829 ± 0.00039 L ☉ .Finally, the relationship between BC K and I − K in Leggett et al. (2001) produces BC K = 2.702 ± 0.053 and L = 0.00870 ± 0.00054 L ☉ .We adopt as the stellar luminosity a weighted average of the three estimates; therefore, L = 0.00862 ± 0.00039 L ☉ .The uncertainty in L was taken to be equal to the lowest of the three uncertainties to account for potential systematic errors.We then proceed to use the Stefan-Boltzmann law to determine the effective stellar surface temperature, obtaining T eff = 3289 ± 83 K.This result was based on the solar values of M bol,☉ = 4.7554 mag and T eff,☉ = 5772 K published by Mamajek (2012).We perform two consistency checks using the Gaia G-band magnitude G = 11.4125± 0.0005 from Gaia DR3.We approximate a G-band bolometric correction of BC G = -0.948based on the methods in Creevey et al. (2023; we assume [Fe/H] = [α/Fe] = 0).This yields L = 0.00811 L ☉ , a value within 1.3σ of our estimate.We also estimate the effective temperature using the T eff -M G relation from Rabus et al. (2019), obtaining T eff = 3301 K, also consistent with our calculated value.
LHS 475 appears to be a magnetically quiet star.The CHIRON spectra (described in Section 3.4) do not show evidence of rotational broadening, and Hα is in absorption.LHS 475 has a previously determined photometric rotation period of P rot = 79.3 days (Newton et al. 2018), consistent with its relative inactivity.This yields an estimated equatorial rotation velocity of 0.18 km s −1 , compared to CHIRON's spectral resolution of 3.75 km s −1 .The star also flares relatively infrequently; specifically, it has an estimated flare rate of R ln 11.90 0.79 31,5 = - where R 31,5 is the number of flares per day with total energy above 3.16 × 10 31 erg in the TESS bandpass (Medina et al. 2020).
While M dwarfs do slowly spin down as they evolve, accurately determining their age is known to be a difficult endeavor.For the slowest-rotating mid-to-late M dwarfs (P rot > 70 days), galactic kinematics suggests a mean age of 5 2 4 -+ Gyr (Newton et al. 2016).More recently, Medina et al. (2022) used galactic kinematics to derive a mean age of 5.6 ± 2.7 Gyr for mid-to-late M dwarfs with rotation periods between 10 and 90 days while such stars with P rot > 90 days would have an estimated age of 12.9 ± 3.5 Gyr.Ultimately, we are unable to ascertain the age of LHS 475, but it is unlikely to be less than a few billion years old.

TESS
TESS (Ricker et al. 2015) gathered photometric measurements of LHS 475 during its Prime Mission (observation sectors 12 and 13) as well as its first Extended Mission (sectors 27 and 39).These observations were conducted between 2019 May and 2021 June and span Barycentric Julian Date (BJD) ranges of 2458625.0-2458682.4 (sectors 12-13), 2459036.3-2459060.2 (sector 27), and 2459361.8-2459389.7 (sector 39).The target was included in TIC with a TIC ID of 369327947 as well as the original TESS Candidate Target List (Stassun et al. 2018).LHS 475 was also included in TESS Guest Investigator Programs G011180 (PI: Courtney Dressing) and G011231 (PI: Jennifer Winters).The observations were made with a twominute cadence during the Prime Mission and a 20 s cadence within the Extended Mission.We utilize photometric data reduced by the NASA Ames Science Processing Operations Center (SPOC) pipeline (Jenkins et al. 2016).Specifically, we adopt the Pre-search Data Conditioning Simple Aperture Photometry (PDCSAP; Smith et al. 2012;Stumpe et al. 2012Stumpe et al. , 2014) ) 2 minute cadence version of the light curve which has been cotrended and corrected for instrument systematics as well as crowding: unresolved light from other sources that fall within the same TESS CCD pixel.We exclude six measurements that were marked through the SPOC Data Quality Flags to correspond to impulsive outliers (cadence quality flag bit 10).We also manually exclude a section of the data at the end of sector 27 spanning the BJD range 2459059.5-2459060.2 (comprising 0.68% of the total number of data points) due to a strong negative trend in the observed flux that is indicative of spacecraft systematics.The remaining data set contains a total of 73,604 individual measurements.
SPOC initially detected a transit-like signal in phase-folded sector 12 data and the TESS team dubbed the planet candidate TOI 910.01.The accompanying data validation report (DVR; Twicken et al. 2018;Li et al. 2019) cited a candidate period of 2.029 days, a transit depth close to 1100 parts per million (ppm), and a transit signal-to-noise ratio (S/N) of 12.0 based on 12 transits from sector 12.The detected signal passed preliminary validation tests (described by Twicken et al. 2018) and had a very low bootstrap false-alarm probability, favoring the hypothesis that TOI 910.01 indeed represents a planet.An updated DVR from SPOC based on all four observation sectors later updated the orbital period to P = 2.029088 ± 0.000006 days, the transit depth to 978 ± 73 ppm, and the S/N to 19.3 based on a total of 45 transits.We note that despite the availability of 20 s cadence data from the Extended Mission, SPOC only performs a transit search in the 2 minute cadence version of those data.Either cadence is much shorter than the fitted transit duration of 41.6 minutes.
Since the estimated rotation period of LHS 475 (P rot = 79.3 days) is substantially longer than the duration of a single TESS sector, we do not attempt to include an explicit model for the quasi-periodic flux variation caused by stellar rotation.Instead, we detrend the light curve using a sliding median with a fixed width of 12 hr.This approach has the benefit of smoothing out any long-term variations while not affecting the light curve on short timescales (e.g., the estimated duration of a transit).We provide a plot of the SPOC PDCSAP light curve as well as the fitted baseline within each observation sector in Figure 1.All the TESS data used in this paper can be found in MAST:10.17909/bmdh-kd60.

MEarth
We obtained follow-up observations for five individual transits of LHS 475 b with the ground-based MEarth-South telescope array (Nutzman & Charbonneau 2008;Berta et al. 2012;Irwin et al. 2015).The MEarth-South array consisted of eight robotically controlled 40 cm telescopes at the Cerro Tololo Inter-American Observatory in Chile.Each telescope was equipped with a CCD camera with a custom 715 nm longpass filter that is sensitive to red optical and near-infrared light.The observations took place in August and September of 2019; the corresponding BJD range is 2458717.5-2458727.8.A total of seven individual telescopes were used concurrently to monitor the star during transit events, and a separate light curve was produced for each telescope as a result of the data reduction process.
We perform aperture photometry with fixed aperture radii of 12, 17, 24, and 34 pixels (at a scale of 0 84 per pixel) and verify that the transit signal persists regardless of the choice of aperture.We also generate light curves for every detected light source within 2 5 of LHS 475 and verify that the transit signal is not present in any of them.We adopt an aperture radius of 12 pixels (or 10 08) for the remainder of the analysis.
The MEarth light curves need to be detrended to account for atmospheric effects and telescope systematics.We begin by estimating a full transit duration of T full = 40.0minutes based on the TESS data.For each observed transit, we fit a baseline to the MEarth light curve, including all points within 2T full of the transit midpoint t 0 while excluding the transit window itself; that is, we include every observation timestamp t such that In order to best describe the behavior of each transit's baseline, we utilize several competing models.Specifically, we fit each baseline with a constant, linear, and quadratic function.We adopt the model with the lowest value for the Bayesian information criterion, which accounts for the number of model parameters.We display the detrended transit light curves in Figure 2. The baselines for the first two of the five transits were fit with a linear function while the remaining three were detrended using a quadratic function.Figure 2 also includes a panel with the combined residuals after subtracting the fitted model from the data.We note that the weighted rms of the residuals for individual observation nights varies between 1.9 and 3.0 parts per thousand (ppt), and the residuals for the combined data set have an rms of 2.5 ppt.Rapid changes in flux seen in Figure 2 during transits correlate with changes in seeing and are likely caused by variations in the precipitable water vapor content of the atmosphere.LHS 475 was monitored as part of the MEarth survey from 2016 October until 2018 August.Over that time, the star accumulated 12,836 photometric observations.However, LHS 475 b was not detected by the MEarth team due to the small transit depth induced by the planet (approx. 1 ppt) and the relatively low cadence of observations.While the planet remained undetected, this data set enabled the critical photometric determination of the stellar rotation period of 79.317 days by Newton et al. (2018).

Las Cumbres Observatory Global Telescope
We  1922022937), which was approximately 7″ northeast of LHS 475 at the time of the observations.We jointly modeled the five LCOGT light curves and find a planet-to-star radius ratio r R 0.033 0.003 0.002

= -+
, which is consistent within 1σ with the value from the global fit in Section 5.4.The combined and phase-folded LCOGT light curves are shown in Figure 3.

CHIRON
We utilized the CHIRON spectrograph (Tokovinin et al. 2013) mounted on the 1.5 m SMARTS telescope at the CTIO to gather seven reconnaissance spectra of LHS 475.Four of these spectra were obtained as part of the volume-complete spectroscopic survey of nearby mid-to-late M dwarfs by Winters et al. (2021).We carried out the observations between 2018 August and 2021 June.We produced radial velocities (RVs) from each of the spectra according to the reduction methods described in detail in Pass et al. (2023).The reduction process involves performing a cross-correlation between the CHIRON spectra and a set of rotationally broadened templates of inactive stars over wavelength ranges in the regime of 6400-7850 Å.We provide a list of relative RVs in Table 1 and display the entire RV time series in Figure 4.Note that the quoted RVs do not include a derived systemic RV of −10.3 ± 0.5 km s −1 , where the RV uncertainty is dominated by the cross-correlation process.Our derived systemic RV is consistent with the value of −10.59 ± 0.24 km s −1 in Gaia DR3 (Gaia Collaboration et al. 2016, 2023).We also adopt a noise floor of 20 m s −1 based on instrument instability for midto-late M dwarfs (Pass et al. 2023).
The RV time series does not show any evidence of an overall trend, which would point to the existence of a massive companion.Fitting a linear trend to the RVs yields an estimate of −2.1 ± 10.8 m s −1 yr −1 , consistent with zero.The rms of the RVs is 22.1 m s −1 , very close to the quoted individual RV uncertainties.In addition, we are able to rule out companions more massive than 0.20 Jupiter masses (M Jup ) at the orbital period of the planet with 99.73% (3σ) confidence.We obtained this value by fitting an RV model with the orbital period, eccentricity, and time of midtransit fixed to the values given in Table 2 and calculating the required RV semiamplitude such that the χ 2 values of the fits, each with N RV − 1 degrees of freedom, have associated p values below 0.0027.Similarly, we can place limits on the masses of potential long-period companions by increasing the fitted semiamplitude at a fixed orbital period while allowing the time of periastron passage to vary.We can rule out companions with masses above 0.9 M Jup , 1.2 M Jup , and 1.9 M Jup at fixed orbital periods of 1 yr, 5 yr, and 10 yr, respectively, with 99.73% (3σ) confidence.
The spectra show Hα in absorption.Medina et al. (2020) obtained an Hα equivalent width of 0.26 ± 0.09 Å using a subset of these spectra.

Zorro
We performed speckle imaging observations of LHS 475 with the Zorro dual-channel imager on the 8.1 m Gemini South telescope in Chile (program number GS-2019A-Q-302, PI: Winters).The star was observed on 2019 July 18 in the 562 and 832 nm bandpasses.The images are diffraction-limited with an estimated FWHM of 0 02.We provide the detection sensitivity curve and the auto-correlation function of the imaging data in Figure 5.No nearby light sources (within 1 2, corresponding to 15 au) are detected in either band down to an estimated contrast Δm of 4.88 mag (562 nm) or 5.87 mag (832 nm) at 0 5 from the star.We adopt an identical light curve and transit model for TESS and MEarth data sets, constraining the modeled parameters with various prior probability distributions.Specifically, the stellar mass and radius are constrained with Gaussian priors; in each case, the location and width of the prior matches the estimated value from Section 2. Gaussian priors are also included for the orbital period P and time of midtransit t 0 , although the posterior distributions for either parameter constrain these values much more tightly.We impose a uniform prior on the planet-to-star radius ratio with a domain of r/R ä [0, 0.1].Conditioned on the value of r/R, the prior distribution for the impact parameter b is distributed uniformly between 0 and 1 + r/R.We also include a loose Gaussian prior centered at a value of 1 on the baseline relative flux of the light curve.

Modeling
We opt for a quadratic limb-darkening model for the stellar surface.The limb-darkening coefficients (u 1 , u 2 ) are taken from Table 15 of Claret (2018) based on the spherical PHOENIX-COND model (Husser et al. 2013).Specifically, we adopt Gaussian priors centered at u 1 = 0.1529 and u 2 = 0.4604, which correspond to a surface gravity of g log 5.0 = and a surface temperature of 3300 K.The priors have a width of 0.02, a characteristic value based on the uncertainty in the estimated surface temperature.We also test a model where u 1 and u 2 are allowed to float freely and confirm that the results are consistent.The metallicity is implicitly assumed to be equal to the Solar value.While these coefficients were computed for the TESS bandpass, the MEarth spectral response is sufficiently similar to the one employed by TESS such that we have not found meaningful differences in the estimated limb-darkening coefficients in previous works (e.g., Ment et al. 2021).
The eccentricity e was fixed to a value of 0 as planets orbiting this close to their stars are expected to be tidally circularized.In order to estimate the tidal circularization timescale τ e , we utilize a simplified equation that Rasio et al. (1996) adopted based on the work by Goldreich & Soter (1966): where Q is the specific dissipation parameter, m and r are the planet's mass and radius, M is the stellar mass, and a is the semimajor axis.Assuming an Earth-mass planet and Q < 500 (an upper limit based on terrestrial solar system planets and satellites), we find τ e < 9 Myr, much less than the probable age of the system.Thus, we expect the orbit to be close to circular.We perform separate model optimization and sampling runs for three different data sets: (1) TESS photometry, (2) MEarth photometry, and (3) a combined set of TESS and MEarth data.The initial optimization is carried out by PyMC3 using a limited memory Broyden-Fletcher-Goldfarb-Shanno algorithm and it returns a local maximum a posteriori (MAP) estimate.We then draw 5000 samples from the posterior, starting at the MAP point, after tuning for 2000 iterations before sampling.We employ the No U-Turn Sampler, which is a Hamiltonian Monte Carlo-based algorithm.We utilize multiprocess sampling with three chains, resulting in a total of 15,000 drawn samples.We assess convergence via a ranknormalized R-test and find that R 1 » for all fitted parameters, indicating that the variance between multiple chains in consistent the variance within each chain.We then adopt the mean and standard deviation from the marginal distributions of each variable.We present the results of the modeling in the following section.

TESS Only
Fitting the model with the four sectors of TESS data alone yields a tightly constrained ephemeris with an orbital period of P = 2.0291018 ± 0.0000025 days and a transit midpoint BJD of t 0 = 2458626.20449± 0.00045.We obtain a fitted planetary radius of r = 0.974 ± 0.059 R ⊕ and an impact parameter of b = 0.739 ± 0.036.We plot a phase-folded version of our bestfit model utilizing TESS data in the top panel of Figure 6.

MEarth Only
Repeating the modeling and sampling process with the MEarth data set (consisting of follow-up light curves of five individual transits), we estimate an orbital period of P = 2.02894 ± 0.00015 days and a transit midpoint BJD of t 0 = 2458626.2105± 0.0066.These values are consistent within 1σ with the TESS-based results in the previous section.We calculate a planet radius of r = 1.008 ± 0.072 R ⊕ ; however, the best-fit MAP estimate is 0.97 R ⊕ .Both of these values are consistent with TESS data.The fitted impact parameter from MEarth data is b = 0.763 ± 0.060.The phase-folded model based on MEarth photometric data is displayed in the second panel of Figure 6.

LCOGT Only
We conduct the same analysis using LCOGT photometric data (follow-up observations of five full transits, described in Section 3.3).We recover an orbital period of P = 2.0290975 ± 0.0000028 days and a transit midpoint BJD of t 0 = 2458626.20509± 0.00074.The transiting planet has a modeled radius of r = 1.030 ± 0.067 R ⊕ and an orbital impact parameter of b = 0.717 ± 0.045.A phase-folded model based only on LCOGT data is shown in Figure 6.

Combined TESS, MEarth, and LCOGT
The calculated parameter values are consistent between the TESS, MEarth, and LCOGT photometric data.We proceed to optimize the model again for the combined data set (TESS, MEarth, and LCOGT light curves) with the goal of producing final estimates for the modeled parameters.This yields an orbital period of P = 2.0291010 ± 0.0000017 days a transit midpoint BJD of t 0 = 2458626.20429± 0.00022.We find a planetary radius of r = 0.975 ± 0.058 R ⊕ and an impact parameter of b = 0.701 ± 0.040, corresponding to an inclination angle of i = 87°.38 ± 0°.19.These values are consistent with the results in previous sections, with the uncertainties in the transit ephemeris largely set by the TESS light curve due to the substantially larger number of observed transits and a longer time baseline within that data set.The best-fit combined model is displayed in the bottom panel of Figure 6.We adopt the values from the combined analysis as final estimates for LHS 475 b.The values for the system parameters are summarized in Table 2.

Discussion and Conclusion
LHS 475 b is an Earth-sized planet orbiting a nearby magnetically quiet M dwarf.Its host star has an estimated luminosity equal to 0.9% of the Solar value and it is located at a distance of 12.5 pc from the Sun.We determine an orbital period of P = 2.0291010 ± 0.0000017 days (corresponding to an a/R ratio of 15.32 ± 0.75), which is short enough for the planet to be tidally locked with a high degree of confidence13 .Thus, the planet is likely to have an uneven surface temperature, with the exact temperature profile dependent on the composition and dynamics of the planetary surface and atmosphere.However, we can make a first-order estimate by assuming that the planet absorbs all incoming stellar radiation hitting its cross section and produces blackbody radiation uniformly from its entire surface according to the Stefan-Boltzmann law.The estimated incident bolometric flux at the planet's orbit is S = 20.8 ± 1.1 S ⊕ , making it a potential Venus analog as defined by Ostberg & Kane (2019).Balancing the input and output energy fluxes yields an estimated equilibrium temperature of T eq = 587 ± 18 K for a zero-albedo surface, T eq = 537 ± 16 K for an Earth-like Bond albedo (A B = 0.3), and T eq = 407 ± 12 K for a Venusian Bond albedo (A B = 0.77).All of these values are too high for the planet to be habitable in the traditional sense.Furthermore, if the absorbed incident radiation is emitted from the dayside only (as would be the case with tidal locking and zero heat redistribution), the dayside temperatures would be 19% higher than the values quoted above.
However, hot effective surface temperatures make the planet more amenable to characterization via emission and transmission spectroscopy during its transits.Adopting the Kempton et al. (2018) framework for a zero-albedo model with full daynight heat redistribution, we obtain a transmission spectroscopic metric (TSM) value of 27.3 (assuming the appropriate scale factor of 0.19 and a planetary mass of 0.91 M ⊕ , derived below).TSM is proportional to the expected transmission spectroscopy S/N.This quoted value places LHS 475 b among a group of other nearby small planets that are promising targets for transit spectroscopy, such as TRAPPIST-1 c (estimated TSM of 24.1; Gillon et al. 2017;Agol et al. 2021) and LHS 1140c (TSM = 25.4;Ment et al. 2019).We note that transmission spectroscopy observations of LHS 475 b were recently conducted with the James Webb Space Telescope during two planetary transits (Lustig-Yaeger et al. 2023); that study yielded a estimated planetary radius and ephemeris that is consistent with ours.We can similarly calculate the emission spectroscopic metric (ESM) and find that ESM = 5.2 for LHS 475 b, comparable to LHS 1445A b (ESM = 5.7; Winters et al. 2019Winters et al. , 2022)).
Due to its tidal locking, LHS 475 b is also a feasible target for photometric thermal emission measurements with JWST during a secondary eclipse.Such observations can quantify the amount of heat redistibution between the dayside and the nightside, which can be used to constrain the presence of an optically thick planetary atmosphere.Among the known terrestrial planets orbiting nearby stars, LHS 3844 b is the only planet where a thick atmosphere has been ruled out (Kreidberg et al. 2019;Diamond-Lowe et al. 2020).The surface insolation of LHS 475 b is lower than that of LHS 3844 b, making the former more likely to retain an atmosphere.If an atmosphere were to be found on LHS 475 b, it would place an important constraint on atmospheric escape in terrestrial planets.Furthermore, we note that LHS 475 b has a very similar radius and surface insolation to TOI 540 b (Ment et al. 2021); however, the latter is orbiting an M dwarf that is still within the magnetically active phase of its evolution.Consequently, a comparative study of the two planets' atmospheres could yield an important before-and-after test for atmospheric escape.
Our best estimate for the radius of LHS 475 b is r = 0.975 ± 0.058 R ⊕ .Similar values can be independently derived from either TESS photometry (see Section 5.1) or MEarth photometry (Section 5.2) alone with remarkable consistency, demonstrating the power of targeted ground-based follow-up observations to confirm the validity of TESS planet candidates.Unfortunately, the existing radial velocity data of LHS 475 does not have the necessary precision to calculate the mass of the planet.As explained in Section 1, planets of this size are highly likely to be terrestrial.Given that the planet is close in size to Earth, it is not unreasonable to suppose that it might also have a similar interior composition.Adopting a simple two-layer composition model with an Earth-like core mass fraction (CMF) of 0.33, we invert the empirical radiusmass relation by Zeng et al. (2016) and derive a planetary mass of 0.91 M ⊕ .14This is consistent with an estimated mass of m 0.92 0.31 0.56 = -+ M ⊕ using Forecaster (Chen & Kipping 2017).Subsequently, we estimate that the radial velocity signal induced by LHS 475 b will have a semiamplitude of 1.1 m s −1 .This level of precision is achievable with current state-of-the-art RV instruments but would likely require a substantial amount of observation time.
observed five full transit windows of LHS 475 b on 2019 August 3, 2019 August 9, 2019 August 17, 2019 August 26, and 2021 August 7 UT in Pan-STARRS z-short band using the Las Cumbres Observatory Global Telescope (LCOGT; Brown et al. 2013) 1.0 m network nodes at Cerro Tololo Inter-American Observatory (CTIO) and South Africa Astronomical Observatory.The images were calibrated by the standard LCOGT BANZAI pipeline (McCully et al. 2018) and differential photometric data were extracted using Astro-ImageJ (Collins et al. 2017).We used circular photometric apertures with radius 5 8 centered on LHS 475.The target star apertures excluded most of the flux from the nearest known neighbor in the Gaia DR3 and TIC v8 catalogs (TIC

Figure 1 .
Figure 1.TESS photometry of LHS 475 in Sectors 12, 13, 27, and 39.The black line represents the fitted baseline as described in Section 3.1.

Figure 2 .
Figure 2. MEarth-South follow-up photometry of five individual transits of LHS 475.The Barycentric Julian Date values (BJD) within the plots denote the times of transit midpoints.The vertical dashed lines encompass the full transit duration.The solid line depicts the best-fit model for all MEarth data; it is identical to the model in the middle panel of Figure 6 and is further described in Section 5.2.The bottom panel displays the residuals after subtracting the best-fit model from the data.
We utilize the exoplanet package (Foreman-Mackey et al. 2021a, 2021b) in Python for transit modeling purposes.exoplanet employs the modeling framework of the PyMC Python library (Salvatier et al. 2016) and uses Theano (Theano Development Team 2016) for computationally efficient Markov chain Monte Carlo sampling.Limb-darkened transit signals are computed analytically by the open-source starry package (Luger et al. 2019).

Figure 3 .
Figure 3. Combined and phase-folded LCOGT light curves of LHS 475 b.Top: gray symbols show the unbinned data and red symbols show the same data in 5 minutes bins.Bottom: the transit model from a joint fit to all five LCOGT light curves is overplotted on the binned data.The model has a planet-to-star radius ratio r R 0.033 0.003 0.002

Figure 4 .
Figure 4. Left: relative radial velocities of LHS 475 from CHIRON.The estimated systemic velocity is −10.3 ± 0.5 km s −1 .Right: Same as the left panel, with the radial velocities phase-folded at the fitted orbital period of the planet.The black dotted line shows the modeled RV signature of the planet using the estimated semiamplitude of 1.1 m s −1 from Section 6.

Figure 5 .
Figure 5. Speckle imaging sensitivity curves of LHS 475 utilizing the 562 nm (blue line) and 832 nm (red line) bandpasses.No nearby light sources are detected.

Figure 6 .
Figure 6.Phase-folded photometry of LHS 475 highlighting the 3 hr window surrounding the planetary transits.Solid lines depict the median model obtained from sampling the posterior distributions whereas the shaded gray areas show the 16th and 84th percentiles.Median values and 1σ uncertainties for the orbital period P and planet radius R are also given.We produce separate subplots for TESS data (top panel), MEarth data (second panel), LCOGT data (third panel), and the combined data set (bottom panel).The combined model is used for the values in Table 2.

Table 2
System Parameters for LHS 475