Transit Depth Research Articles

Asteroseismology of iota Draconis and Discovery of an Additional Long-period Companion

Abstract Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host stars are also some of the brightest in the sky and so enable high signal-to-noise ratio follow-up measurements. Here, we report on new observations of the bright (V ∼ 3.3) giant star ι Draconis (ι Dra), known to host a planet in a highly eccentric ∼511 day period orbit. TESS observations of the star over 137 days reveal asteroseismic signatures, allowing us to constrain the stellar radius, mass, and age to ∼2%, ∼6%, and ∼28%, respectively. We present the results of continued radial-velocity monitoring of the star using the Automated Planet Finder over several orbits of the planet. We provide more precise planet parameters of the known planet and, through the combination of our radial-velocity measurements with Hipparcos and Gaia astrometry, we discover an additional long-period companion with an orbital period of ∼ 68 − 36 + 60 yr. Mass predictions from our analysis place this substellar companion on the border of the planet and brown dwarf regimes. The bright nature of the star combined with the revised orbital architecture of the system provides an opportunity to study planetary orbital dynamics that evolve as the star moves into the giant phase of its evolution.

HST PanCET Program: A Complete Near-UV to Infrared Transmission Spectrum for the Hot Jupiter WASP-79b

Abstract We present a new optical transmission spectrum of the hot Jupiter WASP-79b. We observed three transits with the STIS instrument mounted on the Hubble Space Telescope (HST), spanning 0.3–1.0 μm. Combining these transits with previous observations, we construct a complete 0.3–5.0 μm transmission spectrum of WASP-79b. Both HST and ground-based observations show decreasing transit depths toward blue wavelengths, contrary to expectations from Rayleigh scattering or hazes. We infer atmospheric and stellar properties from the full near-UV to infrared transmission spectrum of WASP-79b using three independent retrieval codes, all of which yield consistent results. Our retrievals confirm previous detections of H2O (at 4.0σ confidence) while providing moderate evidence of H− bound–free opacity (3.3σ) and strong evidence of stellar contamination from unocculted faculae (4.7σ). The retrieved H2O abundance (∼1%) suggests a superstellar atmospheric metallicity, though stellar or substellar abundances remain consistent with present observations (O/H = 0.3–34× stellar). All three retrieval codes obtain a precise H− abundance constraint: log( X H − ) ≈ −8.0 ± 0.7. The potential presence of H− suggests that James Webb Space Telescope observations may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred faculae are ∼500 K hotter than the stellar photosphere, covering ∼15% of the stellar surface. Our analysis underscores the importance of observing UV–optical transmission spectra in order to disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra.

Radius and Mass Distribution of Ultra-short-period Planets

Abstract Ultra-short-period (USP) planets are an enigmatic subset of exoplanets defined by having orbital periods <1 day. It is still not understood how USP planets form, or to what degree they differ from planets with longer orbital periods. Most USP planets have radii <2 R ⊕, while planets that orbit further from their star extend to Jupiter size (>10 R ⊕). Several theories attempt to explain the formation and composition of USP planets: they could be remnant cores of larger gas giants that lost their atmospheres due to photoevaporation or Roche-lobe overflow, or they could have developed through mass accretion in the innermost part of the protoplanetary disk. The radius and mass distribution of USP planets could provide important clues to distinguish between potential formation mechanisms. In this study, we first verify and update the Kepler catalog of USP planet host star properties, incorporating new data collected by the Gaia mission where applicable. We then use the transit depths measured by Kepler to derive a radius distribution and present occurrence rates for USP planets. Using spherical and tidally distorted planet models, we then derive a mass distribution for USP planets. Comparisons between the updated USP planet mass distribution and simulated planetary systems offer further insights into the formation and evolutionary processes shaping USP planet populations.

Multicolor photometry and parameters estimation of three transiting Jupiter-sized exoplanets; TrES-3b, WASP-2b and HAT-P-30b

Precise and frequent photometric follow-up studies of transit light curves are indispensable when accurately characterizing extrasolar planets. We present new multi-wavelength photometry of three transiting “Hot Jupiters” — TrES-3b, WASP-2b and HAT-P-30b (WASP-51b). Data were acquired from an 0.8 meter telescope at Tarleton State University. When combined with data from the literature, this allowed us to redetermine system parameters. GCX reduction pipeline and TAP - modeling and light curve fitting package was used to analyze the extracted light curves. Weighted mean values were used to estimate the parameters from BVRI filters for three exoplanetary systems and compared them with previous published results. From our study, TreS-3b with a mass of Mp = 1.7730±0.02Mjup and Rp = 1.3047±0.04Rjup, appears to be 0.2±0.03Mjup less massive than previously published by O’donovan et al. (2007) and Sozzetti et al. (2009), while HAT-P-30b, which has a mass of Mp = 0.7006±0.041Mjup and Rp = 1.5109±0.0557Rjup appears to be a bloated “Hot Jupiter”. Additionally, we compared the results of our broadband photometric analysis with previous studies to search for transit depth wavelength dependence. We found a flat spectrum across optical wavelengths (except WASP-2b in R-band) for TrES-3b and WASP-2b, indicating the presence of clouds in their atmospheres. HAT-P-30b had a significantly larger radius in B filter with Rp = 1.613±0.05Rjup, resulting from Rp/R∗ = 0.1334±0.004, which was secondarily confirmed by the atmospheric scale height value, H = 1450±38km, indicating that HAT-P-30b is an inflated “Hot Jupiter”.

Ground-based Transmission Spectroscopy with VLT FORS2: Evidence for Faculae and Clouds in the Optical Spectrum of the Warm Saturn WASP-110b

Abstract We present a ground-based optical transmission spectrum for the warm Saturn-mass exoplanet WASP-110b from two transit observations made with the FOcal Reducer and Spectrograph on the Very Large Telescope. The spectrum covers the wavelength range from 4000–8333 Å, which is binned in 46 transit depths measured to an averaged precision of 220 parts per million (ppm) over an averaged 80 Å bin for a Vmag = 12.8 star. The measured transit depths are unaffected by a dilution from a close A-type field dwarf, which was fully resolved. The overall main characteristic of the transmission spectrum is an increasing radius with wavelength and a lack of the theoretically predicted pressure-broadened sodium and potassium absorption features for a cloud-free atmosphere. We analyze archival high-resolution optical spectroscopy and find evidence for low to moderate activity of the host star, which we take into account in the atmospheric retrieval analysis. Using the AURA retrieval code, we find that the observed transmission spectrum can be best explained by a combination of unocculted stellar faculae and a cloud deck. Transmission spectra of cloud-free and hazy atmospheres are rejected at a high confidence. With a possible cloud deck at its terminator, WASP-110b joins the increasing population of irradiated hot-Jupiter exoplanets with cloudy atmospheres observed in transmission.

A search for transiting companions in the J1407 (V1400 Cen) system

Context. In 2007, the young star 1SWASP J140747.93-394 542.6 (V1400 Cen) underwent a complex series of deep eclipses over 56 days. This was attributed to the transit of a ring system filling a large fraction of the Hill sphere of an unseen substellar companion. Subsequent photometric monitoring has not found any other deep transits from this candidate ring system, but if there are more substellar companions and if they are coplanar with the potential ring system, there is a chance that they will transit the star as well. This young star is active, and the light curves show a 5% modulation in amplitude with a dominant rotation period of 3.2 days due to starspots rotating into and out of view. Aims. We model and remove the rotational modulation of the J1407 light curve and search for additional transit signatures of substellar companions orbiting around J1407. Methods. We combine the photometry of J1407 from several observatories, spanning a 19 yr baseline. We remove the rotational modulation by modeling the variability as a periodic signal, whose periodicity changes slowly with time over several years due to the activity cycle of the star. A transit least squares (TLS) analysis is used to search for any periodic transiting signals within the cleaned light curve. Results. We identify an activity cycle of J1407 with a period of 5.4 yr. A TLS search does not find any plausible periodic eclipses in the light curve, from 1.2% amplitude at 5 days up to 1.9% at 20 days. This sensitivity is confirmed by injecting artificial transits into the light curve and determining the recovery fraction as a function of transit depth and orbital period. Conclusions. J1407 is confirmed as a young active star with an activity cycle consistent with a rapidly rotating solar mass star. With the rotational modulation removed, the TLS analysis reaches down to planetary mass radii for young exoplanets, ruling out transiting companions with radii larger than about 1 RJup.

Effect of stellar flares and coronal mass ejections on the atmospheric escape from hot Jupiters

AbstractSpectral observations in the Ly-α line have shown that atmospheric escape is variable and for the exoplanet HD189733b, the atmospheric evaporation goes from undetected to enhanced evaporation in a 1.5 years interval. To understand the temporal variation in the atmospheric escape, we investigate the effect of flares, winds, and CMEs on the atmosphere of hot Jupiter HD189733b using 3D self-consistent radiation hydrodynamic simulations. We consider four cases: first, the quiescent phase including stellar wind; secondly, a flare; thirdly, a CME; and fourthly, a flare followed by a CME. We find that the flare alone increases the atmospheric escape rate by only 25%, while the CME leads to a factor of 4 increments, in comparison to the quiescent case. We also find that the flare alone cannot explain the observed high blue-shifted velocities seen in the Ly-α. The CME, however, leads to an increase in the velocity of escaping atmospheres, enhancing the blue-shifted transit depth.

Speckle Observations of TESS Exoplanet Host Stars. II. Stellar Companions at 1–1000 au and Implications for Small Planet Detection

Abstract We present high-angular-resolution imaging observations of 517 host stars of TESS exoplanet candidates using the ‘Alopeke and Zorro speckle cameras at Gemini North and South. The sample consists mainly of bright F, G, K stars at distances of less than 500 pc. Our speckle observations span angular resolutions of ∼20 mas out to 1.″2, yielding spatial resolutions of <10–500 au for most stars, and our contrast limits can detect companion stars 5–9 mag fainter than the primary at optical wavelengths. We detect 102 close stellar companions and determine the separation, magnitude difference, mass ratio, and estimated orbital period for each system. Our observations of exoplanet host star binaries reveal that they have wider separations than field binaries, with a mean orbital semimajor axis near 100 au. Other imaging studies have suggested this dearth of very closely separated binaries in systems which host exoplanets, but incompleteness at small separations makes it difficult to disentangle unobserved companions from a true lack of companions. With our improved angular resolution and sensitivity, we confirm that this lack of close exoplanet host binaries is indeed real. We also search for a correlation between planetary orbital radii versus binary star separation; but, given the very short orbital periods of the TESS planets, we do not find any clear trend. We do note that in exoplanet systems containing binary host stars, there is an observational bias against detecting Earth-size planet transits due to transit depth dilution caused by the companion star.

The near-UV transit of HD 189733b with the XMM–Newton optical monitor

ABSTRACT We present analysis of XMM–Newton optical monitor observations in the near-ultraviolet of HD 189733, covering 20 primary transits of its hot Jupiter planet. The transit is clearly detected with both the UVW2 and UVM2 filters, and our fits to the data reveal transit depths in agreement with that observed optically. The measured depths correspond to radii of $1.059^{+0.046}_{-0.050}$ and $0.94^{+0.15}_{-0.17}$ times the optically measured radius (1.187 RJ at 4950 Å) in the UVW2 and UVM2 bandpasses, respectively. We also find no statistically significant variation in the transit depth across the 8 yr baseline of the observations. We rule out extended broad-band absorption towards or beyond the Roche lobe at the wavelengths investigated, although observations with higher spectral resolution are required to determine if absorption out to those distances from the planet is present in individual near-UV lines.

ACCESS and LRG-BEASTS: A Precise New Optical Transmission Spectrum of the Ultrahot Jupiter WASP-103b

Abstract We present a new ground-based optical transmission spectrum of the ultrahot Jupiter WASP-103b ( T eq = 2484 K). Our transmission spectrum is the result of combining five new transits from the ACCESS survey and two new transits from the LRG-BEASTS survey with a reanalysis of three archival Gemini/GMOS transits and one VLT/FORS2 transit. Our combined 11-transit transmission spectrum covers a wavelength range of 3900–9450 Å with a median uncertainty in the transit depth of 148 parts per million, which is less than one atmospheric scale height of the planet. In our retrieval analysis of WASP-103b’s combined optical and infrared transmission spectrum, we find strong evidence for unocculted bright regions (4.3σ) and weak evidence for H2O ( 1.9 σ ), HCN ( 1.7 σ ), and TiO ( 2.1 σ ), which could be responsible for WASP-103b’s observed temperature inversion. Our optical transmission spectrum shows significant structure that is in excellent agreement with the extensively studied ultrahot Jupiter WASP-121b, for which the presence of VO has been inferred. For WASP-103b, we find that VO can only provide a reasonable fit to the data if its abundance is implausibly high and we do not account for stellar activity. Our results highlight the precision that can be achieved by ground-based observations and the impacts that stellar activity from F-type stars can have on the interpretation of exoplanet transmission spectra.

Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. II. WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability

Abstract This paper presents simulations of the metastable helium (He*) observations of WASP-107b, so far the highest signal-to-noise ratio detection that is confirmed by three different instruments. We employ full 3D hydrodynamics coupled with coevolving nonequilibrium thermochemistry and ray-tracing radiation, predicting mass-loss rates, temperature profiles, and synthetic He* line profiles and light curves from first principles. We find that a stellar wind stronger than solar is demanded by the observed heavily blueshifted line profile and asymmetric transit light curve. Radiation pressure can be important for Lyα observations, but not He*. Our model finds that WASP-107b is losing mass at a rate of M ̇ ≃ 1.0 × 10 − 9 M ⊕ yr − 1 . Although M ̇ varies by <1% given constant wind and irradiation from the host, shear instabilities still emerge from wind impacts, producing ∼10% fluctuations of He* transit depths over hour-long timescales. The common assumption that He* transit depth indicates the fluctuation of M ̇ is problematic. The trailing tail is more susceptible than planet adjacency to the shear instabilities; thus, the line profile is more variable in the blueshifted wing, while the transit light curve is more variable after midtransit. We stress that the synergy between Lyα (higher altitudes, lower density) and He* (lower altitudes, higher density) transit observations, particularly simultaneous ones, yields better understanding of planetary outflows and stellar wind properties.

Discovery of a young low-mass brown dwarf transiting a fast-rotating F-type star by the Galactic Plane eXoplanet (GPX) survey

ABSTRACT We announce the discovery of GPX-1 b, a transiting brown dwarf with a mass of 19.7 ± 1.6 MJup and a radius of 1.47 ± 0.10 RJup, the first substellar object discovered by the Galactic Plane eXoplanet (GPX) survey. The brown dwarf transits a moderately bright (V = 12.3 mag) fast-rotating F-type star with a projected rotational velocity $v\sin {\, i_*}=40\pm 10$ km s−1. We use the isochrone placement algorithm to characterize the host star, which has effective temperature 7000 ± 200 K, mass 1.68 ± 0.10 $\mathrm{\it M}_\odot$, radius 1.56 ± 0.10 $\mathrm{\it R}_\odot$, and approximate age $0.27_{-0.15}^{+0.09}$ Gyr. GPX-1 b has an orbital period of ∼1.75 d and a transit depth of 0.90 ± 0.03 per cent. We describe the GPX transit detection observations, subsequent photometric and speckle-interferometric follow-up observations, and SOPHIE spectroscopic measurements, which allowed us to establish the presence of a substellar object around the host star. GPX-1 was observed at 30-min integrations by TESS in Sector 18, but the data are affected by blending with a 3.4 mag brighter star 42 arcsec away. GPX-1 b is one of about two dozen transiting brown dwarfs known to date, with a mass close to the theoretical brown dwarf/gas giant planet mass transition boundary. Since GPX-1 is a moderately bright and fast-rotating star, it can be followed-up by the means of the Doppler tomography.

Around Which Stars Can TESS Detect Earth-like Planets? The Revised TESS Habitable Zone Catalog

Abstract In the search for life in the cosmos, NASA’s Transiting Exoplanet Survey Satellite (TESS) mission has already monitored about 74% of the sky for transiting extrasolar planets, including potentially habitable worlds. However, TESS only observed a fraction of the stars long enough to be able to find planets like Earth. We use the primary mission data—the first two years of observations—and identify 4239 stars within 210 pc that TESS observed long enough to see three transits of an exoplanet that receives similar irradiation to Earth: 738 of these stars are located within 30 pc. We provide reliable stellar parameters from the TESS Input Catalog that incorporates Gaia DR2 and also calculate the transit depth and radial velocity semiamplitude for an Earth-analog planet. Of the 4239 stars in the Revised TESS HZ Catalog, 9 are known exoplanet hosts—GJ1061, GJ1132, GJ3512, GJ685, Kepler-42, LHS1815, L98-59, RRCae, and TOI700—around which TESS could identify additional Earth-like planetary companions. Thirty-seven additional stars host yet unconfirmed TESS Objects of Interest: three of these orbit in the habitable zone—TOI203, TOI715, and TOI2298. For a subset of 614 of the 4239 stars, TESS has observed the star long enough to be able to observe planets throughout the full temperate, habitable zone out to the equivalent of Mars’ orbit. Thus, the Revised TESS Habitable Zone Catalog provides a tool for observers to prioritize stars for follow-up observation to discover life in the cosmos. These stars are the best path toward the discovery of habitable planets using the TESS mission data.

Analytic Estimates of the Achievable Precision on the Physical Properties of Transiting Planets Using Purely Empirical Measurements

Abstract We present analytic estimates of the fractional uncertainties on the mass, radius, surface gravity, and density of a transiting planet, using only empirical or semi-empirical measurements. We first express these parameters in terms of transit photometry and radial velocity (RV) observables, as well as the stellar radius R ⋆, if required. In agreement with previous results, we find that, assuming a circular orbit, the surface gravity of the planet (g p ) depends only on empirical transit and RV parameters, namely the planet period P, the transit depth δ, the RV semi-amplitude K ⋆, the transit duration T, and the ingress/egress duration τ. However, the planet mass and density depend on all these quantities, plus R ⋆. Thus, an inference about the planet mass, radius, and density must rely upon an external constraint such as the stellar radius. For bright stars, stellar radii can now be measured nearly empirically by using measurements of the stellar bolometric flux, the effective temperature, and the distance to the star via its parallax, with the extinction A V being the only free parameter. For any given system, there is a hierarchy of achievable precisions on the planetary parameters, such that the planetary surface gravity is more accurately measured than the density, which in turn is more accurately measured than the mass. We find that surface gravity provides a strong constraint on the core mass fraction of terrestrial planets. This is useful, given that the surface gravity may be one of the best measured properties of a terrestrial planet.

Phase-curve Pollution of Exoplanet Transmission Spectra

Abstract The occurrence of a planet transiting in front of its host star offers the opportunity to observe the planet’s atmosphere filtering starlight. The fraction of occulted stellar flux is roughly proportional to the optically thick area of the planet, the extent of which depends on the opacity of the planet’s gaseous envelope at the observed wavelengths. Chemical species, haze, and clouds are now routinely detected in exoplanet atmospheres through rather small features in transmission spectra, i.e., collections of planet-to-star area ratios across multiple spectral bins and/or photometric bands. Technological advances have led to a shrinking of the error bars down to a few tens of parts per million (ppm) per spectral point for the brightest targets. The upcoming James Webb Space Telescope (JWST) is anticipated to deliver transmission spectra with precision down to 10 ppm. The increasing precision of measurements requires a reassessment of the approximations hitherto adopted in astrophysical models, including transit light-curve models. Recently, it has been shown that neglecting the planet’s thermal emission can introduce significant biases in the transit depth measured with the JWST/Mid-InfraRed Instrument, integrated between 5 and 12 μm. In this paper, we take a step forward by analyzing the effects of the approximation on transmission spectra over the 0.6–12 μm wavelength range covered by various JWST instruments. We present open-source software to predict the spectral bias, showing that, if not corrected, it may affect the inferred molecular abundances and thermal structure of some exoplanet atmospheres.

Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b

Abstract The metastable helium line at 1083 nm can be used to probe the extended upper atmospheres of close-in exoplanets and thus provide insight into their atmospheric mass loss, which is likely to be significant in sculpting their population. We used an ultra-narrow band filter centered on this line to observe two transits of the low-density gas giant HAT-P-18b, using the 200″ Hale Telescope at Palomar Observatory, and report the detection of its extended upper atmosphere. We constrain the excess absorption to be 0.46% ± 0.12% in our 0.635 nm bandpass, exceeding the transit depth from the Transiting Exoplanet Survey Satellite (TESS) by 3.9σ. If we fit this signal with a 1D Parker wind model, we find that it corresponds to an atmospheric mass loss rate between 8.3 − 1.9 + 2.8 × 10 − 5 M J Gyr−1 and 2.63 − 0.64 + 0.46 × 10 − 3 M J Gyr−1 for thermosphere temperatures ranging from 4000 K to 13,000 K, respectively. With a J magnitude of 10.8, this is the faintest system for which such a measurement has been made to date, demonstrating the effectiveness of this approach for surveying mass loss on a diverse sample of close-in gas giant planets.

Finding Signs of Life in Transits: High-resolution Transmission Spectra of Earth-line Planets around FGKM Host Stars

Abstract Thousands of transiting exoplanets have already been detected orbiting a wide range of host stars, including the first planets that could potentially be similar to Earth. The upcoming Extremely Large Telescopes and the James Webb Space Telescope will enable the first searches for signatures of life in transiting exoplanet atmospheres. Here, we quantify the strength of spectral features in transit that could indicate a biosphere similar to the modern Earth on exoplanets orbiting a wide grid of host stars (F0 to M8) with effective temperatures between 2500 and 7000 K: transit depths vary between about 6000 ppm (M8 host) to 30 ppm (F0 host) due to the different sizes of the host stars. CO2 possess the strongest spectral features in transit between 0.4 and 20 μm. The atmospheric biosignature pairs O2+CH4 and O3+CH4—which identify Earth as a living planet—are most prominent for Sun-like and cooler host stars in transit spectra of modern Earth analogs. Assessing biosignatures and water on such planets orbiting hotter stars than the Sun will be extremely challenging even for high-resolution observations. All high-resolution transit spectra and model profiles are available online: they provide a tool for observers to prioritize exoplanets for transmission spectroscopy, test atmospheric retrieval algorithms, and optimize observing strategies to find life in the cosmos. In the search for life in the cosmos, transiting planets provide the first opportunity to discover whether or not we are alone, with this database as one of the keys to optimize the search strategies.

HD 219134 Revisited: Planet d Transit Upper Limit and Planet f Transit Nondetection with ASTERIA and TESS

Abstract HD 219134 is a K3V dwarf star with six reported radial-velocity discovered planets. The two innermost planets b and c show transits, raising the possibility of this system to be the nearest (6.53 pc), brightest (V = 5.57) example of a star with a compact multiple transiting planet system. Ground-based searches for transits of planets beyond b and c are not feasible because of the infrequent transits, long transit duration (∼5 hr), shallow transit depths (<1%), and large transit time uncertainty (∼half a day). We use the space-based telescopes the Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA) and the Transiting Exoplanet Survey Satellite (TESS) to search for transits of planets f (P = 22.717 days and ) and d (P = 46.859 days and ). ASTERIA was a technology demonstration CubeSat with an opportunity for science in an extended program. ASTERIA observations of HD 219134 were designed to cover the 3σ transit windows for planets f and d via repeated visits over many months. While TESS has much higher sensitivity and more continuous time coverage than ASTERIA, only the HD 219134 f transit window fell within the TESS survey’s observations. Our TESS photometric results definitively rule out planetary transits for HD 219134 f. We do not detect the Neptune-mass HD 219134 d transits and our ASTERIA data are sensitive to planets as small as 3.6 R ⊕. We provide TESS updated transit times and periods for HD 219134 b and c, which are designated TOI 1469.01 and 1469.02 respectively.

A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c

Abstract Strongly irradiated exoplanets develop extended atmospheres that can be utilized to probe the deeper planet layers. This connection is particularly useful in the study of small exoplanets, whose bulk atmospheres are challenging to characterize directly. Here, we report the 3.4σ detection of C ii ions during a single transit of the super-Earth π Men c in front of its Sun-like host star. The transit depth and Doppler velocities are consistent with the ions filling the planet’s Roche lobe and moving preferentially away from the star, an indication that they are escaping the planet. We argue that π Men c possesses a thick atmosphere with abundant heavy volatiles (≳ 50% by mass of atmosphere) but that needs not be carbon rich. Our reasoning relies upon cumulative evidence from the reported C ii detection, the nondetection of H i atoms in a past transit, modeling of the planet’s interior, and the assumption that the atmosphere, having survived the most active phases of its Sun-like host star, will survive another 0.2–2 Gyr. Depending on the current mass of atmosphere, π Men c may still transition into a bare rocky core. Our findings confirm the hypothesized compositional diversity of small exoplanets, and represent a milestone toward understanding the planets’ formation and evolution paths through the investigation of their extended atmospheres.

HAT-P-68b: A Transiting Hot Jupiter around a K5 Dwarf Star*

Abstract We report the discovery by the ground-based Hungarian-made Automated Telescope Network (HATNet) survey of the transiting exoplanet HAT-P-68b, which has a mass of 0.724 ± 0.043 M J, and radius of 1.072 ± 0.012 R J. The planet is in a circular P = 2.2984 day orbit around a moderately bright V = 13.937 ± 0.030 magnitude K-dwarf star of mass M ⊙, and radius 0.6726 ± 0.0069 R ⊙. The planetary nature of this system is confirmed through follow-up transit photometry obtained with the Fred L. Whipple Observatory (FLWO) 1.2 m telescope, high-precision radial velocities measured using Keck I/High Resolution Echelle Spectrometer (HIRES), FLWO 1.5 m/Tillinghast Reflector Echelle Spectrograph (TRES), and Observatoire de Haute-Provence (OHP) 1.9 m/Sophie, and high-spatial-resolution speckle imaging from WIYN 3.5 m/DSSI. HAT-P-68 is at an ecliptic latitude of +3° and outside the field of view of both the NASA Transiting Exoplanet Survey Satellite primary mission and the K2 mission. The large transit depth of 0.036 mag (r band) makes HAT-P-68b a promising target for atmospheric characterization via transmission spectroscopy.