Over 100 rocky planets orbiting Sun-like stars in very short orbital periods ( 1 day) have been discovered by the Kepler mission. The origin of these planets, known as ultra-short-period (USP) ...planets, remains elusive. Here, we propose that most of these planets, originally at periods of ∼5-10 days, reach their current orbits via high-eccentricity migration. In a scaled-down version of the dynamics that may have been experienced by their high-mass analogs, the hot Jupiters, these planets reach high eccentricities via chaotic secular interactions with their companion planets and then undergo orbital circularization due to dissipation from tides raised on the planet. This proposal is motivated by the following observations: planetary systems observed by Kepler often contain several super-Earths with non-negligible eccentricities and inclinations, possibly extending beyond ∼au distances; by contrast, only a small fraction of USP planets have known transiting companions, which are generally not closely spaced, and we argue that most of them should have companions with periods 10 days. The proposed scenario naturally explains the observation that most USP planets have significantly more distant transiting companions compared to their counterparts at slightly longer periods (1-3 days). Our model predicts that USP planets should have: (i) spin-orbit angles, and inclinations relative to outer planets, in the range of ∼10-50°; (ii) several outer planetary companions extending beyond ∼1 au distances. Both of these predictions may be tested by TESS and its follow-up observations.
The hundreds of multiple planetary systems discovered by the Kepler mission are typically observed to reside in close-in ( AU), low-eccentricity, low-inclination orbits. We run N-body experiments to ...study the effect that unstable outer ( AU) giant planets, whose end orbital configurations resemble those in the Radial Velocity population, have on these close-in multiple super-Earth systems. Our experiments show that the giant planets greatly reduce the multiplicity of the inner super-Earths, and the surviving population can have large eccentricities ( ) and inclinations ( ) at levels that anti-correlate with multiplicity. Consequently, this model predicts the existence of a population of dynamically hot single-transiting planets with typical eccentricities and inclinations of ∼0.1-0.5 and ∼10°-40°. We show that these results can explain the following observations: (i) the recent eccentricity measurements of Kepler super-Earths from transit durations; (ii) the tentative observation that single-transiting systems have a wider distribution of stellar obliquity angles compared to the multiple-transiting systems; (iii) the architecture of some eccentric super-Earths discovered by Radial Velocity surveys such as HD 125612c. Future observations from TESS will reveal many more dynamically hot single transiting planets, for which follow up radial velocity studies will be able to test our models and see whether they have outer giant planets.
The nearby super-Earth 55 Cnc e orbits a bright (V = 5.95 mag) star with a period of ∼18 hr and a mass of ∼8M⊕. Its atmosphere may be water-rich and have a large-scale height; though, attempts to ...characterize it have yielded ambiguous results. Here we present a sensitive search for water and TiO in its atmosphere at high spectral resolution using the Gemini North telescope and the GRACES spectrograph. We combine observations with previous observations from Subaru and CFHT, improving the constraints on the presence of water vapor. We adopt parametric models with an updated planet radius based on recent measurements, and use a cross-correlation technique to maximize sensitivity. Our results are consistent with atmospheres that are cloudy or contain minimal amounts of water and TiO. Using these parametric models, we rule out a water-rich atmosphere (VMR 0.1%) with a mean molecular weight of 15 g mol−1 at a 3 confidence level, improving on the previous limit by a significant margin. For TiO, we rule out a mean molecular weight of 5 g mol−1 with a 3 confidence level for a VMR greater than 10−8; for a VMR of greater than 10−7, the limit rises to a mean molecular weight of 10 g mol−1. We can rule out low mean-molecular-weight chemical equilibrium models both including and excluding TiO/VO at very high confidence levels (>10 ). Overall, our results are consistent with an atmosphere with a high mean molecular weight and/or clouds, or no atmosphere.
Abstract
We present high-resolution near-infrared spectra taken during eight transits of 55 Cancri e, a nearby low-density super-Earth with a short orbital period (<18 hr). While this exoplanet’s ...bulk density indicates a possible atmosphere, one has not been detected definitively. Our analysis relies on the Doppler cross-correlation technique, which takes advantage of the high spectral resolution and broad wavelength coverage of our data, to search for the thousands of absorption features from hydrogen-, carbon-, and nitrogen-rich molecular species in the planetary atmosphere. Although we are unable to detect an atmosphere around 55 Cancri e, we do place strong constraints on the levels of HCN, NH
3
, and C
2
H
2
that may be present. In particular, at a mean molecular weight of 5 amu, we can rule out the presence of HCN in the atmosphere down to a volume mixing ratio (VMR) of 0.02%, NH
3
down to a VMR of 0.08%, and C
2
H
2
down to a VMR of 1.0%. If the mean molecular weight is relaxed to 2 amu, we can rule out the presence of HCN, NH
3
, and C
2
H
2
down to VMRs of 0.001%, 0.0025%, and 0.08%, respectively. Our results reduce the parameter space of possible atmospheres consistent with the analysis of Hubble Space Telescope/WFC3 observations by Tsiaras et al. and indicate that if 55 Cancri e harbors an atmosphere, it must have a high mean molecular weight or clouds.
We present high-resolution optical transmission spectroscopy of two sub-Saturn mass transiting exoplanets, HAT-P-12b and WASP-69b. With relatively low densities and high atmospheric scale heights, ...these planets are particularly well-suited to characterization through transit spectroscopy, and serve as ideal candidates for extending previously tested methods to lower planetary masses. Using a single transit for each planet, we take advantage of the Doppler cross-correlation technique to search for sodium, potassium, and water absorption features. Our analysis reveals a likely (3.2 ) detection of sodium absorption features in the atmosphere of HAT-P-12b, and enables us to place constraints on the presence of alkaline and molecular species in the atmospheres of both planets. With our results, we highlight the efficacy of ground-based campaigns for characterizing exoplanetary atmospheres and pave the way for future analyses of low-mass planets.
Abstract
Recent observations of the ultra-hot Jupiter WASP-76b have revealed a diversity of atmospheric species. Here we present new high-resolution transit spectroscopy of WASP-76b with GRACES at ...the Gemini North Observatory, serving as a baseline for the Large and Long Program “Exploring the Diversity of Exoplanet Atmospheres at High Spectral Resolution” (Exoplanets with Gemini Spectroscopy, or ExoGemS for short). With a broad spectral range of 400–1050 nm, these observations allow us to search for a suite of atomic species. We recover absorption features due to neutral sodium (Na
i
), and report a new detection of the ionized calcium (Ca
ii
) triplet at ∼850 nm in the atmosphere of WASP-76b, complementing a previous detection of the Ca
ii
H and K lines. The triplet has line depths of 0.295% ± 0.034% at ∼849.2 nm, 0.574% ± 0.041% at ∼854.2 nm, and 0.454% ± 0.024% at ∼866.2 nm, corresponding to effective radii close to (but within) the planet’s Roche radius. These measured line depths are significantly larger than those predicted by model LTE and NLTE spectra obtained on the basis of a pressure–temperature profile computed assuming radiative equilibrium. The discrepancy suggests that the layers probed by our observations are either significantly hotter than predicted by radiative equilibrium and/or in a hydrodynamic state. Our results shed light on the exotic atmosphere of this ultra-hot world, and will inform future analyses from the ExoGemS survey.
Abstract
We present high-resolution transmission spectroscopy of WASP-76b with Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES)/Gemini North obtained as part of the ExoGemS survey. With a ...broad spectral range of 400–1050 nm and a relatively high resolution of ∼66,000, these observations are particularly well suited to searching for atomic and molecular atmospheric species via the Doppler cross-correlation technique. We recover absorption features due to neutral iron (Fe
i
), sodium (Na
i
), and ionized calcium (Ca
ii
) at high significance (>5
σ
), and investigate possible atmospheric temperatures and wind speeds. We also report tentative (>3
σ
) detections of Li
i
, K
i
, Cr
i
, and V
i
in the atmosphere of WASP-76b. Finally, we report nondetections of a number of other species, some of which have previously been detected with other instruments. Through model injection/recovery tests, we demonstrate that many of these species are not expected to be detected in our observations. These results allow us to place GRACES and the ExoGemS survey in context with other high-resolution optical spectrographs.
Abstract
Terrestrial exoplanets orbiting M-dwarf stars are promising targets for transmission spectroscopy with existing or near-future instrumentation. The atmospheric composition of such rocky ...planets remains an open question, especially given the high X-ray and ultraviolet flux from their host M dwarfs that can drive atmospheric escape. The 1.3
R
⊕
exoplanet GJ 486b (
T
eq
∼ 700 K), orbiting an M3.5 star, is expected to have one of the strongest transmission spectroscopy signals among known terrestrial exoplanets. We observed three transits of GJ 486b using three different high-resolution spectrographs: IRD on Subaru, IGRINS on Gemini-South, and SPIRou on the Canada–France–Hawai’i Telescope. We searched for atmospheric absorption from a wide variety of molecular species via the cross-correlation method, but did not detect any robust atmospheric signals. Nevertheless, our observations are sufficiently sensitive to rule out several clear atmospheric scenarios via injection and recovery tests, and extend comparative exoplanetology into the terrestrial regime. Our results suggest that GJ 486b does not possess a clear H
2
/He-dominated atmosphere, nor a clear 100% water-vapor atmosphere. Other secondary atmospheres with high mean molecular weights or H
2
/He-dominated atmospheres with clouds remain possible. Our findings provide further evidence suggesting that terrestrial planets orbiting M-dwarf stars may experience significant atmospheric loss.
Abstract
Exoplanet atmosphere studies are often enriched by synergies with brown dwarf analogs. However, many key molecules commonly seen in brown dwarfs have yet to be confirmed in exoplanet ...atmospheres. An important example is chromium hydride (CrH), which is often used to probe atmospheric temperatures and classify brown dwarfs into spectral types. Recently, tentative evidence for CrH was reported in the low-resolution transmission spectrum of the hot Jupiter WASP-31b. Here, we present high spectral resolution observations of WASP-31b’s transmission spectrum from GRACES/Gemini North and UVES/Very Large Telescope. We detect CrH at 5.6
σ
confidence, representing the first metal hydride detection in an exoplanet atmosphere at high spectral resolution. Our findings constitute a critical step in understanding the role of metal hydrides in exoplanet atmospheres.
Abstract
We report on a chemo-dynamical analysis of SPLUS J142445.34−254247.1 (SPLUS J1424−2542), an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process (
...r
-process). This star was first selected as a metal-poor candidate from its narrowband S-PLUS photometry and followed up spectroscopically in medium resolution with Gemini-South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini-South, allowing for the determination of the chemical abundances for 36 elements, from carbon to thorium. At Fe/H = −3.39, SPLUS J1424−2542 is one of the lowest-metallicity stars with measured Th and has the highest
log
ϵ
(
Th
/
Eu
)
observed to date, making it part of the “actinide-boost” category of
r
-process–enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424−2542 formed must have been enriched by at least two progenitor populations. The light-element (
Z
≤ 30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3–13.4
M
⊙
, and the heavy-element (
Z
≥ 38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66
M
⊙
and 1.27
M
⊙
) event. A kinematical analysis also reveals that SPLUS J1424−2542 is a low-mass, old halo star with a likely in situ origin, not associated with any known early merger events in the Milky Way.
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