Precise atmospheric observations have been made for a growing sample of warm Neptunes. Here, we investigate the correlations between these observations and a large number of system parameters to show ...that, at 95% confidence, the amplitude of a warm Neptune's spectral features in transmission correlates with either its equilibrium temperature (Teq) or its bulk H/He mass fraction (fHHe)-in addition to the standard scaling. These correlations could indicate either more optically thick, photochemically produced hazes at lower Teq and/or higher-metallicity atmospheres for planets with smaller radii and lower fHHe. We derive an analytic relation to estimate the observing time needed with JWST/NIRISS to confidently distinguish a nominal gas giant's transmission spectrum from a flat line. Using this tool, we show that these possible atmospheric trends could reduce the number of expected TESS planets accessible to JWST spectroscopy by up to a factor of eight. Additional observations of a larger sample of planets are required to confirm these trends in atmospheric properties as a function of planet or system quantities. If these trends can be confidently identified, the community will be well-positioned to prioritize new targets for atmospheric study and eventually break the complex degeneracies between atmospheric chemistry, composition, and cloud properties.
Abstract
The size of a planet is an observable property directly connected to the physics of its formation and evolution. We used precise radius measurements from the California-
Kepler
Survey to ...study the size distribution of 2025
Kepler
planets in fine detail. We detect a factor of ≥2 deficit in the occurrence rate distribution at 1.5–2.0
. This gap splits the population of close-in (
P
< 100 days) small planets into two size regimes:
and
, with few planets in between. Planets in these two regimes have nearly the same intrinsic frequency based on occurrence measurements that account for planet detection efficiencies. The paucity of planets between 1.5 and 2.0
supports the emerging picture that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5
or smaller with varying amounts of low-density gas that determine their total sizes.
Results from the Kepler mission indicate that the occurrence rate of small planets (<3 R⊕) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the ...conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the 2.6 R⊕ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star ( 1368 − 107 + 114 W m−2) as the Earth receives from the Sun (1361 W m−2), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study we observed nine transits of K2-18b using Hubble Space Telescope/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.
Abstract
The newfound ability to detect SO
2
in exoplanet atmospheres presents an opportunity to measure sulfur abundances and so directly test between competing modes of planet formation. In ...contrast to carbon and oxygen, whose dominant molecules are frequently observed, sulfur is much less volatile and resides almost exclusively in solid form in protoplanetary disks. This dichotomy leads different models of planet formation to predict different compositions of gas giant planets. Whereas planetesimal-based models predict roughly stellar C/S and O/S ratios, pebble-accretion models more often predict superstellar ratios. To explore the detectability of SO
2
in transmission spectra and its ability to diagnose planet formation, we present a grid of atmospheric photochemical models and corresponding synthetic spectra for WASP-39b (where SO
2
has been detected). Our 3D grid contains 11
3
models (spanning 1–100× the solar abundance ratio of C, O, and S) for thermal profiles corresponding to the morning and evening terminators, as well as mean terminator transmission spectra. Our models show that for a WASP-39b-like O/H and C/H enhancement of ∼10× solar, SO
2
can only be seen for C/S and O/S ≲ 1.5× solar, and that WASP-39b’s reported SO
2
abundance of 1–10 ppm may be more consistent with planetesimal accretion than with pebble-accretion models (although some pebble models also manage to predict similarly low ratios). More extreme C/S and O/S ratios may be detectable in higher-metallicity atmospheres, suggesting that smaller and more metal-rich gas and ice giants may be particularly interesting targets for testing planet formation models. Future studies should explore the dependence of SO
2
on a wider array of planetary and stellar parameters, both for the prototypical SO
2
planet WASP-39b, as well as for other hot Jupiters and smaller gas giants.
Abstract
The California-
Kepler
Survey (CKS) is an observational program developed to improve our knowledge of the properties of stars found to host transiting planets by NASA’s
Kepler
Mission. The ...improvement stems from new high-resolution optical spectra obtained using HIRES at the W. M. Keck Observatory. The CKS stellar sample comprises 1305 stars classified as
Kepler
objects of interest, hosting a total of 2075 transiting planets. The primary sample is magnitude-limited (
) and contains 960 stars with 1385 planets. The sample was extended to include some fainter stars that host multiple planets, ultra-short period planets, or habitable zone planets. The spectroscopic parameters were determined with two different codes, one based on template matching and the other on direct spectral synthesis using radiative transfer. We demonstrate a precision of 60 K in
, 0.10 dex in
, 0.04 dex in
, and 1.0
in
. In this paper, we describe the CKS project and present a uniform catalog of spectroscopic parameters. Subsequent papers in this series present catalogs of derived stellar properties such as mass, radius, and age; revised planet properties; and statistical explorations of the ensemble. CKS is the largest survey to determine the properties of
Kepler
stars using a uniform set of high-resolution, high signal-to-noise ratio spectra. The HIRES spectra are available to the community for independent analyses.
Observations of Exoplanet Atmospheres Crossfield, Ian J. M.
Publications of the Astronomical Society of the Pacific,
10/2015, Letnik:
127, Številka:
956
Journal Article
Recenzirano
Odprti dostop
Detailed characterization of an extrasolar planet's atmosphere provides the best hope for distinguishing the makeup of its outer layers, and the only hope for understanding the interplay between ...initial composition, chemistry, dynamics and circulation, and disequilibrium processes. In recent years, some areas have seen rapid progress, while developments in others have come more slowly and/or have been hotly contested. This article gives an observer's perspective on the current understanding of extrasolar planet atmospheres prior to the considerable advances expected from the next generation of observing facilities. Atmospheric processes of both transiting and directly imaged planets are discussed, including molecular and atomic abundances, cloud properties, thermal structure, and planetary energy budgets. In the future we can expect a continuing and accelerating stream of new discoveries, which will fuel the ongoing exoplanet revolution for many years to come.
Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and ...gas disk from which they formed. The precise ages of meteorites indicate that planetesimals—the building blocks of planets—are produced within the first million years of a star’s life. Fully formed planets are frequently detected on short orbital periods around mature stars. Some theories suggest that the in situ formation of planets close to their host stars is unlikely and that the existence of such planets is therefore evidence of large-scale migration. Other theories posit that planet assembly at small orbital separations may be common. Here we report a newly born, transiting planet orbiting its star with a period of 5.4 days. The planet is 50 per cent larger than Neptune, and its mass is less than 3.6 times that of Jupiter (at 99.7 per cent confidence), with a true mass likely to be similar to that of Neptune. The star is 5–10 million years old and has a tenuous dust disk extending outward from about twice the Earth–Sun separation, in addition to the fully formed planet located at less than one-twentieth of the Earth–Sun separation.
Doppler imaging produces 2D global maps of rotating objects using high-dispersion spectroscopy. When applied to brown dwarfs and extrasolar planets, this technique can constrain global atmospheric ...dynamics and/or magnetic effects on these objects in unprecedented detail. I present the first quantitative assessment of the prospects for Doppler imaging of substellar objects with current facilities and with future giant ground-based telescopes. Observations will have the greatest sensitivity in K band, but the H and L bands will also be useful for these purposes. To assess the number and availability of targets, I also present a compilation of all measurements of photometric variability, rotation period (P), and projected rotational velocity (v sin i) for all known brown dwarfs. Several bright objects are already accessible to Doppler imaging with currently available instruments. With the development of giant ground-based telescopes, Doppler imaging will become feasible for many dozens of brown dwarfs and for the few brightest directly imaged extrasolar planets (such as β Pic b). The present set of measurements of P, v sin i, and variability are incomplete for many objects, and the sample is strongly biased toward early-type objects (<L5). Thus, surveys to measure these quantities for later-type objects will be especially helpful in expanding the sample of candidates for global weather monitoring via Doppler imaging.
Abstract
We used high-precision radial velocity measurements of FGKM stars to determine the occurrence of giant planets as a function of orbital separation spanning 0.03–30 au. Giant planets are more ...prevalent at orbital distances of 1–10 au compared to orbits interior or exterior of this range. The increase in planet occurrence at ∼1 au by a factor of ∼4 is highly statistically significant. A fall-off in giant planet occurrence at larger orbital distances is favored over models with flat or increasing occurrence. We measure
14.1
−
1.8
+
2.0
giant planets per 100 stars with semimajor axes of 2–8 au and
8.9
−
2.4
+
3.0
giant planets per 100 stars in the range 8–32 au, a decrease in occurrence with increasing orbital separation that is significant at the ∼2
σ
level. We find that the occurrence rate of sub-Jovian planets (0.1–1 Jupiter masses) is also enhanced for 1–10 au orbits. This suggests that lower-mass planets may share the formation or migration mechanisms that drive the increased prevalence near the water–ice line for their Jovian counterparts. Our measurements of cold gas giant occurrence are consistent with the latest results from direct imaging surveys and gravitational lensing surveys despite different stellar samples. We corroborate previous findings that giant planet occurrence increases with stellar mass and metallicity.
AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79 parsecs and with an age of 22 million years
. AU Mic possesses a relatively rare
and spatially resolved
...edge-on debris disk extending from about 35 to 210 astronomical units from the star
, and with clumps exhibiting non-Keplerian motion
. Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic 'activity' on the star
. Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.
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