Abstract
We explore atmospheric escape from close-in exoplanets with the highest mass-loss rates. First, we locate the transition from stellar X-ray and UV-driven escape to rapid Roche lobe overflow, ...which occurs once the 10–100 nbar pressure level in the atmosphere reaches the Roche lobe. Planets enter this regime when the ratio of the substellar radius to the polar radius along the visible surface pressure level, which aligns with a surface of constant Roche potential, is X/Z ≳ 1.2 for Jovian planets (
M
p ≳ 100
M
⊕
) and X/Z ≳ 1.02 for sub-Jovian planets (
M
p
≈ 10–100
M
⊕
). Around a Sun-like star, this regime applies to orbital periods of less than two days for planets with radii of about 3–14R
⊕
. Our results agree with the properties of known transiting planets and can explain parts of the sub-Jovian desert in the population of known exoplanets. Second, we present detailed numerical simulations of atmospheric escape from a planet like Uranus or Neptune orbiting close to a Sun-like star that support the results above and point to interesting qualitative differences between hot Jupiters and sub-Jovian planets. We find that hot Neptunes with solar-metallicity hydrogen and helium envelopes have relatively more extended upper atmospheres than typical hot Jupiters, with a lower ionization fraction and higher abundances of escaping molecules. This is consistent with existing ultraviolet transit observations of warm Neptunes, and it might provide a way to use future observations and models to distinguish solar-metallicity atmospheres from higher-metallicity atmospheres.
Abstract
Understanding the occurrence of Earth-sized planets in the habitable zone of Sun-like stars is essential to the search for Earth analogs. Yet a lack of reliable Kepler detections for such ...planets has forced many estimates to be derived from the close-in (2 <
P
orb
< 100 days) population, whose radii may have evolved differently under the effect of atmospheric mass-loss mechanisms. In this work, we compute the intrinsic occurrence rates of close-in super-Earths (∼1–2
R
⊕
) and sub-Neptunes (∼2–3.5
R
⊕
) for FGK stars (0.56–1.63
M
⊙
) as a function of orbital period and find evidence of two regimes: where super-Earths are more abundant at short orbital periods, and where sub-Neptunes are more abundant at longer orbital periods. We fit a parametric model in five equally populated stellar mass bins and find that the orbital period of transition between these two regimes scales with stellar mass, like
P
trans
∝
M
*
1.7
±
0.2
. These results suggest a population of former sub-Neptunes contaminating the population of gigayear-old close-in super-Earths, indicative of a population shaped by atmospheric loss. Using our model to constrain the long-period population of intrinsically rocky planets, we estimate an occurrence rate of
Γ
⊕
=
15
−
4
+
6
%
for Earth-sized habitable zone planets, and predict that sub-Neptunes may be ∼ twice as common as super-Earths in the habitable zone (when normalized over the natural log-orbital period and radius range used). Finally, we discuss our results in the context of future missions searching for habitable zone planets.
Abstract
Atmospheric escape is a fundamental process that affects the structure, composition, and evolution of many planets. The signatures of escape are detectable on close-in, gaseous exoplanets ...orbiting bright stars, owing to the high levels of extreme-ultraviolet irradiation from their parent stars. The Colorado Ultraviolet Transit Experiment (CUTE) is a CubeSat mission designed to take advantage of the near-ultraviolet stellar brightness distribution to conduct a survey of the extended atmospheres of nearby close-in planets. The CUTE payload is a magnifying near-ultraviolet (2479–3306 Å) spectrograph fed by a rectangular Cassegrain telescope (206 mm × 84 mm); the spectrogram is recorded on a back-illuminated, UV-enhanced CCD. The science payload is integrated into a 6U Blue Canyon Technology XB1 bus. CUTE was launched into a polar, low-Earth orbit on 2021 September 27 and has been conducting this transit spectroscopy survey following an on-orbit commissioning period. This paper presents the mission motivation, development path, and demonstrates the potential for small satellites to conduct this type of science by presenting initial on-orbit science observations. The primary science mission is being conducted in 2022–2023, with a publicly available data archive coming online in 2023.
Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated ...with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 161 and 403 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm (Na/H = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star (Na/H = 0.485 0.043).
Abstract
Ultraviolet observations of ultrahot Jupiters, exoplanets with temperatures over 2000 K, provide us with an opportunity to investigate if and how atmospheric escape shapes their upper ...atmosphere. Near-ultraviolet transit spectroscopy offers a unique tool to study this process owing to the presence of strong metal lines and a bright photospheric continuum as the light source against which the absorbing gas is observed. WASP-189b is one of the hottest planets discovered to date, with a dayside temperature of about 3400 K orbiting a bright A-type star. We present the first near-ultraviolet observations of WASP-189b, acquired with the Colorado Ultraviolet Transit Experiment (CUTE). CUTE is a 6U NASA-funded ultraviolet spectroscopy mission, dedicated to monitoring short-period transiting planets. WASP-189b was one of the CUTE early science targets and was observed during three consecutive transits in 2022 March. We present an analysis of the CUTE observations and results demonstrating near-ultraviolet (2500–3300 Å) broadband transit depth (
1.08
−
0.08
+
0.08
%
) of about twice the visual transit depth indicating that the planet has an extended, hot upper atmosphere with a temperature of about 15,000 K and a moderate mass-loss rate of about 4 × 10
8
kg s
−1
. We observe absorption by Mg
ii
lines (
R
p
/
R
s
of
0.212
−
0.061
+
0.038
) beyond the Roche lobe at >4
σ
significance in the transmission spectrum at a resolution of 10 Å, while at lower resolution (100 Å), we observe a quasi-continuous absorption signal consistent with a “forest” of low-ionization metal absorption dominated by Fe
ii
. The results suggest an upper atmospheric temperature (∼15,000 K), higher than that predicted by current state-of-the-art hydrodynamic models.
Thermal escape from extrasolar giant planets Koskinen, Tommi T.; Lavvas, Panayotis; Harris, Matthew J. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
04/2014, Letnik:
372, Številka:
2014
Journal Article
Recenzirano
Odprti dostop
The detection of hot atomic hydrogen and heavy atoms and ions at high altitudes around close-in extrasolar giant planets (EGPs) such as HD209458b implies that these planets have hot and rapidly ...escaping atmospheres that extend to several planetary radii. These characteristics, however, cannot be generalized to all close-in EGPs. The thermal escape mechanism and mass loss rate from EGPs depend on a complex interplay between photochemistry and radiative transfer driven by the stellar UV radiation. In this study, we explore how these processes change under different levels of irradiation on giant planets with different characteristics. We confirm that there are two distinct regimes of thermal escape from EGPs, and that the transition between these regimes is relatively sharp. Our results have implications for thermal mass loss rates from different EGPs that we discuss in the context of currently known planets and the detectability of their upper atmospheres.
Gravity wave (GW) signatures have been derived from temperature profiles observed by Cassini/Ultraviolet Imaging Spectrograph in the Saturnian thermosphere during the Grand Finale campaign. They ...demonstrate upward propagation of GW packets, their saturation, and breaking. We determined wave amplitudes, potential energy, and momentum fluxes and estimated the associated wave drag imposed by dissipating harmonics on the ambient flow. The data set of 18 profiles covers the middle and high latitudes of both hemispheres, which allows for exploring the global impact of waves. The diagnostics based on the Transformed Eulerian Mean and modified geostrophy approach reveal that the GW drag induces an equatorward flow in both hemispheres, facilitating transport of heat away from the auroral zones and redistributing energy across latitudes. Like all the outer planets, Saturn's thermosphere is hundreds of degrees hotter than what follows from radiative balance and these results help to explain the observed temperatures at all latitudes.
Plain Language Summary
Gravity waves are small‐scale fluctuations of air density, temperature, and other atmospheric variables, which are dynamically important in the upper atmospheres. During Cassini's Grand Finale, observations with the Ultraviolet Imaging Spectrograph delivered a set of density profiles in the Saturnian thermosphere with a resolution sufficient to detect such waves. We derived various characteristics and estimated the forcing imposed by dissipating waves on the global circulation. It turned out that this “gravity wave drag” causes enhanced flow toward the equator in both hemispheres, transporting heat away from the auroral sources in high latitudes and distributing it over Saturn's thermosphere. Previous studies suggested that such redistribution would be prevented by fast westward jets (the “energy crisis”). We show that the waves observed on Saturn can successfully help winds to overcome this barrier. A similar mechanism can exist on other outer planets, whose thermospheres are also much hotter than what follows from radiative balance.
Key Points
We present evidence for the omnipresence of gravity waves in the Saturnian thermosphere from Cassini/Ultraviolet Imaging Spectrograph Grand Finale measurements
We determine wave amplitudes, energy, momentum fluxes, and forcing imposed on the mean flow along with their spatial distributions
Gravity wave drag enhances pole‐to‐equator flow in both hemispheres, which helps to redistribute energy across latitudes
Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically. Theoretically, it has been shown that such ...escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere. Jupiter's atmosphere is stable, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere-ionosphere model and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.
•We measured benzene absorption cross sections in the temperature range 215–298K.•We presented the first analysis of temperature dependency, which is small.•The cross sections were used to analyze 4 ...stellar occultations measured by UVIS.•We retrieved 4 density profiles of benzene in the upper atmosphere of Titan.•Our results indicate that photochemical models do not produce enough benzene.
Benzene is an important molecule in Titan’s atmosphere because it is a potential link between the gas phase and the organic solid phase. We measured photoabsorption in the ultraviolet by benzene gas at temperatures covering the range from room temperature to 215K. We derived benzene absorption cross sections and analyzed them in terms of the transitions observed. No significant variation with measurement temperature was observed. We discuss the implications of our measurements for the derivation of benzene abundance profiles in Titan’s thermosphere, by the Cassini/Ultraviolet Imaging Spectrograph (UVIS). The use of absorption cross sections at low temperature is recommended to avoid small systematic uncertainties in the profiles. We used our measurements, together with absorption cross sections from other molecules, to analyze four stellar occultations by Titan, measured by UVIS during flybys T21, T41, T41_II, and T53. We derived and compared benzene abundance profiles in Titan’s thermosphere between approximately 530 and 1000km, for different dates and geographical locations. The comparisons of our benzene profiles with each other, and with profiles from models of the upper atmosphere, point to a complex behavior that is not explained by current photochemical models.
Ionization of Extrasolar Giant Planet Atmospheres Koskinen, Tommi T; Cho, James Y-K; Achilleos, Nicholas ...
Astrophysical journal/The Astrophysical journal,
10/2010, Letnik:
722, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Many extrasolar planets orbit close in and are subject to intense ionizing radiation from their host stars. Therefore, we expect them to have strong, and extended, ionospheres. Ionospheres are ...important because they modulate escape in the upper atmosphere and can modify circulation, as well as leave their signatures, in the lower atmosphere. In this paper, we evaluate the vertical location Z{sub I} and extent D{sub I} of the EUV ionization peak layer. We find that Z{sub I{approx}}1-10 nbar-for a wide range of orbital distances (a = 0.047-1 AU) from the host star-and D{sub I}/H{sub p{approx}}>15, where H{sub p} is the pressure scale height. At Z{sub I}, the plasma frequency is {approx}80-450 MHz, depending on a. We also study global ion transport, and its dependence on a, using a three-dimensional thermosphere-ionosphere model. On tidally synchronized planets with weak intrinsic magnetic fields, our model shows only a small, but discernible, difference in electron density from the dayside to the nightside ({approx}9 x 10{sup 13} m{sup -3} to {approx}2 x 10{sup 12} m{sup -3}, respectively) at Z{sub I}. On asynchronous planets, the distribution is essentially uniform. These results have consequences for hydrodynamic modeling of the atmospheres of close-in extrasolar giant planets.
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