Short-period Extrasolar Planets Research Articles

Exoplanet Aeronomy: A Case Study of WASP-69 b’s Variable Thermosphere

Aeronomy, the study of Earth’s upper atmosphere and its interaction with the local space environment, has long traced changes in the thermospheres of Earth and other solar system planets to solar variability in the X-ray and extreme-ultraviolet (collectively, XUV) bands. Extending comparative aeronomy to the short-period extrasolar planets may illuminate whether stellar XUV irradiation powers atmospheric outflows that change planetary radii on astronomical timescales. In recent years, near-IR transit spectroscopy of metastable Hei has been a prolific tracer of high-altitude planetary gas. We present a case study of exoplanet aeronomy using metastable Hei transit observations from Palomar Observatory's Wide Field InfraRed Camera and follow-up high-energy data from the Neil Gehrels Swift Observatory that were taken within 1 month of the WASP-69 system, a K-type main-sequence star with a well-studied hot Jupiter companion. Supplemented by archival data, we find that WASP-69's X-ray flux in 2023 was less than 50% of what was recorded in 2016 and that the metastable Hei absorption from WASP-69 b was lower in 2023 versus past epochs from 2017 to 2019. Via atmospheric modeling, we show that this time-variable metastable Hei signal is in the expected direction given the observed change in stellar XUV, possibly stemming from WASP-69's magnetic activity cycle. Our results underscore the ability of multiepoch, multiwavelength observations to paint a cohesive picture of the interaction between an exoplanet’s atmosphere and its host star.

Formation of rocky super-earths from a narrow ring of planetesimals

The formation of super-Earths, the most abundant planets in the Galaxy, remains elusive. These planets have masses that typically exceed that of the Earth by a factor of a few, appear to be predominantly rocky, although often surrounded by H/He atmospheres, and frequently occur in multiples. Moreover, planets that encircle the same star tend to have similar masses and radii, whereas those belonging to different systems exhibit remarkable overall diversity. Here we advance a theoretical picture for rocky planet formation that satisfies the aforementioned constraints: building upon recent work, which has demonstrated that planetesimals can form rapidly at discrete locations in the disk, we propose that super-Earths originate inside rings of silicate-rich planetesimals at approximately ~1 au. Within the context of this picture, we show that planets grow primarily through pairwise collisions among rocky planetesimals until they achieve terminal masses that are regulated by isolation and orbital migration. We quantify our model with numerical simulations and demonstrate that our synthetic planetary systems bear a close resemblance to compact, multi-resonant progenitors of the observed population of short-period extrasolar planets. A planetary origin model that forms exoplanets from a narrow ring of silicate material at a stellocentric distance of 1 au is able to explain the physical properties of super-Earths and reproduce the ‘peas in a pod’ pattern of uniformity within planetary architecture.

Shallowness of circulation in hot Jupiters

The inflated radii of giant short-period extrasolar planets collectively indicate that the interiors of hot Jupiters are heated by some anomalous energy dissipation mechanism. Although a variety of physical processes have been proposed to explain this heating, recent statistical evidence points to the confirmation of explicit predictions of the Ohmic dissipation theory, elevating this mechanism as the most promising candidate for resolving the radius inflation problem. In this work, we present an analytic model for the dissipation rate and derive a simple scaling law that links the magnitude of energy dissipation to the thickness of the atmospheric weather layer. From this relation, we find that the penetration depth influences the Ohmic dissipation rate by an order of magnitude. We further investigate the weather layer depth of hot Jupiters from the extent of their inflation and show that, depending on the magnetic field strength, hot Jupiter radii can be maintained even if the circulation layer is relatively shallow. Additionally, we explore the evolution of zonal wind velocities with equilibrium temperature by matching our analytic model to statistically expected dissipation rates. From this analysis, we deduce that the wind speed scales approximately as 1/√(Teq−T0), where T0 is a constant that equals T0 ∼ 1000 K − 1800 K depending on planet-specific parameters (radius, mass, etc.). This work outlines inter-related constraints on the atmospheric flow and the magnetic field of hot Jupiters and provides a foundation for future work on the Ohmic heating mechanism.

Signatures of Obliquity in Thermal Phase Curves of Hot Jupiters

Recent work suggests that many short-period extrasolar planets may have spin obliquities that are significantly tilted with respect to their orbital planes. These large obliquities are a natural outcome of ``secular spin-orbit resonance'', a configuration in which the planetary spin precession frequency matches the frequency of orbit nodal regression, or a Fourier component thereof. While exoplanet spin obliquities have not yet been measured directly, they may be detectable indirectly through their signatures in various observations, such as photometric measurements across the full phase of a planet's orbit. In this work, we employ a thermal radiative model to explore how large polar tilts affect full-phase light curves, and we discuss the range of unique signatures that are expected to result. We show that the well-studied short-period planets HD 149026 b, WASP-12 b, and CoRoT-2 b all exhibit phase curve features that may arise from being in high-obliquity states. We also constrain the parameters and assess the detectability of hypothetical perturbing planets that could maintain the planets in these states. Among the three planets considered, CoRoT-2 b has the tightest constraints on its proposed obliquity ($45.8^{\circ} \pm 1.4^{\circ}$) and axial orientation. For HD 149026 b, we find no significant evidence for a non-zero obliquity, and the phase curve of WASP-12 b is too complicated by strong tidal distortions for a conclusive assessment.

Nonlinear tidal flows in short-period planets

I discuss two related nonlinear mechanisms of tidal dissipation that require finite tidal deformations for their operation: the elliptical instability and the precessional instability. Both are likely to be important for the tidal evolution of short-period extrasolar planets. The elliptical instability is a fluid instability of elliptical streamlines, such as in tidally deformed non-synchronously rotating or non-circularly orbiting planets. I summarise the results of local and global simulations that indicate this mechanism to be important for tidal spin synchronisation, planetary spin-orbit alignment and orbital circularisation for the shortest period hot Jupiters. The precessional instability is a fluid instability that occurs in planets undergoing axial precession, such as those with spin-orbit misalignments (non-zero obliquities). I summarise the outcome of local MHD simulations designed to study the turbulent damping of axial precession, which suggest this mechanism to be important in driving tidal evolution of the spin-orbit angle for hot Jupiters. Avenues for future work are also discussed.

HATS-43b, HATS-44b, HATS-45b, and HATS-46b: Four Short-period Transiting Giant Planets in the Neptune–Jupiter Mass Range*

Abstract We report the discovery of four short-period extrasolar planets transiting moderately bright stars from photometric measurements of the HATSouth network coupled to additional spectroscopic and photometric follow-up observations. While the planet masses range from 0.26 to 0.90 , the radii are all approximately a Jupiter radii, resulting in a wide range of bulk densities. The orbital period of the planets ranges from 2.7 days to 4.7 days, with HATS-43b having an orbit that appears to be marginally non-circular (e = 0.173 ± 0.089). HATS-44 is notable for having a high metallicity ( = 0.320 ± 0.071). The host stars spectral types range from late F to early K, and all of them are moderately bright (13.3 < V < 14.4), allowing the execution of future detailed follow-up observations. HATS-43b and HATS-46b, with expected transmission signals of 2350 ppm and 1500 ppm, respectively, are particularly well suited targets for atmospheric characterization via transmission spectroscopy.

Three-dimensional simulations of internal wave breaking and the fate of planets around solar-type stars

We study the fate of internal gravity waves approaching the centre of a non-rotating solar-type star, by performing 3D numerical simulations using a Boussinesq-type model. These waves are excited at the top of the radiation zone by the tidal forcing of a short-period planet on a circular, coplanar orbit. This extends previous work done in 2D by Barker & Ogilvie. We first derive a linear wave solution, which is not exact in 3D; however, the reflection of ingoing waves from the centre is close to perfect for moderate amplitude waves. Waves with sufficient amplitude to cause isentropic overturning break, and deposit their angular momentum near the centre. This forms a critical layer, at which the angular velocity of the flow matches the orbital angular frequency of the planet. This efficiently absorbs ingoing waves, and spins up the star from the inside out, while the planet spirals into the star. We also perform numerical integrations to determine the linearised adiabatic tidal response throughout the star, for solar-type stellar models with masses in the range 0.5 < m_star/M_sun < 1.1, throughout their main sequence lifetimes. The aim is to study the launching region for these waves at the top of the radiation zone in more detail, and to determine the accuracy of a semi-analytic approximation for the tidal torque on the star, that was derived under the assumption that all ingoing wave angular momentum is absorbed in a critical layer. The main conclusions are that this nonlinear mechanism of tidal dissipation could provide an explanation for the survival of all short-period extrasolar planets observed around FGK stars, while it predicts the destruction of more massive planets. This work provides further support for the model outlined in a previous paper by Barker & Ogilvie, and makes predictions that will be tested by ongoing observational studies, such as WASP and Kepler.

SEARCHING FOR FAR-ULTRAVIOLET AURORAL/DAYGLOW EMISSION FROM HD 209458b

We present recent observations from the Hubble Space Telescope-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD 209458b. We obtained medium-resolution (R ∼ 20,000) far-ultraviolet (1150–1700 Å) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8σ) feature at +15(± 20) km s−1 in the rest frame of the planet, at λ1582 Å. This feature is consistent with emission from H2 B–X (2–9) P(4) (λrest = 1581.11 Å); however, the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short-period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.

High-precision photometry by telescope defocussing - II. The transiting planetary system WASP-4

We present and analyse light curves of four transits of the Southern hemisphere extrasolar planetary system WASP-4, obtained with a telescope defocused so the radius of each point spread function was 17 arcsec (44 pixels). This approach minimizes both random and systematic errors, allowing us to achieve scatters of between 0.60 and 0.88 mmag per observation over complete transit events. The light curves are augmented by published observations and analysed using the jktebop code. The results of this process are combined with theoretical stellar model predictions to derive the physical properties of the WASP-4 system. We find that the mass and radius of the planet are Mb= 1.289+0.090−0.090+0.039−0.000MJup and Rb= 1.371+0.032−0.035+0.021−0.000RJup, respectively (statistical and systematic uncertainties). These quantities give a surface gravity and density of gb= 17.03+0.97−0.54 m s−2 and ρb= 0.500+0.032−0.021+0.000−0.008ρJup, and fit the trends for short-period extrasolar planets to have relatively high masses and surface gravities. WASP-4 is now one of the best-quantified transiting extrasolar planetary systems, and significant further progress requires improvements to our understanding of the physical properties of low-mass stars.

A search for electron cyclotron maser emission from compact binaries

Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultracompact binaries with orbital periods of 5.4 and 9.6 min, respectively. The UI model predicts that those two sources may be electron cyclotron maser sources at radio wavelengths. Other systems in which UI has been predicted to occur are short period extrasolar terrestrial planets with conducting cores. If UI is present, circularly polarized radio emission is predicted to be emitted. We have searched for this predicted radio emission from short period binaries using the Very Large Array (VLA) and Australian Telescope Compact Array (ATCA). In one epoch, we find evidence for a radio source, coincident in position with the optical position of RX J0806+15. Although we cannot completely exclude that this is a chance alignment between the position of RX J0806+15 and an artefact in the data reduction process, the fact that it was detected at a significance level of 5.8 sigma and found to be transient suggests that it is more likely that RX J0806+15 is a transient radio source. We find an upper limit on the degree of circular polarization to be similar to 50 per cent. The inferred brightness temperature exceeds 10(18) K, which is too high for any known incoherent process, but is consistent with maser emission and UI being the driving mechanism. We did not detect radio emission from ES Cet, RX J1914+24 or Gliese 876.

Gas at the inner disk edge

AbstractInfrared molecular spectroscopy is a key tool for the observation of gas in the innermost region of disks around T Tauri stars. In this contribution, we examine how infrared spectroscopy of CO can be used to study the inner truncation region of disks around T Tauri stars. The inferred inner gas radii for T Tauri star disks are compared to the inner dust radii of disks, to the expectations of models for disk truncation, and to the orbital distribution of short-period extra-solar planets.

Upper limits on the hot Jupiter fraction in the field of NGC 7789

We describe a method of estimating the abundance of short-period extra-solar planets based on the results of a photometric survey for planetary transits. We apply the method to a 21-night survey with the 2.5-m Isaac Newton Telescope of ∼32 000 stars in a ∼0.5 x 0.5 deg 2 field including the open cluster NGC 7789. From the colour-magnitude diagram, we estimate the mass and radius of each star by comparison with the cluster main sequence. We search for injected synthetic transits throughout the light curve of each star in order to determine their recovery rate, and thus calculate the expected number of transit detections and false alarms in the survey. We take proper account of the photometric accuracy, time sampling of the observations and criteria (signal-to-noise ratio and number of transits) adopted for transit detection. Assuming that none of the transit candidates found in the survey will be confirmed as real planets, we place conservative upper limits on the abundance of planets as a function of planet radius, orbital period and spectral type.

On the Surface Heating of Synchronously Spinning Short‐Period Jovian Planets

We consider the atmospheric flow on short-period extra-solar planets through two-dimensional numerical simulations of hydrodynamics with radiation transfer. One side is always exposed to the irradiation from the host star. The other is always in shadow. The temperature of the day side is determined by the equilibrium which the planetary atmosphere establishes with stellar radiation. Part of the thermal energy deposited on the day side is advected to the night side by a current. The radiation transfer, the night-side temperature distribution and by this the spectroscopic signature of the planet are sensitive functions of the atmospheric opacity. If the atmosphere contains grains with an abundanceand size distribution comparable to that of the interstellar medium, shallow heating occurs on the day side and the night side cools well below the day side. The temperature difference decreases as the abundance of grains is reduced. A simple analytic model of the dissipation of the circulation flow and associated kinetic heating is considered. This heating effect occurs mostly near the photosphere, not deep enough to significantly affect the size of planets. We show that the surface irradiation suppresses convection near the photospheric region on the day side. In some cases convection zones appear near the surface on the night side. This structural modification may influence the response and dissipation of tidal disturbances and alter the circularization and synchronization time scales.

Tidal Dissipation in Rotating Giant Planets

[Abridged] Tides may play an important role in determining the observed distributions of mass, orbital period, and eccentricity of the extrasolar planets. In addition, tidal interactions between giant planets in the solar system and their moons are thought to be responsible for the orbital migration of the satellites, leading to their capture into resonant configurations. We treat the underlying fluid dynamical problem with the aim of determining the efficiency of tidal dissipation in gaseous giant planets. In cases of interest, the tidal forcing frequencies are comparable to the spin frequency of the planet but small compared to its dynamical frequency. We therefore study the linearized response of a slowly and possibly differentially rotating planet to low-frequency tidal forcing. Convective regions of the planet support inertial waves, while any radiative regions support generalized Hough waves. We present illustrative numerical calculations of the tidal dissipation rate and argue that inertial waves provide a natural avenue for efficient tidal dissipation in most cases of interest. The resulting value of Q depends in a highly erratic way on the forcing frequency, but we provide evidence that the relevant frequency-averaged dissipation rate may be asymptotically independent of the viscosity in the limit of small Ekman number. In short-period extrasolar planets, if the stellar irradiation of the planet leads to the formation of a radiative outer layer that supports generalized Hough modes, the tidal dissipation rate can be enhanced through the excitation and damping of these waves. These dissipative mechanisms offer a promising explanation of the historical evolution and current state of the Galilean satellites as well as the observed circularization of the orbits of short-period extrasolar planets.

Scattered Light from Close‐in Extrasolar Planets: Prospects of Detection with theMOSTSatellite

The ultra-precise photometric space satellite MOST (Microvariability and Oscillations of STars) will provide the first opportunity to measure the albedos and scattered light curves from known short-period extrasolar planets. Due to the changing phases of an extrasolar planet as it orbits its parent star, the combined light of the planet-star system will vary on the order of tens of micromagnitudes. The amplitude and shape of the resulting light curve is sensitive to the planet's radius and orbital inclination, as well as the composition and size distribution of the scattering particles in the planet's atmosphere. To predict the capabilities of MOST and other planned space missions, we have constructed a series of models of such light curves, improving upon earlier work by incorporating more realistic details such as: limb darkening of the star, intrinsic granulation noise in the star itself, tidal distortion and back-heating, higher angular resolution of the light scattering from the planet, and exploration of the significance of the angular size of the star as seen from the planet. We use photometric performance simulations of the MOST satellite, with the light curve models as inputs, for one of the mission's primary targets, $\tau$ Bootis. These simulations demonstrate that, even adopting a very conservative signal detection limit of 4.2 $\mu$mag in amplitude (not power), we will be able to either detect the $\tau$ Bootis planet light curve or put severe constraints on possible extrasolar planet atmospheric models.

On the Tidal Inflation of Short‐Period Extrasolar Planets

We examine the consequences of tidal interaction between eccentric short-period extrasolar planets and their host stars and of secular perturbations between planets in a given system. If the planet is within 0.05 AU of the star, the dissipation of the stellar tidal disturbance within the planet provides a significant energy source, which causes the planet to inflate as it adjusts to a thermal equilibrium. We determine the planetary size as a function of the tidal dissipation rate with or without the presence of a core. Inflation intensifies the star-planet tidal interaction and accelerates the pace of the planet's spin synchronization and orbital circularization. We apply our results to three systems with short-period planets: HD 209458, Ups And, and Tau Boo.