Colour-Magnitude Diagrams of Transiting Exoplanets. I - Systems with parallaxes

Broadband flux measurements centred around [3.6 $\mu$m] and [4.5 $\mu$m] obtained with Spitzer during the occultation of seven extrasolar planets by their host stars have been combined with parallax measurements to compute the absolute magnitudes of these planets. Those measurements are arranged in two colour-magnitude diagrams. Because most of the targets have sizes and temperatures similar to brown dwarfs, they can be compared to one another. In principle, this should permit inferences about exo-atmospheres based on knowledge acquired by decades of observations of field brown dwarfs and ultra-cool stars' atmospheres. Such diagrams can assemble all measurements gathered so far and will provide help in the preparation of new observational programs. In most cases, planets and brown dwarfs follow similar sequences. HD\,2094589b and GJ 436b are found to be outliers, so is the nightside of HD 189733b. The photometric variability associated with the orbital phase of HD 189733b is particularly revealing. The planet exhibits what appears like a spectral type and chemical transition between its day and night sides: HD 189733b straddles the L-T spectral class transition, which would imply different cloud coverage on each hemisphere. Methane absorption could be absent at its hot spot but present over the rest of the planet.Comment: Accepted for publication in MNRAS Letters; 4 pages, 2 tables, 1 figur

Prospects for detecting the Rossiter-McLaughlin effect of Earth-like planets: the test case of TRAPPIST-1b and c

The Rossiter-McLaughlin effect is the principal method of determining the sky-projected spin--orbit angle ($\beta$) of transiting planets. Taking the example of the recently discovered TRAPPIST-1 system, we explore how ultracool dwarfs facilitate the measurement of the spin--orbit angle for Earth-sized planets by creating an effect that can be an order of magnitude more ample than the Doppler reflex motion caused by the planet if the star is undergoing rapid rotation. In TRAPPIST-1's case we expect the semi-amplitudes of the Rossiter-McLaughlin effect to be $40-50$ m/s for the known transiting planets. Accounting for stellar jitter expected for ultracool dwarfs, instrumental noise, and assuming radial velocity precisions both demonstrated and anticipated for upcoming near-infrared spectrographs, we quantify the observational effort required to measure the planets' masses and spin--orbit angles. We conclude that if the planetary system is well-aligned then $\beta$ can be measured to a precision of $\lesssim 10^{\circ}$ if the spectrograph is stable at the level of 2 m/s. We also investigate the measure of $\Delta \beta$, the mutual inclination, when multiple transiting planets are present in the system. Lastly, we note that the rapid rotation rate of many late M-dwarfs will amplify the Rossiter-McLaughlin signal to the point where variations in the chromatic Rossiter-McLaughlin effect from atmospheric absorbers should be detectable.Comment: 11 pages, 4 figures. Accepted to MNRAS. Comments welcom

Planets Transiting Non-Eclipsing Binaries

The majority of binary stars do not eclipse. Current searches for transiting circumbinary planets concentrate on eclipsing binaries, and are therefore restricted to a small fraction of potential hosts. We investigate the concept of finding planets transiting non-eclipsing binaries, whose geometry would require mutually inclined planes. Using an N-body code we explore how the number and sequence of transits vary as functions of observing time and orbital parameters. The concept is then generalised thanks to a suite of simulated circumbinary systems. Binaries are constructed from RV surveys of the solar neighbourhood. They are then populated with orbiting gas giants, drawn from a range of distributions. The binary population is shown to be compatible with the Kepler eclipsing binary catalogue, indicating that the properties of binaries may be as universal as the initial mass function. These synthetic systems produce transiting circumbinary planets occurring on both eclipsing and non-eclipsing binaries. Simulated planets transiting eclipsing binaries are compared with published Kepler detections. We obtain 1) that planets transiting non-eclipsing binaries probably exist in the Kepler data, 2) that observational biases alone cannot account for the observed over-density of circumbinary planets near the stability limit, implying a physical pile-up, and 3) that the distributions of gas giants orbiting single and binary stars are likely different. Estimating the frequency of circumbinary planets is degenerate with the spread in mutual inclination. Only a minimum occurrence rate can be produced, which we find to be compatible with 9%. Searching for inclined circumbinary planets may significantly increase the population of known objects and will test our conclusions. Their existence, or absence, will reveal the true occurrence rate and help develop circumbinary planet formation theories.Comment: 19 pages, 14 figures, accepted August 2014 to A&A, minor changes to previous arXiv versio

The Ability of Significant Tidal Stress to Initiate Plate Tectonics

Plate tectonics is a geophysical process currently unique to Earth, has an important role in regulating the Earth's climate, and may be better understood by identifying rocky planets outside our solar system with tectonic activity. The key criterion for whether or not plate tectonics may occur on a terrestrial planet is if the stress on a planet's lithosphere from mantle convection may overcome the lithosphere's yield stress. Although many rocky exoplanets closely orbiting their host stars have been detected, all studies to date of plate tectonics on exoplanets have neglected tidal stresses in the planet's lithosphere. Modeling a rocky exoplanet as a constant density, homogeneous, incompressible sphere, we show the tidal stress from the host star acting on close-in planets may become comparable to the stress on the lithosphere from mantle convection. We also show that tidal stresses from planet-planet interactions are unlikely to be significant for plate tectonics, but may be strong enough to trigger Earthquakes. Our work may imply planets orbiting close to their host stars are more likely to experience plate tectonics, with implications for exoplanetary geophysics and habitability. We produce a list of detected rocky exoplanets under the most intense stresses. Atmospheric and topographic observations may confirm our predictions in the near future. Investigations of planets with significant tidal stress can not only lead to observable parameters linked to the presence of active plate tectonics, but may also be used as a tool to test theories on the main driving force behind tectonic activity.Comment: 34 pages, 3 figures, 3 Tables, accepted to Icaru

The Rossiter-McLaughlin Effect for Planets and Low-Mass Binaries

The Rossiter-McLaughlin effect occurs during the eclipse or transit of an object in front of another one. In our case, it appears as an anomaly on the radial velocity Doppler reflex motion. The modelling of that effect allows one to measure the sky-projected angle between the rotation spin of the primary and the orbital spin of the secondary. In the case of exoplanets, it gave clues about the formation of the hot Jupiters. In this paper, I will talk about how the data are acquired, how models are adjusted to them, and which results have been mad

Warm Jupiters are less lonely than hot Jupiters: close neighbours

Exploiting the Kepler transit data, we uncover a dramatic distinction in the prevalence of sub-Jovian companions, between systems that contain hot Jupiters (periods inward of 10 days) and those that host warm Jupiters (periods between 10 and 200 days). Hot Jupiters, with the singular exception of WASP-47b, do not have any detectable inner or outer planetary companions (with periods inward of 50 days and sizes down to $2 R_{\rm Earth}$). Restricting ourselves to inner companions, our limits reach down to $1 R_{\rm Earth}$. In stark contrast, half of the warm Jupiters are closely flanked by small companions. Statistically, the companion fractions for hot and warm Jupiters are mutually exclusive, particularly in regard to inner companions. The high companion fraction of warm Jupiters also yields clues to their formation. The warm Jupiters that have close-by siblings should have low orbital eccentricities and low mutual inclinations. The orbital configurations of these systems are reminiscent of those of the low-mass, close-in planetary systems abundantly discovered by the Kepler mission. This, and other arguments, lead us to propose that these warm Jupiters are formed in-situ. There are indications that there may be a second population of warm Jupiters with different characteristics. In this picture, WASP-47b could be regarded as the extending tail of the in-situ warm Jupiters into the hot Jupiter region, and does not represent the generic formation route for hot Jupiters.Comment: 12 pages, 7 figures, accepted by Ap

Saltire - A model to measure dynamical masses for high-contrast binaries and exoplanets with high-resolution spectroscopy

High-resolution cross-correlation methods are widely used to discover and to characterise atomic and molecular species in exoplanet atmospheres. The characteristic cross-correlation signal is typically represented as a function of the velocity of the system, and the semi-amplitude of the planet's orbit. We present Saltire, a fast and simple model that accurately reproduces the shape of such cross-correlation signals, allowing a direct fit to the data by using a minimum set of parameters. We show how to use this model on the detection of atmospheric CO in archival data of the hot Jupiter τ Boötis b, and how Saltire can be used to estimate the semi-amplitude and rest velocity of high brightness-ratio binaries. By including the shape of the signal, we demonstrate that our model allows to robustly derive the signal position up to 10 times more accurate, compared to conventional methods. Furthermore, we discuss the impact of correlated noise and demonstrate that Saltire is a robust tool for estimating systematic uncertainties on the signal position. Saltire opens a new door to analyse high signal-to-noise data to accurately study atmospheric dynamics and to measure precise dynamical masses for exoplanets and faint stellar companions. We show, that the phase-resolved shape of the atmospheric CCF signal can accurately be reproduced, allowing studies of phase-dependent signal changes and to disentangle them from noise and data aliases

Dynamical Stability of Imaged Planetary Systems in Formation: Application to HL Tau

A recent ALMA image revealed several concentric gaps in the protoplanetary disk surrounding the young star HL Tau. We consider the hypothesis that these gaps are carved by planets, and present a general framework for understanding the dynamical stability of such systems over typical disk lifetimes, providing estimates for the maximum planetary masses. We collect these easily evaluated constraints into a workflow that can help guide the design and interpretation of new observational campaigns and numerical simulations of gap opening in such systems. We argue that the locations of resonances should be significantly shifted in massive disks like HL Tau, and that theoretical uncertainties in the exact offset, together with observational errors, imply a large uncertainty in the dynamical state and stability in such disks. This presents an important barrier to using systems like HL Tau as a proxy for the initial conditions following planet formation. An important observational avenue to breaking this degeneracy is to search for eccentric gaps, which could implicate resonantly interacting planets. Unfortunately, massive disks like HL Tau should induce swift pericenter precession that would smear out any such eccentric features of planetary origin. This motivates pushing toward more typical, less massive disks. For a nominal non-resonant model of the HL Tau system with five planets, we find a maximum mass for the outer three bodies of approximately 2 Neptune masses. In a resonant configuration, these planets can reach at least the mass of Saturn. The inner two planets' masses are unconstrained by dynamical stability arguments.Comment: Accepted in ApJ. 16 pages 8 figure