Linking the Climate and Thermal Phase Curve of SuperEarth 55 Cancri e

Linking the Climate and Thermal Phase Curve of 55 Cancri e

Authors:

Hammond et al

Abstract: 

The thermal phase curve of 55 Cancri e is the first measurement of the temperature distribution of a tidally locked super-Earth, but raises a number of puzzling questions about the planet's climate. The phase curve has a high amplitude and peak offset, suggesting that it has a significant eastward hot-spot shift as well as a large day–night temperature contrast. We use a general circulation model to model potential climates, and investigate the relation between bulk atmospheric composition and the magnitude of these seemingly contradictory features. We confirm theoretical models of tidally locked circulation are consistent with our numerical model of 55 Cnc e, and rule out certain atmospheric compositions based on their thermodynamic properties. Our best-fitting atmosphere has a significant hot-spot shift and day–night contrast, although these are not as large as the observed phase curve. We discuss possible physical processes that could explain the observations, and show that night-side cloud formation from species such as SiO from a day-side magma ocean could potentially increase the phase curve amplitude and explain the observations. We conclude that the observations could be explained by an optically thick atmosphere with a low mean molecular weight, a surface pressure of several bars, and a strong eastward circulation, with night-side cloud formation a possible explanation for the difference between our model and the observations.

The Discovery and Mass Measurement of a New Ultra-short-period Planet: EPIC 228732031b

The Discovery and Mass Measurement of a New Ultra-short-period Planet: EPIC 228732031b 

Authors:

Dai et al

Abstract:

We report the discovery of a new ultra-short-period planet and summarize the properties of all such planets for which the mass and radius have been measured. The new planet, EPIC 228732031b, was discovered in K2 Campaign 10. It has a radius of ${1.81}_{-0.12}^{+0.16}\,{R}_{\oplus }$ and orbits a G dwarf with a period of 8.9 hr. Radial velocities obtained with Magellan/PFS and TNG/HARPS-N show evidence for stellar activity along with orbital motion. We determined the planetary mass using two different methods: (1) the "floating chunk offset" method, based only on changes in velocity observed on the same night; and (2) a Gaussian process regression based on both the radial velocity and photometric time series. The results are consistent and lead to a mass measurement of $6.5\pm 1.6\,{M}_{\oplus }$ and a mean density of ${6.0}_{-2.7}^{+3.0}$ g cm−3.

EPIC 246393474 b: A 5 Earth Mass super-Earth transiting a K7 V star every 6.7 hours

EPIC 246393474 b: A 5-M⊕ super-Earth transiting a K7 V star every 6.7 hours 

Authors:

Barragán et al

Abstract:

We report on the discovery of EPIC 246393474 b, an ultra-short-period super-Earth on a 6.7-hour orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planet's existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. EPIC 246393474 b has a mass of 5.31±0.46 M⊕ and radius of 1.54+0.10−0.09 R⊕, yielding a mean density of 8.00+1.83−1.45 gcm−3 and suggesting a rocky-iron composition. Models indicate that iron cannot exceed ∼70 % of the total mass. With an orbital period of only 6.7 hours, EPIC 246393474 b is the shortest-period planet known to date with a precisely determined mass.

Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet

Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet 

Authors: 

Mann et al 

Abstract:
Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting EPIC 247589423, a late K dwarf in the Hyades (~800 Myr) cluster, and robust detection limits for additional planets in the system. The planets were identified from their K2 light curves, as part of our survey of young clusters and star forming regions. The smallest planet has a radius comparable to Earth (0.99+/-0.05 Earth radii), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of 2.91+/-0.11 and 1.45+/-0.10 Earth radii, respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m/s, achievable with modern precision RV spectrographs. Because the target star is bright (V=11.2) and has relatively low-amplitude stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best planets known in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.

Asteroid impacts on terrestrial planets: The effects of super-Earths and the role of the ν6 resonance

Asteroid impacts on terrestrial planets: The effects of super-Earths and the role of the ν6 resonance

Authors:

Smallwood et al

Abstract:

With N-body simulations of a planetary system with an asteroid belt we investigate how the asteroid impact rate on the Earth is affected by the architecture of the planetary system. We find that the ν6 secular resonance plays an important role in the asteroid collision rate with the Earth. Compared to exoplanetary systems, the solar system is somewhat special in its lack of a super-Earth mass planet in the inner solar system. We therefore first consider the effects of the presence of a super-Earth in the terrestrial planet region. We find a significant effect for super-Earths with a mass of around 10M⊕ and a separation greater than about 0.7AU. For a super-Earth that is interior to the Earth's orbit, the number of asteroids colliding with Earth increases the closer the super-Earth is to the Earth's orbit. This is the result of multiple secular resonance locations causing more asteroids to be perturbed onto Earth-crossing orbits. When the super-Earth is placed exterior to Earth's orbit, the collision rate decreases substantially because the ν6 resonance no longer exists in the asteroid belt region. We also find that changing the semi-major axis of Saturn leads to a significant decrease in the asteroid collision rate, while increasing its mass increases the collision rate. These results may have implications for the habitability of exoplanetary systems.

Evaporation of Low-Mass Planet Atmospheres: Multidimensional Hydrodynamics with Consistent Thermochemistry

Evaporation of Low-Mass Planet Atmospheres: Multidimensional Hydrodynamics with Consistent Thermochemistry

Authors:

Wang et al

Abstract:

Direct and statistical observational evidences suggest that photoevaporation is important in eroding the atmosphere of sub-Neptune planets. We construct full hydrodynamic simulations, coupled with consistent thermochemistry and ray-tracing radiative transfer, to understand the physics of atmospheric photoevaporation caused by high energy photons from the host star. We identify a region on the parameter space where a hydrostatic atmosphere cannot be balanced by any plausible interplanetary pressure, so that the atmosphere is particularly susceptible to loss by Parker wind. This region may lead an absence of rich atmosphere (substantially H/He) for planets with low mass (M ~ 3 M_earth). Improving on previous works, our simulations include detailed microphysics and a self-consistent thermochemical network. Full numerical simulations of photoevaporative outflows shows a typical outflow speed ~ 30 km/s and Mdot ~ 4e-10 M_earth/yr for a 5 M_earth fiducial model rocky-core planet with 1e-2 of its mass in the atmosphere. Supersonic outflows are not quenched by stellar wind ram pressure (up to 5 times the total pressure at transonic points of the fiducial model). The outflows modulated by stellar wind are collimated towards the night side of the planet, while the mass loss rate is only ~ 25% lower than the fiducial model. By exploring the parameter space, we find that EUV photoionization is most important in launching photoevaporative wind. Other energetic radiation, including X-ray, are of secondary importance. The leading cooling mechanism is ro-vibrational molecular cooling and adiabatic expansion rather than recombination or Ly alpha cooling. The wind speed is considerably higher than the escape velocity at the wind base in most cases, hence the mass loss rate is proportional to the second power of the EUV photosphere size R_euv, instead of the third, as suggested by previous works...

On The Existence of Planets Around the Pulsar PSR B0329+54

On The Existence of Planets Around the Pulsar PSR B0329+54 

Authors:

Starovoit et al

Abstract: 

Results of timing measurements of the pulsar PSR B0329+54 obtained in 1968--2012 using the Big Scanning Antenna of the Pushchino Radio Astronomy Observatory (at 102 and 111 MHz), the DSS 13 and DSS 14 telescopes of the Jet Propulsion Laboratory (2388 MHz), and the 64 m telescope of the Kalyazin Radio Astronomy Observatory (610 MHz) are presented. The astrometric and rotational parameters of the pulsar are derived at a new epoch. Periodic variations in the barycentric timing residuals have been found, which can be explained by the presence of a planet orbiting the pulsar, with an orbital period P1 = 27.8 yr, mass \textit{mc}sin\textit{i} = 2M⊕, and orbital semi-major axis a = 10.26 AU. The results of this study do not confirm existence of a proposed second planet with orbital period P2 = 3 yr.

Average Albedos of Close-in Super-Earths and Super-Neptunes from Statistical Analysis of Long-cadence Kepler Secondary Eclipse Data

Average Albedos of Close-in Super-Earths and Super-Neptunes from Statistical Analysis of Long-cadence Kepler Secondary Eclipse Data
Authors: 

Sheets et al 

Abstract:

We present the results of our work to determine the average albedo for small, close-in planets in the Kepler candidate catalog. We have adapted our method of averaging short-cadence light curves of multiple Kepler planet candidates to long-cadence data, in order to detect an average albedo for the group of candidates. Long-cadence data exist for many more candidates than the short-cadence data, and so we separate the candidates into smaller radius bins than in our previous work: 1–2 ${R}_{\oplus }$, 2–4 ${R}_{\oplus }$, and 4–6 ${R}_{\oplus }$. We find that, on average, all three groups appear darker than suggested by the short-cadence results, but not as dark as many hot Jupiters. The average geometric albedos for the three groups are 0.11 ± 0.06, 0.05 ± 0.04, and 0.23 ± 0.11, respectively, for the case where heat is uniformly distributed about the planet. If heat redistribution is inefficient, the albedos are even lower, since there will be a greater thermal contribution to the total light from the planet. We confirm that newly identified false-positive Kepler Object of Interest (KOI) 1662.01 is indeed an eclipsing binary at twice the period listed in the planet candidate catalog. We also newly identify planet candidate KOI 4351.01 as an eclipsing binary, and we report a secondary eclipse measurement for Kepler-4b (KOI 7.01) of ~7.50 ppm at a phase of ~0.7, indicating that the planet is on an eccentric orbit.

A Case for an Atmosphere on Super-Earth 55 Cancri e

A Case for an Atmosphere on Super-Earth 55 Cancri e

Authors:

Angelo et al

Abstract:

One of the primary questions when characterizing Earth-sized and super-Earth-sized exoplanets is whether they have a substantial atmosphere like Earth and Venus or a bare-rock surface like Mercury. Phase curves of the planets in thermal emission provide clues to this question, because a substantial atmosphere would transport heat more efficiently than a bare-rock surface. Analyzing phase curve photometric data around secondary eclipse has previously been used to study energy transport in the atmospheres of hot Jupiters. Here we use phase curve, Spitzer time-series photometry to study the thermal emission properties of the super-Earth exoplanet 55 Cancri e. We utilize a semi-analytical framework to fit a physical model to the infrared photometric data at 4.5 micron. The model uses parameters of planetary properties including Bond albedo, heat redistribution efficiency (i.e., ratio between radiative timescale and advective timescale of the atmosphere), and atmospheric greenhouse factor. The phase curve of 55 Cancri e is dominated by thermal emission with an eastward-shifted hot spot. We determine the heat redistribution efficiency to be ~1.47, which implies that the advective timescale is on the same order as the radiative timescale. This requirement cannot be met by the bare-rock planet scenario because heat transport by currents of molten lava would be too slow. The phase curve thus favors the scenario with a substantial atmosphere. Our constraints on the heat redistribution efficiency translate to an atmospheric pressure of ~1.4 bar. The Spitzer 4.5-micron band is thus a window into the deep atmosphere of the planet 55 Cancri e.

Steamworlds: atmospheric structure and critical mass of planets accreting icy pebbles

Steamworlds: atmospheric structure and critical mass of planets accreting icy pebbles

Author:

Chambers

Abstract:

In the core accretion model, gas-giant planets first form a solid core, which then accretes gas from a protoplanetary disk when the core exceeds a critical mass. Here, we model the atmosphere of a core that grows by accreting ice-rich pebbles. The ice fraction of pebbles evaporates in warm regions of the atmosphere, saturating it with water vapor. Excess water precipitates to lower altitudes. Beneath an outer radiative region, the atmosphere is convective, following a moist adiabat in saturated regions due to water condensation and precipitation. Atmospheric mass, density and temperature increase with core mass. For nominal model parameters, planets with core masses (ice + rock) between 0.08 and 0.16 Earth masses have surface temperatures between 273 K and 647 K and form an ocean. In more massive planets, water exists as a super-critical convecting fluid mixed with gas from the disk. Typically, the core mass reaches a maximum (the critical mass) as a function of the total mass when the core is 2-5 Earth masses. The critical mass depends in a complicated way on pebble size, mass flux, and dust opacity due to the occasional appearance of multiple core-mass maxima. The core mass for an atmosphere of 50 percent hydrogen and helium may be a more robust indicator of the onset of gas accretion. This mass is typically 1-3 Earth masses for pebbles that are 50 percent ice by mass, increasing with opacity and pebble flux, and decreasing with pebble ice/rock ratio.

A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at Thirty Parsecs

A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at Thirty Parsecs
Authors: 

Rodriguez et al 

Abstract: 

We report the discovery of three small transiting planets orbiting GJ 9827, a bright (K = 7.2) nearby late K-type dwarf star. GJ 9827 hosts a 1.64+0.22−0.20 R⊕ super Earth on a 1.2 day period, a 1.29+0.17−0.16 R⊕ super Earth on a 3.6 day period, and a 2.08+0.28−0.26 R⊕ super Earth on a 6.2 day period. The radii of the planets transiting GJ 9827 span the transition between predominantly rocky and gaseous planets, and GJ 9827 b and c fall in or close to the known gap in the radius distribution of small planets between these populations. At a distance of ∼30 parsecs, GJ 9827 is the closest exoplanet host discovered by K2 to date, making these planets well-suited for atmospheric studies with the upcoming James Webb Space Telescope. The GJ 9827 system provides a valuable opportunity to characterize interior structure and atmospheric properties of coeval planets spanning the rocky to gaseous transition.

Three Small Super-Earths Transiting the nearby star GJ 9827

Three Small Super-Earths Transiting the nearby star GJ 9827

Authors:

Niraula et al

Abstract:

We report on the discovery of three transiting planets around GJ~9827. The planets have radii of 1.75+0.11−0.12, 1.36+0.09−0.09, and 2.10+0.15−0.15~R⊕, and periods of 1.20896, 3.6480, and 6.2014 days, respectively. The detection was made in Campaign 12 observations as part of our K2 survey of nearby stars. GJ~9827 is a V=10.39~mag K6V star at distance of 30.3 parsecs and the nearest star to be found hosting planets by Kepler and K2. The radial velocity follow-up, high resolution imaging, and detection of multiple transiting objects near commensurability drastically reduce the false positive probability. The orbital periods of GJ~9827~b, c and d planets are very close to the 1:3:5 mean motion resonance. Our preliminary analysis shows that GJ~9827 planets are excellent candidates for atmospheric observations. Besides, the planetary radii span both sides of the rocky and gaseous divide, hence the system will be an asset in expanding our understanding of the threshold.

Rocky exoplanets are expected to be eroded by space weather in a similar way as in the solar system. In particular, Mercury is one of the dramatically eroded planets whose material continuously escapes into its exosphere and further into space. This escape is well traced by sodium atoms scattering sunlight. Due to solar wind impact, micrometeorite impacts, photo-stimulated desorption and thermal desorption, sodium atoms are released from surface regolith. Some of these released sodium atoms are escaping from Mercury's gravitational-sphere. They are dragged anti-Sun-ward and form a tail structure. We expect similar phenomena on exoplanets. The hot super-Earth 61 Vir b orbiting a G3V star at only 0.05 au may show a similar structure. Because of its small separation from the star, the sodium release mechanisms may be working more efficiently on hot super-Earths than on Mercury, although the strong gravitational force of Earth-sized or even more massive planets may be keeping sodium atoms from escaping from the planet. Here, we performed model simulations for Mercury (to verify our model) and 61 Vir b as a representative super-Earth. We have found that sodium atoms can escape from this exoplanet due to stellar wind sputtering and micrometeorite impacts, to form a sodium tail. However, in contrast to Mercury, the tail on this hot super-Earth is strongly aligned with the anti-starward direction because of higher light pressure. Our model suggests that 61 Vir b seems to have an exo-base atmosphere like that of Mercury.

Space Weathering of Super-Earths: Model Simulations of Exospheric Sodium Escape from 61 Virgo b

Authors:

Yoneda et al

Abstract:

Rocky exoplanets are expected to be eroded by space weather in a similar way as in the solar system. In particular, Mercury is one of the dramatically eroded planets whose material continuously escapes into its exosphere and further into space. This escape is well traced by sodium atoms scattering sunlight. Due to solar wind impact, micrometeorite impacts, photo-stimulated desorption and thermal desorption, sodium atoms are released from surface regolith. Some of these released sodium atoms are escaping from Mercury's gravitational-sphere. They are dragged anti-Sun-ward and form a tail structure. We expect similar phenomena on exoplanets. The hot super-Earth 61 Vir b orbiting a G3V star at only 0.05 au may show a similar structure. Because of its small separation from the star, the sodium release mechanisms may be working more efficiently on hot super-Earths than on Mercury, although the strong gravitational force of Earth-sized or even more massive planets may be keeping sodium atoms from escaping from the planet. Here, we performed model simulations for Mercury (to verify our model) and 61 Vir b as a representative super-Earth. We have found that sodium atoms can escape from this exoplanet due to stellar wind sputtering and micrometeorite impacts, to form a sodium tail. However, in contrast to Mercury, the tail on this hot super-Earth is strongly aligned with the anti-starward direction because of higher light pressure. Our model suggests that 61 Vir b seems to have an exo-base atmosphere like that of Mercury.

The discovery and mass measurement of a new ultra-short-period planet: EPIC~228732031b

The discovery and mass measurement of a new ultra-short-period planet: EPIC~228732031b

Authors:

Dai et al

Abstract:
We report the discovery of a new ultra-short-period planet and summarize the properties of all such planets for which the mass and radius have been measured. The new planet, EPIC~228732031b, was discovered in {\it K2} Campaign 10. It has a radius of 1.81+0.16−0.12 R⊕ and orbits a G dwarf with a period of 8.9 hours. Radial velocities obtained with Magellan/PFS and TNG/HARPS-N show evidence for stellar activity along with orbital motion. We determined the planetary mass using two different methods: (1) the "floating chunk offset" method, based only on changes in velocity observed on the same night, and (2) a Gaussian process regression based on both the radial-velocity and photometric time series. The results are consistent and lead to a mass measurement of 6.5±1.6 M⊕, and a mean density of 6.0+3.0−2.7~g~cm−3.

Kepler-30: the Cesar of TTV variations

Kepler-30: the Cesar of TTV variations


Authors:

Panichi et al

Abstract:
We study the orbital architecture, physical character of planets, formation and long-term evolution of the Kepler-30 planetary system, detected and announced in 2012 by the KEPLER team. We show that it belongs to a particular class of very compact and quasi-resonant, yet long-term stable planetary systems. We re-analyzed light-curves of the host star spanning Q1-Q17 quarters. A huge variability of the Transit Timing Variations (TTV) exceeding 2 days is induced by a massive Jovian planet located between two Neptune-like companions. The innermost pair is near to the 2:1 mean motion resonance (MMR), and the outermost pair is close to the 19:8 MMR. Our re-analysis of photometric data allows us to constrain better than before the orbital elements, planets' radii and masses, which are 9.2±0.1, 536±5, and 23.7±1.3 Earth masses for Kepler-30b, Kepler-30c and Kepler-30d, respectively. The masses of the inner planets are determined within ∼1% uncertainty. We inferred the internal structures of the Kepler-30 planets and their bulk densities in a wide range from ∼0.19±0.01 g⋅cm−3 for Kepler-30d, ∼0.96±0.15 g⋅cm−3 for Kepler-30b, to ∼1.71±0.13 g⋅cm−3 for Jovian planet Kepler-30c, respectively. We attempt to explain the origin of this unique planetary system and a deviation of the orbits from exact MMRs by the planetary migration scenario. We anticipate that the Jupiter-mass planet plays an important role in determining the present dynamical state of this system.

Transit Timing and Duration Variations for the Discovery and Characterization of Exoplanets

Transit Timing and Duration Variations for the Discovery and Characterization of Exoplanets

Authors:

Agol et al

Abstract:

Transiting exoplanets in multi-planet systems have non-Keplerian orbits which can cause the times and durations of transits to vary. The theory and observations of transit timing variations (TTV) and transit duration variations (TDV) are reviewed. A watershed since the last review is that the Kepler spacecraft has detected several hundred perturbed planets. In a few cases, these data have been used to discover additional planets, which has historical resonance with how Neptune was discovered in our own Solar System. However, the more impactful aspect of TTV and TDV studies has been characterization of planetary systems in which multiple planets transit. After addressing the equations of motion and parameter scalings, the main dynamical mechanisms for TTV and TDV are described, with citations to the observational literature for real examples. We describe parameter constraints, particularly how the mass/eccentricity degeneracy comes about and is overcome by the high-frequency component of the signal. On the observational side, derivation of timing precision and introduction to the timing diagram are given. Science results are reviewed, with an emphasis on mass measurements of transiting sub-Neptunes and super-Earths, which allows access to the mass-radius diagram and hence inference of bulk compositions.

Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b

Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b

Authors:

Mahapatra et al

Abstract:

Clouds form in the atmospheres of planets where they can determine the observable spectra, the albedo and phase curves. Cloud properties are determined by the local thermodynamical and chemical conditions of an atmospheric gas. A retrieval of gas abundances requires a comprehension of the cloud formation mechanisms under varying chemical conditions. With the aim of studying cloud formation in metal rich atmospheres, we explore the possibility of clouds in evaporating exoplanets like CoRoT-7b and 55 Cnc e in comparison to a generic set of solar abundances and the metal-rich gas giant HD149026b. We assess the impact of metal-rich, non-solar element abundances on the gas-phase chemistry, and apply our kinetic, non-equilibrium cloud formation model to study cloud structures and their details. We provide an overview of global cloud properties in terms of material compositions, maximum particle formation rates, and average cloud particle sizes for various sets of rocky element abundances. Our results suggest that the conditions on 55 Cnc e and HD149026b should allow the formation of mineral clouds in their atmosphere. The high temperatures on some hot-rocky super-Earths (e.g. the day-side of Corot-7b) result in an ionised atmospheric gas and they prevent gas condensation, making cloud formation unlikely on its day-side.

Dynamics and Collisional Evolution of Closely Packed Planetary Systems

Dynamics and Collisional Evolution of Closely Packed Planetary Systems


Authors:

Hwang et al

Abstract:
High-multiplicity Kepler systems (referred to as Kepler multis) are often tightly packed and may be on the verge of instability. Many systems of this type could have experienced past instabilities, where the compact orbits and often low densities make physical collisions likely outcomes. We use numerical simulations to study the dynamical instabilities and planet-planet interactions in a synthetically generated sample of closely-packed, high-multiplicity systems. We focus specifically on systems resembling Kepler-11, a Kepler multi with six planets, and run a suite of dynamical integrations, sampling the initial orbital parameters around the nominal values reported in Lissauer et al. (2011a), finding that most of the realizations are unstable, resulting in orbit crossings and, eventually, collisions and mergers. We study in detail the dependence of stability on the orbital parameters of the planets and planet-pair characteristics to identify possible precursors to instability, compare the systems that emerge from dynamical instabilities to the observed Kepler sample (after applying observational corrections), and propose possible observable signatures of these instabilities. We examine the characteristics of each planet-planet collision, categorizing collisions by the degree of contact and collision energy, and find that grazing collisions are more common than direct impacts. Since the structure of many planets found in Kepler multis is such that the mass is dominated by a rocky core, but the volume is dominated by a low-density gaseous envelope, the sticky-sphere approximation may not be valid, and we present hydrodynamic calculations of planet-planet collisions clearly deviating from this approximation. Finally, we rerun a subset of our dynamical calculations using instead a modified prescription to handle collisions, finding, in general, higher multiplicity remnant systems.

Spin dynamics of close-in planets exhibiting large TTVs

Spin dynamics of close-in planets exhibiting large TTVs

Authors:

Delisle et al

Abstract:
We study the spin evolution of close-in planets in compact multi-planetary systems. The rotation period of these planets is often assumed to be synchronous with the orbital period due to tidal dissipation. Here we show that planet-planet perturbations can drive the spin of these planets into non-synchronous or even chaotic states. In particular, we show that the transit timing variation (TTV) is a very good probe to study the spin dynamics, since both are dominated by the perturbations of the mean longitude of the planet. We apply our model to KOI-227b and Kepler-88b, which are both observed undergoing strong TTVs. We also perform numerical simulations of the spin evolution of these two planets. We show that for KOI-227b non-synchronous rotation is possible, while for Kepler-88b the rotation can be chaotic.

Rocky Terrestrial Exoplanets may only go up to 2 Earth Radius

Hydrodynamic Escape of Planetary Atmospheres during a Star's X-ray and Extreme Ultraviolet Saturation May Impose a Size Limit of ~2 Earth Radii on Rocky Exoplanets

Authors:

Lehmer et al

Abstract:
Recent observations and analysis of small exoplanets have found that rocky planets like the Earth only have radii up to 1.5-2R⊕. Two general hypotheses exist for the cause of the dichotomy between rocky and gas-enveloped planets (or possible water worlds): either small planets do not easily form thick atmospheres, or the thick atmospheres on small planets easily escape driven by x-ray and extreme ultraviolet (XUV) emissions from young parent stars. Here we show that a cutoff between rocky and gas-enveloped planets due to hydrodynamic escape is most likely to occur at a mean radius of 1.76±0.38R⊕ (2σ) Earth radii around Sun-like stars. We examine the limit in rocky planet radii predicted by hydrodynamic escape across a wide range of possible model inputs using 10,000 parameter combinations drawn randomly from plausible parameter ranges. We find a cutoff between rocky and gas-enveloped planets that agrees with the observed cutoff. The large cross-section available for XUV absorption in the extremely distended primitive atmospheres of low mass planets results in complete loss of atmospheres during the ~100 Myr phase of stellar XUV saturation. In contrast, more massive planets have less distended atmospheres and less escape, and so retain thick atmospheres through XUV saturation and then indefinitely as the XUV and escape fluxes drop over time. The agreement between our model and exoplanet data leads us to conclude that hydrodynamic escape plausibly explains the observed upper limit on rocky planet size.