Rory Barnes

Observational Evidence for Tidal Destruction of Exoplanets

The distribution of the orbits of close-in exoplanets shows evidence for ongoing removal and destruction by tides. Tides raised on a planets host star cause the planets orbit to decay, even after the orbital eccentricity has dropped to zero. Comparison of the observed orbital distribution and predictions of tidal theory shows good qualitative agreement, suggesting tidal destruction of close-in exoplanets is common. The process can explain the observed cutoff in small semimajor axis values, the clustering of orbital periods near three days, and the relative youth of transiting planets. Contrary to previous considerations, a mechanism to stop the inward migration of close-in planets at their current orbits is not necessarily required. Planets nearing tidal destruction may be found with extremely small semimajor axes, possibly already stripped of any gaseous envelope. The recently discovered CoroT-7 b may be an example of such a planet and will probably be destroyed by tides within the next few Gyrs. Also, where one or more planets have already been accreted, a star may exhibit an unusual composition and/or spin rate.

Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs

We show that terrestrial planets in the habitable zones of M dwarfs older than ∼1 Gyr could have been in runaway greenhouses for several hundred million years following their formation due to the stars extended pre-main sequence phase, provided they form with abundant surface water. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. During this early runaway phase, photolysis of water vapor and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. We find that the amount of water lost scales with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. In addition to undergoing potential desiccation, planets with inefficient oxygen sinks at the surface may build up hundreds to thousands of bar of abiotically produced O2, resulting in potential false positives for life. The amount of O2 that builds up also scales with the planet mass; we find that O2 builds up at a constant rate that is controlled by diffusion: ∼5 bar/Myr on Earth-mass planets and up to ∼25 bar/Myr on super-Earths. As a result, some recently discovered super-Earths in the habitable zone such as GJ 667Cc could have built up as many as 2000 bar of O2 due to the loss of up to 10 Earth oceans of water. The fate of a given planet strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. In general, we find that the initial phase of high luminosity may compromise the habitability of many terrestrial planets orbiting low-mass stars.

Tidal obliquity evolution of potentially habitable planets

Context. Stellar insolation has been used as the main constraint on a planet’s potential habitability. However, as more Earth-like planets are discovered around low-mass stars (LMSs), a re-examination of the role of tides on the habitability of exoplanets has begun. Those studies have yet to consider the misalignment between a planet’s rotational axis and the orbital plane normal, i.e. the planetary obliquity. Aims. This paper considers the constraints on habitability arising from tidal processes due to the planet’s spin orientation and rate. Since tidal processes are far from being understood we seek to understand differences between commonly used tidal models. Methods. We apply two equilibrium tide theories – a constant-phase-lag model and a constant-time-lag model – to compute the obliquity evolution of terrestrial planets orbiting in the habitable zones around LMSs. The time for the obliquity to decrease from an Earth-like obliquity of 23.5 ◦ to 5 ◦ , the “tilt erosion time”, is compared to the traditional insolation habitable zone (IHZ) in the parameter space spanned by the semi-major axis a, the eccentricity e, and the stellar mass Ms. We also compute tidal heating and equilibrium rotation caused by obliquity tides as further constraints on habitability. The Super-Earth Gl581 d and the planet candidate Gl581 g are studied as examples for these tidal processes.

The extreme physical properties of the CoRoT-7b super-Earth

Photospheric stellar activity (i.e. dark spots or bright pl ages) might be an important source of noise and confusion in s tellar radialvelocity (RV) measurements. Radial-velocimetry planet se arch surveys as well as follow-up of photometric transit sur veys require a deeper understanding and characterization of the e ffects of stellar activities to di fferentiate them from planetary signals. We simulate dark spots on a rotating stellar photosphere. The variation s in the photometry, RV, and spectral line shapes are charact erized and analyzed according to the stellar inclination, the latitud e, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requi s a good sampling to be resolved when there are several spot s on the photosphere. The Lomb-Scargle periodograms of the RV varia tions induced by activity present power at the rotational pe riod Prot of the star and its two first harmonics Prot/2 andProt/3. Three adjusted sinusoids fixed at the fundamental period a nd its two-first harmonics allow us to remove about 90% of the RV jitter amplit ude. We apply and validate our approach on four known active p lanethost stars: HD 189733, GJ 674, CoRoT-7, and ιHor. We succeed in fitting simultaneously activity and plane t ry signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoR oT-7b and c of respectively, 5.7± 2.5 MEarth and 13.1± 4.1 MEarth. The larger uncertainties properly take into account the st ellar active jitter. We exclude short-period low-mass exoplanets around ιHor. For data with realistic time-sampling and white Gaussi an noise, we use simulations to show that our approach is e ffective in distinguishing reflex-motion due to a planetary co mpanion and stellar-activityinduced RV variations provided that 1) the planetary orbita l period is not close to that of the stellar rotation or one of i ts two first harmonics, 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal, 3) the rotati nal period of the star is accurately known, and 4) the data cover more than o ne stellar rotational period.

PREDICTING PLANETS IN KNOWN EXTRASOLAR PLANETARY SYSTEMS. I. TEST PARTICLE SIMULATIONS

Recent work has suggested that many planetary systems lie near instability. If all systems are near instability, then at least one additional planet must exist in stable regions of well-separated extrasolar planetary systems to push these systems to the edge of stability. We examine the known systems by placing massless test particles in between the planets and integrating for 1–10 million yr. We find that some systems, HD 168443 and HD 74156, eject nearly all test particles within 2 million yr. However, we find that HD 37124, HD 38529, and 55 Cnc have large contiguous regions in which particles survive for 10 million yr. These three systems, therefore, seem the most likely candidates for additional companions. Furthermore, HD 74156 and HD 168443 must be complete; therefore radial velocity surveys should only focus on detecting more distant companions. We also find that several systems show stable regions that only exist at nonzero eccentricities. Subject headingg celestial mechanics — methods: numerical — planetary systems

THE (IN)STABILITY OF PLANETARY SYSTEMS

We present results of numerical simulations that examine the dynamical stability of known planetary systems, a star with two or more planets. First we vary the initial conditions of each system on the basis of observational data. We then determine regions of phase space that produce stable planetary configurations. For each system we perform 1000 ~ 106 yr integrations. We examine υ And, HD 83443, GJ 876, HD 82943, 47 UMa, HD 168443, and the solar system. We find that the resonant systems, two planets in a first-order mean motion resonance (HD 82943 and GJ 876) have very narrow zones of stability. The interacting systems, not in first-order resonance, but able to perturb each other (υ And, 47 UMa, and the solar system), have broad stable regions. The separated systems, two planets beyond 10 : 1 resonance (we examine only HD 83443 and HD 168443) are fully stable. We find that the best fits to the interacting and resonant systems place them very close to unstable regions. The boundary in phase space between stability and instability depends strongly on the eccentricities and (if applicable) the proximity of the system to perfect resonance. Furthermore, we also find that the longitudes of periastron circulate in chaotic systems but librate in regular systems. In addition to 106 yr integrations, we also examined stability on ~108 yr timescales. For each system we ran ~10 long-term simulations, and find that the Keplerian fits to these systems all contain configurations that are regular on this timescale.

TIDAL LIMITS TO PLANETARY HABITABILITY

The habitable zones (HZs) of main-sequence stars have traditionally been defined as the range of orbits that intercept the appropriate amount of stellar flux to permit surface water on a planet. Terrestrial exoplanets discovered to orbit M stars in these zones, which are close-in due to decreased stellar luminosity, may also undergo significant tidal heating. Tidal heating may span a wide range for terrestrial exoplanets and may significantly affect conditions near the surface. For example, if heating rates on an exoplanet are near or greater than that on Io (where tides drive volcanism that resurfaces the planet at least every 1 Myr) and produce similar surface conditions, then the development of life seems unlikely. On the other hand, if the tidal heating rate is less than the minimum to initiate plate tectonics, then CO2 may not be recycled through subduction, leading to a runaway greenhouse that sterilizes the planet. These two cases represent potential boundaries to habitability and are presented along with the range of the traditional HZ for main-sequence, low-mass stars. We propose a revised HZ that incorporates both stellar insolation and tidal heating. We apply these criteria to GJ 581 d and find that it is in the traditional HZ, but its tidal heating alone may be insufficient for plate tectonics.

The roles of tidal evolution and evaporative mass loss in the origin of CoRoT-7 b

CoRoT-7 b is the first confirmed rocky exoplanet, but, with an orbital semimajor axis of 0.0172 au, its origins may be unlike any rocky planet in our Solar system. In this study, we consider the roles of tidal evolution and evaporative mass loss in CoRoT-7 bs history, which together have modified the planets mass and orbit. If CoRoT-7 b has always been a rocky body, evaporation may have driven off almost half its original mass, but the mass loss may depend sensitively on the extent of tidal decay of its orbit. As tides caused CoRoT-7 bs orbit to decay, they brought the planet closer to its host star, thereby enhancing the mass loss rate. Such a large mass loss also suggests the possibility that CoRoT-7 b began as a gas giant planet and had its original atmosphere completely evaporated. In this case, we find that CoRoT-7 bs original mass probably did not exceed 200 Earth masses (about two-third of a Jupiter mass). Tides raised on the host star by the planet may have significantly reduced the orbital semimajor axis, perhaps causing the planet to migrate through mean-motion resonances with the other planet in the system, CoRoT-7 c. The coupling between tidal evolution and mass loss may be important not only for CoRoT-7 b but also for other close-in exoplanets, and future studies of mass loss and orbital evolution may provide insight into the origin and fate of close-in planets, both rocky and gaseous.

Predicting Planets in Known Extrasolar Planetary Systems. III. Forming Terrestrial Planets

Recent results have shown that many of the known extrasolar planetary systems contain regions that are stable for both Earth-mass and Saturn-mass planets. Here we simulate the formation of terrestrial planets in four planetary systems, 55 Cancri, HD 38529, HD 37124, and HD 74156, under the assumption that these systems of giant planets are complete and that their orbits are well determined. Assuming that the giant planets formed and migrated quickly, terrestrial planets may form from a second generation of planetesimals. In each case, Moon- to Mars-sized planetary embryos are placed in between the giant planets and evolved for 100 Myr. We find that planets form relatively easily in 55 Cnc, with masses up to 0.6 M⊕ and, in some cases, substantial water content and orbits in the habitable zone. HD 38529 is likely to support an asteroid belt, but no terrestrial planets of significant mass. No terrestrial planets form in HD 37124 and HD 74156, although in some cases 1-2 lone embryos survive for 100 Myr. If migration occurred later, depleting the planetesimal disk, then massive terrestrial planets are unlikely to form in any of these systems.

Formation, Habitability, and Detection of Extrasolar Moons

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoons stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.