Prospects for spectroscopic reflected-light planet searches

High-resolution spectroscopic searches for the starlight reflected from close-in extrasolar giant planets have the capability of determining the optical albedo spectra and scattering properties of these objects. When combined with radial velocity measurements they also yield the true mass of the planet. To date, only two such planets have been targeted for reflected-light signals, yielding upper limits on the optical albedos of the planets. Here we examine the prospects for future searches of this kind. We present Monte Carlo estimates of prior probability distributions for the orbital velocity amplitudes and planet/star flux ratios of six bright stars known to harbour giant planets in orbits with periods of less than 5 d. Using these estimates, we assess the viability of these targets for future reflected-light searches using 4- and 8-m class telescopes.     

Magnetospheric radio emission from extrasolar giant planets: the role of the host stars

We present a new analysis of the expected magnetospheric radio emission from extrasolar giant planets (EGPs) for a distance limited sample of the nearest known extrasolar planets. Using recent results on the correlation between stellar X-ray flux and mass-loss rates from nearby stars, we estimate the expected mass-loss rates of the host stars of extrasolar planets that lie within 20 pc of the Earth. We find that some of the host stars have mass-loss rates that are more than 100 times that of the Sun and, given the expected dependence of the planetary magnetospheric radio flux on stellar wind properties, this has a very substantial effect. Using these results and extrapolations of the likely magnetic properties of the extrasolar planets, we infer their likely radio properties.
We compile a list of the most promising radio targets and conclude that the planets orbiting Tau Bootes, Gliese 86, Upsilon Andromeda and HD 1237 (as well as HD 179949) are the most promising candidates, with expected flux levels that should be detectable in the near future with upcoming telescope arrays. The expected emission peak from these candidate radio emitting planets is typically ∼40–50 MHz. We also discuss a range of observational considerations for detecting EGPs.     

太阳系外行星的凌星观测研究

为对太阳系外行星的物理参数进行更精确估算,利用山东大学威海天文台/威海市天文台的1 m反射望远镜,对7颗已知具有行星系统的恒星:TrES-1、TrES-3、XO-2、WASP-1、WASP-2、WASP-3、HAT-P-7,进行了凌星现象的观测研究.介绍观测和数据处理的基本情况,给出凌星光变曲线结果及由之推算出的一些行星参数.在总结结果并加以分析的同时,展望下一步将进行的更为深入细致的研究.     

Transits of extrasolar planets

In this paper, we consider the physical properties and characteristic features of extrasolar planets and planetary systems, those, for which the passage of low-orbit giant planets across the stellar disk (transits) are observed. The paper is mostly a review. The peculiarities of the search for transits are briefly considered. The main attention in this paper is given to the difference in the physical properties of low-orbit giant planets. A comparison of the data obtained during the transits of “hot Jupiters” points to the probable existence of several distinct subtypes of low-orbit extrasolar planets. “Hot Jupiters” of low density (HD 209458b), “hot Jupiters” with massive cores composed of heavy elements (HD 149026b), and “very hot Jupiters” (HD 189733b) are bodies that probably fall into different categories of exoplanets. Dissipation of the atmospheres of low-orbit giant planets estimated from the experimental data is compared with the calculated Jeans atmospheric losses. For “hot Jupiters”, the expected Jeans mass losses due to atmospheric escape on a cosmogonic time scale hardly exceed a few percent. Low-orbit giant planets should have a strong magnetic field. Since the orbital velocity of “hot Jupiters” is close to the magnetosonic velocity (or can even exceed it), the moving planet should actively interact with the “stellar wind” plasma. The possession of a magnetic field by extrasolar planets and the effects of their interaction with plasma can be used to search for extrasolar planets.     

Extrasolar low-orbit planets: Dissipation of their atmospheres and probable magnetic field

Analysis of the data obtained during transits of low-orbit extrasolar planets across the stellar disk yields different estimates of their atmospheric loss rates. Experimental data point to the probable existence of several distinct subtypes of extrasolar giant planets, including “hot Jupiters” of low density (HD 209458b), with massive cores composed of heavy elements (HD 149026b), and others. We show that the expected hot-Jupiter mass losses due to atmospheric escape on a cosmogonic time scale do not exceed a few percent, while the losses through Jeans dissipation are negligible. We also argue that low-orbit giant planets should have a strong magnetic field that interacts with circumstellar plasma with the planet’s supersonic orbital velocity. The magnetic field properties can be used to search for extrasolar planets.     

Simulations for multi-object spectrograph planet surveys

Radial velocity surveys for extrasolar planets generally require substantial amounts of large telescope time in order to monitor a sufficient number of stars. Two of the aspects which can limit such surveys are the single-object capabilities of the spectrograph, and an inefficient observing strategy for a given observing window. In addition, the detection rate of extrasolar planets using the radial velocity method has thus far been relatively linear with time. With the development of various multi-object Doppler survey instruments, there is growing potential to dramatically increase the detection rate using the Doppler method. Several of these instruments have already begun usage in large-scale surveys for extrasolar planets, such as Fibre Large Array Multi Element Spectrograph (FLAMES) on the Very Large Telescope (VLT) and Keck Exoplanet Tracker (ET) on the Sloan 2.5-m wide-field telescope.
In order to plan an effective observing strategy for such a program, one must examine the expected results based on a given observing window and target selection. We present simulations of the expected results from a generic multi-object survey based on calculated noise models and sensitivity for the instrument and the known distribution of exoplanetary system parameters. We have developed code for automatically sifting and fitting the planet candidates produced by the survey to allow for fast follow-up observations to be conducted. The techniques presented here may be applied to a wide range of multi-object planet surveys.     

Giant Metrewave Radio Telescope low-frequency observations of extrasolar planetary systems

Extrasolar planets are expected to emit detectable low-frequency radio emission. In this paper, we present results from new low-frequency observations of two extrasolar planetary systems (Epsilon Eridani and HD 128311) taken at 150 MHz with the Giant Metrewave Radio Telescope (GMRT). These two systems have been chosen because the stars are young (with ages <1 Gyr) and are likely to have strong stellar winds, which will increase the expected radio flux. The planets are massive (presumably) gas giant planets in longer period orbits, and hence will not be tidally locked to their host star (as is likely to be the case for short-period planets) and we would expect them to have a strong planetary dynamo and magnetic field. We do not detect either system, but are able to place tight upper limits on their low-frequency radio emission, at levels comparable to the theoretical predictions for these systems. From these observations, we have a 2.5σ limit of 7.8 mJy for ε Eri and 15.5 mJy for HD 128311. In addition, these upper limits also provide limits on the low-frequency radio emission from the stars themselves. These results are discussed and also the prospects for the future detection of radio emission from extrasolar planets.     

Are stars with planets anomalous?

The chemical-dynamical properties of stars with giant planets are compared to those of a nearby star sample within the framework of a stellar orbital diffusion model. The stars-with-planets sample includes recently discovered extrasolar planets and the Sun. We find that the planet-bearing stars, 14 Her, ρ ¹ Cnc and τ Boo, are much more metal-rich than stars of similar age and this cannot be easily explained by orbital diffusion. We also confirm previous claims that the motion of the Sun relative to the local standard of rest is very small compared to other G dwarfs of similar age, and we offer a possible explanation for this apparent anomaly.     

Obliquity variations of terrestrial planets in habitable zones

We have investigated obliquity variations of possible terrestrial planets in habitable zones (HZs) perturbed by a giant planet(s) in extrasolar planetary systems. All the extrasolar planets so far discovered are inferred to be jovian-type gas giants. However, terrestrial planets could also exist in extrasolar planetary systems. In order for life, in particular for land-based life, to evolve and survive on a possible terrestrial planet in an HZ, small obliquity variations of the planet may be required in addition to its orbital stability, because large obliquity variations would cause significant climate change. It is known that large obliquity variations are caused by spin-orbit resonances where the precession frequency of the planet's spin nearly coincides with one of the precession frequencies of the ascending node of the planet's orbit. Using analytical expressions, we evaluated the obliquity variations of terrestrial planets with prograde spins in HZs. We found that the obliquity of terrestrial planets suffers large variations when the giant planet's orbit is separated by several Hill radii from an edge of the HZ, in which the orbits of the terrestrial planets in the HZ are marginally stable. Applying these results to the known extrasolar planetary systems, we found that about half of these systems can have terrestrial planets with small obliquity variations (smaller than 10°) over their entire HZs. However, the systems with both small obliquity variations and stable orbits in their HZs are only 1/5 of known systems. Most such systems are comprised of short-period giant planets. If additional planets are found in the known planetary systems, they generally tend to enhance the obliquity variations. On the other hand, if a large/close satellite exists, it significantly enhances the precession rate of the spin axis of a terrestrial planet and is likely to reduce the obliquity variations of the planet. Moreover, if a terrestrial planet is in a retrograde spin state, the spin-orbit resonance does not occur. Retrograde spin, or a large/close satellite might be essential for land-based life to survive on a terrestrial planet in an HZ.     

Are extrasolar oceans common throughout the Galaxy?

Light and cold extrasolar planets such as OGLE 2005‐BLG‐390Lb, a 5.5 Earth masses planet detected via microlensing, could be frequent in the Galaxy according to some preliminary results from microlensing experiments. These planets can be frozen rocky‐ or ocean‐planet, situated beyond the snow line and, therefore, beyond the habitable zone of their system. They can nonetheless host a layer of liquid water, heated by radiogenic energy, underneath an ice shell surface for billions of years, before freezing completely. These results suggest that oceans under ice, like those suspected to be present on icy moons in the Solar system, could be a common feature of cold low‐mass extrasolar planets. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Structure of Jupiter, Saturn, and Exoplanets: Key Questions for High-Pressure Experiments

We give an overview of our current understanding of the structure of gas giant planets, from Jupiter and Saturn to extrasolar giant planets. We focus on addressing what high-pressure laboratory experiments on hydrogen and helium can help to elucidate about the structure of these planets.     

The role of planetary formation and evolution in shaping the composition of exoplanetary atmospheres

Over the last twenty years, the search for extrasolar planets has revealed the rich diversity of outcomes from the formation and evolution of planetary systems. In order to fully understand how these extrasolar planets came to be, however, the orbital and physical data we possess are not enough, and they need to be complemented with information about the composition of the exoplanets. Ground-based and space-based observations provided the first data on the atmospheric composition of a few extrasolar planets, but a larger and more detailed sample is required before we can fully take advantage of it. The primary goal of a dedicated space mission like the Exoplanet Characterization Observatory (EChO) proposal is to fill this gap and to expand the limited data we possess by performing a systematic survey of extrasolar planets. The full exploitation of the data that space-based and ground-based facilities will provide in the near future, however, requires knowledge about the sources and sinks of the chemical species and molecules that will be observed. Luckily, the study of the past history of the Solar System provides several indications about the effects of processes like migration, late accretion and secular impacts, and on the time they occur in the life of planetary systems. In this work we will review what is already known about the factors influencing the composition of planetary atmospheres, focusing on the case of gaseous giant planets, and what instead still need to be investigated.     

Formation and evolution of planetary systems: the impact of high-angular resolution optical techniques

The direct images of giant extrasolar planets recently obtained around several main sequence stars represent a major step in the study of planetary systems. These high-dynamic range images are among the most striking results obtained by the current generation of high-angular resolution instruments which will be superseded by a new generation of instruments in the coming years. It is, therefore, an appropriate time to review the contributions of high-angular resolution visible/infrared techniques to the rapidly growing field of extrasolar planetary science. During the last 20 years, the advent of the Hubble Space Telescope, of adaptive optics on 4- to 10-m class ground-based telescopes, and of long-baseline infrared stellar interferometry, has opened a new viewpoint on the formation and evolution of planetary systems. By spatially resolving the optically thick circumstellar discs of gas and dust where planets are forming, these instruments have considerably improved our models of early circumstellar environments and have thereby provided new constraints on planet formation theories. High-angular resolution techniques are also directly tracing the mechanisms governing the early evolution of planetary embryos and the dispersal of optically thick material around young stars. Finally, mature planetary systems are being studied with an unprecedented accuracy thanks to single-pupil imaging and interferometry, precisely locating dust populations and putting into light a whole new family of long-period giant extrasolar planets.     

Ionization fraction in the thermosphere of the exoplanet HD 209458b

Dissociation and ionization of hydrogen molecules and ionization of hydrogen atoms due to extreme UV radiation from the parent star are accompanied by the formation of a concurrent photoelectron flux with excess kinetic energy. These dissociation and ionization processes are the main source of atomic and molecular ions in the thermospheres of extrasolar planets, such as the “hot Jupiter” HD 209458b. The ionization processes are the most important part of contemporary aeronomic models of planetary atmospheres in the Solar System and extrasolar systems (Johnson et al., 2008; Yelle et al., 2008). We estimate the contribution of the dissociation and ionization processes due to the stellar UV radiation and the concurrent photoelectron flux to the formation of extended ionospheres around extrasolar giant planets. As opposed to models of other researchers, we calculated the ionization rates due to the concurrent photo-electron flux for the first time. It is established that, in contrast to a widely used parametrization of the photoelectron contribution (Cecchi-Pestellini et al., 2006; 2009), the rate of secondary ionization due to the photoelectrons depends appreciably on the altitude, approaching the photoionization rate in the lower layers of the thermosphere. The calculated ionization rate in the thermosphere of the extrasolar giant planet (EGP) orbiting close to its parent star is a necessary link when modeling an aeronomic model and estimating the rate of the EGP atmospheric loss.     

The long-term stability of extrasolar system HD 37124. Numerical study of resonance effects

We describe numerical tools for the stability analysis of extrasolar planetary systems. In particular, we consider the relative Poincaré variables and symplectic integration of the equations of motion. We apply the tangent map to derive a numerically efficient algorithm of the fast indicator Mean Exponential Growth factor of Nearby Orbits (MEGNO), a measure of the maximal Lyapunov exponent, that helps to distinguish chaotic and regular configurations. The results concerning the three-planet extrasolar system HD 37124 are presented and discussed. The best-fitting solutions found in earlier works are studied more closely. The system involves Jovian planets with similar masses. The orbits have moderate eccentricities, nevertheless the best-fitting solutions are found in dynamically active region of the phase space. The long-term stability of the system is determined by a net of low-order two-body and three-body mean motion resonances. In particular, the three-body resonances may induce strong chaos that leads to self-destruction of the system after Myr of apparently stable and bounded evolution. In such a case, numerically efficient dynamical maps are useful to resolve the fine structure of the phase space and to identify the sources of unstable behaviour.     

Extrasolar planets and the rotation and axisymmetric mass-loss of evolved stars

I examine the implications of the recently found extrasolar planets on the planet-induced axisymmetric mass-loss model for the formation of elliptical planetary nebulae (PNe). This model attributes the low departure from spherical mass-loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of orbital angular momentum of the planets. Since about half of all PNe are elliptical, i.e., have low equatorial to polar density contrast, it was predicted that about 50 per cent of all Sun-like stars have Jupiter-like planets around them, i.e., a mass about equal to that of Jupiter, M _J, or more massive. In the light of the new findings that only 5 per cent of Sun-like stars have such planets, and a newly proposed mechanism for axisymmetric mass-loss, the cool magnetic spots model, I revise this prediction. I predict that indeed ∼50 per cent of PN progenitors do have close planets around them, but the planets can have much lower masses, as low as ∼0.01 M _J, in order to spin-up the envelopes of AGB stars efficiently. To support this claim, I follow the angular momentum evolution of single stars with main-sequence mass in the range of 1.3–2.4 M_⊙ , as they evolve to the post-AGB phase. I find that single stars rotate much too slowly to possess any significant non-spherical mass-loss as they reach the upper AGB. It seems, therefore, that planets, in some cases even Earth-like planets, are sufficient to spin-up the envelope of these AGB stars for them to form elliptical PNe. The prediction that on average several such planets orbit each star, as in the Solar system, still holds.     

Pixel lensing as a way to detect extrasolar planets in M31

We study the possibility to detect extrasolar planets in M31 through pixel-lensing observations. Using a Monte Carlo approach, we select the physical parameters of the binary lens system, a star hosting a planet, and we calculate the pixel-lensing light curve taking into account the finite source effects. Indeed, their inclusion is crucial since the sources in M31 microlensing events are mainly giant stars. Light curves with detectable planetary features are selected by looking for significant deviations from the corresponding Paczyński shapes. We find that the time-scale of planetary deviations in light curves increase (up to 3–4 d) as the source size increases. This means that only few exposures per day, depending also on the required accuracy, may be sufficient to reveal in the light curve a planetary companion. Although the mean planet mass for the selected events is about     , even small mass planets  ( M _P < 20 M_⊕)  can cause significant deviations, at least in the observations with large telescopes. However, even in the former case, the probability to find detectable planetary features in pixel-lensing light curves is at most a few per cent of the detectable events, and therefore many events have to be collected in order to detect an extrasolar planet in M31. Our analysis also supports the claim that the anomaly found in the candidate event PA-99-N2 towards M31 can be explained by a companion object orbiting the lens star.     

Orbital Migration Of Giant Planets: Using Numerical Integration To Investigate Consequences For Other Bodies

A number of extrasolar planets have been detected in close orbits around nearby stars. It is probable that these planets did not form in these orbits but migrated from their formation locations beyond the ice line. Orbital migration mechanisms involving angular momentum transfer through tidal interactions between the planets and circumstellar gas-dust disks or by gravitational interaction with a residual planetesimal disk together with several means of halting inward migration have been identified. These offer plausible schemes to explain the orbits of observed extrasolar giant planets and giant planets within the Solar System. Recent advances in numerical integration methods and in the power of computer workstations have allowed these techniques to be applied to modelling directly the mechanisms and consequences of orbital migration in the Solar System. There is now potential for these techniques also to be applied to modelling the consequences of the orbital migration of planets in the observed exoplanetary systems. In particular the detailed investigation of the stability of terrestrial planets in the habitable zone of these systems and the formation of terrestrial planets after the dissipation of the gas disk is now possible. The stability of terrestrial planets in the habitable zone of selected exoplanetary systems has been established and the possibility of the accretion of terrestrial planets in these systems is being investigated by the author in collaboration with Barrie W. Jones (Open University), and with John Chambers (NASA-Ames) and Mark Bailey of Armagh Observatory, using numerical integration. The direct simulation of orbital migration by planetesimal scattering must probably await faster hardware and/or more efficient algorithms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Radio wavelength transients: Current and emerging prospects

Known classes of radio wavelength transients range from the nearby stellar flares and radio pulsars to the distant Universe γ‐ray burst afterglows. Hypothesized classes of radio transients include analogs of known objects, e.g., extrasolar planets emitting Jovian‐like radio bursts and giant‐pulse emitting pulsars in other galaxies, to the exotic, prompt emission from γ‐ray bursts, evaporating black holes, and transmitters from other civilizations. A number of instruments and facilities are either under construction or in early observational stages and are slated to become available in the next few years. With a combination of wide fields of view and wavelength agility, the detection and study of radio transients will improve immensely. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)