Earth-like Extrasolar Planets Research Articles

Relevance of Earth-Bound Extremophiles in the Search for Extraterrestrial Life.

The recent discovery of extrasolar Earth-like planets that orbit in their habitable zone of their system, and the latest clues of the presence of liquid water in the subsurface of Mars and in the subglacial ocean of Jupiter's and Saturn's moons, has reopened debates about habitability and limits of life. Although liquid water, widely accepted as an absolute requirement for terrestrial life, may be present in other bodies of the solar system or elsewhere, physical and chemical conditions, such as temperature, pressure, and salinity, may limit this habitability. However, extremophilic microorganisms found in various extreme terrestrial environments are adapted to thrive in permanently extreme ranges of physicochemical conditions. This review first describes promising environments for life in the Solar System and the microorganisms that inhabit similar environments on the Earth. The effects of extreme temperatures, salt, and hydrostatic pressure conditions on biomolecules will be explained in some detail, and recent advances in understanding biophysical and structural adaptation strategies allowing microorganisms to cope with extreme physicochemical conditions are reviewed to discuss promising environments for life in the Solar System in terms of habitability.

Concept of an achromatic stellar coronagraph and its application for detecting extrasolar planets

Imaging the planets that orbit around other stars requires blocking the host star which is usually 8 to 10 orders of magnitude brighter than the planets. This is achieved with the help of a stellar coronagraph. In the current work, a concept of a new type of stellar coronagraph is introduced where the star light is blocked by a linear polarizer in the collimated beam. It is based on differential rotation between the linear polarization state of planet light and that of star light. This is achieved with the help of a set of thick birefringent crystals in the collimated beam of a telescope where the planet light is made to travel extra optical path length compared to star light. By adjusting the orientation and thickness of the crystal, the optical path length can be made to cause a phase difference of π, just enough to rotate the initial plane of polarization by 90 deg for planet-light without affecting the star light. Theoretical calculations involving the phase difference due to birefringent crystals are presented along with the basic configuration and design. It is shown that the design blocks the star light identically at all wavelengths. Application of this concept for detecting Earth-like extrasolar planet is discussed using a 1-m class telescope.

Tracking sub-nanometer thermal structural changes with speckle interferometry.

Based on our results of the James Webb Space Telescope (JWST) center-of-curvature tests where we were able to measure dynamic amplitudes of Zernike terms to the order of a few picometers, we have applied the same approach to determine if it is possible to measure the accuracy of higher-order Zernike terms as a function of time rather than frequency, i.e., static measurements in place of measuring the amplitude of frequency components. We have applied this approach to data taken for the JWST backplane structure test article (BSTA) in 2006 and find that we can measure effects at the sub-nanometer level, as small as 50 pm for Zernike terms over 30. We conclude that these results show it will be possible to use these techniques to ensure that the optics and support structure for large space telescopes can meet the necessary stability requirements for detecting spectral signatures of life on Earth-like extra-solar planets.

Abiotic O2 Levels on Planets around F, G, K, and M Stars: Effects of Lightning-produced Catalysts in Eliminating Oxygen False Positives

Abstract Over the last few years, a number of authors have suggested that, under certain circumstances, molecular oxygen (O2) or ozone (O3) generated by abiotic processes may accumulate to detectable concentrations in a habitable terrestrial planet’s atmosphere, producing so-called “false positives” for life. But the models have occasionally disagreed with each other, with some predicting false positives, and some not, for the same apparent set of circumstances. We show here that photochemical false positives derive either from inconsistencies in the treatment of atmospheric and global redox balance or from the treatment (or lack thereof) of lightning. For habitable terrestrial planets with even trace amounts of atmospheric N2, NO produced by lightning catalyzes the recombination of CO and O derived from CO2 photolysis and should be sufficient to eliminate all reported false positives. Molecular oxygen thus remains a useful biosignature gas for Earth-like extrasolar planets, provided that the planet resides within the conventional liquid water habitable zone and has not experienced distinctly non-Earth-like, irrecoverable water loss.

Studies of the Core Conditions of the Earth and Super-Earths Using Intense Ion Beams at FAIR

Abstract Using detailed numerical simulations, we present the design of an experiment that will generate samples of iron under extreme conditions of density and pressure believed to exist in the interior of the Earth and interior of extrasolar Earth-like planets. In the proposed experiment design, an intense uranium beam is used to implode a multilayered cylindrical target that consists of a thin Fe cylinder enclosed in a thick massive W shell. Such intense uranium beams will be available at the heavy-ion synchrotron, SIS100, at the Facility for Antiprotons and Ion Research (FAIR), at Darmstadt, which is under construction and will become operational in the next few years. It is expected that the beam intensity will increase gradually over a couple of years to its maximum design value. Therefore, in our studies, we have considered a wide range of beam parameters, from the initial beam intensity (“Day One”) to the maximum specified value. It is also worth noting that two different focal spot geometries have been used. In one case, a circular focal spot with a Gaussian transverse intensity distribution is considered, whereas in the other case, an annular focal spot is used. With these two beam geometries, one can access different parts of the Fe phase diagram. For example, heating the sample with a circular focal spot generates a hot liquid state, while an annular focal spot can produce a highly compressed liquid or a highly compressed solid phase depending on the beam intensity.

Galactic cosmic rays on extrasolar Earth-like planets

(abridged abstract) Theoretical arguments indicate that close-in terrestial exoplanets may have weak magnetic fields. As described in the companion article (Paper I), a weak magnetic field results in a high flux of galactic cosmic rays to the top of the planetary atmosphere. We investigate effects that may result from a high flux of galactic cosmic rays both throughout the atmosphere and at the planetary surface. Using an air shower approach, we calculate how the atmospheric chemistry and temperature change under the influence of galactic cosmic rays for Earth-like (N_2-O_2 dominated) atmospheres. We evaluate the production and destruction rate of atmospheric biosignature molecules. We derive planetary emission and transmission spectra to study the influence of galactic cosmic rays on biosignature detectability. We then calculate the resulting surface UV flux, the surface particle flux, and the associated equivalent biological dose rates. We find that up to 20% of stratospheric ozone is destroyed by cosmic-ray protons. The reduction of the planetary ozone layer leads to an increase in the weighted surface UV flux by two orders of magnitude under stellar UV flare conditions. The resulting biological effective dose rate is, however, too low to strongly affect surface life. We also examine the surface particle flux: For a planet with a terrestrial atmosphere, a reduction of the magnetic shielding efficiency can increase the biological radiation dose rate by a factor of two. For a planet with a weaker atmosphere (with a surface pressure of 97.8 hPa), the planetary magnetic field has a much stronger influence on the biological radiation dose, changing it by up to two orders of magnitude.

Galactic cosmic rays on extrasolar Earth-like planets

(abridged abstract) Theoretical arguments indicate that close-in terrestial exoplanets may have weak magnetic fields, especially in the case of planets more massive than Earth (super-Earths). Planetary magnetic fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the planetary atmosphere. We wish to quantify the flux of Galactic cosmic rays to an exoplanetary atmosphere as a function of the particle energy and of the planetary magnetic moment. We numerically analyzed the propagation of Galactic cosmic-ray particles through planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV $\le$ E $\le$ 524 GeV) and as a function of the planetary magnetic field strength (in the range 0 ${M}_\oplus$ $\le$ {M} $\le$ 10 ${M}_\oplus$). Combined with the flux outside the planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the planetary atmosphere as a function of the planetary magnetic moment. We find that the particle flux to the planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting magnetic field. For a weakly magnetized planet (${M}=0.05\,{M}_{\oplus}$), only particles with energies below 512 MeV are at least partially shielded. For a planet with a magnetic moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly magnetized planet ($M=10.0\,{M}_{\oplus}$), partial shielding extends up to 200 GeV. We find that magnetic shielding strongly controls the number of cosmic-ray particles reaching the planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.

3D climate modeling of Earth-like extrasolar planets orbiting different types of host stars

The potential habitability of a terrestrial planet is usually defined by the possible existence of liquid water on its surface, since life as we know it needs liquid water at least during a part of its life cycle. The potential presence of liquid water on a planetary surface depends on many factors such as, most importantly, surface temperatures. The properties of the planetary atmosphere and its interaction with the radiative energy provided by the planet's host star are thereby of decisive importance.In this study we investigate the influence of different main-sequence stars (F, G, and K-type stars) upon the climate of Earth-like extrasolar planets and their potential habitability by applying a state-of-the-art three-dimensional (3D) Earth climate model accounting for local and dynamical processes. The calculations have been performed for planets with Earth-like atmospheres at orbital distances (and corresponding orbital periods) where the total amount of energy received from the various host stars equals the solar constant. In contrast to previous 3D modeling studies, we include the effect of ozone radiative heating upon the vertical temperature structure of the atmospheres. The global orbital mean results obtained have been compared to those of a one-dimensional (1D) radiative convective climate model to investigate the approximation of global mean 3D results by those of 1D models.The different stellar spectral energy distributions lead to different surface temperatures and due to ozone heating to very different vertical temperature structures. As previous 1D studies we find higher surface temperatures for the Earth-like planet around the K-type star, and lower temperatures for the planet around the F-type star compared to an Earth-like planet around the Sun. However, this effect is more pronounced in the 3D model results than in the 1D model because the 3D model accounts for feedback processes such as the ice-albedo and the water vapor feedback. Whether the 1D model may approximate the global mean of the 3D model results strongly depends on the choice of the relative humidity profile in the 1D model, which is used to determine the water vapor profile. Hence, possible changes in the hydrological cycle need to be accounted for when estimating the potential habitability of an extrasolar planet.

MINERAL ECOLOGY: CHANCE AND NECESSITY IN THE MINERAL DIVERSITY OF TERRESTRIAL PLANETS

Four factors contribute to the roles played by chance and necessity in determining mineral distribution and diversity at or near the surfaces of terrestrial planets: (1) crystal chemical characteristics; (2) mineral stability ranges; (3) the probability of occurrence for rare minerals; and (4) stellar and planetary stoichiometries in extrasolar systems. The most abundant elements generally have the largest numbers of mineral species, as modeled by relationships for Earth9s upper continental crust (E) and the Moon (M), respectively: Log ( N E ) = 0.22 Log ( C E ) + 1.70 ( R 2 = 0.34 ) ( 4861 minerals , 72 elements ) Log ( N M ) = 0.19 Log ( C M ) + 0.23 ( R 2 = 0.68 ) ( 63 minerals , 24 elements ) , where C is an element9s abundance in ppm and N is the number of mineral species in which that element is essential. Several elements that plot significantly below the trend for Earth9s upper continental crust ( e.g. , Ga, Hf, and Rb) mimic other more abundant elements and thus are less likely to form their own species. Other elements ( e.g ., Ag, As, Cu, Pb, S, and U) plot significantly above the trend, which we attribute to their unique crystal chemical affinities, multiple coordination and oxidation states, their extreme concentration in some ore-forming fluids, and/or frequent occurrence with a variety of other rare elements—all factors that increase the diversity of mineral species incorporating these elements. The corresponding diagram for the Moon shows a tighter fit, most likely because none of these elements, except Cu and S, are essential constituents in lunar minerals. Given the similar slopes for Earth and the Moon, we suggest that the increase in mineral diversity with element abundance is a deterministic aspect of planetary mineral diversity. Though based on a limited number of collecting sites, the Moon9s observed mineralogical diversity could be close to the minimum for a rocky planet or moon comparable in size—a baseline against which diversity of other terrestrial planets and moons having radii in the same range as Earth and its Moon can be measured. Mineral-forming processes on the Moon are limited to igneous activity, meteor impacts, and the solar wind—processes that could affect any planet or moon. By contrast, other terrestrial planets and moons have been subjected to more varied physical, chemical, and (in the case of Earth) biological processes that can increase mineral diversity in both deterministic and stochastic ways. Total mineral diversity for different elements is not appreciably influenced by the relative stabilities of individual phases, e.g. , the broad pressure-temperature-composition stability ranges of cinnabar (HgS) and zircon (ZrSiO 4 ) do not significantly diminish the diversity of Hg or Zr minerals. Moreover, the significant expansion of near-surface redox conditions on Earth through the evolution of microbial oxygenic photosynthesis tripled the available composition space of Earth9s near-surface environment, and resulted in a corresponding tripling of mineral diversity subsequent to atmospheric oxidation. Of 4933 approved mineral species, 34% are known from only one or two localities, and more than half are known from five or fewer localities. Statistical analysis of this frequency distribution suggests that thousands of other plausible rare mineral species await discovery or could have occurred at some point in Earth9s history, only to be subsequently lost by burial, erosion, or subduction— i.e. , much of Earth9s mineral diversity associated with rare species results from stochastic processes. Measurements of stellar stoichiometry reveal that stars can differ significantly from the Sun in relative abundances of rock-forming elements, which implies that bulk compositions of some extrasolar Earth-like planets likely differ significantly from those of Earth, particularly if the fractionation processes in evolving stellar nebulas and planetary differentiation are factored in. Comparison of Earth9s upper continental crust and the Moon shows that differences in element ratios are reflected in ratios of mineral species containing these elements. In summary, although deterministic factors control the distribution of the most common rock-forming minerals in Earth9s upper continental crust and on the Moon, stochastic processes play a significant role in the diversity of less common minerals. Were Earth9s history to be replayed, and thousands of mineral species discovered and characterized anew, it is probable that many of those minerals would differ from species known today.

The optical communication link outage probability in satellite formation flying

In recent years, several space systems consisting of multiple satellites flying in close formation have been proposed for various purposes such as interferometric synthetic aperture radar measurement (TerraSAR-X and the TanDEM-X), detecting extra-solar earth-like planets (Terrestrial Planet Finder-TPF and Darwin), and demonstrating distributed space systems (DARPA F6 project). Another important purpose, which is the concern of this paper, is for improving radio frequency communication to mobile terrestrial and maritime subscribers. In this case, radio frequency signals from several satellites coherently combine such that the received/transmit signal strength is increased proportionally with the number of satellites in the formation. This increase in signal strength allows to enhance the communication data rate and/or to reduce energy consumption and the antenna size of terrestrial mobile users' equipment. However, a coherent combination of signals without aligning the phases of the individual communication signals interrupts the communication and outage link between the satellites and the user. The accuracy of the phase estimation is a function of the inter-satellite laser ranging system performance. This paper derives an outage probability model of a coherent combination communication system as a function of the pointing vibration and jitter statistics of an inter-satellite laser ranging system tool. The coherent combination probability model, which could be used to improve the communication to mobile subscribers in air, sea and ground is the main importance of this work.

Searching for Faint Exozodiacal Disks: Keck Results and LBTI Status

AbstractThe possible presence of dust in the habitable zone around nearby main-sequence stars is considered as a major hurdle toward the direct imaging of Earth-like extrasolar planets with future dedicated space-based telescopes (e.g., Roberge et al. 2012). In this context, NASA has funded two ground-based mid-infrared nulling interferometers to combine the large apertures available at the Keck Observatory and the Large Binocular Telescope (LBT). In this poster, we present the preliminary results of the extended survey carried out with the Keck Interferometer Nuller (KIN) between 2008 and 2011 and describe the forthcoming LBTI survey.

Infrared radiative transfer in atmospheres of Earth-like planets around F, G, K, and M stars

Context. The atmosphere of Earth-like extrasolar planets orbiting different types of stars is influenced by the spectral dependence of the incoming stellar radiation. The changes in structure and composition affect atmospheric radiation, hence the spectral appearance of these exoplanets.Aims. We provide a thorough investigation of infrared radiative transfer in cloud-free exoplanets atmospheres by not only analyzing the planetary spectral appearance but also discussing the radiative processes behind the spectral features in detail and identifying the regions in the atmosphere that contribute most at a given wavelength. Methods. Using cloud-free scenarios provided by a one-dimensional radiative-convective steady-state atmospheric model, we computed high-resolution infrared transmission and emission spectra, as well as weighting functions for exoplanets located within the habitable zone of F, G, K, and M stars by means of a line-by-line molecular absorption model and a Schwarzschild solver for the radiative transfer. The monochromatic spectra were convolved with appropriate spectral response functions to study the effects of finite instrument resolution.Results. Spectra of the exoplanets of F, G, K, and M stars were analyzed in the 4.5 μ m N2 O band, the 4.3 μ m and 15 μ m CO2 bands, the 7.7 μ m CH4 band, the 6.3 μ m H2 O band, and the 9.6 μ m O3 band. Differences in the state of the atmosphere of the exoplanets clearly show up in the thermal infrared spectra; absorption signatures known from Earth can be transformed to emission features (and vice versa). Weighting functions show that radiation in the absorption bands of the uniformly mixed gases (CO2 , CH4 , N2 O) and (to some extent) ozone comes from the stratosphere and upper troposphere, and also indicate that changes in the atmospheres can shift sources of thermal radiation to lower or higher altitudes. Molecular absorption and/or emission features can be identified in the high-resolution spectra of all planets and in most reduced resolution spectra. Conclusions. Insight into radiative transfer processes is essential for analyzing exoplanet spectral observations; for instance, understanding the impact of the temperature profile (nb. non-existence of an inversion) on the CO2 bands facilitates their interpretation and can help avoid false positive or negative estimates of O3 . The detailed analysis of the radiation source and sink regions could even help give an indication about the feasibility of identifying molecular signatures in cloud-covered planets, i.e. radiation mainly coming from the upper atmosphere is less likely to be hidden by clouds. Infrared radiative transfer and biomarker detectability in cloud-covered exoplanets will be presented in a companion paper.

GLANCING VIEWS OF THE EARTH: FROM A LUNAR ECLIPSE TO AN EXOPLANETARY TRANSIT

It has been posited that lunar eclipse observations may help predict the in-transit signature of Earth-like extrasolar planets. However, a comparative analysis of the two phenomena addressing in detail the transport of stellar light through the planet's atmosphere has not yet been presented. Here, we proceed with the investigation of both phenomena by making use of a common formulation. Our starting point is a set of previously unpublished near-infrared spectra collected at various phases during the August 2008 lunar eclipse. We then take the formulation to the limit of an infinitely distant observer in order to investigate the in-transit signature of the Earth-Sun system as being observed from outside our Solar System. The refraction-bending of sunlight rays that pass through the Earth's atmosphere is a critical factor in the illumination of the eclipsed Moon. Likewise, refraction will have an impact on the in-transit transmission spectrum for specific planet-star systems depending on the refractive properties of the planet's atmosphere, the stellar size and the planet's orbital distance. For the Earth-Sun system, at mid-transit, refraction prevents the remote observer's access to the lower ~12-14 km of the atmosphere and, thus, also to the bulk of the spectroscopically-active atmospheric gases. We demonstrate that the effective optical radius of the Earth in transit is modulated by refraction and varies by ~12 km from mid-transit to 2nd contact. The refractive nature of atmospheres, a property which is rarely accounted for in published investigations, will pose additional challenges to the characterization of Earth-like extrasolar planets. Refraction may have a lesser impact for Earth-like extrasolar planets within the habitable zone of some M-type stars.

ENHANCED INTERFEROMETRIC IDENTIFICATION OF SPECTRA IN HABITABLE EXTRASOLAR PLANETS

An Earth-like extrasolar planet emits light that is many orders of magnitude fainter than that of the parent star. We propose a method of identifying bio-signature spectral lines in light of known extrasolar planets based on Fourier spectroscopy in the infrared, using an off-center part of a Fourier interferogram only. This results in superior sensitivity to narrower molecular-type spectral bands, which are expected in the planet spectrum but are absent in the parent star. We support this idea by numerical simulations that include photon and thermal noise, and show it to be feasible at a luminosity ratio of 10?6 for a Sun-like parent star in the infrared. We also carried out a laboratory experiment to illustrate the method. The results suggest that this method should be applicable to real planet searches.

MODELING THE INFRARED SPECTRUM OF THE EARTH-MOON SYSTEM: IMPLICATIONS FOR THE DETECTION AND CHARACTERIZATION OF EARTHLIKE EXTRASOLAR PLANETS AND THEIR MOONLIKE COMPANIONS

Large surface temperatures on the illuminated hemisphere of the Moon can lead it to contribute a significant amount of flux to spatially unresolved infrared (IR) observations of the Earth-Moon system, especially at wavelengths where Earth's atmosphere is absorbing. We have paired the NASA Astrobiology Institute's Virtual Planetary Laboratory three-dimensional spectral Earth model with a model of the phase dependent IR spectrum of a Moonlike satellite to investigate the effects of an unresolved companion on IR observations of Earthlike extrasolar planets. For an extrasolar twin Earth-Moon system observed at full phase at IR wavelengths, the Moon consistently comprises about 20% of the total signal, approaches 30% of the signal in the 9.6 micron ozone band and the 15 micron carbon dioxide band, makes up as much as 80% of the total signal in the 6.3 micron water band, and more than 90% of the signal in the 4.3 micron carbon dioxide band. These excesses translate to inferred brightness temperatures for Earth that are too large by about 20-40 K, and demonstrate that the presence of an undetected satellite can have a significant impact on the spectroscopic characterization of terrestrial exoplanets. The thermal flux contribution from an airless companion depends strongly on the star-planet-observer angle (i.e., the phase angle), allowing moons to mimic or mask seasonal variations in the host planet's IR spectrum, and implying that observations of exoplanets should be taken when the phase angle is as small as feasibly possible if contributions from airless companions are to be minimized. We show that, by differencing IR observations of an Earth twin with a companion taken at both gibbous phase and at crescent phase, Moonlike satellites may be detectable by future exoplanet characterization missions for a wide range of system inclinations.

Clouds in the atmospheres of extrasolar planets

We study the influence of low-level water and high-level ice clouds on low-resolution reflection spectra and planetary albedos of Earth-like planets orbiting different types of stars in both the visible and near infrared wavelength range. We use a one-dimensional radiative-convective steady-state atmospheric model coupled with a parametric cloud model, based on observations in the Earth's atmosphere to study the effect of both cloud types on the reflection spectra and albedos of Earth-like extrasolar planets at low resolution for various types of central stars. We find that the high scattering efficiency of clouds substantially causes both the amount of reflected light and the related depths of the absorption bands to be substantially larger than in comparison to the respective clear sky conditions. Low-level clouds have a stronger impact on the spectra than the high-level clouds because of their much larger scattering optical depth. The detectability of molecular features in near the UV - near IR wavelength range is strongly enhanced by the presence of clouds. However, the detectability of various chemical species in low-resolution reflection spectra depends strongly on the spectral energy distribution of the incident stellar radiation. In contrast to the reflection spectra the spectral planetary albedos enable molecular features to be detected without a direct influence of the spectral energy distribution of the stellar radiation. Here, clouds increase the contrast between the radiation fluxes of the planets and the respective central star by about one order of magnitude, but the resulting contrast values are still too low to be observable with the current generation of telescopes.

Speckle level suppression using an unbalanced nulling interferometer in a high-contrast imaging system

High-contrast imaging systems with a stellar halo suppression level of 10(-10) are required for direct detection of Earth-like extra-solar planets. We investigated a novel high-contrast imaging system with an unbalanced nulling interferometer (UNI) followed by phase and amplitude correction (PAC), which not only can reduce starlight but also can suppress the speckle level caused by wavefront aberrations. We successfully demonstrated that wavefront aberrations were sufficiently magnified by the UNI and the magnified aberrations were effectively corrected in amplitude and phase with two deformable mirrors. We confirmed that the suppression level of the speckle pattern with the proposed optics was beyond the limit of the adaptive optics performance.

TIDALLY INDUCED THERMAL RUNAWAYS ON EXTRASOLAR EARTHS: IMPACT ON HABITABILITY

We study the susceptibility of extrasolar Earth-like planets to tidal dissipation by varying orbital, rheological, and heat transfer parameters. We employ a three-dimensional numerical method solving the coupled problem of mantle convection and tidal dissipation. A reference model mimicking a plate tectonic regime and reproducing Earth's present-day heat output is considered. Four other models representing less efficient heat transfer regimes are also investigated. For these five initial models, we determine the orbital configurations under which a positive feedback between tidal dissipation and temperature evolution leads to a thermal runaway. In order to describe the occurrence of thermal runaways, we develop a scaling that relates the global dissipated power to a characteristic temperature and to the orbital parameters. For all numerical experiments sharing the same initial temperature conditions, we show that the reciprocal value of the runaway timescale depends linearly on the global dissipated power at the beginning of the simulation. In the plate tectonic-like regime, Earth-like planets in the habitable zone (HZ) of 0.1 M☉ stars experience thermal runaways for 1:1 spin–orbit resonance if the eccentricity is sufficiently high (e>0.02 at a 4 day period, e>0.2 at a 10 day period). For less efficient convective regimes, runaways are obtained for eccentricities as low as ∼0.004 at the inner limit of the HZ. In the case of 3:2 spin–orbit resonance, the occurrence of thermal runaways is independent of eccentricity and is predicted for orbital periods lower than 12 days. For less efficient convective regimes, runaways may occur at larger orbital periods potentially affecting the HZ of stars with a mass up to 0.4 M☉. Whatever the convective regime and spin–orbit resonance, tidal heating within Earth-like planets orbiting in the HZ of stars more massive than 0.5 M☉ is not significant.

The Earth as an extrasolar transiting planet

An important goal within the quest for detecting an Earth-like extrasolar planet, will be to identify atmospheric gaseous bio-signatures. Observations of the light transmitted through the Earth's atmosphere, as for an extrasolar planet, will be the first step for future comparisons. We have completed observations of the Earth during a Lunar eclipse, a unique situation similar to that of a transiting planet. We aim at showing what species could be detected in its atmosphere at optical wavelengths, where a lot of photons are available in the masked stellar light. We present observations of the 2008 August 16 Moon eclipse performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Locating the spectrograph fibers in the penumbra of the eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and solar light that has passed through the Earth's limb. This mixture essentially reproduces what is recorded during the transit of an extrasolar planet. We report here the clear detection of several Earth atmospheric compounds in the transmission spectra, such as ozone, molecular oxygen, and neutral sodium as well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover, we present a method that allows us to derive the thickness of the atmosphere versus the wavelength for penumbra eclipse observations. We quantitatively evaluate the altitude at which the atmosphere becomes transparent for important species like molecular oxygen and ozone, two species thought to be tightly linked to the presence of life. The molecular detections presented here are an encouraging first attempt, necessary to better prepare for the future of extremely-large telescopes and transiting Earth-like planets. Instruments like SOPHIE will be mandatory when characterizing the atmospheres of transiting Earth-like planets from the ground and searching for bio-marker signatures.

A Scientometric Prediction of the Discovery of the First Potentially Habitable Planet with a Mass Similar to Earth

BackgroundThe search for a habitable extrasolar planet has long interested scientists, but only recently have the tools become available to search for such planets. In the past decades, the number of known extrasolar planets has ballooned into the hundreds, and with it, the expectation that the discovery of the first Earth-like extrasolar planet is not far off.Methodology/Principal FindingsHere, we develop a novel metric of habitability for discovered planets and use this to arrive at a prediction for when the first habitable planet will be discovered. Using a bootstrap analysis of currently discovered exoplanets, we predict the discovery of the first Earth-like planet to be announced in the first half of 2011, with the likeliest date being early May 2011.Conclusions/SignificanceOur predictions, using only the properties of previously discovered exoplanets, accord well with external estimates for the discovery of the first potentially habitable extrasolar planet and highlight the the usefulness of predictive scientometric techniques to understand the pace of scientific discovery in many fields.