arXiv subjects

Amri Wandel

Publications and source records attributed to Amri Wandel.

At least 19 recordsLinked to original sources

arXiv2025

Exoplanets beyond the Conservative Habitable Zone: I. Habitability

The Habitable Zone (HZ) is defined by the possibility of sustaining liquid water on a planetary surface. In the Solar System, the HZ for a conservative climate model extends approximately between the orbits of Earth and Mars. We elaborate on earlier HZ models and apply an analytical climate model of the temperature distribution on tidally-locked planets to extend the HZ. We show that planets orbiting M- and K-dwarf stars may maintain liquid water on their night side, significantly closer to their host star than the inner border of the conservative HZ. We calculate the extended borders of the HZ in the flux-effective temperature diagram. This extension may explain the presence of water vapor and other volatile gases in the transmission spectra of warm Super-Earth-sized exoplanets closely orbiting M dwarfs, recently detected by JWST. We also mention the HZ extension outwards, due to subglacial liquid water in the form of intra-glacial lakes or subglacial melting.

astro-ph.EP↗

arXiv2025

Exoplanets beyond the Conservative Habitable Zone: II. Occurrence

We demonstrate that the extension of the Habitable Zone (HZ) due to the presence of liquid water on the night side of tidally locked planets, modelled in this and earlier works, significantly increases the number of potentially habitable planets. We calculate the occurrence of habitable planets orbiting M-, K-, and G-dwarf stars within the conservative and extended HZ, beyond the inner and outer boundaries of the conservative HZ. Integrating over the phase space in the HZ diagram and normalizing our calculation to relatively recent analyses of the Kepler data, we show that potentially habitable planets may be as much as 50 times more abundant than in the lower estimate, limited to the conservative HZ of G-type stars only. For an intermediate heat transport rate on tidally locked planets, we find that the extended HZ could imply more than one habitable planet per star, and hundreds of habitable planets within 10 pc from Earth.

astro-ph.EP↗

arXiv2024

Potential Interior Structures and Habitability of Super-Earth Exoplanets LHS 1140 b, K2-18 b, TOI-1452 b and TOI-1468 c

We analyze four super-Earth exoplanets, LHS 1140 b, K2-18 b, TOI-1452 b, and TOI-1468 c, which orbit M-dwarf stars in the habitable zone. Their relative proximity, within 40 parsecs, makes them prime candidates for follow-up observations and atmospheric and habitability studies. This paper aims to assess their internal structure and habitability, considering their tidal heating, atmospheric heating, and global transport. We model the interior structure of the planets by applying Bayesian inference to an exoplanet's interior model. A constant quality factor model is used to calculate the range of tidal heating, and a one-dimensional analytical model of tidally locked planets is used to assess their surface temperature distribution and habitability. Assuming no or only thin atmospheres, K2-18 b and TOI-1468 c are likely to be water worlds. However, TOI-1452 b and LHS 1140 b may have rocky surfaces. We find that tidal heating is not enough to raise the global mean surface temperature, but greenhouse heating can effectively do so. If the considered planets have retained thick atmospheres, K2-18 b, TOI-1468 c, and TOI-1452 b may, for significant atmospheric heating and heat transport factors, be too hot to sustain liquid water on their surface. However, the lower instellation of LHS 1140 b and the non-zero probability of it having a rocky surface give more space for habitable conditions on the planet.

astro-ph.EP↗

arXiv2023

Extended habitability of exoplanets due to subglacial water

Considering subglacial liquid water, a significant extension of the classical Habitable Zone is obtained. Elaborating on the model of Wandel (2023) it is shown how an atmosphere and liquid water could survive on tidally locked planets closely orbiting an M-dwarf host, extending the Habitable Zone boundary inwards. In addition, subglacial liquid water could extend the Habitable Zone beyond the outer boundary of the conservative Habitable Zone as well. These two results enhance the circumstellar region with a potential for liquid water well beyond the conservative boundaries of the classical Habitable Zone. It is argued that the probable recent JWST detection of atmospheric water vapor on the rocky Earth-sized exoplanet GJ 486 b, along with earlier detections of water on other planets orbiting M-dwarf stars gives an empirical answer to the much-argued question, of whether such planets can support liquid water, organic chemistry and eventually life. It is shown how water on terrestrial planets closely orbiting M-dwarf stars may sustain in a subglacial melting layer. Finally, the model is applied to a few exoplanets demonstrating how water detection may constrain their atmospheric properties.

astro-ph.EP↗

arXiv2023

Habitability and sub glacial liquid water on planets of M-dwarf stars

A long-standing issue in astrobiology is whether planets orbiting the most abundant type of stars, M-dwarfs, can support liquid water and eventually life. A new study shows that subglacial melting may provide an answer, significantly extending the habitability region, in particular around M-dwarf stars, which are also the most promising for biosignature detection with the present and near-future technology.

astro-ph.EP↗

arXiv2022

The Fermi Paradox revisited: Technosignatures and the Contact Era

A new solution to the Fermi Paradox is presented: probes or visits from putative alien civilizations have a very low probability until a civilization reaches a certain age (called the Contact Era) after the onset of radio communications. If biotic planets are common, putative advanced civilizations may preferentially send probes to planets with technosignatures, such as radio broadcastings. The contact probability is defined as the chance to find a nearby civilization located close enough so that it could have detected the earliest radio emissions (the radiosphere) and sent a probe that would reach the Solar System at present. It is found that the current contact probability for Earth is very low unless civilizations are extremely abundant. Since the radiosphere expands with time, so does the contact probability. The Contact Era is defined as the time (since the onset of radio transmissions) at which the contact probability becomes of order unity. At that time alien probes (or messages) become more likely. Unless civilizations are highly abundant, the Contact Era is shown to be of the order of a few hundred to a few thousand years and may be applied not only to physical probes but also to transmissions (i.e. SETI). Consequently, it is shown that civilizations are unlikely to be able to inter-communicate unless their communicative lifetime is at least a few thousand years.

physics.pop-ph↗

arXiv2019

Will recent advances in AI result in a paradigm shift in Astrobiology and SETI?

The steady advances in computer performance and in programming raise the concern that the ability of computers would overtake that of the human brain, an occurrence termed "the Singularity". While comparing the size of the human brain and the advance in computer capacity, the Singularity has been estimated to occur within a few decades although the capacity of conventional computers may reach its limits in the near future. However, in the last few years, there have been rapid advances in Artificial Intelligence (AI). There are already programs that carry out pattern recognition and self-learning which, at least in limited fields such as chess and other games, are superior to the best human players. Furthermore, the quantum computing revolution, which is expected to vastly increase computer capacities, is already on our doorstep. It now seems inevitable that the Singularity will arrive within the foreseeable future. Biological life, on Earth and on extraterrestrial planets and their satellites, may continue as before, but humanity could be 'replaced' by computers. Older and more advanced intelligent life forms, possibly evolved elsewhere in the universe, may have passed their Singularity a long time ago. Post Singularity life would probably be based not on biochemical reactions but on electronics. Their communication may use effects such as quantum entanglement and be undetectable to us. This may explain the Fermi paradox or at least the "Big Silence" problem in SETI.

physics.pop-ph↗

arXiv2019

On the Habitability of Teegarden's Star planets

We study the habitability of the two 1-2 Earth-mass planets, recently detected by the CARMENES collaboration, around the ultra-cool nearby M dwarf Teegarden's Star. With orbital periods of 4.9 and 11.4 days, both planets are likely to be within the Habitable Zone and tidally locked. They are among the most Earth-like exoplanets yet discovered. Applying an analytical habitability model we find that surface liquid water could be present on both planets for a wide range of atmospheric properties, which makes them attractive targets for bio-signature searches. The prospects of the planets retaining such an atmosphere over their history are discussed.

astro-ph.EP↗

arXiv2019

The Bio-habitable Zone and atmospheric properties for Planets of Red Dwarfs

The Kepler data show that habitable small planets orbiting Red Dwarf stars (RDs) are abundant, and hence might be promising targets to look at for biomarkers and life. Planets orbiting within the Habitable Zone of RDs are close enough to be tidally locked. Some recent works have cast doubt on the ability of planets orbiting RDs to support life. In contrast, it is shown that temperatures suitable for liquid water and even for organic molecules may exist on tidally locked planets of RDs for a wide range of atmospheres. We chart the surface temperature distribution as a function of the irradiation, greenhouse factor and heat circulation. The habitability boundaries and their dependence on the atmospheric properties are derived. Extending our previous analyses of tidally locked planets, we find that tidally locked as well as synchronous (not completely locked) planets of RDs and K-type stars may support life, for a wider range of orbital distance and atmospheric conditions than previously thought. In particular, it is argued that life clement environments may be possible on tidally locked and synchronously orbiting planets of RDs and K-type stars, with conditions supporting Oxygenic Photosynthesis, which on Earth was a key to Complex life. Different climate projections and the biological significance of tidal locking on putative complex life are reviewed. We show that when the effect of continuous radiation is taken into account, the Photo-synthetically Active Radiation (PAR) available on tidally locked planets, even of RDs, could produce a high Potential Plant Productivity, in analogy to mid-summer growth at high latitudes on Earth. Awaiting the findings of TESS and JWST, we discuss the implications of the above arguments to the detection of biomarkers such as liquid water and oxygen, as well as to the abundance of biotic planets and life.

astro-ph.EP↗

arXiv2018

On the bio-habitability of M-dwarf planets

The recent detection of Earth-sized planets in the habitable zone of Proxima Centauri, Trappist-1 and many other nearby M-type stars has led to speculations, whether liquid water and life actually exist on these planets. To a large extent, the answer depends on their yet unknown atmospheres, which may though be within observational reach in the near future by JWST, ELT and other planned telescopes. We consider the habitability of planets of M-type stars in the context of their atmospheric properties, heat transport and irradiation. Instead of the traditional definition of the habitable zone, we define the bio-habitable zone, where liquid water and complex organic molecules can survive on at least part of the planetary surface. The atmospheric impact on the temperature is quantified in terms of the heating factor (a combination of greenhouse heating, stellar irradiation, albedo etc.) and heat redistribution (horizontal energy transport). We investigate the bio-habitable domain (where planets can support surface liquid water and organics) in terms of these two factors. Our results suggest that planets orbiting M-type stars may have life-supporting temperatures, at least on part of their surface, for a wide range of atmospheric properties. We apply this analyses to Proxima b and the Trappist-1 system. Finally we discuss the implications to the search of biosignatures and demonstrate how they may be used to estimate the abundance of photosynthesis and biotic planets.

astro-ph.EP↗

arXiv2016

How far are Extraterrestrial Life and Intelligence after Kepler ?

The Kepler mission has shown that a significant fraction of all stars may have an Earth-size habitable planet. A dramatic support was the recent detection of Proxima Centauri b. Using a Drake-equation like formalism I derive an equation for the abundance of biotic planets as a function of the relatively modest uncertainty in the astronomical data and of the (yet unknown) probability for the evolution of biotic life, Fb. I suggest that Fb may be estimated by future spectral observations of exoplanet biomarkers. It follows that if Fb is not very small, then a biotic planet may be expected within about 10 light years from Earth. Extending this analyses to advanced life, I derive expressions for the distance to putative civilizations in terms of two additional Drake parameters - the probability for evolution of a civilization, Fc, and its average longevity. Assuming "optimistic" values for the Drake parameters, (Fb~Fc~1), and a broadcasting duration of a few thousand years, the likely distance to the nearest civilizations detectable by SETI is of the order of a few thousand light years. Finally I calculate the distance and probability of detecting intelligent signals with present and future radio telescopes such as Arecibo and SKA and how it could constrain the Drake parameters.

astro-ph.EP↗

arXiv2015

The Potential of Planets Orbiting Red Dwarf Stars to Support Oxygenic Photosynthesis and Complex Life

We review the latest findings on extra-solar planets and their potential to support Earth-like life. Focusing on planets orbiting Red Dwarf (RD) stars, the most abundant stellar type, we show that including RDs as potential host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbor by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing exoplanet habitability. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones (where liquid water could exist) would be so close as to make planets tidally locked to their star. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative factors are less severe than originally thought. It has been argued that the lesser photon energy of the radiation of the relatively cool RDs would not suffice for Oxygenic Photosynthesis (OP) and other related energy expending reactions. Numerous authors suggest that OP on RD planets may evolve to utilize photons in the infrared. We however argue, by analogy to the evolution of OP and the environmental physiology and distribution of land-based vegetation on Earth, that the evolutionary pressure to utilize infrared radiation would be small. This is because vegetation on RD planets could enjoy continuous illumination of moderate intensity, containing a significant component of photosynthetic 400-700 nm radiation. We conclude that conditions for OP could exist on RD planets and consequently the evolution of complex life might be possible. Furthermore, the huge number and the long lifetime of RDs make it more likely to find planets with photosynthesis and life around RDs than around solar type stars.

astro-ph.EP↗

arXiv2014

On the abundance of extraterrestrial life after the Kepler mission

The data recently accumulated by the Kepler mission have demonstrated that small planets are quite common and that a significant fraction of all stars may have an Earth-like planet within their Habitable Zone. These results are combined with a Drake-equation formalism to derive the space density of biotic planets as a function of the relatively modest uncertainty in the astronomical data and of the (yet unknown) probability for the evolution of biotic life, Fb. I suggest that Fb may be estimated by future spectral observations of exoplanet biomarkers. If Fb is in the range 0.001 -- 1 then a biotic planet may be expected within 10 -- 100 light years from Earth. Extending the biotic results to advanced life I derive expressions for the distance to putative civilizations in terms of two additional Drake parameters - the probability for evolution of a civilization, Fc, and its average longevity. For instance, assuming optimistic probability values (Fb Fc 1) and a broadcasting longevity of a few thousand years, the likely distance to the nearest civilizations detectable by SETI is of the order of a few thousand light years. The probability of detecting intelligent signals with present and future radio telescopes is calculated as a function of the Drake parameters. Finally, I describe how the detection of intelligent signals would constrain the Drake parameters.

astro-ph.EP↗

arXiv2011

On the BH-galaxy relation of AGN and Narrow Line Seyfert 1 Galaxies

Massive black holes (BHs) are detected in the centers of many nearby galaxies are linearly correlated with the luminosity of the host bulge (spheroid), the black hole mass being about 0.1% of the stellar mass. In active galaxies, the BH mass is best measured by the reverberation mapping (light echo) technique. We and others have shown that in AGNs the BH mass follows the same relation with the luminosity of the host galaxy as in ordinary (inactive) galaxies, with the exception of narrow line AGNs which apparently have significantly lower values of the BH/bulge mass (or BH/bulge luminosity) ratio. The BH/bulge ratio is also found to be strongly correlated with the velocity dispersion of the broad line-emitting gas in the active nucleus. However, in the BH-stellar velocity relation the difference between broad- and narrow-line AGNs (in particular NLS1s) seems to be less obvious. We review the subject adding recent updates and suggestions.

astro-ph.CO↗

arXiv2007

The Black Hole Mass of NGC 4151: Comparison of Reverberation Mapping and Stellar Dynamical Measurements

We present a stellar dynamical estimate of the black hole (BH) mass in the Seyfert 1 galaxy, NGC 4151. We analyze ground-based spectroscopy as well as imaging data from the ground and space, and we construct 3-integral axisymmetric models in order to constrain the BH mass and mass-to-light ratio. The dynamical models depend on the assumed inclination of the kinematic symmetry axis of the stellar bulge. In the case where the bulge is assumed to be viewed edge-on, the kinematical data give only an upper limit to the mass of the BH of ~4e7 M_sun (1 sigma). If the bulge kinematic axis is assumed to have the same inclination as the symmetry axis of the large-scale galaxy disk (i.e., 23 degrees relative to the line of sight), a best-fit dynamical mass between 4-5e7 M_sun is obtained. However, because of the poor quality of the fit when the bulge is assumed to be inclined (as determined by the noisiness of the chi^2 surface and its minimum value), and because we lack spectroscopic data that clearly resolves the BH sphere of influence, we consider our measurements to be tentative estimates of the dynamical BH mass. With this preliminary result, NGC 4151 is now among the small sample of galaxies in which the BH mass has been constrained from two independent techniques, and the mass values we find for both bulge inclinations are in reasonable agreement with the recent estimate from reverberation mapping (4.57[+0.57/-0.47]e7 M_sun) published by Bentz et al.

astro-ph↗

arXiv2004

Supermassive Black Holes in Active Galactic Nuclei. II. Calibration of the M-sigma Relationship for AGNs

We calibrate reverberation-based black hole masses in active galactic nuclei (AGNs) by using the correlation between black hole mass, M, and bulge/spheroid stellar velocity dispersion, sigma. We use new measurements of sigma for 6 AGNs and published velocity dispersions for 10 others, in conjunction with improved reverberation mapping results, to determine the scaling factor required to bring reverberation-based black hole masses into agreement with the quiescent galaxy M-sigma relationship. The scatter in the AGN black hole masses is found to be less than a factor of 3. The current observational uncertainties preclude use of the scaling factor to discriminate between broad-line region models.

astro-ph↗

arXiv2003

Relations between Massive Black Holes in AGN and their Host Galaxies

Massive black holes detected in the centers of many nearby galaxies are linearly correlated with the luminosity of the host bulge, the black hole mass being about 0.1% of the bulge mass. An even stronger relation exists between the BH mass (Mbh) and the stellar velocity dispersion in the host bulge. We show that massive BHs of AGNs (measured by reverberation mapping) and their bulge luminosity (measured by using a bulge/disk decomposition) follow the same relations as ordinary (inactive) galaxies, with the exeption of narrow line AGN which apparently have significantly lower BH/bulge ratios. Narrow line AGNs seem to be outstanding also in the Mbh-velocity dispersion relation: the few Seyfert galaxies with measured velocity dispersion seem to indicate that narrow line Seyfert 1 galaxies have a smaller BH mass/velocity dispersion ratio than quiescent galaxies and broad line Syferts. Estimating the velocity dispersion for from the bulge luminosity with the Faber-Jackson relation more than doubles the sample and supports these results.

astro-ph↗

arXiv2001

Black Holes and Hosts of Active and Quiescent Galaxies: I. The Black Hole-Bulge Relation revisited

Massive Black Holes detected in the centers of many nearby galaxies show an approximately linear relation with the luminosity of the host bulge, with the black hole mass being 0.001-0.002 of the bulge mass. Previous work suggested that black holes of active (Seyfert 1) galaxies follow a similar relation, but apparently with a significantly lower value of $M_{\rm BH}/M_{\rm bulge}$ (Wandel 1999). New data show that this difference was mainly due to overestimating the black hole mass in quiescent galaxies and overestimating the bulge magnitude of Seyfert galaxies. Using new and updated data we show that AGNs (Seyfert galaxies and quasars) follow the same BH-bulge relation as ordinary (inactive) galaxies. We derive the BH-bulge relation for a sample of 55 AGNs and 35 quiescent galaxies, finding that broad line AGNs have an average black hole/bulge mass fraction of $\sim 0.0015$ with a strong correlation (Mbh ~ Lbulge^{0.9\pm 0.16}). This BH-bulge relation is consistent with the BH-bulge relation of quiescent galaxies and much tighter than previous results. Narrow line AGNs appear to have a lower ratio, Mbh/Mbulge ~ 10^{-4}-10^{-3}. We find this to be a more general feature, the BH/bulge ratio in AGNs being inversely correlated with the emission-line width, implying a strong linear relation between the size of the broad emission line region and the luminosity of the bulge. Finally, combining AGNs with observed and estimated stellar velocity dispersion, we find a significant correlation (Mbh ~ v^{3.5-5}), consistent with that of quiescent galaxies.

astro-ph↗