The very recent discovery of planets orbiting very low mass stars sheds light on these exotic objects. Planetary systems around low-mass stars and brown dwarfs are very different from our solar system: the planets are expected to be much closer than Mercury, in a layout that could resemble the system of Jupiter and its moons. The recent discoveries point in that direction with, for example, the system of Kepler-42 and especially the system of TRAPPIST-1 which has seven planets in a configuration very close to the moons of Jupiter. Low-mass stars and brown dwarfs are thought to be very common in our neighborhood and are thought to host many planetary systems. The planets orbiting in the habitable zone of brown dwarfs (and very low-mass stars) represent one of the next challenges of the following decades: they are the only planets of the habitable zone whose atmosphere we will be able to probe (e.g. with the JWST).
Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day-night temperature differences, and potentially limited fractional habitability. Previous studies have focused on scenarios where fractional habitability is confined to the substellar or "eye" region, but in this paper we explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between …
Since the launch of Kepler and Hubble more than a decade ago, we have come a long way in the quest to find a potentially habitable exoplanet. To date, we have already discovered more than 4000 exoplanets most of which are not suitable for sustaining life. Of all those that can potentially sustain life, A large number has been found rotating synchronously around their parent star, mostly Red dwarf star. Due to their synchronous rotation, these planets receive very uneven stellar heating. Synchronous rotation of these planets causes one side of the planet to permanently face the parent star whil…
The habitability of a planet depends on various factors, such as delivery of water during the formation, the co-evolution of the interior and the atmosphere, as well as the stellar irradiation which changes in time. Since an unknown number of rocky exoplanets may operate in a one-plate convective regime, i.e., without plate tectonics, we aim at understanding under which conditions planets in such a stagnant-lid regime may support habitable surface conditions. Understanding the interaction of the planetary interior and outgassing of volatiles with the atmosphere in combination with the evolutio…
Eccentricity is an important orbital parameter. Understanding its effect on planetary climate and habitability is critical for us to search for a habitable world beyond our solar system. The orbital configurations of M-dwarf planets are always tidally-locked at resonance states, which are quite different from those around Sun-like stars. M-dwarf planets need to be investigated separately. Here we use a comprehensive three-dimensional atmospheric general circulation model to systematically investigate how eccentricity influences climate and habitability of M-dwarf exoplanets. The simulation res…
Terrestrial planets are more likely to be detected if they orbit M dwarfs due to the favorable planet/star size and mass ratios. However, M dwarf habitable zones are significantly closer to the star than the one around our Sun, which leads to different requirements for planetary habitability and its detection. We review 1) the current limits to detection, 2) the role of M dwarf spectral energy distributions on atmospheric chemistry, 3) tidal effects, stressing that tidal locking is not synonymous with synchronous rotation, 4) the role of atmospheric mass loss and propose that some habitable wo…
Earth-scale planets in the classical habitable zone (HZ) are more likely to be habitable if they possess active geophysics. Without a constant internal energy source, planets cool as they age, eventually terminating tectonic activity and rendering the planet sterile to life. However, for planets orbiting low-mass stars, the presence of an outer companion could generate enough tidal heat in the HZ planet to prevent such cooling. The range of mass and orbital parameters for the companion that give adequate long-term heating of the inner HZ planet, while avoiding very early total desiccation, is …
The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sun-like stars. Over the past decade, significant progress has been made by both space- and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky plan…
We present estimations of dipolar magnetic moments for terrestrial exoplanets using the Olson & Christiansen (2006) scaling law and assuming their interior structure is similar to Earth. We find that the dipolar moment of fast rotating planets (where the Coriolis force dominates convection in the core), may amount up to ~80 times the magnetic moment of Earth, M_Earth, for at least part of the planets' lifetime. For very slow rotating planets (where the force of inertia dominates), the dipolar magnetic moment only reaches up to ~1.5 M_Earth. Applying our calculations to currently confirmed rock…
We consider some implications of the much-discussed circumstellar habitable zones around M-dwarf stars for the conventionally understood radio SETI. We argue that the flaring nature of these stars would further adversely impact local development of radio communication and that, therefore, their circumstellar habitable zones should be preferentially studied by other methods. This is a clear example how diversity of astrobiological habitats is introducing contingency into the cultural evolution, thus undermining the universality of cultural convergence as one of the major premises of the traditi…
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, w…
Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the Continuous Habitable Zone (CHZ) for ~8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarisation due to a terres…
Planets residing in circumstellar habitable zones (CHZs) offer our best opportunities to test hypotheses of life's potential pervasiveness and complexity. Constraining the precise boundaries of habitability and its observational discriminants is critical to maximizing our chances at remote life detection with future instruments. Conventionally, calculations of the inner edge of the habitable zone (IHZ) have been performed using both 1D radiative-convective and 3D general circulation models. However, these models lack interactive three-dimensional chemistry and do not resolve the mesosphere and…
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 retainin…
A couple of dozen Earth-like planets orbiting M dwarfs have been discovered so far. Some of them have attracted interest because of their potential long-term habitability; such a possibility is currently vigorously debated in the literature. I show that post-Keplerian (pK) orbit precessions may impact the habitability of a fictitious telluric planet orbiting an oblate late-type M dwarf of spectral class M9V with $M_\star=0.08\,M_\odot$ at $a=0.02\,\mathrm{au}$, corresponding to an orbital period $P_\mathrm{b}\simeq 4\,\mathrm{d}$, inducing long-term variations of the planetary obliquity $\vare…
Our knowledge of planets' orbital dynamics, which was based on Solar System studies, has been challenged by the diversity of exoplanetary systems. Around cool and ultra cool dwarfs, the influence of tides on the orbital and spin evolution of planets can strongly affect their climate and their capacity to host surface liquid water. We illustrate the role of tides and dynamics with the extreme case of planets orbiting around brown dwarfs. In multiple planet systems, the eccentricity is excited by planet-planet interactions. Planets are therefore heated up from the inside by the tidally-induced f…
The planetary system of TRAPPIST-1, discovered in 2016-2017, is a treasure-trove of information. Thanks to a combination of observational techniques, we have estimates of the radii and masses of the seven planets of this very exotic system. With three planets within the traditional Habitable Zone limits, it is one of the best constrained system of astrobiological interest. I will review here the theoretical constraints we can put on this system by trying to reconstruct its history: its atmospheric evolution which depends on the luminosity evolution of the dwarf star, and its tidal dynamical ev…
Conventional definitions of habitability require abundant liquid surface water to exist continuously over geologic timescales. Water in each of its thermodynamic phases interacts with solar and thermal radiation and is the cause for strong climatic feedbacks. Thus, assessments of the habitable zone require models to include a complete treatment of the hydrological cycle over geologic time. Here, we use the Community Atmosphere Model from the National Center for Atmospheric Research to study the evolution of climate for an Earth-like planet at constant CO2, under a wide range of stellar fluxes …
Planets orbiting low-mass stars such as M dwarfs are now considered a cornerstone in the search for life-harbouring planets. GJ273 is a planetary system orbiting an M dwarf only 3.75 pc away, composed of two confirmed planets, GJ273b and GJ273c, and two promising candidates, GJ273d and GJ273e. Planet GJ273b resides in the habitable zone. Currently, due to a lack of observed planetary transits, only the minimum masses of the planets are known.Despite being an interesting system, the GJ273 planetary system is still poorly studied. We aim at precisely determine the physical parameters of the indi…
The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterising planets in the "Habitable Zone" (HZ). However, Kepler finds that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions ($M_{env}/M_{core}$) $\gtrsim 1$%. If these planets retain such envelopes over Gyr timescales, they will not be "habitable" even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether sufficient envelope mass can be photoevaporated away for th…
Brown dwarfs are intermediate objects between planets and stars. The lower end of the brown-dwarf mass range overlaps with the one of massive planets and therefore the distinction between planets and brown-dwarf companions may require to trace the individual formation process. We present results on new potential brown-dwarf companions of Sun-like stars, which were discovered using CORALIE radial-velocity measurements. By combining the spectroscopic orbits and Hipparcos astrometric measurements, we have determined the orbit inclinations and therefore the companion masses for many of these syste…
Using an energy balance model with ice sheets, we examine the climate response of an Earth-like planet orbiting a G dwarf star and experiencing large orbital and obliquity variations. We find that ice caps couple strongly to the orbital forcing, leading to extreme ice ages. In contrast with previous studies, we find that such exo-Milankovitch cycles tend to impair habitability by inducing snowball states within the habitable zone. The large amplitude changes in obliquity and eccentricity cause the ice edge, the lowest latitude extent of the ice caps, to become unstable and grow to the equator.…
M dwarfs show the highest rocky planet occurrence among all spectral types, in some instances within the Habitable Zone. Because some of them are very active stars, they are often subject to frequent and powerful flaring, which can be a double-edged sword in regard of exoplanet habitability. On one hand, the increased flux during flare events can trigger the chemical reactions that are necessary to build the basis of prebiotic chemistry. On the other hand, sufficiently strong flares may erode exoplanets' atmospheres and reduce their UV protection. Recent observations of flares have shown that …
The closest potentially habitable worlds outside our Solar system orbit a different kind of star than our Sun: smaller red dwarf stars. Such stars can flare frequently, bombarding their planets with biologically damaging high-energy UV radiation, placing planetary atmospheres at risk of erosion and bringing the habitability of these worlds into question. However, the surface UV flux on these worlds is unknown. Here we show the first models of the surface UV environments of the four closest potentially habitable exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b assuming different atm…
In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The~most attractive feature of the realm of exoplanets is that 40\% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1--3.8 R$_\oplus$). Moreover, the recent finding of cyanobacteria able to use far-r…
Introduction: The search for life on distant exoplanets is expected to rely on atmospheric biosignatures detection, such as oxygen of biological origin. However, it is not demonstrated how much oxygenic photosynthesis, which on Earth depends on visible light, could work under spectral conditions simulating exoplanets orbiting the Habitable Zone of M-dwarf stars, which have low light emission in the visible and high light emission in the far-red/near-infrared. By utilizing cyanobacteria, the first organisms to evolve oxygenic photosynthesis on our planet, and a starlight simulator capable of ac…
We present the discoveries of a brown dwarf and a low mass star from the Kepler and K2 missions. The newly discovered brown dwarf is EPIC 212036875b and the low mass star is KOI-607b. EPIC 212036875b has a mass of $ M_{b}=52.3\pm 1.9M_J$, a radius of $ R_{b}=0.874\pm 0.017R_J$, and orbits its host star in $ P=5.169885 \pm 0.000027$ days. Its host star is a late F-type star with $ M_\star=1.288\pm 0.065M_\odot$, $ R_\star=1.498\pm 0.025R_\odot$, and $ T_{\rm eff}=6238 \pm 60$K. KOI-607b has a mass of $ M_{b}=95.1\pm 3.4M_J$, a radius of $ R_{b}=1.089\pm 0.089R_J$, and an orbital period of $ P=5…
A planet's climate can be strongly affected by its orbital eccentricity and obliquity. Here we use a 1-dimensional energy balance model modified to include a simple runaway greenhouse (RGH) parameterization to explore the effects of these two parameters on the climate of Earth-like aqua planets - completely ocean-covered planets - orbiting F-, G-, K-, and M-dwarf stars. We find that the range of instellations for which planets exhibit habitable surface conditions throughout an orbit decreases with increasing eccentricity. However, the appearance of temporarily habitable conditions during an or…
Nautilus: Fast Time-Resolved Spectroscopy of GKM Stellar Flares and Their Implications for Planetary Habitability
The University of Arizona（亚利桑那大学）；Michigan State University（密歇根州立大学）；Space Telescope Science Institute（太空望远镜科学研究所）；National Solar Observatory（国家太阳观测台）；Massachusetts Institute of Technology（麻省理工学院）；Max Planck Institute for Solar System Research（马普太阳系研究所）
We report on our continued efforts to understand and delineate the magnetic dynamo-induced behavior/variability of red dwarf (K5 V - M6 V) stars over their long lifetimes. These properties include: rotation, light variations (from star spots), coronal-chromospheric XUV activity and flares. This study is being carried out as part of the NSF-sponsored Living with a Red Dwarf program. The Living with a Red Dwarf program's database of dM stars with photometrically determined rotation rates (from starspot modulations) continues to expand, as does the inventory of archival XUV observations. When all…
Superflares may provide the dominant source of biologically relevant UV radiation to rocky habitable zone M-dwarf planets (M-Earths), altering planetary atmospheres and conditions for surface life. The combined line and continuum flare emission has usually been approximated by a 9000 K blackbody. If superflares are hotter, then the UV emission may be 10X higher than predicted from the optical. However, it is unknown for how long M-dwarf superflares reach temperatures above 9000 K. Only a handful of M-dwarf superflares have been recorded with multi-wavelength high-cadence observations. We doubl…
The mass domain where massive extrasolar planets and brown dwarfs overlap is still poorly understood due to the paucity of brown dwarfs orbiting close to solar-type stars, the so-called brown dwarf desert. In this paper we collect all of available data about close brown dwarfs around solar type stars and their host stars from literature and study the demographics of the brown dwarf desert. The data clearly show a short period and a medium mass gap in the brown dwarf period-mass distribution diagram ($ 35<m\sin i<55 M_{\rm Jup}$ and $P<100$ days), representing the "driest land" in the brown dwa…
Aims. A summary of the CoRoT brown dwarf investigations are presented. Methods. Transiting brown dwarfs around solar like stars were studied by using the photometric time-series of CoRoT, and ground based radial velocity measurements. Results. CoRoT detected three transiting brown dwarfs around F and G dwarf stars. The occurence rate of brown dwarfs was found to be 0.20 +/- 0.15% around solar-like stars which is compatible with the value obtained by Kepler-data.
M dwarfs host most of the exoplanets in the local Milky Way. Some of these planets, ranging from sub-Earths to super-Jupiters, orbit in their stars' habitable zones (HZs), although many likely possess surface environments that preclude habitability. Moreover, exomoons around these planets could harbor life for long timescales and thus may also be targets for biosignature surveys. Here we investigate the potential habitability, stability, and detectability of exomoons around exoplanets orbiting M dwarfs. We first compile an updated list of known M-dwarf exoplanet hosts, comprising 109 stars and…
The formation of brown dwarfs (BDs) poses a key challenge to star formation theory. The observed dearth of nearby ($\leq 5$ AU) brown dwarf companions to solar-mass stars, known as the brown dwarf desert, as well as the tendency for low-mass binary systems to be more tightly-bound than stellar binaries, have been cited as evidence for distinct formation mechanisms for brown dwarfs and stars. In this paper, we explore the implications of the minimal hypothesis that brown dwarfs in binary systems originate via the same fundamental fragmentation mechanism as stars, within isolated, turbulent gian…
How habitable are M-dwarf Exoplanets? Modeling surface conditions and exploring the role of melanins in the survival of Aspergillus niger spores under exoplanet-like radiation
Recent studies have shown that close-in brown dwarfs in the mass range 35-55 M$_{\rm Jup}$ are almost depleted as companions to stars, suggesting that objects with masses above and below this gap might have different formation mechanisms. We determine the fundamental stellar parameters, as well as individual abundances for a large sample of stars known to have a substellar companion in the brown dwarf regime. The sample is divided into stars hosting "massive" and "low-mass" brown dwarfs. Following previous works a threshold of 42.5 M$_{\rm Jup}$ was considered. Our results confirm that stars w…
We present the discovery of only the third brown dwarf known to eclipse a non-accreting white dwarf. Gaia parallax information and multi-colour photometry confirm that the white dwarf is cool (9950$\pm$150K) and has a low mass (0.45$\pm$0.05~MSun), and spectra and lightcurves suggest the brown dwarf has a mass of 0.067 $\pm$0.006 MSun (70 MJup) and a spectral type of L5 $\pm$1. The kinematics of the system show that the binary is likely to be a member of the thick disk and therefore at least 5 Gyr old. The high cadence lightcurves show that the brown dwarf is inflated, making it the first brow…
We present Spitzer observations at 3.6 and 4.5 microns and a near-infrared IRTF SpeX spectrum of the irradiated brown dwarf NLTT5306B. We determine that the brown dwarf has a spectral type of L5 and is likely inflated, despite the low effective temperature of the white dwarf primary star. We calculate brightness temperatures in the Spitzer wavebands for both the model radius, and Roche Lobe radius of the brown dwarf, and conclude that there is very little day-night side temperature difference. We discuss various mechanisms by which NLTT5306B may be inflated, and determine that while low mass b…
The discovery of 9 new brown-dwarf candidates orbiting stars in the CORALIE and HARPS radial-velocity surveys is reported. New CORALIE radial velocities yielding accurate orbits of 6 previously-known hosts of potential brown-dwarf companions are presented. Including targets selected from the literature, 33 hosts of potential brown-dwarf companions are examined. Employing innovative methods, we use the new reduction of the Hipparcos data to fully characterise the astrometric orbits of 6 objects, revealing M-dwarf companions with masses between 90 M_Jup and 0.52 M_Sun. Additionally, the masses o…
High resolution spectroscopy of the lowest-mass stars and brown dwarfs reveals their origins, multiplicity, compositions and physical properties, with implications for the star formation and chemical evolution history of the Milky Way. We motivate the need for high-resolution, infrared spectroscopic surveys to reach these faint sources.
Radio detection provides unique means to measure and study magnetic fields of the coolest brown dwarfs. Previous radio surveys have observed quiescent and flaring emission from brown dwarfs down to spectral type L3.5, but only upper limits have been established for even cooler objects. We report the detection of sporadic, circularly polarized flares from the T6.5 dwarf, 2MASS J1047+21, with the Arecibo radio telescope at 4.75 GHz. This is by far the coolest brown dwarf yet detected at radio frequencies. The fact that such an object is capable of generating observable, coherent radio emission, …
Brown dwarfs are commonly regarded as easily-observed templates for exoplanet studies, with comparable masses, physical sizes and atmospheric properties. There is indeed considerable overlap in the photospheric temperatures of the coldest brown dwarfs (spectral classes L and T) and the hottest exoplanets. However, the properties and processes associated with brown dwarf and exoplanet atmospheres can differ significantly in detail; photospheric gas pressures, elemental abundance variations, processes associated with external driving sources, and evolutionary effects are all pertinent examples. …
Brown dwarfs constitute a missing link between low-mass stars and giant planets. Their atmospheres display chemical species typical of planets, and one could wonder whether they also have weather-like patterns. While brown dwarf surface features cannot be directly resolved, the photometric and spectroscopic modulations induced by these features, as they rotate in and out of view, provide a wealth of information on the evolution of their atmosphere. A review of brown dwarfs variability through the L, T and Y spectral types sequence is presented, as well as the constraints that they set on the n…
Brown dwarf companions to white dwarfs are rare, but recent infra-red surveys are slowly reveal- ing examples. We present new observations of the post-common envelope binary WD0137-349, which reveals the effects of irradiation on the ~ 0.05M* secondary, and new observations of GD 1400 which show that it too is a close, post-comon envelope system. We also present the lat- est results in a near-infrared photometric search for unresolved ultracool companions and to white dwarfs with UKIDSS. Twenty five DA white dwarfs were identified as having photometric excesses indicative of a low mass compani…
The physical properties of brown dwarf discs, in terms of their shapes and sizes, are still largely unexplored by observations. To what extent brown dwarf discs are similar to scaled-down T Tauri discs is currently unknown, and this work is a step towards establishing a relationship through the eventual modelling of future observations. We use observations of the brown dwarf disc $ρ$ Oph 102 to infer a fiducial model around which we build a small grid of brown dwarf disc models, in order to model the CO, HCN, and HCO+ line fluxes and the chemistry which drives their abundances. These are the f…
