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Earth-like planets in habitable zones around L (and T) dwarfs

José A. Caballero /xó-se ka-ba-jé-ro/ Departamento de Astrofísica Universidad Complutense de Madrid. Earth-like planets in habitable zones around L (and T) dwarfs. Detecting planets around low-mass stars (and brown dwarfs) : the gateway to terrestrial planets.  The Sun has planets

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Earth-like planets in habitable zones around L (and T) dwarfs

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  1. José A. Caballero /xó-se ka-ba-jé-ro/ Departamento de Astrofísica Universidad Complutense de Madrid Earth-like planets in habitable zones around L (and T) dwarfs Detecting planets around low-mass stars (and brown dwarfs): the gateway to terrestrial planets

  2.  The Sun has planets •  Solar-like stars have planets (51 Peg 1.11 Msol, HD 209458 1.01 Msol...) • M-type stars have planets (HO Lib/GJ 581 0.31 Msol, GJ 317 0.24 Msol, OGLE-05-390L 0.22 Msol...)  And in between? (~0.08-0.01 Msol) • Jupiter has giant satellites (0.001 Msol) Earth-like planets in habitable zones around L (and T) dwarfs

  3. “There are things known and there are things unknown and in between are the Doors”(William Blake)“There are things known and there are things unknown and in between are the L (and T) dwarfs”(José A. Caballero)L spectral type: T ~ 2200-1300 KT spectral type: T ~ 1300-700 KTLeuchars(July, 1971-2000) = 292.2 KTJupiter(1 bar) = 165 K ChaHa8 (M6.5, ~3 Ma) (0.085 ± 0.015 Msol) Joergens & Müller (2007, ApJ, 666, 113) MOA-2007-BLG-192-L (?, ?) (0.060+0.028-0.021 Msol) Bennet et al. (2008, ApJ, in press, arXiv:0806.0025)

  4. Earth-like planets in habitable zones around L (and T) dwarfs: a reasonable idea! Very young (1-10 Ma), M-type low-mass stars and brown dwarfs in star-forming regions (Chamaeleon, s Orionis):* have (protoplanetary?) discs[disc fraction ~50%]* will be L and T dwarfs 1 Ga later[age typical of field ultracool dwarfs]Masses of 1 Ga-old field ultracool dwarfs:M = 0.075-0.040 Msol (L)M = 0.040-0.015 Msol (T)

  5. HZ (1): “a region of space where conditions are favourable for life (as it may be found on Earth)”HZ (2): “an interval of orbital separations to a star where liquid water can exist (at a normal pressure)”Tsurf = 273-373 K[but: runaway greenhouse]Kasting et al. (1993): HZ in FGK stars...Joshi et al. (1997): HZ in M stars...HZ in L stars and brown dwarfs? Variability in brown dwarfs: atmospheres and transits (Caballero & Rebolo 2002, ESA SP-485, 261) In: Proceedings of the First Eddington Workshop on Stellar Structure and Habitable Planet Finding, 11-15 June 2001, Córdoba, Spain

  6. Effective vs. surface planetary temperatures: the effective optical thickness • Surface pressure • Greenhouse gases (CO2) • Oceanic and eolic global patterns (orbital locking, ocean-land distribution... • Tropospheric adiabatic gradient, G • Deviations of the energy balance equation (internal energy source –tides-, non-unity atmospheric emissivity, flux factor for a slow-rotating, thin-atmosphere planet) The radiative energy balance equation:(1-A)pR2pS = 4pR2psT4eff,pA, Rp, Teff,p: planetary albedo, radius, and effective temperatureS = L/4pa2 (power per surface area)L: luminosity of central objecta: average separation (semi-major axis)Planetary surface temperature, Tsurf,p:T4surf,p = T4eff,p(1+2t/3)t: effective optical thickness

  7. Low orbital eccentricity e = 0 (high-amplitude) stable tides Lower albedoes  closer planets  easier detection No appreciable activity found in field ultracool dwarfs Peak of photosynthetic efficiency of Bacterioclorophyll a (Chloracidobacterium thermophilum) at 750-800 nm The habitable zones around L (and T) dwarfs: astrobiological restrictionsSynchronous rotation (global circulation vs. DTsurf in both hemispheres)Relative indetermination of the nIR planetary albedo (theoretical models predict very low albedoes)Ultraviolet emission and magnetic fields (flares in M-type dwarfs, Jupiter-Io)Photosynthesis in the near-infrared?

  8. The habitable zones around L (and T) dwarfs: a toy modelTheoretical isochrones of the Lyon groupOrbital separation a Orbital period P (Kepler’s third law)Albedo A = 0.10Effective optical thickness t = 1.0Mp = 5 MEarth A new complication: the Roche radius (from Aggarwal & Oberbeck 1974) RRoche,AO74 = 1.38 (r*/rp)1/3

  9. Detectability of earth-like planets in habitable zones around L (and T) dwarfs: TransitsCaballero (2006, PhD, thesis)Blake et al. (2008): Near infrared monitoring of ultracool dwarfs: prospects for searching for transiting companions Search for transits Search for atmosheric variability (CLOUDS: Continuous Longitude Observations of Ultracool DwarfS)

  10. Detectability of earth-like planets in habitable zones around L (and T) dwarfs: Radial velocityDesidera (1999)Caballero (2006, PhD, thesis)Viki JoergensHugh JonesJamie LloydCullen BlakeA high-resolution near-infrared spectrograph with resolution of 5 m/s would detect 5 MEarth exoplanets around L0-7 dwarfs in less than two nights

  11. The planet hunters:NAHUAL @ 10.4 Gran Telescopio Canarias, JHK, High Resolution + Image Slicer R = 61500 (2nd generation instrument)CARMENES @ 3.5 Calar Alto Teleskop, JH, High Resolution R = 60000? (Phase A)

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