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

Transcription

1 Earth-like planets in habitable zones around L (and T) dwarfs David Montes José A. Caballero Departamento de Astrofísica Universidad Complutense de Madrid Detecting planets around lowmass stars (and brown dwarfs): the gateway to terrestrial planets

2 Earth-like planets in habitable zones around L (and T) dwarfs The Sun has planets Solar-like stars have planets (51 Peg 1.11 M sol, HD M sol...) M-type stars have planets (HO Lib/GJ M sol, GJ M sol, OGLE L 0.22 M sol...) And in between? (~ M sol ) Jupiter has giant satellites (0.001 M sol )

3 Habitable zones around solar-type and M dwarfs For direct imaging studies, the tight habitable zones of M5V stars present an enormous challenge due to the reduction in the angular separation of the planet and star. In contrast, the M5V system is very favorable for transit surveys, since the transits are more probable and more frequent, and both the photometric and radial-velocity signals are much larger than they would be for the G2V system

4 L spectral type: T ~ K T spectral type: T ~ K T Earth = 294 K T Jupiter (1 bar) = 165 K Cha Hα 8 (M6.5, ~3 Ma) UVES/VLT Host: ± M sol Companion: M Jup Joergens & Müller (2007, ApJ, 666, 113) MOA-2007-BLG-192-L (?,?) - Microlensing Host: M sol Companion: 3: M Earth Bennet et al. (2008, ApJ, 684, 663)

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

6 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) T surf = 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, June 2001, Córdoba, Spain

7 The radiative energy balance equation: (1-A)πR 2 p S = 4πR2 pσt 4 eff,p A, R p, T eff,p : planetary albedo, radius, and effective temperature S = L/4πa 2 (power per surface area) L: luminosity of central object a: average separation (semi-major axis) Effective vs. surface planetary temperatures: the effective optical thickness Surface pressure Greenhouse gases (CO 2 ) Oceanic and eolic global patterns (orbital locking, ocean-land distribution... Tropospheric adiabatic gradient, Γ Deviations of the energy balance equation (internal energy source tides-, non-unity atmospheric emissivity, flux factor for a slow-rotating, thin-atmosphere planet) Planet τ Venus 160 Planetary surface temperature, T surf,p : T 4 surf,p = T 4 eff,p(1+2τ/3) Earth 0.80 Mars 0.38 τ: effective optical thickness

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

9 The habitable zones around L (and T) dwarfs: a toy model Theoretical isochrones of the Lyon group Orbital separation a Orbital period P (Kepler s third law) Albedo A = 0.10 Effective optical thickness τ = 1.0 M p = 5 M Earth A new complication: the Roche radius (from Aggarwal & Oberbeck 1974) R Roche,AO74 = 1.38 (ρ * /ρ p ) 1/3

10 Detectability of earth-like planets in habitable zones around L (and T) dwarfs: Transits Caballero (2006, PhD, thesis) Blake et al. (2008, PASP, 120, 860): 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)

11 Detectability of earth-like planets in habitable zones around L (and T) dwarfs: Radial velocity Desidera (1999, PASP 111, 1529) Caballero (2006, PhD, thesis) A high-resolution near-infrared spectrograph with resolution of 5 m/s would detect 5 M Earth exoplanets around L0-7 dwarfs in less than two nights

12 Detectability of planets around L (and T) dwarfs: Radial velocity - Viki Joergens (CS15) - UVES/VLT RV surveys of BDs and the case of ChaHa8 - Hugh Jones (CS15) - PRVS Gemini PRVS - Jamie Lloyd (CS15) TEDI/Palomar 200 Precision Radial Velocities in the Near Infrared with TEDI - Cullen Blake (CS15) NIRSPEC/Keck Hunting for Planetary Companions to Ultracool Dwarfs - Eike Guenther (this meeting) - CRIRES Searching for Planets of BDs with CRIRES

13 Optical vs. NIR: Radial velocity - LP UVES/VLT NIRSPEC/Keck (Martín et al. 2006, ApJ, 644, L75) - TW Hya - CORALIE, HARPS - CRIRES "A young massive planet in a star disk system", (Setiawan, et al., 2008, Nature 451, 38) "TW Hydrae: evidence of stellar spots instead of a Hot Jupiter", (Huélamo, et al., 2008, A&A, in press)

14 The planet hunters: Gran Telescopio Canarias (GTC), JHK, High Resolution + Image Slicer R = RV precision: 1 10 m/s Calar Alto Teleskop, JH, High Resolution R = 60000?