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

Detecting planets around low-mass stars (and brown dwarfs): the gateway to terrestrial planets

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

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)

“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 K

T spectral type: T ~ 1300-700 K

TLeuchars(July, 1971-2000) = 292.2 K

TJupiter(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)

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, 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)

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

The radiative energy balance equation:

(1-A)R2pS = 4R2

pT4eff,p

A, Rp, Teff,p: planetary albedo, radius, and effective temperatureS = L/4a2 (power per surface area)L: luminosity of central objecta: average separation (semi-major axis)

Planetary surface temperature, Tsurf,p:

T4surf,p = T4

eff,p(1+2/3)

: effective optical thickness

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, •Deviations of the energy balance

equation (internal energy source –tides-, non-unity atmospheric emissivity, flux

factor for a slow-rotating, thin-atmosphere planet)

Planet

Venus 160

Earth 0.80

Mars 0.38

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 restrictions

Synchronous rotation (global circulation vs. Tsurf 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?

A new complication: the Roche radius (from Aggarwal &

Oberbeck 1974)

RRoche,AO74 = 1.38 (*/p)1/3

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 = 1.0

Mp = 5 MEarth

Detectability of earth-like planets in habitable zones around L (and T) dwarfs: Transits

Caballero (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)

Detectability of earth-like planets in habitable zones around L (and T) dwarfs: Radial velocity

Desidera (1999)

Caballero (2006, PhD, thesis)

Viki JoergensHugh JonesJamie LloydCullen Blake

A 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

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)