Finding habitable earths around white dwarfs with a robotic telescope transit survey Eric Agol Associate Professor Department of Astronomy University of Washington (UW) Feb 16, 2011 1
Evolution of a Sun-Like Star 1 AU Earth flux distance L 1/2 Red giant M initial =1.2 M M final =0.6 M Main sequence 2 nd generation planets? White dwarf radius of Earth Data from Jimenez et al. (2004), Renedo et al. (2010)
Formation of short period planets Reform planets in WDHZ: (1) disk formation? (2) migration via instability + tidal circularization? (3) companion evaporation? Hints: (1) pulsar planets; (2) polluted WD; (3) dust disks around WD. For now, assume these planets form with a frequency: η = fraction of WD w/ 0.1-10 M planets & a < 0.02 AU Transit probability is (R p +R WD )/a 1%, but transit depth can be up to 100% Requires 200η -1 white dwarfs to be surveyed for 32 hr each to detect 1 planet: requires robotic telescopes
Habitable planet transits
White dwarf transit survey Assume global network of 1 meter telescopes, e.g. Las Cumbres Observatory Global Telescope Network or Follow each white dwarf for 32 hours to cover 3 Gyr continuously habitable zone ( 0.02 AU) If few minute transit not detected, move to next white dwarf
WD detected planet distribution Mass decreases as dn/dm M -4/3 Search out to 100 pc Few hot WD (20,000 WD): 73 yr on sky (!) Peak is near planets with radius and Low S/N Few massive temperature of Earth Detect 1-10 planets @ >6σ: Low probability η 1-10%
Survey of WD CHZ with LSST: LSST will identify 10 7 White dwarfs w/ RPM Probability in transit: (R p +R WD )/(πa) 1/300 With 10 3 epochs, 3 points in transit ephemeris Binary WD contamination (>10 3 ) requires follow-up: secondary eclipse, Doppler beaming, lensing, Roemer delay, eclipse shape LSST can detect 1-10 HZ Earths if η 0.05-0.5%
Properties of WDHZ Planets should be tidally locked: permanent day/night; rotation period 1 day Star will appear similar in size & color to Sun Longest duration in WDHZ is 8 Gyr Energy source is thermal + gravitational + crystallization, not nuclear ( dead star)
Conclusions White dwarfs have a potentially habitable zone from.005-.02 AU lasting few Gyr If planets could (re)-form close to white dwarfs, easy to detect via transit (p 1%) Ground-based robotic surveys could find these planets, and next generation ground/space telescopes could characterize them; LSST may reach small η 0.05% Would have some properties similar to Earth
Extra slides
Liquid water is essential for life Clever biochemists have suggested that noncarbon-based, non-waterdependent life could possibly exist Nonetheless, the best place to begin the search for life is on planets like the Earth This means that we should look within the conventional habitable zone around nearby stars This does not necessarily mean these must be Sunlike stars (as we know it)
Finding the boundaries of the habitable zone In the Kasting et al. (Icarus, 1993) model, planets are assumed to develop dense atmospheres near either boundary of the habitable zone Dense H 2 O atmosphere near the inner edge (runaway greenhouse) Dense CO 2 atmosphere near the outer edge (from the carbonate-silicate cycle feedback) Stars must have steady or slowly varying luminosities for planet to spend a long duration in the habitable zone
Disk White dwarf luminosity function Liebert et al. (2007)
Simulation of survey Mario Juric et al. s catalog of white dwarfs in LSST (thanks Rob) Impose r < 24.5 cutoff; require at least 3 epochs observed with >7 σ detection of transit LSST can detect CHZ Earths if 0.05-0.5% of WD
LSST WD/planet properties White dwarf temperature Planet mass Detected Intrinsic Planet semimajor axis
Life Track of a Sun-Like Star
White dwarfs in distant globulars Typically 1/10,000 Luminosity of Sun
White dwarf cooling Hubble data White dwarfs cool by emitting neutrinos or photons Interiors are highly conductive: nearly isothermal As they cool, surface temperature decreases, so cooling rate slows: L 4 R 2 T 4 10 4 M L M 8Gyr 7 /5 The age of the disk of our Galaxy is about 5-10 Gyr, so this formula predicts the faintest white dwarfs have L 10-4 L for 8 Gyr
White dwarf luminosity Figure from Renedo et al. (2010)
Continuously habitable zone
White dwarf mass distribution
Radius in units of Sun White dwarf mass-radius relation Provencal et al. (1998) Mass in units of Sun
Extrasolar planet discovery & characterization: why? 1. Comparative planet formation 2. Planetary physics: EOS at high pressure; Atmospheric physics 3. Uniqueness of Earth & signposts for life: Earth-sized & temperature planets η = fraction of stars w/ 0.1-10 M planets & T < T < T
White dwarfs White dwarfs are the remaining cores of dead stars, but size of earth Electron degeneracy pressure supports them against gravity White dwarfs, once they cool, crystallize; carbon white dwarfs are cosmic diamonds : 10 34 carats Sirius B Largest diamond on earth: Star of Africa 530 carats
White dwarfs White dwarfs are the remaining cores of dead stars, but size of earth Electron degeneracy pressure supports them against gravity White dwarfs, once they cool, crystallize; carbon white dwarfs are cosmic diamonds : 10 34 carats Sirius B Largest diamond on earth: Star of Africa 530 carats
Gravitational interaction Electromagnetic interaction Gmu =-8pTmu Radial velocity Astrometry Microlensing Transit Secondary eclipse Imaging
Gravitational interaction Electromagnetic interaction Gmu =-8pTmu Radial velocity Astrometry Microlensing Transit Secondary eclipse Imaging
Winn (2009) 29
White dwarf temperature distribution
What if planet frequency is small? Then need to survey more stars... More stars = fainter stars Time prohibitively large... and requires larger (=more expensive) telescope At some point becomes more efficient to survey many stars at once, with a single, wide-field telescope... Pan STARRS or LSST
Robotic telescope survey
Is LSST * a Terrestrial Planet Finder? * Large Synoptic Survey Telescope
White Dwarf Habitable Zone (WDHZ) Agol (2011, ApJL, submitted) White dwarfs Earth 1% of the Sun s radius Most common white dwarfs have temperature of the Sun and 1/10,000 the luminosity So habitable zone is 0.01 AU (Kasting et al. 1993) Transit probability is (R p +R WD )/a 1%, but transit depth can be up to 100% Habitable Earth-size planets could be detected from the ground! But, need to form after red giant phase...
Nuclear burning habitable zone Kasting et al., Icarus (1993)
Luminosity of a Sun-Like Star M=1.2 M Red giant Main sequence 2 nd generation planets? White dwarf Data from Jimenez et al. (2004), Renedo et al. (2010)