finding habitable earths around white dwarfs with a...
TRANSCRIPT
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
radius of Earth
Minitial=1.2 M⊙
Mfinal =0.6 M⊙
Data from Jimenez et al. (2004), Renedo et al. (2010)
Main sequence
Red giant
White dwarf
Earth flux distance ∝L1/2
2nd generation planets?
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 ≈(Rp+RWD)/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
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 (20,000 WD): 73 yr on sky (!) • Peak is near planets with radius and temperature of Earth • Detect ≈1-10 planets @ >6σ:
η⨁ ≈1-10%
Few massive
Low probability
Low S/N
Few hot WD
Survey of WD CHZ with LSST:
• LSST will identify ≈107 White dwarfs w/ RPM
• Probability in transit: ≈(Rp+RWD)/(πa) ≈1/300
• With ≈103 epochs, ≈3 points in transit ➭ ephemeris
• Binary WD contamination (>103) 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
Liquid water is essential for life (as we know it)
• Clever biochemists have suggested that non-carbon-based, non-water-dependent 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 Sun-like stars
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 H2O atmosphere near the inner edge
(runaway greenhouse) – Dense CO2 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
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
Intrinsic
Detected
White dwarf temperature Planet mass
Planet semi-major axis
White dwarf cooling
• White dwarfs cool by emitting neutrinos or photons
• Interiors are highly conductive: nearly isothermal
• As they cool, surface temperature decreases, so cooling rate slows:
Hubble data
L 4R2T 4 104L
M
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 mass-radius relation
Provencal et al. (1998)
Rad
ius
in u
nit
s o
f Su
n
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’: 1034 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’: 1034 carats
Sirius B
Largest diamond on earth: Star of Africa
530 carats
Gravitational interaction
Electromagnetic interaction
• Transit • Secondary eclipse • Imaging
• Radial velocity • Astrometry • Microlensing
Gmu =-8pTmu
Gravitational interaction
Electromagnetic interaction
• Transit • Secondary eclipse • Imaging
• Radial velocity • Astrometry • Microlensing
Gmu =-8pTmu
• 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
What if planet frequency is small?
White Dwarf Habitable Zone (WDHZ)
• 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 ≈(Rp+RWD)/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...
Agol (2011, ApJL, submitted)