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The Search for Habitable Worlds in Our Galaxy

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The Search for Habitable Worlds in Our Galaxy THE SEARCH FOR HABITABLE WORLDS IN OUR GALAXY RAVI KOPPARAPU NASA GODDARD RADIAL VELOCITY TECHNIQUE Courtesy: ESO Large mass, short period planets are easier to detect. TRANSIT DETECTION https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11428 TRANSIT SPECTROSCOPY KEPLER MISSION DIRECT IMAGING MISSION STUDIES From Kepler mission From Kepler mission STELLAR SPECTRAL TYPES F: ~7000K - 6000K (~2 Gyr) G: ~6000K - 5000K (~10 Gyr) K: ~5000K - 4000K (~30 Gyr) M: ~4000K - 2600K (~100 Gyr) Circumstellar region where a terrestrial size/mass planet with a “suitable” atmosphere can maintain liquid-water on it’s surface. This definition of HZ has little to do with habitability. It is a subset of a larger definition of “The” habitable zone where life can arise (irrespective of detectability) REQUIREMENT OF SURFACE LIQUID WATER Life on Earth requires liquid water during its life cycle. So,our first choice is to look for other planets like Earth It is unlikely that a subsurface biota could modify a planetary atmosphere in a way that could be observed remotely WHY “LIQUID WATER”? Oxygen Hydrogen Hydrogen Water is a “polar” molecule. And a universal solvent! Can form bonds allowing chemical reactions. “WATER ON THE SURFACE OF A PLANET”? Oxygen Methane Green plants and algae (and cyanobacteria) produce oxgyen from photosynthesis. Bacteria produce methane. A combination of these two gases could be a potential life signature. Currently, we can only see the outer atmospheres of planets remotely, using telescopes. So, we look for signatures that affect a planet’s atmosphere. HYDROGEN HABITABLE ZONES “Habitability doesn’t mean much if there is no star” 1-D climate model assuming an Earth-like planet with H2O dominated atmospheres at the inner edge, and CO2 dominated atmospheres at the outer edge (N2 background gas) Kasting, Whitmire & Reynolds (1993) WHAT ARE THE HZS AND WHERE ARE THEY? Inner edge Moist greenhouse: The ocean remains liquid, but the stratosphere becomes wet, leading to rapid photodissociation of water and escape of hydrogen to space. Runaway greenhouse: The atmosphere becomes opaque to outgoing long-wave radiation, preventing the planet to cool. Thus climate warms uncontrollably until all surface water has evaporated. For habitability purpose, Moist greenhouse (or “water-loss” ) limit is more relevant to the inner edge of the HZ runaway GH Moist GH WHAT ARE THE HZS AND WHERE ARE THEY? Outer edge Maximum greenhouse: Atmospheric CO2 should build up as the planet cools. There is a limit at which the CO2 starts to condense, and Rayleigh scattering by CO2 raises the planet’s albedo competing with the greenhouse effect This leads to the “maximum” greenhouse limit that CO2 can produce. Maximum GH UPDATED CLIMATE MODEL Updated Absorption coefficients updated from LBL databases ,HITRAN 2008 &HITEMP 2010 Inner edge: 0.97~0.99AU Outer edge: 1.68 AU SUN-LIKE (G DWARF) STARS VS COOL STARS (M-DWARFS) The radiation from M-dwarfs is peaked in the near-infrared H2O has absorption bands in the near-IR HABITABLE ZONE EXOPLANETS ONLINE HABITABLE ZONE CALCULATOR http://depts.washington.edu/naivpl/sites/default/files/hz.shtml http://www3.geosc.psu.edu/~ruk15/planets/ EXOPLANET PORTRAIT 3-D CLIMATE MODELING OF HZ BOUNDARIES 1-D climate models are not able to reliably predict distributions of re l a t i v e h u m i d i t y o r clouds 3-D climate models can a c c o u n t f o r g l o b a l distribution of clouds and heat transport. LMD, NASA Goddard, NASA GISS, Chicago, Colorado-Boulder, Peking U., UK Met office HZ BOUNDARIES AROUND G-DWARFS Wolf & Toon(2015) Moist greenhouse at 0.92 AU (19% S0, Red) Heat Stress/climate transition at 0.94 AU (12.5% S0, Blue) 3-D MODELS FOR M AND K-DWARF HZS • Planets around M & mid-K dwarfs are tidally locked. • The inner HZ can extend ~ 2 times closer to the star for synchronously rotating planets around MK-dwarfs. Yang et al.(2014) • Substellar clouds dominate the sunny side of tidally locked planets orbiting M and late-K stars, raising their albedos. • The substellar cloud is the result of weak Coriolis force for slowly rotating planets. TRAPPIST-1 Teff ~ 2559K, 7 terrestrial size planets, 3 of them in the HZ, orbital periods between 4 to 12 days. PROXIMA CEN-B HOW FAR IS PROXIMA CENTAURI? “HABITABILITY" OF PLANETS AROUND LOW MASS STARS HABITABILITY OF PLANETS AROUND M- DWARFS Due to the high luminosity in the pre-main-sequence phase of M-dwarfs, a planet in the (current) HZ may have lost water through runaway greenhouse. Luger & Barnes(2014), Ramirez & Kaltenegger(2014) “HABITABILITY” OF PLANETS AROUND LOW MASS STARS TESS (TRANSITING EXOPLANET SURVEY SATELLITE) Ricker et al.(2015) TESS DETECTIONS TESS field ~ 13 days TESS poles ~ 175 days Planned launch 2020. Atmospheric characterization of exoplanets. MOMENT OF TRUTH Image stolen from Kevin Stevenson CURRENT HZ ESTIMATES FROM 1-D AND 3-D MODELS TAKE HOME POINTS • There are ‘real-world’ consequences to the estimates of the Habitable Zone (HZ) • Modeling of HZ by different groups need to reach consensus (following IPCC) • A planet within the HZ may not be “habitable”, but HZ is our first cut to identify target systems of potential habitable planets WHICH HZS TO USE? • What is the purpose? • Every model has a utility depending on the need. • Advances in modeling techniques means an evolution of HZ estimates. HZ models = APPLICATIONS OF HZS • Discover potential habitable planets (around which star)? • Climate studies (atmospheric circulation, dynamics, convection, energy transport)? • Calculate occurrence rates of Earth-size/mass planets (again, around what spectral types?) • What do we want to do with the occurrence rates? Design a direct imaging mission for bio-signature detection. HOW COMMON ARE POTENTIAL HABITABLE PLANETS? HOW COMMON ARE POTENTIAL HABITABLE PLANETS? Survey: 1. Gather information from a small population that is uniformly mixed. 2. Apply the result to a larger population HOW COMMON ARE POTENTIAL HABITABLE PLANETS? For small, cool stars ~ 20% (conservative estimate!) About 14 billion habitable zone planets in our Galaxy! HOW COMMON ARE POTENTIAL HABITABLE PLANETS? For Sun-like stars ~ 22% About 2 billion in our Galaxy! DO OCCURRENCE RATE ESTIMATES AGREE? APPLICATIONS OF HZS • Discover potential habitable planets (around which star)? • Climate studies (atmospheric circulation, dynamics, convection, energy transport)? • Calculate occurrence rates of Earth-size/mass planets (again, around what spectral types?) • What do we want to do with the occurrence rates? Design a direct imaging mission for bio-signature detection. UTILITY OF ETAEARTH Stark et al. 2014 Stark et al. 2015 (Assumes eta_Earth = 0.1) UTILITY OF ETAEARTH Planets Stark et al. 2014 Stark et al. 2015 1 planet per star (Cassan et al.(2012)) Monitoring Solar System ocean moons Europa jets observed Europa jets observed with HST with 15-m LUVOIR Roth et al. (2014) UV hydrogen emission Credit: G. Ballester November 16, 2017 Big Bang to Biosignatures: The LUVOIR Mission Concept 4 EXO-EARTH YIELDS 4m 16m HOW MUCH WE KNOW ABOUT DRAKE EQUATION? • We know R*, fp, ne. LUVOIR will tell us f_L , and as a possible consequence, f_i and L. How? • LUVOIR-A (15m) can detect and characterize 10s of habitable planets. Statistics on f_L, particularly for advanced life, may put constraints on f_i and L. EXOPLANET DIVERSITY WITH DIRECT IMAGING TELESCOPES Habitable Zone Discoveries Exo-Earth yield occurrence rate WHERE IS EVERYBODY? HOW COMMON ARE (POTENTIAL) HABITABLE WORLDS • η⨁: The fraction of stars with at least one terrestrial size/mass planet in the Habitable Zone M-dwarf stars ~ 16% (1 - 1.5 RE) G&K-dwarf stars: 2% to 22% 12% (1.5 - 2 RE) (conservative estimate!) WHERE IS EVERYBODY? • Life is hard to arise ? • Intelligent life is uncommon ? • They want to be anonymous • Hard to travel/communicate expensive • Prime Directive ? • Did not search long enough? • They are us ! • Great Filter ? • We are not listening properly. CONTACT Ravi Kopparapu Ph: 225-678-0058 email: [email protected] webpage: https://science.gsfc.nasa.gov/sed/bio/ ravikumar.kopparapu http://www3.geosc.psu.edu/~ruk15/ .
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