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Search for Life in the Universe Chapter 11 Extrasolar Habitability (Part 2) 5/22/2017 AST 248, Spring 2007 1 Outline • Extrasolar Planets – Transit – Direct Detection • Nature of Extrasolar Planets – – – – Observational Summary Solar-System Formation Implications for Habitability Signatures of Habitability & Life • Earth-like Planets: Rare or Common? – Frequency of Earth-size Planets – Impacts – Stable Climate 5/22/2017 AST 248, Spring 2007 2 Transit • Transit: fancy name for a (very) partial eclipse • HD209548: – 1.7% decline in brightness during transit – Due to Jupiter-size planet – Measurable by inexpensive photometers • Earth-size planet: – 0.01% decline in brightness – Very hard to measure from ground-based telescopes – Kepler mission (2008?): monitor 105 stars from space to detect even smaller than Earth-size transits 5/22/2017 AST 248, Spring 2007 3 5/22/2017 AST 248, Spring 2007 4 5/22/2017 AST 248, Spring 2007 5 5/22/2017 AST 248, Spring 2007 6 Direct Detection • Angular resolution – Need to see dim planet near a bright star – Angular resolution limited by diffraction and atmosphere – Overcome atmosphere by going to space • Infrared observations – Improve the luminosity ratio between star and planet by observing in the infrared – Diffraction blurring , stronger in the infrared • Interferometers – Nulling: directly measure a difference instead of subtracting full observations after the fact – Interferometers: utilize the physical interference of light waves – Terrestrial Planet Finder (TPF, schedule??) and/or Darwin (schedule ??): Search for Earth-size planets around ~150 nearby stars 5/22/2017 AST 248, Spring 2007 7 5/22/2017 AST 248, Spring 2007 8 Observational Summary • Masses – Minimum:median:maximum = 0.12:1.7:16.9 Jupiter masses – Solar system: minimum:median:maximum = 7x106:3x103:1 • Close orbits – Semi-major axis: minimum:median:maximum = 0.02:1.0:5.9 AU – Solar system: Mercury:Earth:Jupiter = 0.4:1.0:5.2 AU • Elliptical orbits – Eccentricities: minimum:median:maximum = 0.00:0.28:0.93 – Solar system: 0.01:0.05:0.25 • Systems with multiple planets – 18 out of 146 (1 in 8) • Terrestrial planets – Possible, but transit shows Jovian size for that planet 5/22/2017 AST 248, Spring 2007 9 5/22/2017 AST 248, Spring 2007 10 5/22/2017 AST 248, Spring 2007 11 5/22/2017 AST 248, Spring 2007 12 5/22/2017 AST 248, Spring 2007 13 5/22/2017 AST 248, Spring 2007 14 Solar System Formation • Standard theory: Jovian planets form at large distances ~ 5 AU and more • So why are they closer? • Theory of planet formation wrong? – We have not found a flaw, even when looking hard after the discovery of the extrasolar planets • Planet migration – The obvious way out, but how? – Drag by a residual disk favors circular orbits – Multiple encounters with planetessimals (evidence that a little of that occurred in the solar system 5/22/2017 AST 248, Spring 2007 15 5/22/2017 AST 248, Spring 2007 16 Implications for Habitability • Jovian planets – Planets themselves: unlikely, as in the solar system – Moons of Jovian planets: a possibility, particularly if the planet stays in the habitable zone • Terrestrial planets: – Migration of Jovian planets disrupts terrestrial planets in the habitable zone during the migration – Final elliptic orbits long-term disruption • Statistic – Migration in most of the systems found – Partly explained as a selection effect – Need a complete sample of nearby systems 5/22/2017 AST 248, Spring 2007 17 Signatures of Habitability & Life • What will we look for once we find Earth-like planets? • Distance from star – Is it in the habitable zone? • Imaging – Clouds – Diurnal changes (oceans v. continents) – Seasonal changes (snow and/or ice) • Spectroscopy – – – – – Surface temperature Surface composition Atmospheric composition (from IR spectra) O2 CH4 5/22/2017 AST 248, Spring 2007 18 5/22/2017 AST 248, Spring 2007 19 Frequency of Earth-size Planets • Need heavy elements – We think that terrestrial planets are formed from rocky planetessimals • Low heavy element abundance – Low heavy element abundance: some regions, e.g., globular clusters – Solar heavy element abundance: most of the disk stars and the interstellar medium • Formation process – Looks pretty straightforward, but we don’t know the details • Bottom line – Earth-size planets are very likely, unless we are unaware of something special in the formation process of our solar system 5/22/2017 AST 248, Spring 2007 20 Impacts • Earth: – Bombardment lasted ~ 0.5 byr and then dropped off, allowing life to form – Could impacts last much longer elsewhere? • Asteroids – Mostly around Lagrange points of Sun and Jupiter (equilateral triangles formed by Sun, Jupiter, and Lagrange point) • Comets – Mostly at Oort cloud, ~10,000 AU, but originated around Jupiter • Jupiter – Responsible for aligning asteroids along circular orbits between Mars and Jupiter – Responsible for ejecting comets to the Oort Cloud – Do other stars have such a “Jupiter”, and what about migration? 5/22/2017 AST 248, Spring 2007 21 Stable Climate • Stable climate for several byr – Essential for life – Has to adapt to the rising luminosity of the star over several byr • Plate tectonics – Essential for the CO2 cycle which regulates the climate – Nothing unique about plate tectonics on Earth: depends on liquid mantle and convection due to radioactive heating • Moon – – – – Stabilizes the Earth’s tilt at 2025, moderating the seasons How rare is a moon due to impact?: cf., Charon, Pluto’s moon Other ways to stabilize seasons: e.g., winds Can life migrate? 5/22/2017 AST 248, Spring 2007 22
