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Search for Life in the Universe Chapter 10 Search for Habitable Worlds (Part 2) 5/24/2017 AST 248, Fall 2005 1 Outline • 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 5/24/2017 AST 248, Fall 2005 2 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 5/24/2017 AST 248, Fall 2005 3 5/24/2017 AST 248, Fall 2005 4 5/24/2017 AST 248, Fall 2005 5 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 5/24/2017 AST 248, Fall 2005 6 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 5/24/2017 AST 248, Fall 2005 7 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 5/24/2017 AST 248, Fall 2005 8 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 5/24/2017 AST 248, Fall 2005 9 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? 5/24/2017 AST 248, Fall 2005 10 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? 5/24/2017 AST 248, Fall 2005 11