Search for Life in the Universe

– 0.01% decline in brightness – Very hard to measure from ground-based telescopes – Kepler mission (2008?): monitor 10 5 stars from space to detect even smaller than Earth-size transits 4/30/2020 AST 248, Spring 2007 3

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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 4/30/2020 AST 248, Spring 2007 7

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Observational Summary

• Masses – Minimum:median:maximum = 0.12:1.7:16.9 Jupiter masses – Solar system: minimum:median:maximum = 7x10  6 :3x10  3 :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 4/30/2020 AST 248, Spring 2007 9

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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 4/30/2020 AST 248, Spring 2007 15

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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 4/30/2020 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) – O 2 – CH 4 4/30/2020 AST 248, Spring 2007 18

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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 4/30/2020 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?

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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 CO 2 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 20  25  , 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?

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Search for Life in the Universe

Transcript Search for Life in the Universe