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Search for Life in the Universe Chapter 10 Search for Habitable Worlds (Part 1) 5/22/2017 AST 248, Fall 2005 1 Outline • Are Habitable Planets Common? • Distant Suns – – – – Properties of Stars Stellar Types Which Stars Make Good Suns? Multiple Stars • Extrasolar Planets – – – – – Detecting Extrasolar Planets Astrometry Doppler Technique Transit Direct Detection 5/22/2017 AST 248, Fall 2005 2 Are Habitable Planets Common? • Disks of young stars: – Theory: disks inevitable planets – Observations: 2550% of stars have disks with a few percent of the stellar mass (cf., <0.2% for our planets) • Extrasolar planets (around main-sequence stars): – 146 planetary systems (11/24/2005) • 170 planets • 18 multiple planet systems – Encyclopedia: http://www.obspm.fr/encycl/encycl.html • Characteristics of discovered extrasolar planets: – – – – – Massive (mainly a selection effect): Jovian planets Unlikely to be habitable Orbit close to the parent stars Highly elliptical orbits common Affect habitability of Earth-like planets 5/22/2017 AST 248, Fall 2005 3 Properties of Stars • Evolution – Protostar main sequence giant or supergiant white dwarf, neutron star, or black hole – Habitability: in main sequence phase, because it lasts longest • Types – – – – Universal composition: >98% H + He Universal burning: H He C + O Mass: only free parameter Types: OBAFGKM (politically incorrect mnemonic: “Oh, Be A Fine Girl, Kiss Me”) • Lifetimes – Luminosity: steep function of mass M(3.54) – Lifetime: steep inverse function of mass M(2.53) 5/22/2017 AST 248, Fall 2005 4 Stellar Types Type O Percent age 0.001% Temp. [C] 50,000 Lumin. [solar] 106 Mass [solar] 60 Lifetime [years] 5x105 B 0.1% 15,000 1,000 6 5x106 A 1% 8,000 20 2 109 F 2% 6,500 7 1.5 2x109 G 7% 5,500 1 1 1010 K 15% 4,000 0.3 0.7 2x1010 M 75% 3,000 0.003 0.2 6x1011 5/22/2017 AST 248, Fall 2005 5 Which Stars Make Good Suns? • O: much too short • B: little life past accretion phase • A and F: – Life short but manageable – UV light a potential problem – Ways around that: e.g., more ozone • G: OK (cf., the Sun) • K and M: – – – – – Long life Very common Less luminous smaller habitable zone Synchronous orbit: problem, unless the atmosphere rotates Flares: main danger is UV, but it also produces more ozone 5/22/2017 AST 248, Fall 2005 6 Multiple Stars • Orbit – Planet pulled by two or more stars can have a complicated orbit, moving in and out of the habitability zones of the stars • Stable cases – Stars close, planet orbiting both at a safe distance – Stars far, planet orbiting close to one of them • Unstable cases – Any other combination 5/22/2017 AST 248, Fall 2005 7 Detecting Extrasolar Planets • Direct detection of the planet – Difficulty: observe a dim planet near a bright star space missions > 10 yrs from now – Absorption spectrum in a planetary atmosphere during transit? • Indirect detection of effect on parent star – Extrasolar planets around main sequence stars discovered in 1995 – Planets around neutron stars discovered previously – Spectroscopy (>100 cases): detect Doppler shift of stellar motion around center of mass – Astrometry (1 case): detect angular motion 5/22/2017 AST 248, Fall 2005 8 Astrometry • Binary or multiple stars – Very practical, if they can be resolved • Planets: – Resolution: not an issue, observe only the star – Amplitude: problem, e.g., at a distance of 10 ly, the amplitude of the solar motion due to Jupiter is 0.003 – Angular motion 1/distance harder for distant stars – Angular motion orbital radius easier for outlying planets, but orbital period longer and region less habitable • Space Interferometer Mission (SIM, launch 2011?) – Angular precision 10-6 – Earth-like planets: detect stellar wobble ~12 nearby stars – Jupiter-size planets: detect wobble 3,000 ly away 5/22/2017 AST 248, Fall 2005 9 5/22/2017 AST 248, Fall 2005 10 Doppler Technique • Doppler effect – / = vlos/c (nonrelativistic) • Detection of planets – Velocities ~30 m/s = 10-7c – Accurate spectroscopy possible with iodine cells instead of an arc – Velocity independent of distance – Velocity Mplanet: favors massive planets – Velocity 1/a: favors small orbits – Orbital period 1/a3: shorter period favors small orbit (observations take less time) 5/22/2017 AST 248, Fall 2005 11 5/22/2017 AST 248, Fall 2005 12 5/22/2017 AST 248, Fall 2005 13 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, Fall 2005 14 5/22/2017 AST 248, Fall 2005 15 5/22/2017 AST 248, Fall 2005 16 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, Fall 2005 17