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Exoplanets Astrobiology Workshop June 29, 2006 Exoplanets: Around Solar-Type Stars Discovery (since 1995) by Doppler shifts in spectral lines of stars Transits of stars by planets Microlensing Maybe imaging Web Sites exoplanet.org exoplanet.eu Solar System Planets Earth Neptune Terrestrial Gas Giant Ice Giant Saturn Jupiter Exoplanets: Around Solar-Type Stars Characteristics All (or almost all?) are gas or ice giants • Masses from 7ME up to > 13MJ (MJ = 320 ME) Orbits are mostly unlike the Solar System • “Hot Neptunes” & “Hot Jupiters” (a < 0.4 AU) are common • Orbits are often very eccentric Earths cannot be detected yet Numbers (>180) Probably at least 10-15% of nearby Sun-like Stars 18 Planetary Systems (stars with 2 or more planets) Doppler Shift due to Stellar Wobble Doppler Shift due to Stellar Wobble Doppler Shift for a Star Orbited by a Planet So How Hard Is It? Difficulty of Doppler Searches Jupiters • C.O.M. of Jupiter-Sun system (5.2 AU orbit radius) is near the Sun’s surface (M = 1,000 MJ) • Jupiter orbits the C.O.M. at 13 km/s • The Sun’s speed is smaller by the ratio of Jupiter’s mass to the mass of the Sun (10-3) • The Sun’s wobble due to Jupiter is only 13 m/s • The speed of light is 3x108 m/s • For the Doppler effect: / = v/c • So, we have to detect changes in wavelength of spectral lines of less than one part in 107 to measure this! • Massive, close-in gas giants are much easier to detect So How Hard Is It? Difficulty of Doppler Searches Earth • The Sun’s wobble due to the Earth is only about 10 cm/s ! Requirements for Any Planet • Very stable reference spectrum • Use of all the spectral lines in the spectrum • Problem: Velocity “noise” from motions in the star’s atmosphere is typically 1 to10 m/s ! Exoplanets from Doppler Shifts: General Picture M V E M J Latest Version Extrasolar Planet Discovery Space brown dwarfs gas giant planets Right of the blue line, the orbit period is more than the time these systems have been observed. Below the dashed line, the stellar wobbles are less than 10 m/s. First Detection of an Exoplanet: 51 Pegasi First Exo-Planetary System: Upsilon Andromedae F8V 4.2 MJ 0.7MJ 1.9MJ Eccentric Orbit Example: 16 Cygni b 1.7 MJ G5V S.S. Analog: 47 Ursa Majoris 47 Ursa Majoris 0.76MJ 2.5MJ 55 Cancri: A Four Planet System QuickTi me™ and a Sor enson Video 3 decompressor are needed to see thi s pi cture. Planet Msini = 4.05 MJ a = 5.9 AU (5,360 days) Planet Msini = 0.21 MJ a = 0.24 AU (44.3 days) Planet Msini = 0.84 MJ a = 0.12 AU (14.7 days) Planet Msini = 0.045 MJ (14 ME) a = 0.038 AU (2.81 days) Star Mass = 0.95 M G8V Gliese 876 System: Gas Giants in 2:1 Resonance Gliese 876 System: 6 to 8 Earth Mass Planet Gliese 876 System: Three Known Planets Planet Msini = 1.89 MJ a = 0.21 AU (61.0 days) Planet Msini = 0.56 MJ a = 0.13 AU (30.1 days) Planet Msini = 5.9 ME a = 0.021 AU (1.94 days) Star Mass = 0.32 M M4V Gliese 876 System: The Movie QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture. Systems Where Planets Transit the Star Transiting Planet HD209458b Planet Mass = 0.69 0.05 MJ Planet Radius = 1.43 0.04 RJ Orbit a = 0.045 AU Orbit Period = 3.52 days Star Mass = 1.05 M (F8V) Transiting Planet HD209458b Transiting Planet HD209458b: Absorption Line of Sodium Transit Surveys Transiting Planet HD149026b: A Massive Heavy Core Transiting Planet HD149026b: A Massive Heavy Core Planet Mass = 0.36 MJ Planet Radius = 0.72 0.025 RJ Orbit a = 0.042 AU Orbit Period = 2.88 days Star Mass = 1.31 M G0IV Image of a Planet? Doppler-Shift Exoplanets: Masses, Eccentricities, & Orbits Brown Dwarf Desert Doppler-Shift Exoplanets: Masses & Orbits Highest Mass ALL Average Mass 30 m/s 10 m/s NEPTUNES JUPITERS Doppler-Shift Exoplanets: Eccentricities & Orbit Periods Doppler-Shift Exoplanets: Metallicity of the Host Star Some statistics [Fe/H] is the log10 of Fe/H in the star divided by the Sun’s value. Transiting Exoplanets: Are They Like Jupiter and Saturn? 1.3 g/cc J S 0.3 g/cc Issues and Concerns: Planet Formation Planet Formation Gas Giant Formation Theories • Solid Core Accretion followed by gas capture – Pro: Mechanism that can work – Con: Slow, expect formation at > few AU, may not be able to make super-Jupiters • Disk Instability due to self-gravity of the protoplanetary disk – Pro: Fast formation – Con: Real protoplanetary disks may not cool fast enough to fragment, may be hard to explain large solid cores • Hybrid: Core Accretion sped up by Disk Instability? Evidence • Metallicity correlation may favor Core Accretion Issues and Concerns: Planet Formation Hot Neptunes & Jupiters? Formation in Place • Probably not possible Planet “Migration” • Planets can drift inward due to planet-disk interaction Eccentricities? How Are They Attained? • Multi-body interactions • Perturbations by nearby stars • Planet-disk interactions • Migration into orbital resonances Overall Incredible Diversity of Planetary Systems! Formation of the Solar System: The “Solar Nebula” Theory Dense, Cold, Rotating Interstellar Cloud Collapses and Flattens 105 yrs Sun Forms with “Solar Nebula” (Protoplanetary Disk) 106-107 yrs Solid Planetesimals and Gas Giant Planets Form, Then Gas Dissipates 107-3x107 yrs Terrestrial Planets Form by Accretion of Planetesimals Gas Giant Planet Formation: The Two Theories Core Accretion few x106 yrs Disk Instability 102 - 103 yrs Issues and Concerns: Life Why Are Hot Jupiters Bad? Origin • Probably exist due to inward “migration” during planet formation Effects • Sweep terrestrial planet material into the star as they migrate • Gas Giants near or inside the habitable zone make stable orbits for terrestrial planets difficult or impossible Why Are Eccentric Gas Giants Bad? Effects • Tend to disrupt terrestrial planet formation • Tend to destabilize terrestrial planet orbits and/or force the orbits to be eccentric, producing extreme seasons Issues and Concerns: Life Hope? There ARE Solar System Analogs! • Gas giants at > few AU in nearly circular orbits • Over the next decade, more are likely to be found Incredible Diversity of Environments! And… Maybe Close-In Gas Giants Have Earth-Like Moons