Exoplanets Astrobiology Workshop June 29, 2006 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  Terrestrial  Gas Giant  Ice Giant Jupiter Earth Saturn Neptune

Exoplanets: Around Solar-Type Stars Characteristics ?  All (or almost all?) are gas or ice giants 7M E > 13M J (M J = 320 M E )Masses from 7M E up to > 13M J (M J = 320 M E )  Orbits are mostly unlike the Solar System a <0.4 AU“Hot Neptunes” & “Hot Jupiters” (a < 0.4 AU) are common Orbits are often very eccentric  Earths cannot be detected yet Numbers (>180) 10-15%  Probably at least 10-15% of nearby Sun-like Stars  18  18 Planetary Systems (stars with 2 or more planets)

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 5.2 AU (M  = 1,000 M J )C.O.M. of Jupiter-Sun system (5.2 AU orbit radius) is near the Sun’s surface (M  = 1,000 M J ) 13 km/sJupiter orbits the C.O.M. at 13 km/s 10 -3The Sun’s speed is smaller by the ratio of Jupiter’s mass to the mass of the Sun (10 -3 ) 13 m/sThe Sun’s wobble due to Jupiter is only 13 m/s 3x10 8 m/sThe speed of light is 3x10 8 m/s  / = v/cFor the Doppler effect:  / = v/c less thanone part in 10 7So, we have to detect changes in wavelength of spectral lines of less than one part in 10 7 to measure this! Massive, close-in gas giants are much easier to detect

So How Hard Is It? Difficulty of Doppler Searches  Earth 10 cm/s !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: 1 10 m/s !Problem: Velocity “noise” from motions in the star’s atmosphere is typically 1 to10 m/s !

Exoplanets from Doppler Shifts: General Picture M VE M J

Latest Version

brown dwarfs gas giant planets Extrasolar Planet Discovery Space 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 4.2 M J 1.9M J 0.7M J F8V

Eccentric Orbit Example: 16 Cygni b 1.7 M J G5V

S.S. Analog: 47 Ursa Majoris 0.76M J 2.5M J 47 Ursa Majoris

55 Cancri: A Four Planet System Planet Msini = 4.05 M J Msini = 4.05 M J a = 5.9 AU (5,360 days) a = 5.9 AU (5,360 days)Planet Msini = 0.21 M J Msini = 0.21 M J a = 0.24 AU (44.3 days) a = 0.24 AU (44.3 days)Planet Msini = 0.84 M J Msini = 0.84 M J a = 0.12 AU (14.7 days) a = 0.12 AU (14.7 days)Planet Msini = M J (14 M E ) Msini = M J (14 M E ) a = AU (2.81 days) a = 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 M J Msini = 1.89 M J a = 0.21 AU (61.0 days) a = 0.21 AU (61.0 days)Planet Msini = 0.56 M J Msini = 0.56 M J a = 0.13 AU (30.1 days) a = 0.13 AU (30.1 days)Planet Msini = 5.9 M E Msini = 5.9 M E a = AU (1.94 days) a = AU (1.94 days) Star Mass = 0.32 M  M4V

Gliese 876 System: The Movie

Systems Where Planets Transit the Star

Transiting Planet HD209458b Planet Mass = 0.69  0.05 M J Planet Radius = 1.43  0.04 R J Orbit a = 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

Planet Mass = 0.36 M J Planet Radius = 0.72  R J Orbit a = 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 NEPTUNES JUPITERS ALL Highest Mass Average Mass 30 m/s 10 m/s

Doppler-Shift Exoplanets: Eccentricities & Orbit Periods

Doppler-Shift Exoplanets: Metallicity of the Host Star Some statistics [Fe/H] is the log 10 of Fe/H in the star divided by the Sun’s value.

Transiting Exoplanets: Are They Like Jupiter and Saturn? J S 1.3 g/cc 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: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 interactionEccentricities?  How Are They Attained? Multi-body interactions Perturbations by nearby stars Planet-disk interactions Migration into orbital resonancesOverall  Incredible Diversity of Planetary Systems!

Formation of the Solar System: The “Solar Nebula” Theory Dense, Cold, Rotating Interstellar Cloud Collapses and Flattens Sun Forms with “Solar Nebula” (Protoplanetary Disk) Solid Planetesimals and Gas Giant Planets Form, Then Gas Dissipates Terrestrial Planets Form by Accretion of Planetesimals 10 5 yrs yrs x10 7 yrs

Gas Giant Planet Formation: The Two Theories Core Accretion Disk Instability few x10 6 yrs 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