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

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 V E M J

First Detection of an Exoplanet: 51 Pegasi

First Exo-Planetary System: Upsilon Andromedae F8V 4.2 MJ 1.9MJ 0.7MJ

Eccentric Orbit Example: 16 Cygni b 1.7 MJ G5V

S.S. Analog: 47 Ursa Majoris 2.5MJ 0.76MJ

55 Cancri: A Four Planet System Msini = 4.05 MJ a = 5.9 AU (5,360 days) Msini = 0.21 MJ a = 0.24 AU (44.3 days) Msini = 0.84 MJ a = 0.12 AU (14.7 days) 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 Msini = 1.89 MJ a = 0.21 AU (61.0 days) Msini = 0.56 MJ a = 0.13 AU (30.1 days) Msini = 5.9 ME a = 0.021 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 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

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?

Issues and Concerns: 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!

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? 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