Atmospheres of Earths and Super Earths - NExScI
Atmospheres of Earths and Super Earths Jonathan Fortney, UC Santa Cruz Thanks to: Eliza Kempton, Kevin Zahnle, Mark Marley
Atmospheres: Primary vs. Secondary • When most people think of Earths & Super Earths, they think of Secondary Atmospheres, those not accreted directly from the protoplanetary nebula (which are H2 and He rich). • The secondary atmospheres come from outgassing from the planet’s interior. The refractory “interior” elements lose their volaIles upon heaIng. • This can occur during accreIon and subsequently, due to tectonic acIvity e.g., Earth, Venus, Mars, Titan • Jupiter, Saturn, Uranus, & Neptune are our Primary Atmospheres, with a relaIve enhancement of metals compared to solar increasing from ~3X (Jupiter) to ~50X (Uranus and Neptune) • Takeaway Messages: • Even relaIvely simple Primary Atmospheres around Super Earths / Sub Neptunes can be complicated to Characterize • People are working towards an understanding of the rich processes that effect the formaIon, evoluIon, and loss of Secondary
Kepler: Planets Somewhat Smaller Than Neptune Much More Common than “Actual Neptunes”
Howard et al. (2011)
2-‐3 Earth Radii Strongly Suggests a H-‐ dominated Atmosphere for Many Planets Fortney, from SS Decadal Survey
Kepler-‐11 • The most densely-‐packed planetary system yet found • 5 planets within the orbit of Mercury • RelaIvely low density for all planets implies thick H/He atmospheres for most
Lissauer et al. (2011)
Kepler-‐11: The Mass-‐Radius View
GJ 1214b
Lissauer et al. (2011)
• Modeled as rock-‐iron cores with water or H/He envelopes • Atmospheric escape with Ime is ignored
Atmospheric Gain and Loss CoRoT-‐7b
Jackson et al. (2010), also Valencia et al. (2010)
Alibert et al. (2005)
• The tremendous number of failed core-‐ accreIon planets in the “super Earth” mass regime will tell us much about accumulaIng H2 from the nebula, if we can understand the H-‐loss processes well. • (And possible H-‐outgassing-‐-‐-‐see later)
Strongly Super-‐Solar Abundances Can Affect CharacterizaIon • Metal enrichments of ~100X solar are quite reasonable for low-‐mass primary atmospheres • Will strongly affect gaseous opacity, cloud opIcal depths, but more modestly on atmospheric mean molecular weight
X Imes solar abundances
Necelmann et al. (2011)
FracIon of Planet Mass that is H-‐atmosphere Fortney (2005)
Secondary Atmospheres: Imagine the PossibiliIes Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Planet FormaIon: ComposiIon Raymond et al. (2006)
• What is the planet made of? • What are the raw materials from which the atmosphere will be made? • What are the complements of volaIle and refractory elements?
Planet FormaIon: ComposiIon
Bond, O’Brien, & Laureca (2010)
Low C/O Solar C/O
High C/O
Planet FormaIon: ComposiIon
Asphaug (2010)
• However, the formaIon models generally assume perfect sIcking • This means that the resulIng composiIon predicIons will be imperfect
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Outgassing from a Planet’s Interior
The raw materials that make up your planet, and the chemistry within the planet, lead to what kind of outgassed atmosphere?
Elkins-‐Tanton & Seager (2008)
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Equilibrium Chemistry of the Outgassed VolaIles
Schaefer & Fegley (2010)
• Also, what about outgassing from samples NOT found in our meteorite samples? • Outgassing RATE will be a funcIon of age and planet mass
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Once Outgassed, What is Lost? Using a hydrodynamic escape model, Tian et al. (2005) found that previous assumpIons regarding Earth’s loss rate of H2 were ~100X too large A reducing atmosphere for the early Earth Origin of Life?
Tian et al. (2005)
• This is a hydrodynamic escape model, similar to what has been applied to the hot Jupiters
The “Cosmic Shoreline” Lighter atoms and molecules preferenIally lost, which depends strongly on the atmospheric temperature (irradiaIon level) and surface gravity / escape velocity Jeans escape only gives one a crude esImate, as well established in the solar system and for hot Jupiters
Elkins-‐Tanton & Seager (2008)
Kevin Zahnle, personal communicaIon
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Surface Gravity Effects are Apparent
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
We Should Also Think of Atmospheres as a FuncIon of log (Age) • What is abundant and observable at 106, 107, 108, 109, 1010 years?
Zahnle et al. (2007): “Emergence of a Habitable Planet,” Space Sci Rev (2007) 129: 35-‐78
Hot Super-‐Earths, Post Collision
Miller-‐Ricci (now, Kempton) et al. (2009)
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
DramaIc Incident Flux VariaIons over Wavelength and Time
Zahnle et al. (2007)
This will obviously differ for different parent stars
High Incident UV Fluxes in the Neighborhood
OB associaIons near the young parent star?
Conclusions • Given that it should be much easier to characterize the H-‐rich atmospheres of low-‐mass planets, that will be an important next fronIer • Don’t expect all primordial atmospheres to be the same • What a planet has for your atmospheric raw materials is the product of the stochasIc planet formaIon process • What happens to the materials once liberated depends on • their relaIve abundances • the surface gravity of the planet • the environment of the planet • the environment of the planetary system • I clearly haven’t covered all possibiliIes! • A modest proposal: A super-‐Monte Carlo? • N-‐body planet formaIon, keeping track of composiIon, and migraIon • Planetary cooling and outgassing • Atmospheric chemistry and and mass loss • Various parent stars, cluster environments, as a funcIon of age • A view of the what we may see?
Atmospheres: Primary vs. Secondary • When most people think of Earths & Super Earths, they think of Secondary Atmospheres, those not accreted directly from the protoplanetary nebula (which are H2 and He rich). • The secondary atmospheres come from outgassing from the planet’s interior. The refractory “interior” elements lose their volaIles upon heaIng. • This can occur during accreIon and subsequently, due to tectonic acIvity e.g., Earth, Venus, Mars, Titan • Jupiter, Saturn, Uranus, & Neptune are our Primary Atmospheres, with a relaIve enhancement of metals compared to solar increasing from ~3X (Jupiter) to ~50X (Uranus and Neptune) • Takeaway Messages: • Even relaIvely simple Primary Atmospheres around Super Earths / Sub Neptunes can be complicated to Characterize • People are working towards an understanding of the rich processes that effect the formaIon, evoluIon, and loss of Secondary
Kepler: Planets Somewhat Smaller Than Neptune Much More Common than “Actual Neptunes”
Howard et al. (2011)
2-‐3 Earth Radii Strongly Suggests a H-‐ dominated Atmosphere for Many Planets Fortney, from SS Decadal Survey
Kepler-‐11 • The most densely-‐packed planetary system yet found • 5 planets within the orbit of Mercury • RelaIvely low density for all planets implies thick H/He atmospheres for most
Lissauer et al. (2011)
Kepler-‐11: The Mass-‐Radius View
GJ 1214b
Lissauer et al. (2011)
• Modeled as rock-‐iron cores with water or H/He envelopes • Atmospheric escape with Ime is ignored
Atmospheric Gain and Loss CoRoT-‐7b
Jackson et al. (2010), also Valencia et al. (2010)
Alibert et al. (2005)
• The tremendous number of failed core-‐ accreIon planets in the “super Earth” mass regime will tell us much about accumulaIng H2 from the nebula, if we can understand the H-‐loss processes well. • (And possible H-‐outgassing-‐-‐-‐see later)
Strongly Super-‐Solar Abundances Can Affect CharacterizaIon • Metal enrichments of ~100X solar are quite reasonable for low-‐mass primary atmospheres • Will strongly affect gaseous opacity, cloud opIcal depths, but more modestly on atmospheric mean molecular weight
X Imes solar abundances
Necelmann et al. (2011)
FracIon of Planet Mass that is H-‐atmosphere Fortney (2005)
Secondary Atmospheres: Imagine the PossibiliIes Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Planet FormaIon: ComposiIon Raymond et al. (2006)
• What is the planet made of? • What are the raw materials from which the atmosphere will be made? • What are the complements of volaIle and refractory elements?
Planet FormaIon: ComposiIon
Bond, O’Brien, & Laureca (2010)
Low C/O Solar C/O
High C/O
Planet FormaIon: ComposiIon
Asphaug (2010)
• However, the formaIon models generally assume perfect sIcking • This means that the resulIng composiIon predicIons will be imperfect
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Outgassing from a Planet’s Interior
The raw materials that make up your planet, and the chemistry within the planet, lead to what kind of outgassed atmosphere?
Elkins-‐Tanton & Seager (2008)
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Equilibrium Chemistry of the Outgassed VolaIles
Schaefer & Fegley (2010)
• Also, what about outgassing from samples NOT found in our meteorite samples? • Outgassing RATE will be a funcIon of age and planet mass
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Once Outgassed, What is Lost? Using a hydrodynamic escape model, Tian et al. (2005) found that previous assumpIons regarding Earth’s loss rate of H2 were ~100X too large A reducing atmosphere for the early Earth Origin of Life?
Tian et al. (2005)
• This is a hydrodynamic escape model, similar to what has been applied to the hot Jupiters
The “Cosmic Shoreline” Lighter atoms and molecules preferenIally lost, which depends strongly on the atmospheric temperature (irradiaIon level) and surface gravity / escape velocity Jeans escape only gives one a crude esImate, as well established in the solar system and for hot Jupiters
Elkins-‐Tanton & Seager (2008)
Kevin Zahnle, personal communicaIon
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
Surface Gravity Effects are Apparent
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
We Should Also Think of Atmospheres as a FuncIon of log (Age) • What is abundant and observable at 106, 107, 108, 109, 1010 years?
Zahnle et al. (2007): “Emergence of a Habitable Planet,” Space Sci Rev (2007) 129: 35-‐78
Hot Super-‐Earths, Post Collision
Miller-‐Ricci (now, Kempton) et al. (2009)
Planet FormaIon
Equilibrium Chemistry
Cooling History
Planet Temperature
Cluster Environment
Outgassing of VolaIles
Mass Loss
Planet LocaIon Planet Surface Gravity
Parent Star Spectrum
DramaIc Incident Flux VariaIons over Wavelength and Time
Zahnle et al. (2007)
This will obviously differ for different parent stars
High Incident UV Fluxes in the Neighborhood
OB associaIons near the young parent star?
Conclusions • Given that it should be much easier to characterize the H-‐rich atmospheres of low-‐mass planets, that will be an important next fronIer • Don’t expect all primordial atmospheres to be the same • What a planet has for your atmospheric raw materials is the product of the stochasIc planet formaIon process • What happens to the materials once liberated depends on • their relaIve abundances • the surface gravity of the planet • the environment of the planet • the environment of the planetary system • I clearly haven’t covered all possibiliIes! • A modest proposal: A super-‐Monte Carlo? • N-‐body planet formaIon, keeping track of composiIon, and migraIon • Planetary cooling and outgassing • Atmospheric chemistry and and mass loss • Various parent stars, cluster environments, as a funcIon of age • A view of the what we may see?
