Luminosity and Spectra of Young Jupiters

Jonathan J. FortneyUniversity of California, Santa Cruz

Mark Marley (NASA Ames)Olenka Hubickyj (NASA Ames)Peter Bodenheimer (UC Santa Cruz)Jack Lissauer (NASA Ames)Didier Saumon (Los Alamos Nat’l Lab)

GPI NYCMay 24, 2010

Talk Outline

• Jupiter and Saturn• Nucleated collapse models (Core accretion – Gas capture)• Alternate early evolution• Limits of applicability of the core accretion-start models

• When can models be trusted?• Can initial conditions be improved?

• Spectra of young giant planets• Effects of Enhanced Metallicity• Effects of Nonequilibrium chemistry

Observations of Jupiter & Saturn

• Composition is not like the Sun• Structure models show enhancement of “heavy elements” (atoms heavier than helium -- ice and rock)

• Jupiter: 1.5 - 6 X solar• Saturn: 6 - 14 X solar

• Heavy element cores• Jupiter: 0-11 M

• Saturn: 9 - 22 M

• Atmospheres show similar enrichment

• Jupiter: 2 - 4 X solar• Saturn: 4 - 10 X solar

• Appears most consistent with the core-accretion formation mechanism (Pollack et al., 1996, Alibert et al., 2005)

Saumon & Guillot (2004)

Standard cooling models for giant planets (and brown dwarfs) make simplifying assumptions:• Planets begin evolution fully formed • Planets are adiabatic at all ages• Initially arbitrarily large and hot• Initial model is unimportant as long as it is quite hot (tKH is very short at large L and R), and models are only plotted for t >1 Myr

“Hot Start” Models

“Although all these calculations may reliably represent the degenerate cooling phase, they cannot be expected to provide accurate information on the first 105-108 years of evolution because of the artificiality of an initially adiabatic, homologously contracting state. --Stevenson (1982)

Saumon et al. (1996)

Hubickyj, Bodenheimer, & Lissauer implementation of the core-accretion model

1. Planetesimals→core2. Gas accretion rate grows

and surpasses solid accretion rate

3. Runaway gas accretion4. Limiting gas accretion→how

fast can nebular gas be supplied? Gas arrives at a shock interface.

5. Accretion terminates→ isolation stage (cooling & contraction)

Stahler et al. (1980a)

Post-Formation Entropy

•Internal specific entropy 1 Myr after formation

•Entropy monotonically decreases with age

•Low post-formation entropy → small radii & low luminosity

•Quite dependent on the treatment of the accretion shock!

•At higher masses, a higher % of mass has passed through shock

Marley, Fortney, et al. (2007)

1. Core-accretion planets are formed with significantly smaller entropy and radii

2. tKH 1/LR e-2.8S, meaning evolution is initially much slower for the core-accretion planets

3. Initial conditions are not forgotten in “a few million years,” but rather, 10 million to 1 billion.

4. Initial Teff values cluster around 600-800 K

A Tiny Bit of Progress

Ayliffe & Bate (2009)

Energetics of Accretion: Hard

SPH radiation hydrodynamics

Must look at gas accretion for a few million years—lots of computing time

Spectra of “Planets” vs.Low Mass Brown Dwarfs

• Effects of increased metallicity are somewhat subtle• Brightening in K band is clearest signature

• Opacity of CH4 and H2O scale with metallicity, but H2 collision-induced absorption (CIA) does not• K band (strongly affected by H2 opacity) is relatively more transparent at high metallicity

• Redder J-K and H-K colors may indicate enhanced metallicity

•CO2 opacity may also become important, because its abundance scales quadratically with metallicity

Fortney, Marley, et al. (2008)

(cloud free)

Redder NIR Colors Due to Enhanced Metallicity

At higher metallicity:H-K redder by 0.5-1.5J-K redder by 0.7-1.0

Fortney, Marley, et al. (2008)

MetallicityExpectations?

Are super Jupiters metal enriched?

Does this vary with orbital distance?

Planet Mass (MJ)

Car

bon

X s

olar

Cold L Dwarfs?

Fortney, Marley, et al. (2008)

Burrows et al. (2006)

L-to-T Transition: Clouds hang around later at lower gravity?

Around L-T transition, low-gravity objects are more CO-rich, at a given Teff

1200 K

1. New core-accretion thermal evolution models from 1-15 MJ, including D-fusion where appropriate (w/ Bodenheimer & Lissauer)

2. Grids of Teff/gravity/metallicity/clouds in preparation for GPI planets:Some additions to Fortney et al. (2008):

CO2 – band at ~2μm in K-bandFiner metallicity gridClouds at higher T (silicates)Clouds at lower T (water)

Next Year or Two

Nonequilibrium Chemistry: HR8799