The First Planets Jarre/ Johnson with Hui Li Los Alamos Na8onal Laboratory credit: NASA/JPL-­‐Caltech/T. Pyle Planet Forma8on in the Early Universe First (Pop III) stars form ~ 100 Myr aKer the Big Bang (e.g. Bromm & Yoshida 2011) ? First galaxies typically form ~500 Myr aKer the Big Bang When and how do the first planets form? credit: firstgalaxies.org First Supernovae Precede First Planets •  For terrestrial planet forma8on, metal enrichment by the first supernovae must occur before planets can form Density Metallicity •  A?er how many stellar generaCons can planets finally form? Dalla Vecchia et al. in prep The Metallicity-­‐Planet Occurrence Rela8on •  Observa8ons find fewer planets around more metal-­‐
poor stars (Santos et al. 2001; Fischer & Valen8 2005; Sozze_ et al. 2009; Mayor et al. 2011) •  This is consistent with the core accreCon model of planet forma8on (Gonzalez et al. 2001; Ida & Lin 2004; Kornet et al. 2005; Pino_ et al. 2005; Johansen et al. 2009) Maldonado et al. 2012 The Core Accre8on Model Four stages: (1)  Dust seJles into the mid-­‐plane of the disk (2)  Dust grains grow larger through (sCcky) collisions (3)  Planetesimals are assembled – merge to form planetary cores (4)  Cores accrete gas – to form gas giants Total planet mass Solid core mass Gas mass accreted For first planets, step (1) is key! Pollack et al. (1996) Dust Se/ling (1) Timescale at Low Metallicity •  Density profile of disk Dust falling to disk mid-­‐plane •  Dust grain equa8on of mo8on •  Dust grain growth rate •  Dust se/ling takes longer at lower dust-­‐to-­‐gas ra8o fdg (lower metallicity) Dust grain growth due to collisions Dullemond & Dominik 2005 Disk Life8mes at Low Metallicity •  Observa8ons of low metallicity star-­‐forming regions reveal younger disks at low metallicity •  Suggests that disk lifeCmes are shorter at lower metallicity Yasui et al. 2009 Disk Photoevapora8on •  High energy radia8on emi/ed from the host star heats the disk and blows it away Credit: James Owen et al. Disk Photoevapora8on Timescale at Low Metallicity •  Disk photoevapora8on 8me is metallicity-­‐
dependent (e.g. Gor8 & Hollenbach 2009; Ercolano & Clarke 2010), due to shielding by heavy elements and dust Here, Σ(r) = Σ(1 AU) x (r/ 1 AU)-­‐p Ercolano & Clarke 2010 Dust Se/ling Timescale vs. Disk Life8me •  Dust se/ling into the disk mid-­‐plane slower at low metallicity (e.g. Kornet et al. 2005; Dullemond & Dominik 2005; Johansen et al. 2009) •  Disk photoevapora8on is faster at low metallicity (e.g. Gor8 & Hollenbach 2009; Yasui et al. 2009; Ercolano & Clarke 2010) •  Key to planetesimal forma8on: dust seJling must occur before the disk is photoevaporated away JLJ & Li 2012, ApJ,751, 81 The Cri8cal Metallicity for Planet Forma8on JLJ & Li 2012, ApJ,751, 81 Implica8ons for the First Earth-­‐like Planets •  Implies a maximum radius rmax within which planets can form: •  Integrate surface density profile out to rmax to find the minmimum metallicity at which an Earth mass of heavy elements can be assembled: For Earth-­‐like planets: Z > 0.1 Zsun JLJ & Li 2012, ApJ,751, 81 Implica8ons for the First Life •  This implies that the first Earth-­‐mass terrestrial planets would have formed at r ~ 3 AU credit: e-­‐educa8on.psu.edu •  Only in habitable zone (liquid water) if host star is > 1.5 Msun •  Such stars live for only ~ 4 Gyr ( < Hubble Cme) First life may have come and gone with death of host star
The GalacCc Habitable Zone Life cannot form if there are: (1)  Close-­‐in gas giants (at high metallicity) (2)  Frequent supernovae (3)  Not enough metals to form a terrestrial planet ~ 0.1 Zsun •  Our minimum metallicity for terrestrial planet forma8on (~0.1 Zsun) is what was assumed for the Galac8c Habitable Zone First galacCc life formed < 10 Gyr ago Lineweaver et al. 2004 Comparison to the Solar System •  Nice model which explains the orbits of the gas giants, invokes a planetesimal-­‐
populated disk out to only r ~30-­‐35 AU Neptune Uranus Saturn Jupiter •  This is just what we find for the rmax~32 AU of disks formed at solar metallicity! Our criCcal metallicity is precisely consistent with the Solar System!
Levison et al. 2008; see also Tsiganis et al. 2005 Discovery of a ‘Forbidden’ Planet: HIP 11952 •  Se8awan et al. (2012) announce discovery of a very metal-­‐poor planetary system •  [Fe/H] = -­‐1.9 r = 0.81 AU •  Gas giant with a mass > 2.93 MJupiter •  Likely formed via gravitaConal instability (e.g. Boss 1997; Rafikov 2005) instead of by core accre8on HIP 11952 violates the cri8cal metallicity! Se8awan et al. 2012 The Gravita8onal Instability (GI) Model •  Planets form via direct fragmenta8on of the disk •  For gravity to overwhelm pressure support need Q ~< 1, where •  Disk must therefore be cool and dense for GI model to work •  Only produces gas giants, not terrestrial planets (core accreCon) Credit: Alan B1
oss Boss 997 How cold must disks be? •  Toomre condi8on for disk fragmenta8on (Q~1) •  Minimum fragment (planet) mass is formed on scale of most unstable mode – must be less than 13 MJupiter! •  At higher disk temperatures, either -­‐ Fragmenta8on is suppressed due to gas pressure support or -­‐ Super-­‐planet mass fragments form JLJ & Li submi/ed Planet Forma8on around the First Stars? Density Temperature •  Disk temperatures are high (2,000 K), due to weak cooling by H2 molecules Greif et al. 2012 First Planet Forma8on via Gravita8onal Instability •  Disks around the first stars are much too hot for planet forma8on! •  But fragments with masses > 13 MJupiter may have formed •  Primordial brown dwarfs? (e.g. Uehara & Inutsuka 2000) Tmax,planet Tmax,brown dwarf Regula8on of Early Planet Forma8on by the CMB •  The CMB sets a temperature floor below which disks cannot radia8vely cool: •  The maximum disk temperature for planet forma8on: •  A maximum distance at which planets can form in the early universe (high z) -­‐ At larger radii only super-­‐planet mass fragments can form! JLJ & Li submi/ed A ‘Forbidden Zone’ for the Gravita8onal Instability Model •  Almost all known gas giants lie outside the Forbidden zone – could have formed via gravita8onal instability JLJ & Li submi/ed Gravita8onal Instability at Low Metallicity •  Disk cooling more efficient at lower metallicity – due to lower opacity to cooling radiaCon (e.g. Cai et al. 2006; Meru & Bate 2010) Low opacity 100 x higher opacity •  Lowers Q – favors disk fragmentaCon at low metallicity Meru & Bate 2010 Is the Early Evidence form Kepler Consistent? •  Kepler data show a weak dependence on metallicity for low-­‐mass planet forma8on – and a stronger dependence for high-­‐mass planet forma8on Planets with radii < 4 REarth Planets with radii > 4 REarth •  Opposite of the expected trend for gravita8onal instability! •  But they do not probe metallici8es below [Fe/H] ~ -­‐0.4 •  These data support the core accreCon model for gas giant planet formaCon – higher metallicity implies more mass accreted onto large planets Buchhave et al. 2012 Conclusions •  The cri8cal metallicity is a useful tool for tes8ng the theory of core accre8on •  The First Earth-­‐like planets can only form once metallicity exceeds 0.1 Zsun For Earth-­‐like planets: Z > 0.1 Zsun •  The first planets may have been gas giants formed from gravitaConal instability – on 8ght orbits around their host stars, due to disk hea8ng by the CMB