CarbonCarbon PlanetsPlanets
Dr. Peter Woitke
St Andrews University
Advanced Topics in Modern Physics
16. October 2013
TalkTalk outlineoutline ● Observations of “carbon-rich planets” / “diamond stars” − BPM 37093 (Metcalfe+ ApJ 2004) − WASP-12b (Hebb+ ApJ 2009, Madhusudhan+ Nature 2010) − PSR J1719-1438 b (Bailes+ Science 2011) − 55 Cancri e (Madhusudhan+ ApJ 2012, however: Nissen+ A&A 2013) ● Press Resonance ● Protoplanetary Discs − chemical and temperature structure − mid-plane conditions ● Planet Formation Models ● Chemical Mid-plane Evolution − gas and ice segregation − cosmic ray CO unblocking
BPMBPM 3709337093 (Metcalfe(Metcalfe etet al.al. ApJApJ 605,605, 2004)2004)
● massive, old white dwarf M* ~ 1.1 Msun, Teff ~ 11000 K, log g ~ 8.8 ● astro-seismology + crystallization theory − “... all (fitting models) have a crystallized mass fraction of 0.9” − “The best of our six fits is the 1.1 Msun (pure) O-core model” − “Realistic stellar models do not predict pure compositions but rather a C/O mixture that varies as a function of radius. For the large degrees of crystallization that we have found ... the liquid mantle above the crystallized core is expected to be fully mixed and significantly enriched in carbon.”
● not a planet at all PressPress ResonanceResonance
● BBC news, 16. February 2004
− “Twinkling in the sky is a diamond star of 10 billion trillion trillion carats”
− “Astronomers have decided to call the star 'Lucy' after the Beatles song, Lucy in the Sky with Diamonds”
− “Our Sun will become a diamond that truly is forever”
WASP-12bWASP-12b (Hebb et al. 2009, ApJ 693; Madhusudhan et al. 2010, Nature 469)
● very hot Jupiter P = 1.1 days, Rpl ~ 1.8 MJup, Tpl ~ 2500 K ● Spitzer photometry + parametric atmosphere model − “... WASP-12b has a dayside atmosphere depleted in water vapor and enhanced in methane by over two orders of magnitude compared to a solar-abundance chemical equilibrium model at the expected temperatures” ● however (Swain+2012) HST WFC3-IR grism: no evidence for C/O>1 ● this is a gas giant
PressPress ResonanceResonance
● Scientific American, January 12, 2013
● “On other worlds ... carbon might be as common as dirt. In fact, carbon and dirt might be one and the same.” −
● “Life-forms on a carbon planet — if they exist — would little resemble the oxygen- dependent organisms of Earth. Precious oxygen would prove valuable as a fuel in much the same way that humans covet hydrocarbon fuels on Earth"
PSRPSR J1719-1438J1719-1438 bb (Bailes(Bailes etet al.al. 2011,2011, SienceSience 333)333) ● PSR J1719-1438 b and PSR J1719-1438 were formerly two stars in a binary star system − after PSR J1719-1438 went supernova and became a milli-sec pulsar, PSR J1719-1438 b evolved into its red giant phase and became a white dwarf − mass transfer has stripped off most of the companion's mass (ordinary Jupiter would not fit into Roche lobe) ● pulsar timing
− P = 2h:10min, Mpl ~ 1 MJup, ρpl ~ 20 ρJup, Tpl ~ 4500 K − “The chemical composition, pressure and dimensions of the companion make it certain to be crystallized (i.e. diamond).” ● exotic object – a planet at all?
5555 CancriCancri ee (Madhusudhan+ ApJ 2012, based on Delgado Mena+ 2010) ● double star with 5 planets (!) − 55 Cancri e is a “superearth”, detected via radial velocity and transit methods P ~ 18h, Mpl ~ 8 MEarth, Rpl ~ 2 REarth − particular planet density extensively discussed, also as “water planet” − host star has C/0 = 1.12 ± 0.19 and metal- rich, Fe/H = 1.8 (carbon abundances were determined from high-excitation CI lines and oxygen abundances from the zero excitation, forbidden [OI] line at 6300Å − C/O(planet) = C/O(star) is assumed ● however (Nissen+ 2013): − “Instead of the [OI] line, the OI triplet at 7774Å may be used to determine oxygen abundances” − “the C/O values found in this paper lend no support to the existence of carbon-rich planets” host-stars PressPress ResonanceResonance ● National Geographic Daily News, October 11, 2011
− “Science fiction has dreamed of diamond planets for many years, so it's amazing that we finally have evidence of its existence in the real universe”
− "It's the first time we know of such an exotic planet that we think was born mostly of carbon - which really makes this a fundamental game- changer in our understanding of what's possible in planetary chemistry" However:However: SaturnSaturn largestlargest moonmoon TitanTitan (!)(!)
ongoing Cassini-Huygens mission
– “On Titan it is so cold that water plays the role of rock and lava, and flowing methane carves river channels and fills great lakes with liquid natural gas.”
– “Vast regions of tall dunes stretch across the landscape – dunes whose "sand" is composed of dark hydrocarbon grains.”
Ligeia Mare, the second largest known body of liquid on Titan.
ProtoplanetaryProtoplanetary DiscsDiscs
GasGas andand DustDust TemperaturesTemperatures
GasGas HeatingHeating && CoolingCooling
ChemistryChemistry
PlanetPlanet FormationFormation ModelsModels
● Core Accretion Model (Safronov 1969) − ~1μm dust grains undergo sticky collisions to form ~1 km rocky planetesimals − larger planetesimals gravitationally accrete smaller planetesimals (“oligarchic growth”), until planet’s “feeding zone” (4-5 Hill-radii) is depleted of smaller planetesimals − runaway gas accretion − growth continues until a gap is opened in the disk, planet feeding zone is now empty of both planetesimals and gas → almost complete segregation of gas and solids, then putting it back together
● Gravitational Disc Instability (Boss 2004, Rice+ 2011) can occur in any region that becomes sufficiently dense, cool and cooling: − spiral waves − self-gravitating turbulence − mass and angular momentum transport through long-range torques − fragmentation into clumps and subtructure (given extreme cooling)
→ no segregation of gas and solids PlanetPlanet FormationFormation ModelsModels
● Core Accretion Model (Safronov 1969) − ~1μm dust grains undergo sticky collisions to form ~1 km rocky planetesimals − larger planetesimals gravitationally accrete smaller planetesimals (“oligarchic growth”), until planet’s “feeding zone” (4-5 Hill-radii) is depleted of smaller planetesimals − runaway gas accretion − growth continues until a gap is opened in the disk, planet feeding zone is now empty of both planetesimals and gas → almost complete segregation of gas and solids, then putting it back together
● Gravitational Disc Instability (Boss 2004, Rice+ 2011) can occur in any region that becomes sufficiently dense, cool and cooling: − spiral waves − self-gravitating turbulence − mass and angular momentum transport through long-range torques − fragmentation into clumps and subtructure (given extreme cooling)
→ no segregation of gas and solids ChemicalChemical EvolutionEvolution inin MidplaneMidplane
● gas-ice segregation:
in the cold midplane, into which no UV and X-rays can penetrate, elements can almost completely vanish from the gas phase, to form ices like H2O#, CO#, N2#. The elements bound into those ices should follow the dust rather than the gas evolution … … and so become part of the planet cores? ● time-dependent disk midplane chemistry
can predict how gas and ice separate, and develop in the disk midplane as function of time and position (slow evolution!) ● chemistry in the observable upper layers …
… is different. The upper layers, which are observable through gas emission lines, have an active chemistry, with short chemical relaxation timescales
MolecularMolecular CloudCloud ChemistryChemistry n~3.E+4 cm-3, T~15K, Av~20, after 5.E+5 yrs (how it all starts: all you need is H2, cosmic rays, atoms and electrons)
CR H2 e- H2 H2+ H3+ O H2O+ H2 H3O+ H2O H2O# H3+ e- C O H2 H H OH+ OH OH# CH2 N O O O2 O2# CO CO#
H3+ e- NO N N2 N2# HCO+ H3+ H3+ e- e- HNO+ HN2+ = stable end product
DiscDisc chemistrychemistry disk: r=2.5AU, n~2.E+13 cm-3, T~70K, Av~3000, after 1E+6 yrs
He CR 5.3(-5) reaction rates [1/s/cm3] NOTE: CR-induced, mainly linear He+ reaction chains, leading from CO → H2O# CO O and C+ 3.5(-5) τ (CO) ~ 1.8 Myrs
1.4(-5) H+, CR or He+ O2+ O2 H2 H2 H2 C+ or He+ O+ OH+ H2O+ H3O+ 2.4(-5) 2.4(-5) 2.5(-5) 2.5(-5) PAH or PAH- 2.5(-5) H2O H2O# 3.3(-5) CRphot NH2 or He+ H2+M O O 7(-6) N2 N NH2 HNO NO 1.2(-5) 3.1(-5) 2.1(-5) 2.1(-5) O2+ 1.4(-5) 1.6(-5) e- NO+ midplanemidplane evolutionevolution ofof TOTALTOTAL GASGAS && ICEICE
midplanemidplane evolutionevolution ofof GASGAS
MidplaneMidplane evolutionevolution ofof ICEICE
SummarySummary
● observational evidence for carbon planets is … thin (just be careful with press releases)
● core-accretion planet formation → large variety of C/O in planetary atmospheres to be expected, but details are unclear - efficiency and time-dependence of gas/solid segregation in disc - survival of ice during oligarchic growth - re-mixing of dust, ice and gas during/after run-away gas accretion ● gas chemistry in midplane should switch from O-rich→C-rich after a couple of Myrs, due to CR unblocking of CO and subsequent H2O# formation, leading to organic molecules
● interesting region is between H2O ice-line (~100K, 0.5-2 AU) and CO ice-line (~20K, 10-40 AU), relevant for planet formation
● paper with Christiane Helling in prep.