Yasunori HORI Are Super-Earths Orbiting Cool Stars

Are Super-Earths Orbiting Cool Stars Water Worlds Surrounded by Thick Atmospheres?

Yasunori HORI University of California, Santa Cruz

Thanks to Shigeru Ida (Tokyo Tech.) D.N.C. Lin (UCO/Lick, UCSC) M. Ikoma (Univ. of Tokyo) H. Genda (Tokyo Tech.) Exoplanetary Sciences (Apr.20-26, 2014) in Quy Nhon, Vietnam The First Earth-sized Habitable Planet Candidate (see also Lunine’s talk) NASA has released an amazing news on Apr.17, 2014:

Kepler-186 f with 1.1 Earth-radius orbiting a cool star is in the habitable zone This system has 5 Earth-sized planets (1.07-1.4 ) in almost coplanar orbits

(Quintana et al. 2014, Science) 0.356AU

Kepler-186 f Ensemble of Super-Earths From the Kepler Around low-mass stars and Sun-like stars, (see also Dong’s & Howard’s talks) low-mass planets are common

(Dressing & Charbonneau,2013) (Petigura et al,2013)

Late-K, early-M dwarfs

= 5-100 days Sun-like stars P having Fraction of stars w/ planets Number of planets per stars 0.5 0.7 1.0 1.4 2.0 2.8 4.0 5.7 8.0 11.3 16.0 22.6 32.0 1.0 1.4 2.0 2.8 4.0 5.7 8.0 11.3 16.0 Planet radius ( ) Planet radius ( )

For 0.5 - 4 planets w/ P < 50days For 1 - 4 planets w/ P < 50days +4 Occurrence rate = Occurrence rate = 90 -3 % 35.8±4.8% Diverse Bulk Composition of Close-in Super-Earths high-density super-Earths composed of iron/rocky material (e.g.) CoRoT-7b, Kepler-10b low-density super-Earths contain abundant H/He and/or volatile elements (e.g.) GJ3470b, Kepler-11e 8 7 Low-density SEs Kepler-18d Kepler-87c HAT-P-26b 6

Kepler-18c 5 HAT-P-11b Kepler-11e GJ3470b GJ436b Neptune Kepler-4b 4 Kepler-36c Uranus Kepler-11d Kepler-20c H2O 100% 3 GJ1214b Kepler-20d MgSiO3 Kepler-11c Kepler-11f Kepler-68b HD97658b Kepler-18b 55 Cnc e Earth-like Planetary radius ( ) 2 Kepler-11b Kepler-20b Kepler-36b CoRoT-7b Fe 1 Kepler-10b Earth Kepler-68c High-density SEs 0 Mercury 0 5 10 15 20 25 30 Planetary mass ( )

Transmission spectra from 5 Planets are obtained via primary eclipses (GJ 1214, GJ3470b, GJ436, HD97658b, HAT-P-11b) Peering into Atmospheres of Super-Earths

GJ 1214b

No Rayleigh scattering ?

(Narita et al.2013)

(Fukui+13;Nacimbeni+13;Crossfield+13)

Rayleigh slope low-µ atmosphere (e.g.) H2-rich or Flat features (featureless) GJ 3470b Rayleigh scattering ? high-µ atmosphere (e.g.) H2O vapor Transmission Spectra from HST/WFC3

GJ 1214b GJ 436b (Kreidberg et al.2014) solar + 1mbar cloud (Knutson et al.2014) 1900 x solar solar

solar 100% H2O

100% CH4 depth (ppm) Transit 100% H2O 100% CO2 1.0 1.2 1.4 1.6 1.8 Wavelength (µm) Relative transit depth (ppm)

HD 97658b (Knutson et al.2014) 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (µm)

Super-Earths/mini-Neptunes have likely or solar + 1mbar cloud high-µ atmospheres depth (ppm) Transit hydrogen-rich atmospheres solar 50x solar 100% H2O w/(o) and/or 1.0 1.2 1.4 1.6 1.8 hazes clouds Wavelength (µm) Monte Carlo Simulations of Planet Formation (see also Kokubo’s & Ida’s talks)

② Planet-planet scattering (e.g. Rasio & Ford,1996;Weidenschilling & Marzari,1996) ① Giant impacts

unstable

Type I migration ③ ④ Type II migration (Goldreich & Tremaine,1980 (Lin & Papaloizou,1986;Ward,1997) Ward,1986; Tanaka et al.2002)

Gap

water水の存在量 content (wt%) (wt%) 100 100 (YH, Ida, & Lin, ApJ submitted) ■ close-in super-Earth Planetary migration+giant impacts 80

10 in situ formation 60

40 1 惑星質量 ( )

Planetary mass ( ) 20

0.1 0 0.01 0.1 1 10 100 Semimajor軌道長半径 axis (AU) (AU) Super-Earths Orbiting Cool Stars are Water worlds Less than 30 planets within 0.01-0.1AU → close-in super-Earths = water worlds (>10wt%) (e.g.) GJ1214b, GJ 436b, GJ 3470b 100

0.1

Water content (wt%) 0.01 0.01-0.1AU 0.1-1AU 1-10AU 0.001 0.01 0.1 1 10 30 Planetary mass ( ) Super-Earths Surrounded H/He Blankets Sub-Earths : little primitive atmosphere (< 0.1wt%) (Close-in) super-Earths (1-10 ) --- 0.1-30wt% atmospheres H/He-dominated 100 50wt%

1 0.01-0.1AU

H-He atmosphere (wt%) 0.01-0.1AU0.1-1AU 0.1-1AU 1-10AU1-10AU 0.1 0.1 1 10 100 Planetary mass ( ) Impact'Erosion'of'Water'MantleMantle Stripping of Water-dominated Planets Rocky-frac7on fraction SPH-simula7on- Rocky ...-5-Earth

] 3.0 ) 80

2.8 esc 2 70 H O 75%

Water-75% Water-75% V 2.6 60 Rock Rock25% Rock25%H2O 75% 2.4 50 25% θ 2.2 40 Rock 2.0 25% 1.8 30 1.6 1.4 Impact-Velocity-[ 1.2 Ordinary-Impact Impact velocity ( 1.00102030405060708090 CollisionImpact-Angle-[ angledeg (deg)] Li#le&change&in&composi0on& ...hard&to&make&varia.on&

θ = 15° (Genda & Ikoma, in prep.) Giant impacts change little the ratio Escape of H/He Atmospheres via Giant Impacts

SPH$simula+on$ rocky$frac+onRocky fraction ...$5$Earth$mass$planets(Two super-Earth) 90% 100%

H/He$10% H/He$10% ] 3.0 )

H/He 10% 2.8

esc 96

V Rock$90%Rock $Rock90% 2.6 H/He 10% 90% 2.4 98969492 θ Rock 2.2 90% 2.0 1.8 1.6 1.4 Impact$Velocity$[ Impact velocity ( 1.2 Ordinary$Impact 1.00102030405060708090 CollisionImpact$Angle$[deg] angle (deg) Genda$&$Ikoma$(in$prep.) can remove θ = 15° (Genda & Ikoma, in prep.) Giant impact a large amount of H/He atmosphere Can SEs Accumulate Disk Gas during Post-GIs? 100 0.1AU low-density super-Earth (SE) 0.25AU (e.g.) Kepler-11e, GJ3470b 2AU 10

high-density SE 1 (e.g.) Earth, Kepler-10b accreted atmospheres (%) 獲得大気の質量割合 (%) (Ikoma & Hori, 2012)

Mass fraction of critical points 0.1 1 10 MassSuper-Earthの質量 of Super-Earth ( ( ) ) Bimodal pattern : (a) less than 10% H-He atmosphere (b) (more than) 40-50% H-He atmosphere Mass Loss of Close-in Planets via Thermal/Non-Thermal Escape (see also Lecavelier des Etangs’s review) (a) Intense stellar X-ray/EUV irradiation → Hydrodynamic escape from a heated upper atmosphere (e.g. Lammer+03;Lecavelier des Etangs+04;Yelle,04;Erkaev+07) Penz & Micela 2008; Lopez+12; Owen & Wu 2013; Lopez & Fortney,2013)

(b) H+ , O- pick-up due to stellar wind／coronal mass ejection in particular, around weakly- or non-magnetized planets (Khodachenko et al.2007;Lammer et al.2009; Terada+09)

(a) Thermal process (b) Non-thermal process

X-ray mass loss EUV Star Star Planet Energy-limited hydrodynamic escape (e.g. Yelle,2008)

SEs inside 0.1AU are likely to experience “destructive hydrogen escape” (e.g. Owen & Wu 2013; Lopez & Fortney,2013) Survival Limit of Close-in Water Planets

Close-in SEs orbiting cool stars can retain water during 1Gyr!

(Kurosaki, Ikoma & YH, 2014) (Kurosaki, Ikoma & YH, in prep.) 10 10 55 Cnc e 2000 G-dwarfs 5 CoRoT-7b Kepler-10b [K] 1500 M-dwarfsretain H2O retain H2O 平衡温度

惑星質量 ( ) 1 HD10180b

1 α Centauri Bb 1000 惑星質量 [地球質量] Equilibrium temperature (K) Planetary mass ( ) 0.5 lose H2O completely

0.3 500 0.010.01 0.03 0.05 0.1 0.1 Semimajor軌道長半径 [AU]axis (AU) Take-home Messages

For (close-in) super-Earths orbiting cool stars,

Waterworlds? (>20-30w%) ̶ Likely but water loss via GIs and stellar XUV ? ̶ Inefficient

Primitive H/He atmospheres? (>1w%) ̶ Likely but mass loss via GIs and stellar XUV ? ̶ Efficient

Close-in SEs are common but HJs are rare Yes as suggested by exoplanet statistics ̶

Metal-rich atmosphere model proposed for Possible transmission spectra of transiting SEs ̶ How Do We Explain Metal-rich Atmospheres of SEs ?

(A) Pollution of Icy planetesimal heavy-elements supplied via evaporation/melting and break-up of ablated icy planetesimals in the atmosphere (e.g., Pollack+1986;Podolak+1988; YH & Ikoma, 2012;Fortney+13)

1 (B) Dynamical mixing 10-1

10-2 Planet radius planet radius Planet radius 10-3

10-4

10-5 Break-up 10m-sized icy planetesimals

Nomalized planetesimal mass planetesimal Nomalized 100m-sized icy planetesimals 10-6 1 10 102 103 Radius (core radius)

(see also Lunine’s, Ehrenreich’s, & Zhou’s talk) 6.5m-telescope launched in 2018? (NASA)

30cm-telescope launched in 2017 (Bern Univ. )

4 x telescopes launched in 2017 (MIT)

TESS (ESA S-class mission) JWST

3.5m-telescope 34 telescopes launched in 2022? (JAXA) launched in 2022-25 PLATO

SPICA (ESA M-class mission) Summary To characterize sub-/super-Earths orbiting cool stars, we generated a series of 103 theoretical population synthesis models

Estimates on the asymptotic amount of water and hydrogen-rich atmospheres of sub-/super-Earths

(1) Super-Earths inside 1AU (including, in a HZ) are likely to be waterlogged planets (more than 20-30wt% water mantle) have thick H2-rich atmospheres (2) Water mantle stripping of giant impacts change water content (3) Super-Earths beyond ~0.1AU may not be in danger of complete water loss due to intense XUV irradiation “ExoEarths around cool stars = water world (multiple-planet system)” NIR RV Transmission Spectra Thank you for your attention