How inner planetary systems relate to inner and outer debris belts - PowerPoint PPT Presentation

How inner planetary systems relate to inner and outer debris belts Mark Wyatt Institute of Astronomy, University of Cambridge

The Solar System’s outer and inner debris belts Outer debris: Kuiper belt Inner debris: Asteroid belt + comets Mars The Sun has belts of planetesimal debris at ~40au and 2-3au that collide and fragment into dust observable from Earth as the zodiacal cloud

Some nearby stars have outer and inner debris Cold dust at 150au (Wyatt+05; Duchene+14; Panic+in prep) Herschel 70µm η Corvi is a nearby 18pc ~1Gyr main sequence F2 star exhibiting dust emission at a two- temperatures, from two belts Hot dust at ~0.7au (Smith+09; Lisse+12; Defrere+14) VISIR 18µm LBTI 10µm

Why we think debris systems have planets Fomalhaut (Kalas et al. 2008; 2013) Dust replenished by km-sized planetesimals Debris disks stirred somehow Cleared inner regions Some disks are asymmetric β Pic (Lagrange et al. 2010; Dent et al. 2014; Apai et al. 2014) Some systems actually have planets 2003 2010

Relation of outer and inner debris to inner planets? 10 4 Inner planets Minimum planet or disk mass (M Earth ) Outer J planets 10 2 S U N 10 0 V E Inner M debris + M RadVel terrestrial 10 -2 KB Imaging Outer planets Transit debris Other Disks AB 10 -2 10 -1 10 0 10 1 10 2 10 3 Semi-major axis (AU)

Exoplanet stars don’t always have debris Spitzer survey of stars with planets from radial velocity studies found no difference in fractional luminosity distributions of the disks around stars with and without planets (Bryden et al. 2009) Explained as debris is >>10AU and planets are <<10AU, but conditions that form detectable planets could have implications for remaining debris (Kenyon & Bromley 2008; Raymond et al. 2012)

DEBRIS: unbiased Herschel survey Herschel imaged debris 30-100au from 8.5pc G2V star 61 Vir (Wyatt et al. 2012) , which also hosts two sub-Saturn-mass planets <1au (Vogt et al. 2010) 6M earth at 0.05AU Radial velocity (m/s) 19M earth at 0.2AU -0.25 0 0.25 Orbital phase While the disk was known by Spitzer, these planets were not known when disk-planet correlations were last considered – reanalyse!

Debris disk low-mass planet correlation Of nearest 60 G stars, 4/6 with low mass Kennedy+15 planets have debris, but 0/5 with high mass planets have debris (Wyatt et al. 2012) Star Planets Debris Wyatt+12 HD20794 3x 2-5M earth <0.4au Dust 25au 61 Vir 3x 5-24M earth <0.5au Dust 30-100au HD69830 3x 10-20M earth <0.7au Dust 1au HD38858 1x 32M earth 1au Dust 30-200au HD102365 1x 17M earth 0.5au No dust HD136352 3x 5-12M earth <0.5au No dust Kennedy+15 Red = discovered since 2010 ! Planet searches around debris disk stars may be fruitful (di Folco et al.) !

Debris disk low-mass planet correlation persists SKARPS sample (99 FGK stars with Correlation also extends >1 RV planet, K<6, b>3 o ) (PI G. Bryden) to M stars confirms tentative correlation 100 μ m 23/85 =27% Of 60 nearest M stars only GJ581, an M3V at 6.3pc 6/12= with four 2-18M earth planets 50% <0.3au (Forveille et al. 2011) has debris, at 25-60au (Lestrade et al. 2012)

Origin of low-mass planet-debris correlation? The formation of a system with low-mass planets is also conducive to the formation of a debris disk that is bright after Gyr – why? If planets start at 8au then migrate in (Alibert et al. 2006), many planetesimals end up outside outermost planet (Payne et al. 2009), which would be dynamically stable if no giant planets to remove it

Kennedy+15 Planets at 1-30au? Planets 1-30au also favoured Minimum planet mass, M earth as secular perturbation timescales from known planets are >60Gyr (Mustill & Wyatt 2009) , so >6au planets may be required to stir the disk RV already sets significant constraints on planets in 1-30au region, but doesn’t rule out disk stirring planets Disk Known See Matt Read’s poster for planets more details on constraints on HARPS detection such planets threshold Radius (au)

Another debris disk – planet correlation Agrees with prediction from planet - Fe/H correlation that detectable SKARPS also found a higher planets form in more massive disk luminosity for stars with protoplanetary disks which also planets (Matthews et al. 2014; Bryden result in higher initial debris et al. in prep) luminosity (Wyatt, Clarke & Greaves 2007) Planet stars Non-planet stars

Planetary systems with inner debris The mid-IR spectrum of 2Gyr HD69830 is similar to Hale- Bopp; ~400K suggests dust at ~1au with no outer belt (Beichman et al. 2005, 2011; Smith et al. 2009) <2.4au Planet mass, M earth 0.1 1 10 100 Radius (au) The dust is just outside (?) 3 Neptune mass planets discovered in radial velocity studies (Lovis et al. 2006)

Exozodi luminosity function Define as fraction of stars with 12 μ m excess greater than R 12 = F disk /F * Correlating Hipparcos FGKs with WISE WISE detection threshold quantifies rarity of large excesses (Kennedy & Wyatt 2013): <120Myr Most 12 μ m excess sources are <120Myr Old stars: R 12 >0.1 is >1Gyr 1:1000, R 12 >10 is 1:10,000 Implications for origin of hot dust and its relation to inner planets?

Models'for'origin'of'extrasolar'hot'dust' In situ origin: Steady state: • • Asteroid belt Only if young • Terrestrial planet formation • Stochastic: • Giant impact External origin: Steady state: • Comets from outer belt • Requires outer belt Dust brought in by P-R drag • Stochastic: • Recent dynamical instability •

Some likely have external origin Keck nulling interferometry at 8-13µm for 47 nearby main sequence stars detected 5 systems with >3 σ significance at 1-2% excess (Mennesson et al. 2014) All were around stars with outer belts (5/20 detected cf 0/20 for those without outer belts) Apart from η Corvi, these levels fit expectations from a model in which dust migrates in from the outer belt by P-R drag (Wyatt 2005)

Dust distribution inside outer belt Dust created in outer belt migrates in by P-R drag getting destroyed in collisions on way, so an outer belt dense enough to detect ( τ >>10 -5 ) has little dust in inner regions (Wyatt 2005) … however at KIN-detectable levels Alternatively the hot dust detections could originate in comets scattered in from outer belt

Good news: inner dust aids Earth-detection As zodiacal dust spirals past Earth it Spitzer flew through the encounters Earth’s resonances and some clump confirming the gets trapped causing a clump of dust that model predictions (Reach follows the Earth (Dermott et al. 1994) 2010) Model of non-axisymmetric zodiacal cloud structure (Shannon et al. 2015) Earth Spitzer’s Imaging structures in exozodis orbit can be a planet-finding tool (clumps or inner gaps)

Bad news: inner dust also hinders Earth detection If inner reaches of planetary systems are permeated with dust, that creates noise that hinders direct detection of Earth-like planets Stark et al. (2014) Pale blue dot detection not limited by zodi brightness, but exodot detections will be if “exozodis” are >10x Solar System level (Beichman et al. 2006; Roberge et al. 2012; Stark et al. 2014)

How common is faint inner dust? Nulling mid-IR interferometry needed to detect <few% excesses HOSTS: NASA- funded project (PI: Phil Hinz) using LBTI to search ~60 of the nearest stars for 11µm emission from inner dust approaching the 10-zodi limit

Conclusions (1) Outer debris is more frequent around stars with low mass inner planets (maybe as no giant planets to disrupt the disk?) (2) Outer debris is more luminous around stars with inner planets (as planets form in high mass PPD which also makes more debris?) (3) Inner debris is co-located with inner planets, but is rare around old stars (4) Inner debris correlates with outer debris (dust may be dragged in from outer belt, so expect structures indicative of planets) (5) Inner debris could hinder direct imaging of Earth-like planets, but we don’t know how prevalent it is