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 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

Mars

Inner debris: Asteroid belt + comets

η Corvi is a nearby 18pc ~1Gyr main sequence F2 star exhibiting dust emission at a two- temperatures, from two belts

VISIR 18µm Hot dust at ~0.7au (Smith+09; Lisse+12; Defrere+14) Cold dust at 150au (Wyatt+05; Duchene+14; Panic+in prep)

Some nearby stars have outer and inner debris

Herschel 70µm LBTI 10µm

Why we think debris systems have planets

Dust replenished by km-sized planetesimals Debris disks stirred somehow Cleared inner regions Some disks are asymmetric Some systems actually have planets

β Pic (Lagrange et al. 2010; Dent et al. 2014; Apai et al. 2014) 2003 2010 Fomalhaut (Kalas et al. 2008; 2013)

Relation of outer and inner debris to inner planets?

10-2 10-1 100 101 102 103 Semi-major axis (AU) 10-2 100 102 104 Minimum planet or disk mass (MEarth) Disks Other Transit Imaging RadVel

M V E M J S U N AB KB

Inner planets Outer debris Outer planets Inner debris + terrestrial planets

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)

6Mearth at 0.05AU 19Mearth at 0.2AU While the disk was known by Spitzer, these planets were not known when disk-planet correlations were last considered – reanalyse! Orbital phase

• 0.25 0 0.25

Radial velocity (m/s)

Star Planets Debris HD20794 3x 2-5Mearth <0.4au Dust 25au 61 Vir 3x 5-24Mearth <0.5au Dust 30-100au HD69830 3x 10-20Mearth <0.7au Dust 1au HD38858 1x 32Mearth 1au Dust 30-200au HD102365 1x 17Mearth 0.5au No dust HD136352 3x 5-12Mearth <0.5au No dust

Of nearest 60 G stars, 4/6 with low mass planets have debris, but 0/5 with high mass planets have debris (Wyatt et al. 2012)

Red = discovered since 2010

Debris disk low-mass planet correlation

! Planet searches around debris disk stars may be fruitful (di Folco et al.) !

Kennedy+15 Kennedy+15 Wyatt+12

Debris disk low-mass planet correlation persists

SKARPS sample (99 FGK stars with >1 RV planet, K<6, b>3o) (PI G. Bryden) confirms tentative correlation

6/12= 50% 23/85 =27%

Correlation also extends to M stars 100μm Of 60 nearest M stars only GJ581, an M3V at 6.3pc with four 2-18Mearth planets <0.3au (Forveille et al. 2011) has debris, at 25-60au (Lestrade et

• al. 2012)

Origin of low-mass planet-debris correlation?

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 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?

Planets at 1-30au?

Planets 1-30au also favoured 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

• ut disk stirring planets

See Matt Read’s poster for more details on constraints on such planets

HARPS detection threshold Known planets

Disk

Minimum planet mass, Mearth Radius (au)

Kennedy+15

Another debris disk – planet correlation

SKARPS also found a higher disk luminosity for stars with planets (Matthews et al. 2014; Bryden

et al. in prep)

Agrees with prediction from planet - Fe/H correlation that detectable planets form in more massive protoplanetary disks which also result in higher initial debris 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)

The dust is just outside (?) 3 Neptune mass planets discovered in radial velocity studies (Lovis et al. 2006) Radius (au) Planet mass, Mearth

0.1 1 10 100

<2.4au

Exozodi luminosity function

Define as fraction of stars with 12μm excess greater than R12 = Fdisk/F* Correlating Hipparcos FGKs with WISE quantifies rarity of large excesses

(Kennedy & Wyatt 2013):

Most 12μm excess sources are <120Myr Old stars: R12>0.1 is 1:1000, R12>10 is 1:10,000 Implications for origin of hot dust and its relation to inner planets?

<120Myr >1Gyr WISE detection threshold

Models'for'origin'of'extrasolar'hot'dust'

In situ origin:

• Steady state:
• Asteroid belt
• Terrestrial planet formation
• Stochastic:
• Giant impact

External origin:

• Steady state:
• Comets from outer belt
• Dust brought in by P-R drag
• Stochastic:
• Recent dynamical instability

Only if young Requires outer belt

Some likely have external origin

Keck nulling interferometry at 8-13µm for 47 nearby main sequence stars detected 5 systems with >3σ 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

• uter 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 encounters Earth’s resonances and some gets trapped causing a clump of dust that follows the Earth (Dermott et al. 1994) Spitzer flew through the clump confirming the model predictions (Reach

2010)

Imaging structures in exozodis can be a planet-finding tool (clumps or inner gaps)

Model of non-axisymmetric zodiacal cloud structure (Shannon et al. 2015) Spitzer’s

• rbit

Earth

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 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) 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

• f 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

• ld 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