Is there evidence of planets in debris disks?

Mark Wyatt

Institute of AstronomyUniversity of Cambridge

La planètmania frappe les astronomes Kalas, P. 1998, La Recherche 314, 38

Is there evidence for planets in debris disks?

Yes! Eridani has both a dust disk (Greaves et al. 1998) and a planet detected by radial velocity surveys (Hatzes et al. 2000)

But radial velocity planets and debris disks are at different locations and it is unclear to what extent the two phenomena are related (Greaves et al. 2004; Beichman et al. 2005)

Do debris disks contain evidence for planets?

• What signature would a planet impose on a debris disk?

• Have these signatures been observed?

• Is there any other possible cause of these signatures?

• Can we make further testable predictions?

Central cavities

Central cavities were inferred from the lack of mid-IR emission in the SED:

Telesco et al. (2000) Kalas et al. (2005)

HR4796 Fomalhaut

But it was imaging which proved the existence of the inner holes:

Walker & Wolstencroft (1998)

Wavelength, m

Log(F

, Jy)

1 10 100

Central cavities: without planets P-R drag would fill in the hole?

Without planets to scatter or trap dust in resonance, P-R drag would fill in the inner hole in tpr = 400r2/Mstar years

Roques et al. (1994)

With and Without Planets

Kuiper Belt dust distribution

Liou & Zook (1999)

Num

ber

densi

ty

Distance, AU

This was the model proposed to explain the hole in the Pictoris disk (>5Mearth planet at 20AU)

Central cavities: no, P-R drag is insignificant

The balance of P-R drag and collisions results in a surface density that depends only on o = 5000(ro)[ro/M*]0.5/

Wyatt (2005)

Tenuous disks

Dense disks

Tenuous disks 0 < 1 flat density distribution P-R drag dominatedDense disks 0 > 1 dust confined to planetesimal belt collision dominated

P-R drag is insignificant in all detectable debris disks

o is an observable parameter, which for the known debris disks is >10

Origin of the inner holes?

• Lack of mid-IR emission implies few colliding planetesimals in inner regions (Wyatt 2005)

• Few planetesimals expected in middle of planetary systems as planets clear gaps along their orbits (Wisdom 1980)

• Growth of planetesimals into planets is faster closer to the star resulting in the formation of inner holes (Kenyon & Bromley 2002)

• Could the early evolution of circumstellar disks also produce inner holes?

• Radial transport of dust (Takeuchi & Artymowicz 2001)

• Viscous draining of inner disk (Clarke et al. 2001)

Inner holes are weak, though credible, evidence of planets

Secular perturbations: warps

A planet aligns planetsimals to its orbital plane so that a disk is warped if

• one planet is misaligned with the disk (Augereau et al. 2001)

• two planets with different orbital planes

Augereau et al. (2001)

Secular perturbations are the long term effect of the planet’s gravity and act on all disk material over >0.1 Myr timescales

A planet's gravity affects the orbits of planetesimals and dust in a debris disk.Perturbations from a planet can be secular or resonant (Murray & Dermott 1999).

Secular perturbations: spirals and offsets

Planets on eccentric orbits impose eccentricities on nearby planetesimals causing:

Wyatt et al. (1999)

spiral structure offset centre of symmetry

Wyatt (2005)

Resonant perturbations: clumpy rings

Resonances cover small regions of parameter space, but can be filled:

• Inward migration of dustDust spirals in due to P-R drag andresonances halt inward migration

• Outward migration of planet Planet migrates out sweeping planetesimals into its resonances

Resonant filling causes a clumpy ring to form along the planet’s orbit

Pl

Resonance

Star

Pl

Resonance

Star

Resonances affect material at locations where orbital periods are a ratio of two integers times that of planet: Pres = Pplanet*(p+q)/p

Why resonances are clumpy

Dust migration into resonance

Dust created in the asteroid belt spirals in toward the Sun over 50 Myr, but resonant forces halt the inward migration…

Earth

Sun

Ozernoy et al. (2000) Wilner et al. (2002) Quillen & Thorndike (2002)

Dermott et al. (1994)

…causing a ring to form along the Earth’s orbit

Time

Semimajor axis, AU

Models of dust migration into planetary resonances have also been applied to debris disks

Summarising dust migration structures

The type of structure expected when dust migrates into planetary resonances depends on the planet’s mass and eccentricity (Kuchner &

Holman 2003):

I low mass, low eccentricity e.g., Dermott et al. (1994),

Ozernoy et al. (2000) Eri

II high mass, low eccentricity e.g., Ozernoy et al. (2000) Vega

III low mass, high eccentricity e.g., Quillen & Thorndike (2002)

IV high mass, high eccentricity e.g., Wilner et al. (2002), Moran et al. (2004)

Resonant structures due to planet migration

Wyatt (2003)

Resonant structures due to planet migration

Wyatt (2003)

Have these signatures been observed?

Warps

Spirals

Offsets

Brightness asymmetries

Clumpy rings

Yes!!

Other causes of signatures? collisions

Could this be the cause of the clumps?

Yes for clumps seen in the mid-IR around young systems (e.g., Pictoris):• smaller colliding objects, ~100km• witnessed at special point in time

No for clumps seen in the sub-mm (e.g., Fomalhaut):• the collision would have to involve two >1400 km objects• too few can coexist in the disk for this to be likely

Wyatt & Dent (2002)

Telesco et al. (2005)

Other causes of signatures? ISM sandblasting

If ISM sandblasting of a debris disk is important, substantial asymmetries can arise…

… however, the ISM contribution is only important >400 AU from the star

Motion relative to the ISM

Artymowicz & Clampin (1997)

Other causes of signatures? binary companions

As well as truncating disks, binary companions can also impose spiral structure and asymmetries…

Augereau & Papaloizou (2003)

Quillen et al. (2005)

Secular perturbations cause asymmetric extended structure

Tidal perturbations cause open two armed structure

… but the binary companions cannot explain all the spiral structure in the HD141569A disk (Wyatt 2005)

Other causes of signatures? stellar flybys

Stellar flybys induce perturbations which excite eccentricities which cause spiral structure which collapses into nested eccentric rings

Such an event may explain clumps seen in the NE of the Pictoris disk

Kalas et al. (2000)

Larwood & Kalas (2001)

However, flyby encounters of field stars at an appropriate distance to perturb the disk (<1000 AU) are extremely rare

Exoplanet statistics?Is it too early to consider the statistics of these putative planets in debris disks?

Perhaps it is, but these planets occupy a region of parameter space unexplorable with other techniques

Thus it is vital to confirm their existence

Debris disk planet predictions

• Detect planet itself directly or indirectly: hard

• Multi-epoch imaging:

• Resonant structures• orbit with planet• decade timescales• 2 detection of rotation in Eri (Greaves et al. 2005)

• Secular structures• >0.1Myr timescales

• Multi-wavelength imaging:• can be done now!

Summary of the Vega planet migration model

Orbit Distribution Spatial Distribution Emission Distribution

• Vega’s two asymmetric clumps seen in the sub-mm can be explained by the migration of a 17Mearth planet from 40-65AU in 56 Myr

• Most planetesimals end up in the planet’s 2:1(u) and 3:2 resonances

Observed

Model

Wyatt (2003)

Dynamics of small bound grains• Radiation pressure alters the orbital periods of small dust grains and so their relation to the resonance• Libration widths increase for small grains until they fall out of resonance for

>0.0020.5 or D<200m (Lstar/Mstar)-0.5

where =Mpl/Mstar in Mearth/Msun

Wyatt (submitted)

3:2

2:1

Dynamics of small unbound grains• Radiation pressure puts small (>0.5) grains on hyperbolic trajectories• The collision rate of planetesimals in resonance is higher in the clump region

Wyatt (submitted)

3:2 2:1

Particle populations in a resonant disk

Population Spatial distributionI Same clumpy distribution as planetesimalsII Axisymmetric distributionIII r-1 distribution IIIa Spiral structure emanating from resonant clumps IIIb Axisymmetric distribution

Wyatt (submitted)

3:2

2:1

Application to Vega

SED modelling used to convert Su et al. 3 component model into continuous size distribution to assess contribution of different grain sizes to observations in different wavebands

Wyatt (submitted)

Observations in different wavebands sample different grain sizes, thus multi-wavelength images should have different structures and can be used to test models

For Vega:• Sub-mm observations sample population I grains• Mid- to far-IR observations sample population III grains

Application to VegaSize distribution used to derive collisional lifetimes of different grain sizes

Wyatt (submitted)

Conclusions:• Population II reduced by collisions with blow-out grains (Krivov et al. 2000)

• Population III grains removed at 2M/Myr• Population II destroyed at 0.02M/Myr• Population III is type IIIa and mid- to far-IR images should exhibit spiral structure emanating from clumps

Size distribution Collisional lifetime

Conclusions

• Planets would impose structures on debris disks ranging from clumps to warps, offsets, brightness asymmetries and spirals

• All of these structures have been observed in debris disks and (in most cases) there is no other explanation

• Planets also cause holes, but this is weak evidence of planets

• This is a credible and unique exoplanet detection technique

• We need to confirm planetary interpretation through• multi-epoch imaging • multi-wavelength imaging