Exoplanets: a dynamic field Alexander James Mustill Amy Bonsor, - - PowerPoint PPT Presentation

Alexander James Mustill

Amy Bonsor, Melvyn B. Davies, Boris Gänsicke, Anders Johansen, Dimitri Veras, Eva Villaver

Exoplanets: a dynamic field

Solar System

0.1 au 1 au

The (transiting) exoplanet population

Lissauer et al 14 Hot Jupiters:

• ccurrence ~1%

Super-Earths/ Neptunes: occurrence ~50%

Orbital eccentricities often high: evidence of dynamical interactions

Planet–planet scattering

Chambers+96

Time until first close encounter in 3-planet systems

• Planet’s gravity dominates in its

Hill sphere (~Roche lobe) rH = a(Mpl/3M★)1/3

• This is a region where strong

scattering is possible: orbits change radically on < orbital timescales

• Closely-spaced planets can

experience this, with their separation (in Hill radii) setting the timescale for the onset of strong scattering

Scattering explains eccentricity distribution of giant exoplanets

Raymond+11

• Distribution well-

reproduced if ~75-80%

• f giant planets form in

unstable multi-planet systems (e.g., Juric & Tremaine 08, Raymond+11)

Lidov–Kozai effect

• Subclass of secular interactions
• ccurring at high inclination
• Inclination and eccentricity couple

together, conserving Lz = [a(1 – e2)]1/2 cos I

• Can drive e to very high values if

starting from low-e, high-I state

• Important for planets with a wide,

inclined, binary stellar companion; and planets after scattering excites

• rbital eccentricities or inclinations

Fabrycky & Tremaine 07 HD80606b

Tidal effects

• When bodies (planets, stars,

moons,…) are close, they feel a differential gravitational force across their volume

• Distortion can lead to energy

dissipation and an exchange

• f angular momentum

between orbit and spin

• Extremely strong function of

physical radius/orbital separation

• Severe distortion can result in

total disruption of a body at the Roche limit

aRoche = (3M★/Mpl)1/3Rpl = (3ρ★/ρpl)1/3R★

Star–planet tides: circularisation and orbital decay

semi-major axis eccentricity

Tide raised on planet by star dominates: orbital energy lost but

• rbital angular momentum constant

Tide raised on star by planet dominates: orbital energy and angular momentum lost Planet disrupted by strong tidal force High e excited by perturber

Solar System

0.1 au 1 au

Why hot Jupiters are single

Lissauer et al 14 Hot Jupiters:

• ccurrence ~1%,

usually single or with wide-orbit (≳1au) companion Super-Earths/ Neptunes: occurrence ~50%, single or multiple

High-eccentricity Hot Jupiter Migration

Scattering/Kozai/secular Tidal circularisation Rasio & Ford 96, Wu & Murray 03, Fabrycky & Tremaine 07, Wu & Lithwick 11, Petrovich 15…

Destruction of any inner planets by high-eccentricity giants

~0.1 au 3 au 100 au

Mustill+15, 17

3 super-Earths Giant planet Binary star

Loss of planets by stellar collision: potential cause of chemical enrichment

• HD80606: star with eccentric

proto-hot Jupiter (a = 0.45 au, e = 0.93) and binary companion HD80607 (~1000au)

• Binary can drive Kozai cycles
• n planet (Fabrycky &

Tremaine 07)

• HD80606 appears slightly

metal-rich compared to HD80607 (Liu et al submitted)

Fabrycky & Tremaine 07

Loss of planets by stellar collision: potential cause of chemical enrichment

Liu et al submitted

Indirect evidence for planetary systems around white dwarfs (review: Farihi 16)

• Spectroscopic signatures of

metals accreted into ~40%

• f WD atmospheres
• Dust discs detected

through IR excesses

• Gas discs detected through

Keplerian emission features

• Transits of disintegrating

asteroids

time flux λ flux λ flux λ flux

White Dwarf atmospheric abundances allow determination of planetary/asteroidal bulk composition

Transit + RV = radius + mass ??? WD spectroscopy: detailed elemental breakdown

Jura & Young 2014

3.0 2.5 2.0 1.5 1.0 0.5 0.0

Mass of Star [Solar Mass]

10

• 3

0.01 0.1 1 10 100 103 100 10 1 0.1 0.01

Semi-Major Axis [Astronomical Units (AU)] Planet Mass [Earth Mass]

exoplanets.org | 3/28/2017

Survive engulfment Engulfed by giant star

⊕ ☿ ♀ ♂ ♃ ♄ ⛢ ♆

Present-day Solar radius Mustill & Villaver 2012 survival limit

Effects of stellar evolution on planetary orbits

• Mass loss causes orbits to expand
• Conserve angular momentum L = [GM★a(1 - e2)]1/2
• If mass loss slow compared to orbital timescale, e is an

adiabatic invariant

• Orbits expand with afinal/ainitial = M★initial/M★final
• Factor ~3 expansion for a typical 2M⦿ progenitor
• WD should be surrounded by a cleared volume of several au

radius: how to get material to the surface?

Effects of stellar evolution on planetary orbits

• Mass loss destabilises

formerly stable systems (Debes & Sigurdsson 02)

• Planetary dynamics

set by planet:star mass ratio Mpl/M★

• Planets’ Hill spheres

increase in size faster than orbits expand: rH = a (Mpl/3M★)1/3

Mustill+14

Delivery of asteroids/comets to WD during and after planetary instability

AGB tip instability

Mustill+ submitted

Scattering by super-Earth planets well reproduces observed accretion rates

Red: inner belt particles Blue: outer belt particles

Mustill+ submitted

Are super-Earths as common on wide (>several au) orbits as they are on close- in (<1 au) orbits?

Open questions

• How representative are the simulated systems of real ones?
• Effects of non-gravitational forces (radiation, gas drag,…) on the

planetesimals

• Circularisation of planets and planetesimals: compare to occurrence of

transiting close-in bodies

• How do the simulated scattering rates relate to the observed accretion

rates? Material will collisionally process, and pass through the dust and gas discs…

• Can all phenomena (accretion rates, disc properties, occurrence rates
• f transiting bodies) be quantitatively reproduced?
• Postdoc wanted! Deadline 22nd February!

Conclusions

• Planetary systems are evolving, dynamic entities
• Planet–planet or planet–star interactions can significantly

change systems after their formation

• High-eccentricity migration of hot Jupiters explains

why they do not commonly have close companions

• Stellar evolution can trigger qualitative changes in

planetary dynamics

• Wide-orbit super-Earth planets are a good candidate

for delivering material to white dwarfs