Exoplanets: a dynamic field Alexander James Mustill Amy Bonsor, Melvyn B. Davies, Boris Gänsicke, Anders Johansen, Dimitri Veras, Eva Villaver
The (transiting) exoplanet population Solar System Hot Jupiters: occurrence ~1% Super-Earths/ Neptunes: occurrence ~50% 0.1 au 1 au Lissauer et al 14
Orbital eccentricities often high: evidence of dynamical interactions
Planet planet scattering Planet s gravity dominates in its Hill sphere (~Roche lobe) r H = a(m pl /3M ) 1/3 Time until first close encounter in 3-planet systems 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 Chambers+96
Scattering explains eccentricity distribution of giant exoplanets Distribution wellreproduced if ~75-80% of giant planets form in unstable multi-planet systems (e.g., Juric & Tremaine 08, Raymond+11) Raymond+11
Lidov Kozai effect Subclass of secular interactions occurring at high inclination Inclination and eccentricity couple together, conserving L z = [a(1 e 2 )] 1/2 cos I HD80606b 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 orbital eccentricities or inclinations Fabrycky & Tremaine 07
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 of 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) 1/3 Rpl = (3ρ /ρpl) 1/3 R
Star planet tides: circularisation and orbital decay High e excited by perturber eccentricity Tide raised on planet by star dominates: orbital energy lost but orbital angular momentum constant Tide raised on star by planet dominates: orbital energy and angular momentum lost Planet disrupted by strong tidal force semi-major axis
Why hot Jupiters are single Hot Jupiters: occurrence ~1%, usually single or with wide-orbit ( 1au) companion Solar System Super-Earths/ Neptunes: occurrence ~50%, single or multiple 0.1 au 1 au Lissauer et al 14
High-eccentricity Hot Jupiter Migration Rasio & Ford 96, Wu & Murray 03, Fabrycky & Tremaine 07, Wu & Lithwick 11, Petrovich 15 Scattering/Kozai/secular Tidal circularisation
Destruction of any inner planets by high-eccentricity giants 3 au ~0.1 au Binary star Giant planet 100 au 3 super-earths Mustill+15, 17
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 on 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% of WD atmospheres Dust discs detected through IR excesses Gas discs detected through Keplerian emission features Transits of disintegrating asteroids flux flux flux flux λ λ λ time
White Dwarf atmospheric abundances allow determination of planetary/asteroidal bulk composition Transit + RV = radius + mass??? Jura & Young 2014 WD spectroscopy: detailed elemental breakdown
exoplanets.org 3/28/2017 3.0 10 3 Planet Mass [Earth Mass] 100 10 1 0.1 Present-day Solar radius Engulfed by giant star Survive engulfment 0.01 10-3 0.01 0.1 1 10 100 Mustill & Villaver 2012 survival limit Semi-Major Axis [Astronomical Units (AU)] 0.0 0.5 1.0 1.5 2.0 2.5 Mass of Star [Solar Mass]
Effects of stellar evolution on planetary orbits Mass loss causes orbits to expand Conserve angular momentum L = [GM a(1 - e 2 )] 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 M pl /M Planets Hill spheres increase in size faster than orbits expand: rh = a (Mpl/3M ) 1/3 Mustill+14
Mustill+ submitted 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 Are super-earths as common on wide (>several au) orbits as they are on closein (<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 of transiting bodies) be quantitatively reproduced? Postdoc wanted! Deadline 22 nd 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