Time: a new dimension of constraints for planet formation and evolution theory

Christoph Mordasini

S. Jin, P. Mollière Max Planck Institut for Astronomy, Heidelberg, Germany Y. Alibert & W. Benz University of Bern, Switzerland

PLATO meeting Taormina 3.12.2014

Adding the temporal dimension with PLATO

“Stellar masses and ages will be tightly constrained by the systematic use of asteroseismology . … Ages will be known to 10% for solar-like stars.”

For the first time with PLATO: population-wide and accurate planet evolution in time as a measurable quantity on the observational side

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On the theoretical side: temporal evolution: a fundamental and omnipresent concept

Radius evolution and formationRadius evolution in time depends on

New constraints to better understand planet formation

Implication for planet formation

Formation location, migration, opacity in the protoatmosphere, planetesimal / pebble accretion

• Bulk composition (H/He, silicates, Fe, ices)

Accretion history, planetesimal size• Energy transport in the interior (convection)

Enrichment, formation location• Atmosphere & opacity

Efficiency of H/He accretion• Atmospheric escape/evaporation

Migration mechanism (Kozai, disk migration)

• Bloating mechanisms

(Giant) planet formation mode, gas accretion shock structure

• initial thermodynamic state (how hot initially / entropy)

Mordasini 2013Composition Init. cond.

Temporal evolution: giant planetsFo

rtney

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11 Atmospheric model

Guillot & Show

man 2002

Inflation

Leco

nte

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habr

ier 2

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Energy transport

Temporal evolution low-mass planet

All these R(time) curves are not directly observationally constrained

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Rad

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[Ear

th r

adii]

Time since formation [Myr]

total radius w/ evap

core radius 1

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0 2 4 6 8 10 12

Rad

ius

[Ear

th r

adii]

Time since formation [Myr]

total radius w/o evap Envelope mass

Time since disk dissipation [Myr]

10-5

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10-3

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10-1

0 2 4 6 8 10 12E

nvel

ope

Mas

s [E

arth

mas

ses]

Time since formation [Myr]Time since disk dissipation [Myr]

Venus atmosphere mass

Mini-Neptune a=0.05 AU Mcore=4 MEarth Menv=0.1 MEarth

Linit=0.1 LJ Earth-like core (0.33 iron, 0.66 silicate) X-rays & EUV evaporation Solar-composition opacity

-fast decrease from ~2 REarth to R=Rcore when envelope totally lost

Temporal evolution of the a-M diagram

Mstar=1 Msun , isothermal type I migration rate x 0.1, cold accretion. 1 embryo/disk, no special inflation mechanisms.

a-M does not change much. (a>0.06 AU)

Close-in, low-mass loose the envelope.

Most of the action early on.

Black: Bare rocky coresJin et al. 2013

Output of core accr. population synthesis

Thermodynamic evolution (cooling & contraction) in time w. atmos. escape

Only thermodynamic evolutionNot dynamics

Mstar=1 Msun Isothermal Type I rate x 0.1. Cold accretion. 1 embryo/disk, no special inflation mechanisms.

Black: Bare rocky cores

Artifact of using Mmin=1 MEarth

Temporal evolution of the a-R diagram

-Contraction -Evaporation -Empty valley below 1-2 REarth

a-R does change!

Radius distribution as a function of time

0.1 Gyr 1 Gyr

10 Gyr

Radius distribution as function of time:

Key constraint for planet formation and evolution theory

Radius alone sufficient No mass needed

2

and

Observed version by PLATO come back in 2030 :-)

Have this in 2030 not from theory, but from PLATO data!

Transition solid - gas rich planets• Kepler: numerous close-in small planets: 49% of stars (P<100 d, R<4 REarth)

• Many are low-density planets (likely with a few percent of H/He)

• Important for formation theory, but also habitability

• At which mass and orbital distance is the transition? In principle simple solution with time dimension:

• Solid planets: mean density ~constant in time

• Gaseous planets: mean density increases: contraction, evaporation

• Comparison with spectroscopy and dynamics of multiple planet systems

Not only giants planets

1 Msun star. Initial conditions from population synthesis.

Transition solid - gas rich planets

Search for the transition in the M-rho-t space

Giants: hotter less dense: bloating

Low mass: hotter denser: evaporation

1 10 100 1000 10000Mass [Earth masses]

0.1

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Mea

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[g/c

m3 ]

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Msin(i) [Earth Mass]

Plan

etar

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ram

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entim

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s3 ]exoplanets.org | 11/1/2013

exoplanets.org cf. Rauer et al. 2013

Earth like

Pure ice

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Msin(i) [Earth Mass]

Plan

etar

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ensi

ty [G

ram

s/C

entim

eter

s3 ]exoplanets.org | 11/1/2013

exoplanets.org cf. Rauer et al. 2013

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/cm

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10 Gyr

Origin of close-in low-mass planets

• Essential open question for planet formation theory • Rapid migration from beyond the iceline: ice-rich interior • In situ formation or slower migration: rocky interior

• Mass-radius relation degenerate: iron/silicate/ice/H-He

• If we can rule out (observationally) that a sub-population of planets can have a significant H/He atmosphere, break degeneracy

Origin of close-in low-mass planets

Planets with density of 2-3 g/cm3 in the “static” part of the a-R (no time evolution i.e. no H/He envelope) must be icy

Marcy et al. 2014

ObservationSynthetic

Conclusions-PLATO’s ability to accurately determine the ages of the host stars adds a new class of constraints for formation theory

-With the time dimension, it should become possible to observationally see -the transition from solid to gas-rich planets -the composition of close-in low-mass planets

-This constrains several key concepts of planet formation theory like -efficiency of H/He accretion -orbital migration

-The mass is sometimes not necessary for these new constrains: the radius and the age are sufficient