EXOPLANET LECTURE PLANET FORMATION

Dr. Judit Szulagyi - ETH Fellow ([email protected]) I. YOUNG STELLAR OBJECTS AND THEIR DISKS

(YSOs) Star Formation

4 6 • Young stars born in 10 – 10 MSun Giant Molecular Clouds. • Massive clumps form in Giant Molecular Clouds, these clumps fragments into smaller cores, which then collapse and star formation begins

Spectral Energy Distribution (SED)

• At each wavelength, we measure the young stellar object brightness: • Star is a blackbody • If there is dust around it, then there is infrared excess

Classification of YSOs Evolutional Sequence of YSOs Debris disk (only Debris disk dust) Disk Envelope + disk Envelope In class activity

• SED components In class activity

When do planets form?

• What do you think? • Help: what we need to build planets? Evolutional Sequence of YSOs When do planets form?

• We need: dust (+gas for giant planets), low enough temperatures Class I and II • Observations tell us: already starts within the first million year Observed Planetary Mass Histogram Observed Planetary Mass Histogram

• Lot of terrestrial planets • Lot of Super-Earths/mini-Neptunes • Lot of ice giants (like Neptune and Uranus) • Few gas giant planets Building Blocks of Planets

• Sticking, bouncing, fragmenting, mass-transfer, depending on the relative velocities • Many problems with how to build planetesimals (very active research field currently) THE TWO REGIMES:

Terrestrial and Giant Planet Formation Snowline

• Terrestrial planets (rocky, no water originally): form within snowline • Giant planets (contain water): form outside the snowline Snowline

• Easier to build larger bodies from icy objects (they stick together easier, like the wet sand on the beach)  formation beyond the snowline is quicker • These icy aggregates continuously migrate to the inner planetary system due to the interaction with the gas and due to the star’s gravity • Within the snowline: dust is dry, takes longer time to build planetesimals • On the other hand: formation timescale scales with orbital timescale, therefore further away from the star the formation timescale is longer • Overall still the outer planetary system is better to build building blocks  continuous supply of pebbles to the inner planetary system Timescale

• Constraints: • Gaseous giant planets need gas for their formation; when gas dissipates from the disk, then giant planet formation stops (3-5 Myr) – upper limit • Rocky planets form mainly by collisions, they only need (dry) dust: they continue forming during the debris disk stage (Class III objects), up to ~20 Myr

Probably giant planets form faster than terrestrial ones TERRESTRIAL PLANET FORMATION • The circumstellar disk cools  more dust condensates out • Dust grains aggregate + stick together building up larger objects. • These objects collide (and fragment), thus some become more and more massive, climbing up the size leather. • These larger bodies then gravitationally attract the smaller ones around them. The region, where the planetesimal's gravity is larger than of the star's is called the Hill-sphere • The feeding zone of the planetesimal is a few times of the Hill-sphere. • Once it accreted all the available material in the feeding zone, the embryo reaches the so called isolation mass (0.01-0.1 MEarth in the inner disk) • As the amount of gas is decreasing in the protoplanetary disk (moving toward the debris disk phase /Class III/), the collisions become more frequent among the bodies. This is when giant impacts start. • Our Moon existence is a proof of this era • Water delivery to Earth GIANT PLANET FORMATION Giant Planet Formation Scenarios

Disk Instability Core Accretion • Direct gravitational • First a solid core forms collapse then it accretes the • Planets form like stars gaseous envelope • • Probably no solid core Bottom-up formation mechanism • Top-down scenario DISK INSTABILITY Gravitational Instability

• Massive circumstellar disk that is gravitationally unstable • spiral arms form and then clumps within • (((same physical mechanism that builds spiral galaxies and trigger star formation within))) • These collapse into proto-planets Pros and Cons

+ Quick (104 – 105 years) – Need gravitationally unstable, massive disk which + Easy to form planets far we rarely observe  away from the star (> 30 (probably it is not the most AU) common formation + We do observe planets even scenario) at few hundreds AU away from their star – Need short enough cooling time: clumps only collapse if they are cool – Works only far away from the star (most planets we observe are within 10 AU) But: planets do not necessarily form in-situ

• Migration: interaction with the gas, changing their orbit, usually getting closer to the star • Type I: • Type II: But: planets do not necessarily form in-situ

• Planet-planet scattering: dynamical interaction between planets that can throw on planet to wider orbit or to eject from the system • Free-floating planets • Captured planets CORE ACCRETION Core Accretion Stages

• Pollack et al. 1996 • 1D model • 3 main stages • Solid core

Pollack+96 Accretion to the planet via disk (in Phase III)

• Circumplanetary disk acts like a bottle-neck for accretion: slows down by a factor of 40 (Szulagyi+14)

-6 • 2x10 MJupiter/year: 500,000 years mass doubling time for Jupiter • This is where the satellites can form Pros and Cons

+ Works within 50 AU – Slow (couple Myrs), + Works in low-mass longer than the gaseous disks (the majority of disk lifetime disks we observe) – Does not work beyond 50 AU Update: Pebble Accretion

• Accretion of cm-dm sized pebbles can enormously speed up the first (two) phases of the core accretion (in contrast to the classical, km sized planetesimals) • Directly form embryos from pebbles • The timescale of accretion can be shortened by a factor of 30-1000 at 5 AU, and 100-10000 at 50 AU  no more timescale problem • Necessities: large amount of dm sized pebbles in the midplane, km sized embryos needed as seeds • These embryos can be formed e.g. by the streaming instability Streaming Instability

• Local dust-overconcentration, that collapses into planetesimals Streaming Instability

Anders Johansen MASTER THESIS?

Creating Scattered Light Synthetic Images of Disks with Embedded Planets

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Simulation Observation with SPHERE