EXOPLANET LECTURE PLANET FORMATION. Dr. Judit Szulagyi - ETH Fellow

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1 EXOPLANET LECTURE PLANET FORMATION Dr. Judit Szulagyi - ETH Fellow (judits@ethz.ch)

2 I. YOUNG STELLAR OBJECTS AND THEIR DISKS (YSOs)

3 Star Formation Young stars born in M Sun Giant Molecular Clouds. Massive clumps form in Giant Molecular Clouds, these clumps fragments into smaller cores, which then collapse and star formation begins

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5 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

6 Evolutional Sequence of YSOs Envelope Envelope + disk Disk Debris disk (only dust)

7 SED components In class activity

8 In class activity

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11 When do planets form? What do you think? Help: what we need to build planets?

12 Evolutional Sequence of YSOs

13 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

14 Observed Planetary Mass Histogram

15 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

16 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)

17 THE TWO REGIMES: Terrestrial and Giant Planet Formation

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

19 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

20 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

21 TERRESTRIAL PLANET FORMATION

22 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.

23 Once it accreted all the available material in the feeding zone, the embryo reaches the so called isolation mass ( M Earth 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

24 GIANT PLANET FORMATION

25 Giant Planet Formation Scenarios Disk Instability Direct gravitational collapse Planets form like stars Probably no solid core Top-down scenario Core Accretion First a solid core forms then it accretes the gaseous envelope Bottom-up formation mechanism

26 DISK INSTABILITY

27 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

28 Pros and Cons + Quick ( years) + Easy to form planets far away from the star (> 30 AU) + We do observe planets even at few hundreds AU away from their star Need gravitationally unstable, massive disk which we rarely observe (probably it is not the most common formation scenario) 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)

29 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:

30 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

31 CORE ACCRETION

32 Core Accretion Stages Pollack et al D model 3 main stages Solid core Pollack+96

33 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) 2x10-6 M Jupiter /year: 500,000 years mass doubling time for Jupiter This is where the satellites can form

34 Pros and Cons + Works within 50 AU + Works in low-mass disks (the majority of disks we observe) Slow (couple Myrs), longer than the gaseous disk lifetime Does not work beyond 50 AU

35 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 at 5 AU, and 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

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

37 Streaming Instability Anders Johansen

38 MASTER THESIS?

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44 Creating Scattered Light Synthetic Images of Disks with Embedded Planets You! Simulation Observation with SPHERE