Observational constraints from the Solar System

and fromExtrasolar Planets

Lecture Universität Heidelberg WS 11/12Dr. Christoph Mordasini Partially based on script by Prof. W. Benz

Lecture 1 Part I

Mentor Prof. T. Henning

Lecture overview

1. Introduction

2. Planet formation paradigm

3. Structure of the Solar System

4. The surprise: 51 Peg b

5. Detection techniques: radial velocity, transits, direct imaging, (microlensing, timing, astrometry)

6. Properties of extrasolar planets: mass, distance, eccentricity distributions, metallicity effect, mass-radius diagram, ...

1. Introduction

Galaxies, stars and planets

10 billion galaxies

100 billion stars

how many planets?

How many harbor life?

How frequent? Life?First generation of human beings with technology to answer this.

Important questions - Planet formation: From dust to planets

10-100 million10 μm

years

- Planet evolution: Habitability

Mars

Venus

Earth

Moon

Terrestrial planets in the solar system: similar initial conditions very different outcome.

The characterization also of exoplanets has just started.

How?

Sun Stars

Herschel’s 1789

For many centuries

Solar System Exoplanets

La Silla Obs. ESO

For a decade

Cloud collapseHertzsprung RusselNuclear FusionStellar Mass Funct.

Formation in disksCollisionsGas accretionMigration

Life Extraterrestrial Life ?

Darwin ESA

In a decade ?

AstrobiologyHabitable ZoneBiomarkersComplex Life

Ways to understanding

2. Planet Formation Paradigm

Core

Accretion

Gravitational

Instability

Planet formation: The paradigm

- remote observations- in-situ measurements- sample returns- laboratory analysis- theoretical modeling

Party line

A satisfactory theory should explain the formation of planets in the solar system as well as around other stars.

Minority line

dust

planetesimals protoplanets

in presence of gas in absence of gas

giant impacts

terrestrial planets

107 years

giantplanets

Sta

r &

pro

topl

anet

ary

disk

migration

type I type II

107 years

108 years

dynamical re-arrangement

Planet formation: Sequential picture

Planet formationInitial conditions, task and orders of magnitude

Task•follow the evolution of the gas and dust for a period of about 100 Million years.

Orders of magnitude to remember•Msun ~2 1033 g•MJ ~ 2 1030 g ~ 1/1000 Msun ~ 318 ME

•ME ~ 6 1027 g•RJ ~ 7.19 109 cm ~ 1/10 Rsun

•RE ~ 6.4 108 cm ~ 1/10 RJ

•AU ~1.5 1013 cm•Lsun ~ 3.8 1033 erg/s

Initial condition•disk of dust and gas orbiting a new born star•total mass of the disk: ~1-10 % of stellar mass•total mass of dust: ~2% of mass of gas

dust(μm)

planetesimals(∼km)

protoplanets

Earth-sized(∼1000 km)

gas giants(∼10000 km)

105-107

years107-108

years

dust sticking

Self.Gravity

runaway growth

oligarchicgrowth

runaway gas accretion

late stagesgiant impacts

size

time104-105 years

Challengesin planet formation

Difficulty: -huge dynamical rage in size/mass- dynamical range in time: 100 million orbital timescales-lots of physics involved, changing over time: gravity, drag, hydrodynamics, radiation transfer, magnetic fields,..- non-linearities (runaway growth)-feedback mechanism (grav. scattering)

3. Structure of the Solar System

Solar systemSystem architecture

Orbital data major planets

Note•Sun has 99.96% of the mass, but only 0.6% of the angular momentum. Solar Prot ~25 d.•LJ/Ltot: 0.61, Lsaturn/Ltot: 0.25•Jupiter is dominating the dynamics. Important during formation (small mars, Asteroids)•mostly circular orbits, all prograde (same rotation direction as the sun)•nearly co-planar orbits: formation in a disk•spacing: Titius-Bode law an=aMercury+0.3 2n-1 n=1,2,...: Orbital stability in Hill units

Inner systemOuter system

Asteroids

Rocky planets

gas giants

ice giants

Solar systemSystem architecture II

Minor bodies

Asteroids•rocky composition, some with significant water content•a few 100’000 known.•total mass 1/30 of lunar mass (1 lunar mass ~1/81 ME): not a destroyed planet.•26 with diameters larger than 200 km. Largest: Ceres 900 km.•2.2 AU < a < 3.2 AU for 95%: between Mars and Jupiter •existence of families (groups with similar orbits and reflectance properties)•All prograde, most have e<0.3 and i<25 deg.•leftovers from formation phase: important obs. constraint on e.g. migration.

Solar systemSystem architecture III

Minor bodies cont.

Trans-Neptunian Objects (TNO) and Kuiper Belt objects (KBO)•icy composition, not much altered (slow evolution). Low albedo (<coal).•estimated 70’000 with diameter >100 km. Larger than typical asteroids.•located beyond Neptune: 30 AU< a < 70 AU.•3 classes:

•classical KBO: 42-47 AU, mean eccentricity ~ 0.07 (small), i < 30 deg.•scattered KBO: large e, total M 0.5-1.5 ME ,source of short period comets, perihel at ~35 AU•Plutinos: 3:2 resonance with Neptune, as Pluto, 0.1<e<0.34, 0<i<2 deg.

Oort Cloud•hypothetical spherical cloud surrounding the sun, extending out 100’000 AU.•Source of long period comets.•Not (yet) directly observed.•Weak gravitationally bound: effect of passing stars.•Objects scattered outwards during planet formation.

Solar systemPhysical properties

Physical data major planets

•Stars: burn hydrogen: M>~75 MJ

•Brown dwarfs: burn deuterium ~13<M/MJ<75•Planet definition (IAU 2006) :A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Approximately to scale

Solar systemPhysical properties II

Composition terrestrial planets

•Inner structure determination: observations (seismic waves, gravitational moments, surface temperature and abundances) combined with modeling.•Terrestrial planets: Iron core, silicate mantle.

•Size of core vs mantle varies: impact history•Earth: core 1/3, mantle 2/3 (in mass). Close to chondritic (primitive meteorites) composition

Earth

Solar systemPhysical properties III

Composition giants

•X=Hydrogen, Y=Helium, Z=”Metals”•Solar composition (primordial): X0 0.71, Y0 0.27, Z0 0.015•The gas giant planets (Jupiter, Saturn) are clearly enriched compared to solar composition. Expected Jupiter solar: 4.8 ME, Saturn solar: 1.4 ME. This is much less than the inferred values. They didn’t form like the sun from the same collapsing cloud. Important constraint•The ice giants consist of ~25% rock, ~60-79% ice, and ~5-15% H/He

Significant uncertainties: equation of state (EOS) of H/He under extreme p and T badly known.

Amount of metals [ME]MJ=~318 ME, MS=~95 ME

Possible J,S compositions Guillot 1999

Historical perspective

Selected discoveries in the Solar System•until 1600 only six planets were known: Mercury, Venus, Earth, Mars, Jupiter and Saturn. Extensively studied since antiquity. •Aristarchus from Samos (270 BC): heliocentric system.•beginning of 17th century: discoveries of satellites of Jupiter and Saturn by Galilei (1564-1642), Huygens (1629-1659) and Cassini (1625-1712).•1781 discovery of Uranus by William Herschel •1846 discovery of Neptune by Johann Galle. Neptune was first theoretically predicted by John Adams and Urbain Le Verrier who studied the perturbations of the orbit of Uranus.•1930 discovery of Pluto by Clyde Tombaugh•1978 discovery of Charon, Pluto’s moon by James Christy•1992 discovery of the first TNO object (QB1) by Jane Luu and Jewitt

Herschel’s big telescope

Historical perspective IISome early formation theories•Rene Decartes (1594-1650)

•space is filled with a universal substance. Planets form in vortices which form at locations of least motions•secondary vortices form around the vortices which make the moons.

•Georges L. L. Buffon (1707-1804)•catastrophe hypothesis: a huge comet hits the sun and ejects material which form the planet. Conceptually similar to the giant hypothesis for Earth’s moon

•Immanuel Kant (1724-1804)•nebula hypothesis (building on similar early work of Emanuel Swedenborg).•nebula composed of gas and dust is flattened by rotation, particles are colliding, loose energy and drift to the center to form the sun•planets form out of local density enhancements which orbit the sun.

•Pierre Simon de Laplace (1749-1829)•planets are formed during the contraction of the sun.•the sun ejects rings of material which cool and form planets.

Swedenborg Kant Laplace

Science, 267, 360 (January 1995)

Oops

Knowledge is evolving. What is believed correct today can turn up wrong tomorrow!

Planet formation theoryState of the art t<1995. Only one example to study..

4. The surprise: 51 Peg b

Nature, 378, 355 (October 6, 1995)

confirmation by Marcy & Butler(October 12, 1995)

A giant planet with a 4.15 days period!

The discovery

G2 IV, d=15 pc, 5.49 mag• First planet mass object in orbit around a solar like star: 51 Pegasi b.

• Very different from theoretical expectations:

• a = 0.052 AU

• P = 4.23 days• M sin i = 0.468 MJ

• Such planets are now called “Hot Jupiters” or Pegasi planets / Pegasids.

Spektrometer ELODIEObservatoire deHaute-Provence193 cm Teleskop

The wake-up call

• About 0.5 -1 % of sun like planets have such a hot Jupiter (as we know now).

Mayor & Queloz

ApJ, 241, 425 (October 1, 1980)

discovered 15 yearsearlier... by theorists!

Migration: was not new after all

5. Planet detection methods

Current status

 692 planetsCandidates detected by radial velocity or astrometry524 planetary systems640 planets76 multiple planet systems

Transiting planets171 planetary systems184 planets14 multiple planet systems

Candidates detected by microlensing12 planetary systems13 planets 1 multiple planet systems

Candidates detected by imaging22 planetary systems25 planets1 multiple planet systems

Candidates detected by timing9 planetary systems14 planets4 multiple planet systems

+ 1235 planet candidates from the KEPLER satellite (transit)

Extra-solar planet encyclopedia (http://exoplanet.eu/)9.11.2011

Planet Detection MethodsMichael Perryman, Rep. Prog. Phys., 2000, 63, 1209 (updated April 2007)

[corrections or suggestions please to michael.perryman@esa.int]

Planet Detection Methods

Magneticsuperflares

Accretionon star

Self-accretingplanetesimals

Detectableplanet mass

Pulsars

Slow

Millisec

Whitedwarfs

Radial velocity

Astrometry

Radio

Optical

GroundSpace

Microlensing

PhotometricAstrometric

Space Ground

Imaging

Disks

Reflected/blackbody

Ground

Space

Transits

Miscellaneous

Ground(adaptive

optics)

Spaceinterferometry

(infrared/optical)

Detectionof Life?

Resolvedimaging

MJ

10MJ

ME

10ME

Binaryeclipses

Radioemission

??

3

206 planets(178 systems,

of which 20 multiple)4 planets2 systems

Dynamical effects Photometric signal

2? 1?

Timing(ground)

Timingresiduals

Existing capabilityProjected (10-20 yr)Primary detectionsFollow-up detectionsn = systems; ? = uncertain

11

Freefloating1?

44

Large number of methods, but only few can detect and allow the study of Earth-like planets!

5.1 Radial velocity (RV) method

Indirect detection - radial velocityStar and planet move around common center of mass. The stars move also (a little bit).Use optical Doppler effect to measure motion along the line of sight: → measure (periodic) shifts of spectral lines i.e. the stellar radial velocities.

But....- motion of the Sun due to Jupiter: 12 m/s → shift of spectral line by ~50 angstroms or 10 Si atoms

on the CCD → average velocity of cyclist at the Tour de France...- motion of the sun due to Earth: 8 cm/s → difficult to detect because of surface fluctuations

Shape and amplitude of the curve give the Msini (minimal mass), period, eccentricity and T0.

Control room

Instrument: High-precision spectrographLocation: 3.6 m ESO at La Silla Observatory (Chile)Consortium: Universities of Geneva and Bern (CH), Observatoire de Haute Provence (F), Service d'Aéronomie (F), ESO.

Vaccum chamber

The most precise RV instrument:

Telescope

Precision: down to 0.6 m/s.Super-Earth planets in the habitable

zone of K dwarfs.

Progress in ground-based RV detections

Earth-like planet detection from the ground by 2012? → still indirect observations → only close-by planets

51 Peg b

HARPS

Detection probability for a first generation instrument (ELODIE)Instrumental precision =10 m/s

Detection bias RV: The less massive, and the further out, the more difficult to find. Don’t forget when interpreting discoveries!

Mordasini et al. 2009

5.2 Transits (Photometry)

Transit detection

Jupiter in front of the sun

1% change in luminosity

Earth in front of the sun

0.01% change in luminosity

But... Transits measure radius not mass. Follow-up is necessary to measure mass (by RV).Many false positives (look photometrically like planets, but are not.)

transit detection principle

Simple in theory, difficult in practice.

=>Miniforschungsprojekt at MPIA

(Rp/Rstar)2

Characterization from transits + RV

- radius of planets: From transit measurements

R = 1.27 ± 0.02 RJ

M = 0.63 MJ ρ = 0.40 g/cm3 gaseous planet(Jupiter: 1.34)

↓↓

- mass of planet: From radial velocity measurements

Example HD209458b (first transiting planet, :

Mass-radius relation for extrasolar

planets

After the indirect detection of Hot Jupiters by RV, some doubts persisted about the origin of these observations (Stellar pulsations?). Transits showed unambiguously the planetary origin.

HD209458b: first measured transit

Charbonneau et al. 2000

Transit detection from space•Detection of planets with a radius of only a few Earth radii is very difficult form the ground, due to the noise in the photometric data introduced by the atmosphere.•To detect such planets photometrically, one must go to space.

•Kepler has revolutionized the transit method by finding more than 1200 candidates. •Warning: maybe ~10% are false positives (no RV confirmation)

Kepler candidates (Feb. 2011)

Launch: 2006Launch: 2008

5.3 Direct imaging

Direct imaging:massive giant planets far out

Very special systems can be imaged from the ground today...far from terrestrial planets!

8 AU from star 6 – 12 MJ

Fomalhaut b M≈ <3 MJ d= 119 AU

Kalas  et  al.  2008

HR 8799 b,c,d,e: M≈ 5-13 MJd= 15-70 AUDynamical constraints

Marois  et  al.  2008

Beta Pictoris b M≈ 6-12 MJ d= 8 AU

Reappeared!

Lagrange  et  al.  2008

Direct detection: (dis)advantages

• Advantages

• Allows physical characterization: Temperature, log g, chemical composition

• Direct detection, no other explanations possible (must exclude background star chance alignment.)

• Disadvantages

• Very difficult, only young objects. Huge brightness contrast, tiny projected separation.

• Measures intrinsic (or reflected) luminosity L. Not mass M. L-M relation is model dependent and very uncertain.

Direct detection: resolution

Difficulty: Resolution

typical numbers: stars ~ 10-100 pc, planet 1 AU → θ = 0.01’’- 0.1” (seeing limits to ~ 0.5”)

Solution:→ use adaptive optics

Direct detection: brightness ratio

visible to near-IRreflected light mid-IR

intrinsic emission

Difficulty: Brightness ratio

Typical numbers: visible: Fplanet / Fstar ≈ 10-9

infrared: Fplanet / Fstar ≈ 10-6

Solution:→ remove star light

- nulling - coronograph

Favorable cases:infrared observationsplanets orbiting less luminous stars→ M dwarfs

young planets→ planet formation

Other techniques: Microlensing, timing, astrometry

Questions?