Lecture 1 Part I Observational constraints from the Solar System and from Extrasolar Planets Lecture Universität Heidelberg WS 11/12 Dr. Christoph Mordasini Partially based on script by Prof. W. Benz 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, massradius diagram,...
1. Introduction
Galaxies, stars and planets 10 billion galaxies How many harbor life? 100 billion stars how many planets? How frequent? Life? First generation of human beings with technology to answer this.
- Planet formation: From dust to planets Important questions 10 μm 10-100 million years How? - Planet evolution: Habitability Mars Earth Terrestrial planets in the solar system: similar initial conditions very different outcome. Moon The characterization also of exoplanets has just started. Venus
Ways to understanding Herschel s 1789 For many centuries Cloud collapse Hertzsprung Russel Nuclear Fusion Stellar Mass Funct. Sun Stars Solar System La Silla Obs. ESO Darwin ESA For a decade In a decade? Formation in disks Collisions Gas accretion Migration Exoplanets Astrobiology Habitable Zone Biomarkers Complex Life Life Extraterrestrial Life?
2. Planet Formation Paradigm
Planet formation: The paradigm Gravitational Core - remote observations - in-situ measurements - sample returns - laboratory analysis - theoretical modeling Minority line Party line Accretion Instability A satisfactory theory should explain the formation of planets in the solar system as well as around other stars.
Planet formation: Sequential picture in presence of gas in absence of gas dust 10 7 years 10 7 years Star & protoplanetary disk planetesimals protoplanets migration giant planets giant impacts 10 8 years type I type II terrestrial planets dynamical rearrangement
Planet formation Initial conditions, task and orders of magnitude 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 Task follow the evolution of the gas and dust for a period of about 100 Million years. Orders of magnitude to remember Msun ~2 10 33 g MJ ~ 2 10 30 g ~ 1/1000 Msun ~ 318 ME ME ~ 6 10 27 g RJ ~ 7.19 10 9 cm ~ 1/10 Rsun RE ~ 6.4 10 8 cm ~ 1/10 RJ AU ~1.5 10 13 cm Lsun ~ 3.8 10 33 erg/s
Challenges in planet formation gas giants ( 10000 km) size Earth-sized ( 1000 km) runaway gas accretion late stages giant impacts protoplanets planetesimals ( km) Self. Gravity runaway growth oligarchic growth 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) dust (μm) dust sticking time 10 4-10 5 10 5-10 7 10 7-10 8 years
3. Structure of the Solar System
Orbital data major planets Solar system System architecture Rocky planets gas giants Inner system Outer system Asteroids ice giants 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 2 n-1 n=1,2,...: Orbital stability in Hill units
Solar system Minor bodies System architecture II 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.
Minor bodies cont. Solar system System architecture III 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.
Physical data major planets Solar system Physical properties Approximately to scale 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 selfgravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
Solar system Physical properties II Composition terrestrial planets Earth 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
Solar system Physical properties III Composition giants Possible J,S compositions Amount of metals [ME] Guillot 1999 MJ=~318 ME, MS=~95 ME Significant uncertainties: equation of state (EOS) of H/He under extreme p and T badly known. 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
Historical perspective Herschel s big telescope 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
Historical perspective II Some 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
Planet formation theory State of the art t<1995. Only one example to study.. Science, 267, 360 (January 1995) Oops Knowledge is evolving. What is believed correct today can turn up wrong tomorrow!
4. The surprise: 51 Peg b
The discovery Nature, 378, 355 (October 6, 1995) confirmation by Marcy & Butler (October 12, 1995) A giant planet with a 4.15 days period!
The wake-up call 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. About 0.5-1 % of sun like planets have such a hot Jupiter (as we know now). G2 IV, d=15 pc, 5.49 mag Mayor & Queloz Spektrometer ELODIE Observatoire de Haute-Provence 193 cm Teleskop
Migration: was not new after all ApJ, 241, 425 (October 1, 1980) discovered 15 years earlier... by theorists!
5. Planet detection methods
Current status 692 planets Candidates detected by radial velocity or astrometry 524 planetary systems 640 planets 76 multiple planet systems Transiting planets 171 planetary systems 184 planets 14 multiple planet systems Candidates detected by microlensing 12 planetary systems 13 planets 1 multiple planet systems + 1235 planet candidates from the KEPLER satellite (transit) Candidates detected by imaging 22 planetary systems 25 planets 1 multiple planet systems Candidates detected by timing 9 planetary systems 14 planets 4 multiple planet systems Extra-solar planet encyclopedia (http://exoplanet.eu/) 9.11.2011
Planet Detection Methods Michael Perryman, Rep. Prog. Phys., 2000, 63, 1209 (updated April 2007) [corrections or suggestions please to michael.perryman@esa.int] Existing capability Projected (10-20 yr) Primary detections Follow-up detections n = systems;? = uncertain Detectable planet mass 10M J M J 10M E M E Pulsars Slow Millisec White dwarfs 4 planets 2 systems Timing (ground) Binary eclipses 206 planets (178 systems, of which 20 multiple) Dynamical effects Radial velocity Optical Astrometry Radio Planet Detection Methods Astrometric Microlensing Photometric signal Imaging Photometric Space interferometry (infrared/optical) Miscellaneous Disks Space Accretion on star Self-accreting planetesimals Magnetic superflares Reflected/ blackbody Transits 2? 4 4 Ground (adaptive 1? Space Ground optics) 11 3 Ground Resolved Ground Space imaging Detection of Life? 1? Timing residuals Free floating?? Radio emission 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 velocity Star 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. Shape and amplitude of the curve give the Msini (minimal mass), period, eccentricity and T0. 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
The most precise RV instrument: Instrument: High-precision spectrograph Location: 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. Precision: down to 0.6 m/s. Super-Earth planets in the habitable zone of K dwarfs. Vaccum chamber Telescope Control room
Progress in ground-based RV detections Mordasini et al. 2009 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! 51 Peg b HARPS Earth-like planet detection from the ground by 2012? still indirect observations only close-by planets
5.2 Transits (Photometry)
Transit detection transit detection principle Simple in theory, difficult in practice. =>Miniforschungsprojekt at MPIA Jupiter in front of the sun Earth in front of the sun (Rp/Rstar) 2 1% change in luminosity 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.)
Characterization from transits + RV 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 - radius of planets: From transit measurements - mass of planet: From radial velocity measurements Example HD209458b (first transiting planet, : R = 1.27 ± 0.02 RJ M = 0.63 MJ ρ = 0.40 g/cm 3 gaseous planet (Jupiter: 1.34) Mass-radius relation for extrasolar planets
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 candidates (Feb. 2011) Launch: 2008 Launch: 2006 Kepler has revolutionized the transit method by finding more than 1200 candidates. Warning: maybe ~10% are false positives (no RV confirmation)
5.3 Direct imaging
Direct imaging:massive giant planets far out Fomalhaut b M <3 MJ d= 119 AU HR 8799 b,c,d,e: M 5-13 MJ d= 15-70 AU Dynamical constraints Kalas et al. 2008 Marois et al. 2008 Beta Pictoris b M 6-12 MJ d= 8 AU Reappeared! 8 AU from star 6 12 MJ Lagrange et al. 2008 Very special systems can be imaged from the ground today... far from terrestrial planets!
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 Difficulty: Brightness ratio Typical numbers: visible: Fplanet / Fstar 10-9 infrared: Fplanet / Fstar 10-6 visible to near-ir reflected light mid-ir intrinsic emission Solution: remove star light - nulling - coronograph Favorable cases: infrared observations planets orbiting less luminous stars M dwarfs young planets planet formation
Other techniques: Microlensing, timing, astrometry
Questions?