Outline• Exoplanets: Current knowledge–Overview of detection methods–Statistics–Formation mechanisms• The stellar habitable zone: classical definition• Influence of the high-energy radiation ong>andong>particle environments–Evaporation of planetary atmospheres–Planets around stars of other spectral types• The future of exoplanet research (observations)–COROT–Darwin/TPF
Exoplanets: Current knowledge• The history of exoplanets goes back just 10 yr agowith the discovery of 51 Peg b by Mayor & Queloz in1995 (besides pulsar planets…)• Currently there are 133 “bona fide” planets in 117planetary systems• The definition of a planet is somewhat fuzzy:formation mechanism ong>andong> deuterium burning limit• A big surprise came with the first detections giantplanets very close to the host stars (~0.05 AU) “Hot Jupiters”• Several methods can be used to detect exoplanetsong>andong> some have been more effective than others…
Exoplanet detection methodsRadial velocityIndirectmethodsTransitsAstrometryMicrolensingDirectmethodsDirect imagingDirect spectroscopy
Radial velocities• Doppler shifts of stellar lines• Since we can only measure the lines of the star ong>andong>M p
First confirmed transit in 2000 (planet already discoveredby RVs): HD 209458Charbonneau et al. (2000)‣ Planet data:– M=0.7 M Jup– R=1.35 R Jup– i=86.6°‣ Star data:– G0 V– V=7.64– R=1.15 R Depth of 1-2% for jovian planets around solar-type stars• Observational biases:‣ Giant planets‣ Close orbits (higher probability)‣ Around dwarf starsCurrently 6planets detectedwith this method
Solar System example...Planet P (yr) a (au) Dur.(h)Depth(%)Geom.prob. (%)Mercury 0.241 0.39 8.1 0.0012 1.19Venus 0.615 0.72 11.0 0.0076 0.65Earth 1.000 1.00 13.0 0.0084 0.47Mars 1.880 1.52 16.0 0.0024 0.31Jupiter 11.86 5.20 29.6 1.01 0.089Saturn 29.5 9.5 40.1 0.75 0.049Uranus 84.0 19.2 57.0 0.135 0.024Neptune 164.8 30.1 71.3 0.127 0.015‣ Very high accuracy photometry needed todetect terrestrial planets!‣ Many stars to be observed because of lowprobability
AstrometryObservation of the reflexmotion of the star in the sky Very high precision needed(µas)!It is easy to demonstrate that:M* a*=Mpap2 3P M*=(ap+a*)⎛MM = ⎜⎝ P• Observational biases:‣ Massive planets⎞⎟⎠2/3αd M*p * jup‣ Intermediate orbits (short period but noticeable effect)‣ Low-mass stars‣ Nearby distances
Some statistics…• Mass distribution compatiblewith expectations (lowestmass planet 14 M ⊕ ). But…‣ Many planets with high orbital eccentricities (?) Contradiction with current formation models‣ Apparent cutoff at semi-major axes < 0.03-0.04 AU (?) What explains the lack of planets closer to the star?
Soon was found (Gonzalez et al.) that stars withplanets seemed to be richer in metals than others!!The explanation is not clear yet:‣ The star forms planets because the available material is richerin metals‣ The star engulfed formed planets enriching its atmosphere
Formation mechanismsObservations indicate arelative richness ofmassive planets in closeorbits: How did they form?• In situ formation requires verysevere assumptions (Wuchterl etal. 2000)• Currently: migration (inward &outward) (Lin et al. 1996)• Migration is caused by planetdiskinteractions• Problem: how to stop it!
The stellar habitable zone:classical definition• Requisite of liquid water on the planet’s surface (stellarradiation or internal heat!)• Early works in the 70s but Kasting, Whitmire &Reynolds (1993) set the stong>andong>ards:– Simple climate model– Internal limit water loss via photolysis ong>andong> hydrogen escape(after runaway greenhouse effect ong>andong> wet stratosphere)– External limit formation of CO 2 clouds (enhanced albedoong>andong> lower temperature)– CO 2 is the principal stabilizing agent of the atmosphereagainst positive feedbacks (carbonate-silicate cycle)• Excellent review by Kasting & Catling (2003, ARA&A,41, 429)
• The habitable zone (HZ) depeds on the star’s luminosity(mass ong>andong> age) ong>andong> on the planet’s mass (ability to sustainan atmosphere)• Present solar values:• Inner limit: ~0.95 AU• Outer limit: 1.37-2.4 AU (CO 2cloud properties still unknown!)• Mars (1.52 AU) ong>andong> Venus (0.72 AU) are quite close to theHZ!; Mars: too small, no plate tectonics!; Venus: runawaygreenhouse, water lost, all CO 2 in the atmosphere!• For massive stars the HZ becomes narrow ong>andong> short-lived• For very low mass stars, tidal synchronization becomes aproblem (atmosphere freezes in the dark side)• The continuously habitable zone (CHZ) is defined as theregion that remains habitable for a finite period of time.Also, galactic habitable zone (GHZ) (Lineweaver et al. 2004)!
An important ingredient: water deliveryPlanets in the HZ• It is well understood that water-rich planetesimals onlyexist beyond the so-called “snow line” (Sun: 5 AU)• Recent study by Raymond et al. (2004, Icarus, 168, 1)with simulations of planetary systems ong>andong> waterdelivery
Influence of the high-energyradiation ong>andong> particle environments• The HZ calculations of Kasting et al. assume a stableatmosphere with basically fixed composition…• … but what if the host stars influenced the planetaryatmospheres in more ways than just the overall flux?• High-energy (quiescent & flares) ong>andong> particle fluxes(stellar wind & CMEs) could also affect theatmospheres• These fluxes are triggered by the stellar magneticactivity• We know that the Sun is a relatively inactive startoday… but, has it always been this way?
NO! The young Sun rotatedabout 10 times faster thantoday ong>andong> had enhancedmagnetic activityEK Dra(100 Myr)π 1 UMa(300 Myr)κ 1 Cet(650 Myr)Sun(4.56 Gyr)
The young post-ZAMS Sun had stronger emissions:‣ 100-1000x in X-rays‣ 10-100x in the EUV-FUV‣ 5-10x in the UVRibas et al. (2004, in press)
‣The flux density evolution scales well with power-lawrelationships of different slopes‣The overall XUV flux (1-1200 Ǻ) decrease has a slope of −1.2 3x higher than today 2.5 Gyr ago, 6x 3.5 Gyr ago, 100x ZAMS!
The Young Sun: : A Summary of propertiesX-Ray, EUV:100-1000xpresent valuesVisible: 70%present valuesFUV, UV: 5-60xpresent valuesFlares: more frequentong>andong> energetic (~2-5 perday)Solar wind:500-1000xpresent values
Evaporation of planetary atmospheres• Most straightforward application to pure-H atmospheres Hot Jupiters• XUV radiation deposits its energy in the exosphere,which heats up ong>andong> expong>andong>s• The exosphere temperature (ong>andong> not T eff ) drives theevaporation• Well-known formalism in most cases (Jeans escape):particles with velocity above escape are lost to space• When escape rates are very high Jeans escape is notapplicable ong>andong> hydrodynamic treatment must be used• This mass loss from Hot Jupiters has been measured inHD 209458 to be >10 10 g s -1 (Vidal-Madjar et al. 2003, 2004) !
Our predictions...High exospheretemperatures ong>andong> strongerosion at close distances!Significant massloss over longperiods of time ong>andong>possible fullevaporation (exceptfor planetary core)Lammer et al. (2003)For HD 209458 weobtain ≈10 12 g s -1
• In addition, non-thermal loss processes also play a role• These are driven by the stellar particle flux (wind),which causes erosion by sputtering ong>andong> ion pickup• Planets have a protecting magnetic field but this can beweaker if synchronized• The stellar particle fluxwas much higher in thepast ong>andong> the resultingpressure may havepushed themagnetopause belowthe exosphere radius• In those conditions the non-thermal loss process may begreatly enhanced (>10 10 g s -1 )• All these calculations could explainthe cutoff at 0.03-0.05 AU! Grieβmeier et al. (2004)
Other stellar types...In principle low-massstars are prime cong>andong>idatesfor searches of planets inthe HZ: Long lived (>10Gyr) Very abundant in thesolar neighborhood Better contraststar/planetFstarsGstarsKstarsMstarsHowever, solar-type starsare active when young, butlower mass stars stay activefor longer periods of time!! Potential for very severeerosion of atmospheres
The future of exoplanet research(observations)• Exoplanet detection efforts are now directedtowards:– Detecting Earth-sized planets (5 yr time-frame)– Characterizing Earth-like exoplanet atmospheres ong>andong> lookfor biosignatures (10-15 yr time-frame)• Both ESA/Europe ong>andong>NASA are setting upambitious missions towork in this direction:Date ESA/Europe NASA2006 COROT −2009 Eddington Kepler2015+ Darwin TPF2012 Gaia SIM
COROT (CNES/ESA)• Mission led by CNES (France) with participation fromESA, Austria, Belgium, Germany, Spain ong>andong> Brazil• High-precision photometry with two-fold goal:– Asteroseismology of bright stars– Exoplanet detection• Expected photometric accuracy 1-10 ppm (10 -5 -10 -6 )in the seismo-field ong>andong> 0.5-1 mmag in the exo-field• Observing program divided into long (150 d) ong>andong>short (20-30 d) runs over a 3 yr nominal mission• The exo-field will permit monitoring of about 6000stars down to mag. 15.5• 27 cm ∅ telescope in polar orbit (900 km), with 42k×4k CCDs (frame transfer) ong>andong> 3×3º FOV
COROT (CNES/ESA)• The mission probably will not be able to detectEarth-sized planets but just slightly larger ones• If our models are correct, COROT may find thecores of Neptune-sized planets that migrated closeto their parent stars ong>andong> evaporated down to theircore sizes (a few M ⊕ )• Mission design is at an advancedstage ong>andong> launch is planned forJune 2006• The Co-Is of the mission will haveprivileged access to the coreprogramdata• Eddington = super COROT
Darwin (ESA/NASA?)A very challenging mission todirectly image planets aroundnearby stars ong>andong> perform lowresolutionspectroscopy:The star’s light is 10 6 times (IR)brighter than the planet’s...Simulation of theinner Solar Systemas observed withnulling interferometry(to eliminate sunlight)The solution is nullinginterferometry but it needsmetrology with accuracyof ~1 nm!
It will even be possible to obtain lowresolutionspectroscopy of the planets Characterization of their atmospheresong>andong> detect presence of life through O 3 !But some studies (Selsis et al. 2002) caution thatpurely abiotic processes can also produce O 3More sophisticated biomarkers need to bedevised
• Many technical aspects of Darwin are still to be defined• Orbital location: L 2 (1.5 million km from Earth)• Free-flying configuration with 6 elements• Precursor technology demonstration: GENIE @ VLT• The Darwin sample will consist of some tens of nearby(
This is a very activeresearch area! Many spacemissions ong>andong>ground-basedprojects during thecoming yearsNASA ong>andong> ESA futureroadmaps give greatweight to this fieldChallenge:Detect planets with masssimilar to the Earth...... located in the habitablezone of the star ...... detection of life... characterize their atmospheres ...