Extra-solar Planets
Solar System Planets
Small, rocky planets on the inside
Large, gas-giant and ice-giant planets on the outside
Relative sizes of Solar System planets
Relative sizes of Sun and planets
 The Sun is ~ 1000 times more massive than Jupiter
 Jupiter is ~ 300 times more massive than the Earth
 Neptune is ~ 17 times more massive than the Earth
The planets orbit the Sun approximately in a plane. Small-body populations orbit between Mars and Jupiter (the asteroid belt) and beyond the orbit of Neptune (Edgeworth-Kuiper belt). These small-body populations are generally regarded as being the debris left over from the formation of the Solar System Chemical gradients exist within Solar System. An important fact is that meteorites whose parent bodies lie further out in the asteroid belt show greater signs of aqueous alteration. This suggests that water (and other volatiles) condensed out from the gas phase to form solids beyond a certain orbital radius in the primordial solar nebula. This radius is often referred to as the snowline (or ice- condensation radius)
Extrasolar Planetary Systems
The Ancient View "There cannot be more worlds than one.” - Aristotle (384-332 BC) "There are innumerable worlds which differ in size. In some worlds there is no sun and moon, in others they are larger than in our world, and in others more numerous. They are destroyed by colliding with each other. There are some worlds without any living creatures, plants, or moisture.” - Hippolytus of Rome (c. 170 - 236) on Democritus (460-370 BC) “There is an infinite number of worlds, some like this world, some unlike it… For the atoms out of which a world might arise, or by which a world might be formed, have not all been expended on one world or a finite number of worlds, whether like or unlike this one. Hence there will be nothing to hinder an infinity of worlds." - Epicurus of Samos (342-270 BC) “…T’must be confessed in other realms there are Still other worlds, still other breeds of men, And other generations of the wild.” - Lucretius (99-55 BC)
Planet Finding Methods Five methods of detecting exoplanets have been successful: i.
Radial velocity searches
ii.
Transit monitoring
iii.
Microlensing
iv.
Detection of IR thermal emission
v.
Direct imaging
Radial Velocity Method When a planet orbits a star it causes the star to wobble 
 back and forth since both planet andstar are orbiting their
 common centre of mass
Looking down on planet and star
Looking at planet and star edge-on
As star moves towards observer, starlight is blueshifted. As it recedes the starlight is red-shifted.
Absoption lines in the starʼs spectrum are seen to move back and forth in phase with the starʼs radial motion relative to the observer.
Radial velocity diagrams
Radial velocity measurements tell us: • Orbital period • Planet mass - mp sin (i) • Eccentricity - from the shape of the curve • Number of planets in system - a composite radial velocity diagram can be decomposed into a number of superposed Keplerian fits Radial velocity searches have detected 399 planets The first was in 1995 - 51 Pegasi
The transit method
For ʻedge-onʼ planet systems the transit method detects the dimming of the starlight as the planet passes between star and observer during each orbit:
From transit can determine radius from decrease in stellar flux (proportional to Rp2 / Rstar2) Combining radius with mass from radial velocity → average density 69 transiting planets detected so far…
Transit method is used to target individual objects and also to survey large areas of sky. The probability of transit depends on size of star and distance of planet from star (ignoring finite size of planet). In survey mode telescope stares at dense stellar regions in Galaxy. Problems arise with confusion due to stellar crowding, false detections due to grazing binaries, blends between forground stars and transiting background stars in same image/pixel. Radial velocity follow-ups require nearby stars to obtain spectra with reasonable signal-to-noise.
SuperWASP is a ground-based project looking for transits by staring at large area of sky CoRoT and KEPLER are space based transit searches – KEPLER has capability to detect transits by Earth-analogues
SuperWasp 
 field in Orion
Microlensing
Foreground star passes in front of background star Note that derived planet parameters depend on knowing distance to lens, Foreground star acts as gravitational lens distance to background star, - brightening image Light curve distorted when foreground star has planet and lens star mass. These are not known quantities and so must be which enters the Einstein ring estimated from Galaxy models. Hence 10 planets have been discovered by this method all planet parameters are based on 1 multiple planet system probabilities.
Hot-Jupiters detected in infrared
Hot-Jupiters have surface temperatures ~ 1500K and emit strongly in the infrared. Spitzer Space Telescope has detected IR emission from 6 planets by measuring secondary eclipse as they disppear
 behind their host stars:
 This represents the first light detected from a planet outside Solar System
Direct imaging of planets
Planets are now being found by direct imaging using ground and space based telescopes. Above image shows 5 Jupiter mass planet apparently orbiting the “brown dwarf star” 2M1207. 11 planets detected by direct imaging so far
An overview of the exoplanet population 429 planets have been discovered since 1995 • Smallest mass is 4.5 Mearth (CoRoT 7-b) Largest mass is 12 Mjupiter - by definition (Deuterium burning limit) • Planet which is closest to its star has distance 0.016 AU Planet which is furthest from its star found by radial velocity surveys has distance of 6 AU Most distant planet from direct imaging has distance ~ 100 AU Solar radius = 0.005 AU • Orbital eccentricities range between e=0 and e=0.92 Data is available at the Extrasolar Planets Encycopedia:
http://exoplanet.eu/
Almost all planets found to date are very close to Sun
 compared with most stars in the Galaxy
The number of planets versus planet mass We see that planets with lower masses (< 1 Jupiter mass) are more common - planet formation process produces lower mass planets Planets with masses > 5 Jupiter masses are rare - the brown dwarf desert Data for planet masses < 1 Jupiter mass are highly incomplete ! But, there do appear to be many planets with masses < 1 Jupiter mass
Jupiter Saturn
Earth
Planet masse versus orbital period Dashed lines represent radial velocity detection limits of 2 m s-1 for Solar mass and 0.33 Solar mass stars, respectively. Detection limits are caused by instrumental noise & jitter caused by convective motions in stellar photospheres. Signal-noise can be improved by integrating over long time periods since the noise amplitude scales with 1/sqrt(time) if the noise obeys Gaussian statistics
Planet number versus orbital eccentricity Unlike the planets in the solar system, extrasolar planets usually have elliptical/ eccentric orbits. Suggests significant gravitational scatteringâ&#x20AC;¨ within planetary systems after formation.
Planetary eccentricity does not obviously correlateâ&#x20AC;¨ with planet mass - which may rule out some modelsâ&#x20AC;¨ for the origins of exoplanet eccentricities
Planets more abundant around metail-rich stars ⇒ provides circumstantial evidence for the core-instability model of planet formation that we will discuss later in the course
Some interesting planets and planetary systems
HD 209458 b • Originally discovered using radial velocity technique • Found to transit in front of its star in 1999 • Combining transit data and radial velocity measurements gives the planet mass and radius:
Mass=0.69 Jupiter masses Radius=1.347 Jupiter radii ⇒ Gas giant planet with mean density of about 1 g/cm3
Small fraction of starlight passes through planet atmosphere during transit Absorption features due to sodium observed in the spectrum - in agreement with theoretical predictions… Using a similar technique water has been detected in atmosphere of the planet HD 189733b Recent observations using the Hubble Space Telescope suggest that the atmosphere of HD 209568b is boiling off - producing a long ʻcometary tailʼ
As HD 209458b goes behind the star, the infrared radiation emitted by the planet is blocked out. Using this method we can determine how much infrared radiation is given out - and how hot the planet is (about 1300 K)
Upsilon Andromeda b
Spitzer observations indicate strong day-night temperature difference in the atmosphere of Upsilon Andromeda b: Tday ~ 1700 K Tnight ~ 400 K - first “weather” observation for a planet outside the Solar System.
GL436b - a transiting ʻhot Neptuneʼ
Discovered using radial velocity method but also found to transit across host star → mass, radius & mean density Computer models indicate that planet has internal structure very similar to Neptune and Uranus - but much hotter surface layers (800 K)
Direct images of Fomalhaut and HR8799
Fomalhaut (above images) has planet weighing in at ~3 Jupiter masses orbiting at 100 AU from star - creating a ring in the dust disc (Kuiper belt) Observations carried using NICMOS + coronograph on HST
HR8799 (left image) has 3 planets orbiting at 24, 38 and and 68 AU. Planet masses are estimated to be between 7-10 Jupiter masses. Observed with Keck + Gemini telescopes
A low mass planet detected via microlensing OGLE2005-BLG-390Lb - discovered through microlensing A 5.5 Earth mass planet orbiting a 0.2 Solar mass star at a distance of 3 AU
The above diagrams show the probability distributions for variousâ&#x20AC;¨ parameters relating to the OGLE2005-BLG-390Lb system
GL581(c+d) - extrasolar planets in the habitable zone ? System of 3 planets orbiting low mass star: 15 Earth mass with 5 day period 5 Earth mass with 13 day period 8 Earth mass with 84 day period Radius of 5 Earth mass planet is 1.5 Earth radii
Initially thought to be in the “habitable zone” of this low mass star - but probably too close to its star to support liquid water on surface GL581d is more likely to be in habitable zone
CoRoT - 7B – the lowest mass exoplanet Transit detected by CoRoT Orbital period 20 hours 29 minutes Radius = 1.7 Earth radii Radial velocity measurements M=4.8 Earth masses Density ~ 5.6 g/cm3 is approximately the same as the Earth Composed of rock + iron The planet is expected to be tidally locked (as with all other close orbiting planets) Maximum surface temperature between 2000-2800 K
Method
Advantages
Disadvatages
RV
Many stars surveyed Multiple planets Broad range of semimajor axes Constrains masses
Cannot detect very low masses Must wait for orbital period Actual masses unknown (sin i) Need nearby stars
Transit
Know mass through RV Know planet size ⇒ density Spectroscopy gives chemistry Large surveys will produce many planet candidates. Transit time gives eccentricity.
Low probability of occurrence Confusion and false alarms in surveys Stellar variability will make detecting small planets hard
IR detection
Provides information about temperature, and temperature distribution around planet
Only useful for close-in, hot planets
Direct detection
Provides information about planet mass, orbital radius, and creates possibility for future spectroscopy.
Cannot yet detect close-in or low mass planets. No information about planet eccentricities.