What do we know about extrasolar planets? Neill Reid STScI

The big questions Over 145 extrasolar planets in more than 125 stellar systems have been discovered since this includes several multiplanet systems  What are the properties of those planetary systems?  What is the likely frequency of planetary systems in the Milky Way Galaxy?

Outline A brief introduction to the Milky Way How to find planets What do we know about extrasolar planetary systems

The Milky Way Spiral galaxy: halo – old metal-poor population near-spherical, R~80,000 l.y. disk – flattened, rotating system radius ~ 40,000 l.y. Sun lies ~27,000 l.y. from Centre bulge – central condensation radius ~ 3000 l.y.

The astronomer’s periodic table In astronomy, metals = everything but H, He Metals are formed by stars  Up to Fe by fusion  Above Fe, supernovae a elements (O, Mg, Ti, etc) are enhanced in stars with low overall metal abundance  capture elements formed preferentially by massive supernovae  enhanced a indicates early formation

The Halo First stars to form in the Milky Way during the initial gravitational collapse near spherical distribution with no net rotation – “pressure supported” halo stars have metal content from to 10% that of the Sun May be an inner halo (initial collapse) + an outer halo (accreted satellite galaxies) Halo contributes less than 1% of the total mass of the Galaxy

The disk The disk is a flattened, rotating population  Formed by dissipational collapse ~10% total mass of Milky Way All current star formation located in Galactic disk Approximately half gas/dust & half stars

Building the Milky Way 1.Big Bang, t ~ 13 Gyrs 2.Density perturbations (regions with enhanced dark matter density) start to collapse shortly after initial expansion 3.First stars in the Milky Way form during early stages of collapse at t ~ 12 Gyrs – globulars, halo stars and central bulge 4.Galactic disk forms at t ~ Gyrs; conservation of angular momentum gives a flattened, rotating system; high gas density, socontinuing star formation 5.Merger with major satellite at t ~ 9-10 Gyrs; initial disk is heated to form a “thick disk” – hiatus in star formation 6.Disk reforms at t ~ 8-9 Gyrs and star formation resumes 7.Continued accretion of small satellite galaxies as Milky Way builds its outer halo Total mass ~ few x solar masses: <1% halo+bulge; 10% disk; ~90% dark matter

The Solar Neighbourhood The Sun lies in a relatively low density region, diameter ~300 l.y. Average stellar separation ~ 3 l.y. Almost all stars known to have planets lie within 100 l.y. of the Sun

Planet formation (classical) 1.Collapse of protostellar core in giant molecular cloud 2.Central star surrounded by protoplanetary disk of gas and dust 3.Fragmentation within the disk leads to formation of planetismals, then planetary embryos, then planetary cores. 4.Cores beyond the ice line (~ 4 AU) accrete further material to become gas giants (time scale < 10 7 years); rocky terrestrial planets in inner Solar system 5.Gas and dust clears (save for residual zodiacal dust) to leave current Solar System

Making planets Considerable evidence for dusty protoplantary disks around young stars  Gas & dust to age ~10 Myrs  Residual dust disks to ~100 Myrs  Zodiacal dust in older stars (Sun)

The Solar System Dimensions: Inner solar system, r < 4 AU Terrestrial planets and the asteroid belt: rocks Masses < 0.3% M J Gas giants, 5 < r < 30 AU Jupiter to Neptune Masses: 7% M J to 1 M J Kuiper belt, 30 < r < 120 AU Pluto and beyond: icy bodies Masses < 0.05% M J Oort comet cloud, r ~ 10 5 AU

Extrasolar expectations  Terrestrial planets at small radii; gas giants beyond the “snow line” Planets on low eccentricity coplanar orbits  Pluto has highest inclination at ~ 17 o.

Finding planets: Doppler surveys Star+planets orbit common centre of gravity  Detect reflex motion of primary  Requires high resolution (Jupiter induces ~12 m/s velocity in Sun)  Require bright target stars (< 9 th magnitude even with Keck 10-metre)

Doppler planets Main limitation – only measure projected mass, semi-major axis

Unexpected planets: Hot Jupiters First planetary discovery confounded expectations: Instead of jovian analogue, 51 Peg b is a gas giant, orbiting around its parent star with a period of 4.2 days  Orbit lies well within Mercury’s in the Solar System Gas giants form by accreting ices (water ice, CO 2, etc) onto a rocky core – but ices can’t exist within the “ice line”  ~ 4-5 AU How can a gas giant form where the ambient temperature is ~1300K?

Hot Jupiters: migration Answer: hot jupiters don’t form at their present radii: formation occurs beyond the ice line subsequent migration through the disk due to angular momentum exchange with the disk i.e. energy loss in viscous disk

Multiple systems Several systems have more than one known planet  At least 15 with 2 planets Lowest mass planet detected so far has M~30 Earth masses

Finding planets: Transits Main alternative to Doppler surveys  Use photometric monitoring to search for periodic variations indicating an eclipsing planet Advantage  known inclination Disadvantage  need high precision Jupiter gives ~1% dip; Earth ~0.01%

HD Planet detected by Doppler motions  a hot Jupiter Photometry revealed that the orbital inclination is low enough that the planet transits the primary star.  Mass = 0.69 M jupiter HST observations show  Radius ~ 1.34 R jupiter  Sodium detected in the planetary atmosphere

Other transit surveys Photometric surveys need to cover tens of thousands of stars per night with millimagnitude accuracy to optimise chances of detecting a handful of transiting systems  CCDs are detector of choice (relatively large format + reliable data)  Mounted on moderate-sized telescopes (~1 metre), can achieve field of view of a few square degrees (~10 times the size of the Moon)  Target stars are much fainter than Doppler surveys, and therefore at larger distances e.g. TrES-1 ~12 th magnitude ~100 times fainter than typical Doppler and d~600 l.y. OGLE Galactic Bulge surveys – 15 th -17 th magnitude, d ~ 4000 l.y. Different types of stars in different environments  also fainter, so more difficult to get detailed observations, but  Radii are closer to R jupiter Future: KEPLER satellite will observe 100,000 stars with sufficient photometric accuracy to detect terrestrial transits

Astrometric searches Planetary companions also produce a `wobble’ in the parent star’s position on the sky  As viewed from ~30 l.y., Jupiter causes the Sun to move by 1 milliarcsecond (< diameter of the full moon)  requires extremely precise positional measurement No planets detected to date (but HST verification of 2 systems) Future: space missions, notably SIM Planetquest

Microlensing Detect planets by their ability to focus light gravitationally  Planet produces short-period spike in microlensing light curve One candidate discovered to date  probably a jovian-mass planet around an M dwarf halfway beftween us and the Galactic Centre Advantage: sampling different environments Disadvantage: Detailed follow-up observations impossible

Current statistics 143 planets in 125 ‘normal’ planetary systems 5 discovered in transit surveys 1 discovered through gravitational microlensing Remaining 119 systems from radial velocity surveys Lowest mass planet: Gl 436b, M~0.067 M J or ~21 M E – Neptunian mass companion of a nearby M dwarfs Longest period systems: P ~ 8 years, a ~ 4.2 AU

What stars have planets? Most stars with detected planets are similar to the Sun – Masses ~ 0.8 to 1.2 M sun Most are H-burning main sequence stars – some giants and 2 low- mass M dwarfs  Mainly reflects biases in radial velocity surveys – we’re looking for planetary systems like our own (and solar type stars are good targets for Doppler surveys)

Planets or brown dwarfs? Early planetary detections attracted some skepticism  are they just very low- mass stars and brown dwarfs in low inclination orbits? No ! Solar type stars show evidence for a ‘brown dwarf desert’ at small separations  Planets are distinct from stellar/BD companions

The mass distribution Mass distribution of known planets rises sharply in number as mass falls below ~20 M jupiter ; flattens at ~4 M jupiter (M a M -1 ); and declines below 1 M jupiter (almost certainly a selection effect).

Planetary orbits Solar system planets have low eccentricity Extrasolar planets show much wider range of e, save at short period (tidal circularisation) Probably reflects dynamical interactions in multi-planet systems  Solar system may be slightly unusual

Planets and metals Planets are much more common around metal-rich stars  Not unexpected, since planets have higher metal content than the parent stars Even so, ~5% of sun-like stars have planetary companions  ~1.1 million sub-like stars with planets at Galactic radii from 24,000 to 30,000 l.y.

Habitability The conventional Habitable Zone: Ambient temperature allows liquid water on planetary surface –~0.8 to 1.5 AU for the Sun, Earth-like atmosphere –Depends on temperature of central star & planetary atmosphere

Ambient temperatures Planetary temperatures depend on: 1.Temperature of central star 2.Distance from central star 3.Planetary atmosphere 1+2  sub-stellar temperature ~40 known planets spend at least part of their orbit within the habitable zone of their primary All of these planets are gas giants – but what about satellites?

Summary 1.~150 planets around ~135 stars currently known 2.Most are companions of stars within the immediate Solar Neighbourhood 3.Metal-rich stars are significantly more likely to have jovian-mass planetary companions. Are lower-mass planets more common at lower [m/H]? 4.Despite this trend, planetary companions to solar-like stars are not rare  ~5% locally

A place in the Sun