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 1995 -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 starsUp to Fe by fusionAbove Fe, supernovae

elements (O, Mg, Ti, etc) are enhanced in stars with low overall metal abundancecapture elements formed preferentially by massive supernovae enhanced 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 0.001 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 populationFormed by dissipational collapse ~10% total mass of Milky WayAll current star formation located in Galactic disk Approximately half gas/dust & half stars

Building the Milky Way1. Big Bang, ~ 13 Gyrs2. Density perturbations (regions with enhanced dark matter density)

start to collapse shortly after initial expansion3. First stars in the Milky Way form during early stages of collapse at

~ 12 Gyrs – globulars, halo stars and central bulge4. Galactic disk forms at ~ 10-11 Gyrs; conservation of angular

momentum gives a flattened, rotating system; high gas density, socontinuing star formation

5. Merger with major satellite at ~ 9-10 Gyrs; initial disk is heated to form a “thick disk” – hiatus in star formation

6. Disk reforms at ~ 8-9 Gyrs and star formation resumes7. Continued accretion of small satellite galaxies as Milky Way builds

its outer halo Total mass ~ few x 1011 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 < 107 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 MyrsResidual dust disks to ~100 MyrsZodiacal dust in older stars (Sun)

The Solar SystemDimensions: Inner solar system, r < 4 AU Terrestrial planets and the asteroid belt: rocks Masses < 0.3% MJ

Gas giants, 5 < r < 30 AU Jupiter to Neptune Masses: 7% MJ to 1 MJ

Kuiper belt, 30 < r < 120 AUPluto and beyond: icy bodiesMasses < 0.05% MJ

Oort comet cloud, r ~ 105 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 ~ 17o.

Finding planets: Doppler surveys

Star+planets orbit common centre of gravityDetect reflex motion of primaryRequires high resolution (Jupiter induces ~12 m/s velocity in Sun)Require bright target stars (< 9th magnitude even with Keck 10-metre)

Doppler planets

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

Unexpected planets: Hot JupitersFirst 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 daysOrbit lies well within Mercury’s in the Solar SystemGas giants form by accreting ices (water ice, CO2, etc) onto a rocky core – but ices can’t exist within the “ice line” ~ 4-5 AUHow 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 planetAt least 15 with 2 planets

Lowest mass planet detected so far has M~30 Earth masses

Finding planets: TransitsMain alternative to Doppler surveysUse photometric monitoring to search for periodic variations indicating an eclipsing planetAdvantage known inclinationDisadvantage need high precisionJupiter gives ~1% dip; Earth ~0.01%

HD 209458

Planet detected by Doppler motions a hot JupiterPhotometry revealed that the orbital inclination is low enough that the planet transits the primary star. Mass = 0.69 Mjupiter

HST observations showRadius ~ 1.34 Rjupiter

Sodium detected in the planetary atmosphere

Other transit surveysPhotometric surveys need to cover tens of thousands of stars per night with millimagnitude accuracy to optimise chances of detecting a handful of transiting systemsCCDs 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 ~12th magnitude ~100 times fainter than typical Doppler and d~600 l.y. OGLE Galactic Bulge surveys – 15th-17th magnitude, d ~ 4000 l.y.Different types of stars in different environments also fainter, so more difficult to get detailed observations, butRadii are closer to 1.05-1.1 Rjupiter

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 skyAs viewed from ~30 l.y., Jupiter causes the Sun to move by 1 milliarcsecond (< 10-6 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

MicrolensingDetect planets by their ability to focus light gravitationallyPlanet produces short-period spike in microlensing light curveOne candidate discovered to date probably a jovian-mass planet around an M dwarf halfway beftween us and the Galactic Centre

Advantage: sampling different environmentsDisadvantage: 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 MJ or ~21 ME – 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 Msun

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 Mjupiter; flattens at ~4 Mjupiter (M M-1); and declines below 1 Mjupiter (almost certainly a selection effect).

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

Planets and metalsPlanets are much more common around metal-rich starsNot 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 star2. Distance from central star3. 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 known2. Most are companions of stars within the immediate

Solar Neighbourhood3. 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