Extra Solar Planets

ASTR 1420

Lecture 17

Sections 11.2

Searching for Extra Solar Planets

• Extra Solar Planets Exo-planets (Exoplanets)

• 0th order expectation: o exoplanets should be moderately common

theoretical : planet formation theory… Disk forming is inevitable followed by temperature dependent condensation.

observation : flat disks… and actual discovery of them since mid 90s.

• Main difficulty o Planets are much fainter than the star : even the brightest (largest and youngest)

planets are 1 million times fainter than a typical star.o Planet—star separation is very small Solar system at ~10 light years = 1 second

of arc (the extent of a penny seen ~2.5 miles away).

History

1st discovery of exo-planet : “Pulsar Planet”

• 1992, Aleksander Wolszczan and Dale Frail discovered two planets orbiting around a pulsar, PSR 1257+12

• 1995, Mayor and Queloz, 51 Peg b first radial velocity planet

• To date, 429 exo-planets were discovered as of today.

Definition of Exoplanet

• Objects with masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they were formed).

• The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.o Substellar objects with true masses above the limiting mass for thermonuclear

fusion of deuterium are "brown dwarfs", no matter how they formed.o Free-floating objects in young star clusters with masses below the limiting mass

for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

Planetary mass objects orbiting around other stars!

Search MethodsNot a planet

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase

Search Methods

1. Direct Imaging2. Astrometry3. Doppler shift4. Pulsar Timing5. Transit6. Microlensing7. Orbital phase (?)

Naming exoplanets

Name is given relative to the central star…• Naming stars::o Relative to constellation name (in Latin adjective)o In a constellation, the brightest star is “alpha”, then, beta, …, omega, 1, 2, 3,…o Stars in multiple system are named relative to the brightest star with prefix capital

alphabets

Examples:brightest star in Orion = α Orioni (α Ori)brightest star in Cygnus = α Cygni2nd companion of the 3rd brightest star in Pegasus = γ Peg C

• planets around a star is given a prefix in small alphabet…o 1st detected planet around 51 Peg = 51 Peg bo planetary system HR 8799 : HR8799 + HR8799 b + HR8799 c + HR8799 d

Doppler Effect

stationary source moving source

Doppler effect and sonic boom

Doppler Effect

Light = Wave

• Light = wave : blue light (high frequency), red light (low frequency)

Solar Spectrum

Each of the 50 slices covers 60 angstroms, for a complete spectrum across the visual range from 4000 to 7000 angstroms

Doppler shift of Stellar spectrum due to unseen planet(s)

blue shift : getting closer

red shift : getting farther away

Effect of Planets on the Stellar Motion

http://www.astro.sunysb.edu/mzingale/software/astro/radial_velocity.avi

Magnitude of reflex motion ≈ mass of the planet + orbital distance

For non-circular motion

http://www.astro.sunysb.edu/mzingale/software/astro/radial_velocity_ell.avi

First Discovery : 51 Pegasi b

Line of sight motion only…

Doppler shifts reveal only the part of a star’s motion directed toward or away from us (i.e., line of sight motion)!Therefore, face-on orbits do not cause any shift in the spectrum…

Real Data

Currently known RV Exoplanets

413 RV planetsas of today!

Hot Jupiters…

• Doppler technique is best suited for identifying massive planets close to their stars

• Hot Jupiters many gas planets close to the central stars were discovered!

143/696 are hot Jupiters

Astrometry

• Had been used many decades to discover binary stars• Similar to the Doppler technique looking for reflex motions.o astrometry : tangential motionso Doppler : line of sight motions

• Therefore, more massive planets, planets at wide orbits are easier to be detected!

• For a given planet mass and orbit, lower mass central stars showing larger reflex motions.

• However, planets take longer time to make one orbit at wide orbits (Period2 = Distance3)

• Current limit : ~10 milli arc second (1/100 arc second)

• Not many discoveries (a handful)…

Sun’s Reflex Motion seen at 33 light-years away

http://planetquest.jpl.nasa.gov/SIM/images/SolarWobbleWhiteSmall.mov

Similar to measure the width ofhuman hair about 10 miles away!

European Mission

GAIA• Launch 2012• 320 Million Euros• 1.45m telescope

• catalog 1 billion stars 3D map of Milky Way

• ~10,000 planets

Mercury Transits (2006, Nov 8)

• Transit of Venuso 2012 June 6

• Transit of Mercuryo 2016 May 9o 2019 Nov 11

HST Observation of HD 209458 b

• Could detect 0.01% variation

Transit Power!

with inclination + RV• Orbital period• Planet Radius• Planet mass• Planet’s density• Planet temperature• Stellar radius• Limb darkening…

Can even detect the effect of Planetary Atmosphere!

Exoplanetary atmosphere

• H2O, CH4…

Currently 258 transiting exoplanets were discovered!

Kepler Mission• Successfully launched on

03/06/2009o 1.4m telescopeo 95 mega pixelso 3.5 years

• search for Earth-sized habitable planets

Kepler FoV

• Will look at 100,000 stars for 3.5 years…

• 20ppm for 12th mag star

~50 Earths to be found!

Kepler discoveries to date : 114 confirmed

Great sensitivity from Kepler!!

One example : Kepler 68c (4.8 Earth-mass planet)

Gravitational Lensing

Microlensing

~6 exoplanets were found as of 2009

In 2006, a 5 Earth mass planet was discovered.

Lensing by the same planet can never(?) be observed again!

But, can detect planets furthest from Earth and sensitive to discover planet on wide orbits.

Optical Gravitational Lensing Experiment (OGLE) toward the Galactic center (~100 million stars)

In summary…

Important Concepts• Exoplanet detection methodso direct & indirect

• Nomenclature of exoplanets• Definition of exoplanets

• Doppler shift• Doppler shift planet searcho line of sight motion only!

• Pros and cons of different exoplanet detection methods

Important Terms• Hot Jupiters • Transit• Astrometry• Microlensing

Chapter/sections covered in this lecture : 11.2Direct Imaging Detection of Exoplanets : next class