Methods for the detection of exosolar planets

Astronomical Seminar

January 2004

Erik Butz

Overview Introduction Today‘s methods Future prospects Summary

Introduction Big question of mankind:

Are we alone in the universe?

Many speculations: Mars, Venus and other planets in the solar system

Search for exosolar planets was hopeless for several centuries because of insufficient sensitivity of instrumentation and because of enormous distances

Introduction 2 Key technique for first discoveries

developed by Christian Doppler in 1842:

c

v

Through shift in spectral lines, velocity of an object can be determined

Introduction 3

1992 first planet around pulsar found (OBS! Not using radial velocity method!)

Pulsar timing: Systematic variation in arrival of pulsar pulses

Introduction 4

1995 first planet around Main Sequence(MS) Star (51 Peg)

Since then: More than 110 planets found!

Today‘s methods Several searches ongoing using

Radial Veclocity technique

Measurement of Doppler shift in the spectrum of the star due to gravitational influence of the planet.

Planetary Doppler Shift 1

Source: www.Extrasolar.net

Planetary Doppler Shift 2

Planetary Doppler Shift 3

Planetary Doppler Shift 4

Long struggle to reach sufficient sensitivities

Jupiter causes shift of 15 m/s

Compare to line-width ~km/s

Planetary Doppler Shift 5

Breakthrough hoped for at 10 m/s

First groups were unlucky: Did not find planet with sensitivity

of 10 m/s 51 Peg: 50 m/s => would not have

been a problem

Planetary Doppler Shift 6

Planetary Doppler Shift 7 Todays precision: ~2 m/s

Compare to: Earth: 0.1 m/s

No Earth finder with present doppler methods!

Planetary Doppler Shift 8 Advantages:

Enables finding of planet with comparably low effort

Can be used on smaller telescopes as well

Disadvantages: Deviation is ~MP => easier to find

larger planets and smaller periods =>shorter observation times

Due to orbital inclination no direct determination of MP

Planetary Doppler Shift 9

Only determination of MP sin i

Todays methods 2 Complementary searches using

astrometric measurements

Measurement of systematic variations of star position also because of gravitational influence of planet

Astrometry 1 Star‘s apperent path due to planet

is ellipse with major half axis :

d

a

M

M P *

Effect is larger for nearby stars

If M*, a and d known: Determination of MP

Deviation for Jupiter in d ~ 10 pc is of order of milliarcsec or lower

Astrometry 2

Astrometry 3

Astrometry 4 Problem: milliarcsec precision only

reached in radio

Hipparcos: ~1 milliarcsec

VLTI(not yet available): 10-100µarcsec

Astrometry 5 Future: µarcsec astrometry possible,

but:

Earth moves sun about 500 km 0,03 %

Sunspots and other dynamic instabilities

0.5 %

Today‘s methods 3

Luminosity variation during Transits of planets

Problems: Situation is highly improbableEffect is small:

Sun/Jupiter in 10 pc: ~2% (0.02m)

Transits 1 Advantages:

Feasible with low effortCan be done with many stars in short time

Transits 2

Transits 3

Future prospects

Imaging of planets in IR and VIS

Problems:

!!10~ 9

*

L

LP

Interferometric imaging 1

Ratio in IR (i.e. at maxPlanet) 105 better

Furthermore: interferometry to further reduce starlight

Nulling interferometry: destructive interference at star position but not at planet position

Summary Several methods

for discovery of exosolar planets

More then 110 planets found

Future methods will enable more discoveries and deeper investigation

With Transits:

Atmosphere‘s=>signatures of life

With extremely large(150 mirrors of 1 m 150 km baseline) space telescope

imaging on exosolar planets