ExtrasolarExtrasolar PlanetsPlanets

Is there a twinIs there a twinof our Homeof our HomePlanetPlanetsomewheresomewhereout there?out there?

Gero Rupprecht, ESOGero Rupprecht, ESOBrandys, 07.05.2004Brandys, 07.05.2004

Giordano Bruno1548-1600

“On the Infinite Universe andthe Worlds"

Every major scientific truth passes throughthe following three stages:

1. People say it contradicts the Bible2. People say it is already known3. People say they always knew it right away…

Michel MayorDidier Queloz(Obs. Geneva)

6.10.1995Public announcement of the discovery of the first extrasolar planet:

51 Pegasi

This was one of the most eagerly awaited discoveries - Searches ongoing for decades:

• many teams• many stars• different techniques

Without success!

Why?

• Were the techniques not sensitive enough?• Perhaps there were no planets after all?

The new planet: a mystery!

• orbital period: 4.2 d• Mass: ~0.5 MJupiter

This is against allthat was expected!

„Hot Jupiter"circling its star inside the orbit of Mercury

Consequences:• Extreme temperatures• Extreme climate• IF it is a gas planet: very short lifetime!

New discoveries followed suit

Today (May 2004): 123 Exoplanets known in 108 systems

with 13 multiple planets

The most successful teams:

• Geneva (Mayor, Queloz)• California/Carnegie (Marcy, Butler)• Anglo-Australian Telescope• Pennsylvania State University (Wolszczan)

More teams at different observatories

Where are they located?

How can Exoplanets be discovered?

1. Direct imaging2. Observation of induced proper motion3. Variation of the star‘s radial velocity4. Variation of the star‘s brightness:

a.Gravitational lens effectb.Transit!

Ad 1: Direct imaging of the planet

VERY difficult due to• extreme contrast: about 1:10^9 in the case of Jupiter/Sun at 5pc distance• very small angular separation: <1"

51 Pegasi

• only from space: “nulling interferometry”• GENIE (ESA/ESO VLTI 2008)• DARWIN (ESA)• TPF (NASA)

Using the effects of gravity

Depending on our relativePosition we see either• a “wobble” in the star’s position or• a “wobble” in the star’s velocity

Period = orbital period of the companion

Ad 2: Observation of a star‘s proper motion

• Superposition of space and orbital motion.• More difficult with increasing distance and decreasing mass of the planet. • Requires at least observation of one full orbit.• So far no planet discovered but one confirmed by HST• ESO VLTI: 10^-4‘‘ – ok for Jupiter, not Uranus in 10pc• ESA mission GAIA to measure precise PM in 2012

Ad 3: Variation of the star‘s radial velocity

• Dependent on the mass ratio star/planet• Independent of the distance from the observer• Jupiter causes 13m/s variation, Earth only 0.1m/s• Measurement limit 1m/s (HARPS at ESO 3.6m)

Geneva team

HARPS: High Accuracy RV Planet Searcheron ESO/La Silla 3.6m All you need is … Stability!

• Coude focus• fibre fed• image scrambler• no focusing mechanism• climatized room• vacuum vessel• heating blanket• special calibration method

• long-term stability: 1m/s• short-term: ~30cm/s• good for Uranus-like planets

HARPS: echelle spectrograph, RS=120000 CCD mosaic 4k*4k

HARPS inside

Echelle spectrum

HARPS echellegrating

Cross dispersed

Ad 4a: Gravitational lens effect

Very recent: published in April 2004

Observation Interpretation

Ad 4b: Variation in brightness due to a transit

One case known:

HD 209458 – 1% dip

Accessible even for amateurs:Nirölä Obs/FinlandSep. 2000

Meade 16’’+ Focal reducerSBIG ST7E CCD

Transit method applied by HST

Precise photometry

Detection of aNa “atmosphere”

Transit method: the future• only for a short fraction of the orbital period (hours)• only if Earth is nearly exactly in the orbital plane• Jupiter: 1% dimming, but Earth: 0.01% dimming!Currently the only method to discover Earth sized planets: only from space!• COROT (ESA, >2006; 30cm telescope)

• Kepler (NASA, >2007; 95cm telescope, differential photometer)

Some examples for exoplanet systems

All exoplanetsknown to date

--- E

arth

--- Ju

pite

r

For comparison:Our solar system

Exoplanets – what do we know about them?

Obviously: they are clearly different from our own system! But who is typical: we or them?

Multiple planetsRadial velocity curveshows 2 periods

Their orbits comparedto Earth’s orbit

A strange case: cannibal star – detected by VLT!

HD 82943• 2 planets• Li in stellar spectrum!

explanation: cannibalism!

Exoplanet summary

• a new, “hot” field in astronomy• within reach of amateurs!• appealing for the general public• large potential for exciting discoveries

>> Europe in the lead <<