The Search The Search for Extra-for Extra-Solar PlanetsSolar Planets

With thanks to Dr Martin Hendry

http://www.astro.gla.ac.uk/users/martin/teaching/

Prof Webster CashAstrophysical & Planetary Sciences

THE BIG QUESTIONS

• What is Reality?

• What are we?

• Are we alone?

How do we even get a handle on these questions?

Extra-Solar Planets

One of the most active and exciting areas of astrophysics

Nearly 4000 exoplanets discovered since 1995

Some important questions

o How common are planets?How common are planets?

o How did planets form?How did planets form?

o Can we find Earth-like planets?Can we find Earth-like planets?

o Do they harbor life?Do they harbor life?

1. How can we detect extra-solar planets?

Planets don’t shine by themselves; they just reflect light from their parent star

Exoplanets are very faint

Earth is about 10Billion times fainter than the Sun

25 Aug 2008 5

The Basic Problem:

Stars are very bright and their glare makes it difficult to see fainter objects

near them

1. How can we detect extra-solar planets?

They cause their parent star to ‘wobble’, as they orbit their common centre of gravity

Johannes Kepler Isaac Newton

Star + planet in circular orbit about centre of mass, to line of sight

Star + planet in circular orbit about centre of mass, to line of sight

Star + planet in circular orbit about centre of mass, to line of sight

Can see star ‘wobble’, even when planet is unseen.

But how large is the wobble?…

Star + planet in circular orbit about centre of mass, to line of sight

Can see star ‘wobble’, even when planet is unseen.

But how large is the wobble?…

Centre of mass condition

2211 rmrm

P

SSPS m

mrrrr 1

e.g. ‘Jupiter’ at 30 l.y.

radiansd

rSS

kg109.1

kg100.227

30

P

S

m

m

deg105.1 7

The Sun’s “wobble”, mainly due to Jupiter, seen from 30 light years away = width of a tennis ball in London

SIM Planet Quest Just Cancelled!

Suppose line of sight is in orbital plane

Direction to Earth

Direction to Earth

Suppose line of sight is in orbital plane

Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally

Suppose line of sight is in orbital plane

Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally

Detectable via the Doppler Effect

Can detect motion from shifts in spectral lines

Absorption

e -

e -

Electron absorbs photon of the precise energy required to jump to higher level.

Light of this energy (wavelength) is missing from the continuous spectrum from a cool gas

Star

Laboratory

Limits of current technology:

-1sm1v

c

v

0

Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours

Doppler formula

Wavelength of light as measured in the laboratory

Change in wavelength

Radial velocity

Speed of light

millionth3000

51 Peg – the first new planetDiscovered in 1995

Doppler amplitude

How do we deduce planet’s data from this curve?

-1sm55v

PSS mmT

G 3/23/1

2v

We can observethese directly

We can infer this from spectrum

Complications

Elliptical orbits

Complicates maths a bit, butotherwise straightforward

radius semi-major axis

Orbital plane inclined to line of sight

We measure only

If is unknown, then we obtain a lower limit to

( as )

Multiple planet systems

Again, complicated, but exciting opportunity (e.g. Upsilon Andromedae)

Stellar pulsations

Can confuse signal from planetary ‘wobble’

obssinv iS

i

obssinvv iSS Pm

1sin i

Change in brightness from a planetary transit

Brightness

Time

Star

Planet

Another method for finding planets is gravitational lensing

The physics behind this method is based on Einstein’s General Theory of Relativity, which predicts that gravity bends light, because gravity causes spacetime to be curved.

This was one of the first experiments to test GR: Arthur Eddington’s 1919 observations of a total solar eclipse.

Another method for finding planets is gravitational lensing

If some massive object passes between us and a background light source,

it can bend and focus the light from the source, producing multiple,

distorted images.

Background stars

Gravitational lens

Lens’ gravity focuses the light of the background star on the Earth

So the background star briefly appears brighter

Even if the multiple images are too close together to be resolved

separately, they will still make the background source appear (temporarily)

brighter.

We call this case gravitational microlensing. We can plot a light curve

showing how the brightness of the background source changes with time.

Time

The shape of the curve tells about the mass and position of the object which does the lensing

Even if the multiple images are too close together to be resolved

separately, they will still make the background source appear (temporarily)

brighter.

We call this case gravitational microlensing. We can plot a light curve

showing how the brightness of the background source changes with time.

If the lensing star

has a planet which also

passes exactly between

us and the background

source, then the light

curve will show a second

peak.

Even low mass planets can

produce a high peak (but for

a short time, and we only

observe it once…)

Could in principle detect Earth mass planets!

What have we learned about exoplanets?Discovery of many ‘Hot Jupiters’:

Massive planets with orbits closer to their star than Mercury is to the Sun

Very likely to be gas giants, but with surface temperatures of several thousand degrees.

Mercury

Artist’s impression of ‘Hot Jupiter’ orbiting

HD195019

‘Hot Jupiters’ produce Doppler wobbles of very large amplitude

e.g. Tau Boo:

-1ms474sinv iS

1. The Doppler wobble technique will not be sensitive enough to

detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets

2. The ‘position wobble’ (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after

2010

(done with HST in Dec 2002 for a 2 x Jupiter-mass planet)

3. The Kepler mission (launch 2008?) will detect transits

of Earth-type planets, by observing the brightness dip of stars

Right Now

Transit Detection by OGLE III program in 2003

But the Future is in Direct Imaging….

External Occulters• Let’s Resurrect an Old Idea

– Spitzer (1962) appears to be the first

• Just Keep the Starlight Out of the Telescope

Occulter Diagram

Telescope big enough to collect enough light from planetOcculter big enough to block star

– Want low transmission on axis and high transmission off axisTelescope far enough back to have a properly small IWANo outer working angle: View entire system at once

NWD Starshade JWSTTarget Star

Planet

Fly the Telescope into the Shadow

Dropping It In

Note: No Outer Working Angle

New Worlds Observer

Simulated Solar System

The First Image of Solar System

JupiterSaturn

Uranus

Neptune

Zodiacal Light

Galaxies

10 arcseconds

Simulated Image of Earth

Planet Finding with Starshades

Five Random Systems from Raymond Database

The higher resolution of ATLAST brings weak signals out of the noise

ATLAST

JWST

Spectroscopy

• R > 100 spectroscopy will distinguish terrestrial atmospheres from Jovian with modeling

O2

H2O

CH4

NH3

S. Seager

Earth Viewed at Improving Resolution

100 km300 km3000 km 1000 km

TRUE PLANET IMAGING

Conclusion By 2025

O2

H2O

By 2013

Demonstration Program 2010-2013Study Planets with Small Starshade 2018Full Up New Worlds Observer 2027Planet Imager – 2035?

Lectures Complete• Final Exam

• 1:30-4:00pm Wednesday 17th Here.

• Just like the mid-terms except twice as long

• Covers everything (comprehensive)

• A bit extra on last four lectures

• One or two longer essays

• Review Session by Josh Monday 5:30-6:30 here

• I will do office hours 12:00-1:30 Wednesday for last minute questions (Duane F913)