direct imaging of planets - princeton universitygk/a205old/lecture11.pdf · 2003. 10. 20. ·...

Direct Imaging Of Planets

AST 205David Spergel

Advantages

•Much faster detections

•Immediate detection of entire system

•Enormous additional science

•Size and Albedo

•Spectroscopy

•Biomarkers

Why is this hard?• Detecting light from planets

beyond solar system is hard:– Planet signal is weak but

detectable (few photons/sec/m2)

– Star emits million to billionmore than planet

– Planet within 1 AU of star– Dust in target solar system ¥300

brighter than planet• Finding a firefly next to a

searchlight on a foggy night

>109>106

ScatteredLight

ThermalEmission

Fourier Transform

Any function can berepresented as a sum ofcosine (and sine) waves

=

Telescope Size Sets Resolving Power

q = l/d

d=1

d=2

d=4

Ringing

Airy Rings

Linear Scale

Log Scale(1e-10 is black)

The Diffraction Problem(Visible)

Focal plane

Diameter (D)

The image in thefocal plane is thespatial Fouriertransform of theentrance field

Wavelength (l)

Entrance Pupil

0 5 10 1510-12

10-10

10-8

10-6

10-4

10-2

100

Angle (lambda/D)

1

1e-10

Pupil Coronagraph

What if we don’tgive equal weight toeach Fourier mode?

Pupil Design

Concentric Rings

Starshaped Mask

Starshaped mask

20-point star

150-point star

Wavefront Sensing andControl

What is the biggest problem?

Surface Errors Variations in Reflectivity

Interferometery

• Enables precision astrometry,high resolution imaging,starlight nulling

• Make astrometric census of planets • Detect “Hot Jupiter’s” • Detect exo-zodiacal dust clouds• Image protostellar disks

• Break link between diameter, baseline

IRInterferometer

• Interferometer with cooled two to four 3~4 mmirrors– 30 m boom– 75-1000 m baseline using formation flying

• Operate at 1 AU for 5 years to survey 150 stars

Nulling Interferometryl /b

2-arm Nulling Interferometerb

y

x

Null depth integrated overfinite star diameter ~8x10-5

4-arm Nulling InterferometerIntensity:

b

y

x

Null depth integrated overfinite star diameter ~6x10-10

InterferometerDetects and

CharacterizesPlanetary Systems

• TPF produces image of planetarysystem

– Orbital location– Temperature and radius

• TPF produces spectrum to searchfor biomarkers• 1-2 m telescopes to find Jupiters,nearest Earths•3-4 m telescopes for full TPF goals

The Angular Resolution Challenge

+

• Coronagraphs at >3l/D• Interferometers at > 1 l/B

10 mm, 28 mCoronagraph

Cost ($$), L

aunc h Dat e

Conclusions

• Interferometry and Coronagraphy are twopromising but difficult techniques

• They have the potential of yielding directdetections of Earth-like planets

• Planets can not only be detected but alsocharacterized