Lecture 21 Life beyond our Solar system Extrasolar planets

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Lecture 21 Life beyond our Solar
system(Extrasolar planets)
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Extrasolar planets

• Planets are Petri dishes for the origin and
  evolution of life.
• Giordano Bruno There are countless suns and
  earths all rotating around their suns in exactly
  the same way as the seven planets of our
  system.. (1584)
• First confirmed planet 51 Pegasi on October
  1995 by Mayor and Queloz (1995)

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G. Marcy and P. Butler (circa 2002)
Mayor and Queloz (1996)
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• By now, 329 mostly Jupiter-sized planets
• (300 Earth masses) have been discovered
• Several of these are Super-Earth 1-10 times
• more massive than Earth
• The smallest one so far is 3.3 Earth masses
• --Discovered by gravitational microlensing
• About 20 percent of these planets are within
• the HZs of their parent stars
• First super-Earth in habitable zone Gliese 581c
  discovered in April 2007

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Hertzsprung-Russell (HR) Diagram
http//observe.arc.nasa.gov/nasa/space/stellardeat
h/stellardeath_1ai.html
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Prospects for finding habitable planets

• Best candidates are F, G, and early K-type stars,
  i.e., stars not too different from the Sun
• Early-type stars (blue stars)
• High UV fluxes
• Short main sequence lifetimes
• Late-type stars (red dwarfs) M-class
• Tidal locking
• Lots of flares

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Earth would look like this
The star near it would be 10,000 times
brighter than this, and separated from Earth by
the small dot width.
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Detection of the Extrasolar Planets

• Direct detection Large and hot planets with
  significant separation from a parent star
• Indirect detection
• 1) Astrometry (changes in a stars position)
• 2) Radial velocity (variation in stellar speed)
• 3) Transits (changes in stellar luminosity)
• 4) Gravitational microlensing

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Planets discovered by direct detection
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Transit Method
When a planet passes in front of its parent star
the observed brightness of the star drops. 52
planets have been discovered by this method
Advantages a) Relatively cheap b) Can determine
the size of the planet Disadvantages a) Bias
towards large planets and in short period
orbits b) False detections due to stellar
variability
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Planets discovered by transit
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Kepler Mission

• This space-based telescope
• will point at a patch of the
• Milky Way and monitor the
• brightness of 100,000 stars,
• looking for transits of Earth-
• sized (and other) planets
• Scheduled to launch March 4, 2009

http//www.nmm.ac.uk/uploads/jpg/kepler.jpg
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Radial velocity (Doppler) method
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305 planets were detected by radial velocity
technique as of November 2008
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Planets discovered by radial velocity
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• Foreground star acts like a lens, magnifying the
  light of a distant background star.
• This effect occurs only when the two stars are
  almost exactly aligned.
• If the foreground lensing star has a planet, then
  that planet's own gravitational field can affect
  the lensing effect. 8 planets detected.

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Planets discovered by gravitational microlensing
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How Will We Know A Planet Supports Life?
Look for evidence of oxygen
Look for liquid water
Look for signs of biological activity (methane)
Analyze the reflected light from the planet to
see if the planet has an atmosphere
And Rule Out Other Explanations?
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NIMS Data (from Galileo)
(A band)
But credit Toby Owen for pointing this out
(1980)
Sagan et al. (1993)
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NIMS data in the near-IR

• Simultaneous presence
• of O2 and a reduced
• gas (CH4 or N2O) is
• the best evidence for
• life
• Credit Joshua Lederburg
• and James Lovelock for
• the idea (1964)

Sagan et al. (1993)
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Thermal IR spectra
Source R. Hanel, Goddard Space Flight Center
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Terrestrial Planet Finder (TPF)

• Visible or thermal-IR?
• Contrast ratio
• 1010 in the visible
• 107 in the thermal-IR
• Resolution ? ?/D
• Required aperture
• 8 m in the visible
• 80 m in the IR

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Courtesy Chas Beichman, JPL
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Evolution of the TPF Flight Design Concepts

• The original idea was to fly a thermal infrared
  interferometer on a fixed 80-m boom, similar to
  SIM, but bigger (and cooled)
• Disadvantages
• Vibrations
• Fixed baseline

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TPF-I (or Darwin) Free-flying IR interferometer

• This idea has now evolved into a free-flying
  interferometer, similar to ESAs proposed Darwin
  mission
• Advantages good contrast ratio, excellent
  spectroscopic biomarkers
• Disadvantages needs cooled, multiple spacecraft

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TPF-C Visible/near-IR coronagraph

• It may be easier, however, to do TPF in the
  visible, using a single telescope and spacecraft
• Advantages single spacecraft and telescope
• Disadvantages high contrast ratio between
  planet and star

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An Integrated Program of Planet Finding Science
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KEPLER
Survey of distant stars for Earths
Optical signs of habitable worlds
KECK
TPF-C
Survey of nearby stars for dust and giant planets
Mid-infrared signs
of habitable worlds
SIM
Masses and orbits of large terrestrial planets
TPF-I
ARE THERE OTHER HABITABLE WORLDS?
LBTI
JWST
PLANET CHARACTERIZATION Planet chemistry in
visible and infrared
Presence of water
Radius
Surface gravity and temperature
Atmospheric conditions
Biomarkers
ARE THERE OTHER SOLAR SYSTEMS LIKE OUR OWN?

• Young Jupiters
• Transit Follow-up
• Debris Disks

PLANET DETECTION Nearby giant planets
Young, hot Jupiter's
2005
2010
2015
2020
2025
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TPF summary

• NASAs proposed TPF mission (or ESAs Darwin
  mission) may eventually be able to locate
  Earth-sized planets around other stars and take
  either visible or thermal-IR spectra of their
  atmospheres
• O2 (or O3) and CH4 have absorption bands in both
  wavelength regions that may be used as potential
  indicators of extraterrestrial life
• But,
• We need other bioindicators for reduced,
  early-Earth type atmospheres