Detectability of Habitable Planets with the Space Interferometry Mission

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Detectability of Habitable Planets with the
Space Interferometry Mission

• Evan Bierman, Chris McCarthy,
• Debra Fischer, Geoff Marcy
• San Francisco State University

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Overview

• What is SIM and why is it important?
• How does SIM find Planets?
• What is a Habitable Planet?
• Can SIM find a habitable planet the size of earth
  around another star?

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The Space Interferometry Mission (SIM)
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SIM has a 5 year mission in a heliocentric earth
trailing orbit
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Unparalleled Precision

• SIM has an astrometric precision of 1?as or
  1/3600000000th of a degree
• 1000 times better than HIPPARCOS and current
  ground based technologies
• 100 times better than Hubble
• Will be able to determine actual masses of
  extrasolar planets

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How does SIM find planets?

• Astrometry

• An observing technique to measures the precise
  Position of star.
• The Stars Wobble is caused by a gravitational
  perturbation as a planet orbits the star

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Most of the Astrometric Signature is NOT due to
Earth
Signature at 10pc (?as)
Planet
0.01
Mercury
0.18
Venus
0.31
Earth
0.05
Mars
512.62
Jupiter
281.25
Saturn
89.28
Uranus
158.57
Neptune
0.03
Pluto
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What does the Suns Astrometric Wobble look like?
The wobble of the Sun's projected position due to
influence of all the planets in the Solar System
as it would appear from 10pc, courtesy of NASA
ORIGNS
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What is a Habitable Planet?

• Liquid water must exist on the planet
• Temperatures 0ºC-100ºC

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• Given the Luminosity of a star, it is possible
  to compute an approximate Habitable Zone (HZ)

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First Order Calculation of a Habitable Zone

• Stars Luminosity
• Planets Bond Albedo
• Temperature range of the habitable zone
  0ºC-100ºC
• Neither the size nor mass of the planet is a
  factor

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Astrometric Signature of a Habitable Planet
Increases with Star Mass
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Given the 1?as precision, what planets could be
detected???

• The Two SIM Key Science projects (PIs Shao,
  Marcy) selected 120 optimal target stars for low
  mass planet detection
• The habitable zones were calculated for all of
  these stars
• Assume that there is a planet orbiting in the
  middle of the habitable zone what is the minimum
  detectable habitable planet mass

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The Five Best Candidates
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Conclusion

• SIM should be able to find several large
  habitable earths and dozens of habitable
  Neptunes although the likelihood of finding a
  habitable Earth is low
• SIM will also discover and confirm the existence
  of hundreds of planets not restricted to the
  habitable zone!!

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References

• California Carnegie Planet Search,
  exoplanets.org
• Hart, M. H., "Habitable Zones about Main Sequence
  Stars", Icarus, vol. 37, 351-357.
• Kastings, J.F., Whitmire, D.P., Reynolds, R.T.
  (1993) "Habitable Zones around Main Sequence
  Stars", Icarus 101, 108-128
• SIM Key Science Project Discovery of Planetary
  Systems with SIM, www.physics.sfsu.edu/SIM
• http//www.unm.edu/astro1/ET109/zones/zones.html

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Second Order Corrections

• Stars Luminosity and Temperature over the entire
  lifetime of the main sequence stage
• Greenhouse Effect (GE) Warms the planet by
  trapping IR radiation
• Wavelength dependent and time varying Albedo
• Atmospheric Composition and Pressure The effects
  on the GE, atmospheric lapse rates, and
  atmospheric convection rates
• Whether the planet started frozen and had to be
  thawed or whether the planet was always warm
• Ocean Cover and Cloud formation

Kasting et. al 1993
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Other Considerations on the HZ

• Tidal Locking of close planets
• Spin Axis tilt
• Companion Stars
• Carbonate-Silicate cycle as a feedback mechanism
  on the Greenhouse Effect (CS cycle)
• Organic feedback mechanisms
• Plate Tectonics (PT) A key component of the CS
  cycle
• Planets Mass, Radius, and composition determine
  the length of time and magnitude of PT
• Jupiter Sized Objects Can stunt the formation of
  planets. Example Mars
• Planetary Formation Zone vs. Habitable Zone

Weatherill 1996