The Galactic Exoplanet Survey Telescope GEST

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The Galactic Exoplanet Survey Telescope (GEST)
PI D. Bennett (Notre Dame), J. Bally
(Colorado), I. Bond (Auckland), E. Cheng (AC, ex
GSFC), M. Clampin (GSFC), K. Cook (LLNL), D.
Deming, (GSFC), P. Garnavich (Notre Dame), M.
Greenhouse (GSFC), K. Griest (UCSD), D. Jewitt
(Hawaii), N. Kaiser (Hawaii), T. Lauer (NOAO), J.
Lunine (Arizona), G. Luppino (Hawaii), J. Mather
(GSFC), D. Minniti (Catolica), S. Peale (UCSB),
B. Rauscher (GSFC), S. Rhie (Notre Dame), J.
Rhodes (GSFC), K. Sahu (STScI), J. Schneider
(Paris Obs.), R. Stevenson (Notre Dame), C.
Stubbs (UW), D. Tenerelli (Lockheed), N. Woolf
(Arizona) and P. Yock (Auckland)
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Talk Outline

• What do we need to know to determine the
  abundance of habitable or Earth-like planets?
• What does Earth-like mean?
• The basics of microlensing
• The Scientific Return
• Simulated planetary light curves
• planet detection sensitivity
• Lens star detection
• What we learn from the planets that are detected
• GEST Mission Design
• Why is a Space mission needed for microlensing?
• Resolve main sequence stars
• continuous light curve coverage

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Requirements for a Habitable Planet

• A 1 M? planet at 1 AU orbiting a G or K-star?
• How about a 1 M? planet at 1.5 or 2 AU?
• with a greenhouse atmosphere
• Is a gas giant at 5 or 10 AU needed, as well?
• Are planets orbiting M-stars more or less
  habitable than those orbiting G-stars?
• Is there a Galactic Habitable Zone?
• Moons of giant stars?
• Is a large moon important for the development of
  life?
• Could life be based upon NH3 instead of H2O?
• It seems likely that we cannot understand
  habitability until we understand the basic
  properties of planetary systems.

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The Physics of ?-lensing

• Foreground lens star planet bend light of
  source star
• Multiple distorted images
• Total brightness change is observable
• Sensitive to planetary mass
• Low mass planet signals are rare not weak
• Peak sensitivity is at 2-3 AU the Einstein ring
  radius, RE

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Planetary Microlensing Light Curve

• Top panel shows stellar images at 1 mas
  resolution centered on lens star
• Einstein ring in green
• Magnified stellar images shown in blue
• Unmagnified image is red outline
• The observable total magnification is shown in
  the bottom panel
• A planet in the shaded region gives a detectable
  deviation

Video from B. S. Gaudi (CfA)
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Microlensing Rates are Highest Towards the
Galactic Bulge
High density of source and lens stars is required.
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GEST Mission Simulation

• From Bennett Rhie (2002) ApJ 574, 985
• Continuous observations of 2.4 sq. deg. central
  Galactic bulge field 108 stars
• Simulated images based on HST luminosity function
  from Holtzman et al (1998)
• mass function from Kroupa (2000), Zoccali et al
  (2000)
• 15,000 events in 4 seasons
• microlensing probability, ? 3.0?10-6, assumed
• at Galactic coordinates l 1.3?, b -2.2?

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Simulated Planetary Light Curves

• Planetary signals can be very strong
• There are a variety of light curve features to
  indicate the planetary mass ratio and separation
• Exposures every 10 minutes

moon signal
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more light curves
Low S/N
visible G-star lenses with typical S/N
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GESTs Double Planet Detections
? 3?10-6, 10-3 a 1, 5.2 AU 10 events

• ? 10-3, 3?10-4 a 5.2, 9.5 AU 100 events

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Planet Detection Sensitivity Comparison

• Sensitivity to all Solar System-like planets
• Except for Mercury Pluto
• most sensitive technique for a ? 1 AU
• Good sensitivity to outer habitable zone
  (Mars-like orbits) where detection by TPF is
  easiest
• Mass sensitivity is 1000 ? better than vr
• Assumes 12.5? detection threshold
• GEST is complementary to Kepler

Updated from Bennett Rhie (2002) ApJ 574, 985
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Lens Star Identification

• Flat distribution in mass
• assuming planet mass ? star mass
• 33 are visible
• within 2 I-mag of source
• not blended w/ brighter star
• Solar type (F, G or K) stars are visible
• 20 are white, brown dwarfs (not shown)
• Visible lens stars allow determination of stellar
  type, distance, and relative lens-source proper
  motion

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Planetary Semi-major Axes
For faint lens stars, separation determination
yields a to factor-of-2 accuracy, but the
brightest 30 of lens stars are detectable. For
these stars, we can determine the stellar type
and semi-major axis to 10-20.
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GESTs Planetary Results

• Planets detected rapidly - even in 20 year
  orbits
• average number of planets per star down to Mmars
  0.1M?
• Separation, a, is known to a factor of 2.
• planetary mass function, f(?Mplanet/M?,a)
• for 0.3Msun ? M? ? 1 Msun
• planetary abundance as a function of M and
  Galactocentric distance
• planetary abundance as a function of separation
  (known to 10)
• abundance of free-floating planets down to Mmars
• the ratio of free-floating planets to bound
  planets.
• Abundance of planet pairs
• high fraction of pairs gt near circular orbits
• Abundance of large moons (?)
• 50,000 giant planet transits

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GEST Mission Design

• 1.2-1.5m telescope 3 mirror anastigmat
• 1.24 sq. deg. FOV
• 2 fields gives 2.48 sq. deg. survey area
• NO shutter for camera
• 0.2/pixel gt 4?108 pixels
• continuous view of Galactic bulge
• for 8 months per year
• 60 degree Sun avoidance
• high Earth Orbit
• 40 degree Earth limb avoidance
• Image downloaded once every
  minute
• 100 Mbits/sec mean data rate
• lt0.02 pointing stability
• maintained gt95 of the time

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Wide FOV Camera
Focal Plane layout 72 IR and 24 Si 2k ? 2k CMOS
detectors CCDs, 18?m pixels 400 Mpix total. 2
field coverage
Bulge stars are highly reddened, so HST-WFC3
style IR detectors improve sensitivity.
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GEST Orbit
Inclined Geosynchronous Orbit allows continuous
view of the Galactic bulge for 8 months/year
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A Similar Mission SNAP

• The SuperNovae Acceleration Probe (SNAP)
• wide FOV optical/near IR telescope
• TMA design
• Mixture of Si and HgCdTe detectors
• 500 Mpixels
• Passively cooled to 140K
• High Earth orbit
• Under study
• DOE High Energy Physics
• NASA JDEM
• Time sharing w/ GEST?

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Microlensing From the Ground vs. Space

• Target main sequence stars are not resolved from
  the ground.
• Poor photometry for unresolved stars, except for
  very high magnification events
• Poor light curve coverage
• Ground surveys can only find events with a ? RE
• No measurement of planetary abundance vs.
  semi-major axis

Ground-based Images of a Microlensing Event
GEST Single Frame
GEST Dithered Image
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Ground-based Challenge for GEST

• 1995 ExNPS study recommended a ground-based
  microlensing program to find Earths
• Sensitivity overestimated because blending was
  ignored!
• Provoked backlash brief anti-microlensing
  campaign from Kepler team
• Some other papers have been quite optimistic
• It is easier to detect a microlensing signal from
  an Earth-like planet than to be able to show that
  the signal can only be explained by an Earth
• A Space mission must have a large advantage (a
  factor of 100) over ground-based observations to
  be approved
• The Microlensing Community must agree on the
  capabilities of space and ground-based programs

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Ground-Based Simulations

• Same parameters as GEST simulations
• Real seeing and weather data from Paranal, La
  Silla (as a proxy for SAAO), and MSSSO (MACHO
  seeing 0.8 contribution from optics)
• Sky brightness model includes proximity of moon
• Photometric accuracy limited to 0.7 of total
  flux at stellar position
• Flux includes blends and sky
• OGLE-3 DIA results for bright stars
• Little advantage for very large telescopes
• Assume large wide FOV telescopes
• Like original VISTA or individual PAN-STARS
  telescopes
• Single site (Paranal) survey as advocated by
  Sackett (1997)
• 3-site survey Paranal, SAAO, Siding Springs
  hardware cost 50M
• High magnification events (Amax gt 200) ignored

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Light curves from a single site survey (Paranal)
Signals of Earth mass planets can be detected,
but characterization is difficult with data from
a single site.
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Light curves from a single 3-site survey
3-site light curve coverage can allow good
coverage, but often, poor seeing at some sites
makes additional coverage useless.
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Light curves from a single 3-site survey
Coverage can be lost due to poor weather, and
some detectable planets have stellar light curves
that cannot be accurately characterized.
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Ground vs. Space-Based Planet Discoveries
Comparison of planetary discovery rates for GEST
and ambitious ground-based surveys. Most
ground-based discoveries are high magnification
events with low mass lenses. Does not include
high mass planets which can be detected with
giant source stars.
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Ground vs. Space

• We need consensus to get future projects funded
• Current near future ground-based
• Giant source stars high magnification events
  are best targets
• Good sensitivity down to MNeptune 15 M?
• Limit for giant source stars
• High magnification events
• Good sensitivity to most massive planet in a
  system
• Earths in a system with Jupiters and Saturns
  probably cant be detected and characterized
• Separation info is limited
• 50M ground-based survey has 1 sensitivity of
  GEST
• A Space-based mission like GEST is the only way
  to get good statistics on Earths

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GEST Summary

• Straight-forward technique with
  existing technology
• Discovery class mission
• Low-mass planets detected with
  strong signals
• Cannot be done from the ground
• Sensitive to planetary mass
• Sensitive to a wide range of separations
• Venus-Neptune
• Free floating planets, too
• Combination with Kepler gives planetary abundance
  at all separations
• Should be done!