Ground-Based Direct Detection of Exoplanets with the Gemini Planet
  Imager (GPI)
Microsoft Word - gpi_wp_astroph.doc
GROUND-BASED DIRECT DETECTION OF EXOPLANETS WITH
THE GEMINI PLANET IMAGER (GPI)
James R. Graham1, Bruce Macintosh2, Rene Doyon3, Don Gavel4, James Larkin5,
Marty Levine6, Ben Oppenheimer7, David Palmer2, Les Saddlemyer8,
Anand Sivaramakrishnan7, Jean-Pierre Veran8, & Kent Wallace6
Abstract
The Gemini Planet (GPI) imager is an “extreme” adaptive optics system being designed
and built for the Gemini Observatory. GPI combines precise and accurate wavefront
control, diffraction suppression, and a speckle-suppressing science camera with integral
field and polarimetry capabilities. GPI’s primary science goal is the direct detection and
characterization of young, Jovian-mass exoplanets. For systems younger than 2 Gyr
exoplanets more massive than 6 MJ and semimajor axes beyond 10 AU are detected
with completeness greater than 50%. GPI will also discover faint debris disks, explore
icy moons and minor planets in the solar system, reveal high dynamic range main-
sequence binaries, and study mass loss from evolved stars. This white paper explains
the role of GPI in exoplanet discovery and characterization and summarizes our
recommendations to the NSF-NASA-DOE Astronomy and Astrophysics Advisory
Committee ExoPlanet Task Force.
                                                  
1 UC Berkeley
2 Lawrence Livermore National Lab
3 Université de Montréal
4 UC Santa Cruz
5 UCLA
6 JPL
7 American Museum of Natural History
8 NRC/Herzberg Institute
Introduction
Direct detection affords access to exoplanet
atmospheres, which yields fundamental
information including effective temperature,
gravity, and composition.  Mid-IR exoplanetary
light has been detected in secondary eclipses by
SPITZER (Grillmair et al. 2007; Richardson et
al. 2007).  However, such information is
necessarily limited by the overwhelming
photon shot noise of the primary. Direct
detection separates the exoplanet light from
that of its primary so that this noise source is
suppressed.
When the fundamental atmospheric
properties are known for a population of
planets spanning a range of mass and age it will
be possible to conduct critical tests of
formation scenarios and evolutionary models.
The goals of direct detection include
measurement of abundances and reconstruction
of the thermal history.  Such information will
discriminate between idealized adiabatic
contraction (Burrows et al. 2004), more
realistic accretion models that occurs in core-
accretion (Marley et al. 2007), and formation
by gravitational instability (Boss 2006).
Ultimately, the impact of composition and
equation of state on evolution can also be
investigated.
Figure 1: Planet-to-star contrast for a 1 MJ exoplanet
orbiting a G2 V star at 4 AU for ages 0.1–5 Gyr
(Burrows et al. 2004). Planets shine by reflected light in
the visible. Young exoplanets (< 1 Gyr for 1 MJ) are
detectable by their intrinsic luminosity. Note strong J
(1.25 µm) and H (1.65 µm) emission where ground-
based facilities have sensitive operation. Old exoplanets
(> 1 Gyr) shine only by reflected light at short
wavelengths (< 2 µm).
Direct detection has several practical
advantages.  Planets at large semimajor axes
can be found without waiting for an orbit to
complete—a condition that renders indirect
detection of Neptune like planets (a = 30 AU, P
= 160 yr) impractical. The Fourier
decomposition that underlies Doppler and
astrometric detection are also subject to
aliasing and beat phenomena, and suffer
confusion when multiple planets are present.
Direct imaging gives an immediate portrait of a
planetary system, including sensitivity to the
analogs of Zodiacal and Kuiper dust belts.
Figure 2: Contrast for a 5 Gyr system orbiting a G2 V
star at 4 AU. Masses are 0.5–8 MJ. Massive, solar-system
age exoplanets shine only by reflected light at visible
wavelengths, but remain sufficiently self-luminous to be
detectable at contrast ratios > 3 x 10-8 in the near-IR.
Surveys at large semimajor axes are necessary
for several fundamental reasons.  The structure
of our own solar system implies that a full
picture of planet formation cannot be
constructed without reaching out to 30 or 40
AU. Moreover, if planets form as a
consequence of disk instabilities, they are most
likely to occur at large semimajor axis
separation (> 20 AU for solar type stars) where
the cooling time exceeds the Keplerian shear
time scale.
Indirect searches continue to hint that
the semimajor axis distribution is at least flat or
even rising in dN/dlog(a) beyond 5 AU.  With
only 200 Doppler planets known there are too
few to make a statistically significant study of
trends with planetary and stellar mass,
semimajor axis, and eccentricity. Because
Doppler methods have reached the domain of
diminishing returns (a = P2/3), improving the
statistics will be challenging.  A direct search
that can efficiently tap into the dominant
population of giant exoplanets beyond 5 AU
can bring statistical significance to these
studies.
Figure 3: A schematic of the Gemini Planet Imager
showing the major subsystems.
Detecting Self-Luminous
Exoplanets
Direct detection of Jovian-mass planets via
their reflected sunlight requires a contrast ratio
of order 
2 ×10−9(a /5AU)−2  relative to their
parent star. Because of the inverse square law,
reflected light searches are an impractical way
to explore the outer regions of solar systems.
GPI seeks to detect the energy radiated by the
planet itself, which is independent of a, except
for very small semimajor axis separations. Old
planets are cool and dim, but young planets are
hot and therefore bright in the infrared relative
to their parent star (Figure 1 & Figure 2). For
example, at 1.6 µm it is possible to detect a 10
Myr-old 3 MJ planet, or a 100 Myr-old 7 MJ
planet, orbiting a G2V star at a contrast ratio of
only 4 × 10-6. With improved contrast, an
increasingly large phase space of planets
becomes accessible.  Better contrast is
obviously preferable, but it comes at a penalty.
For example, Angel (1994) described an AO
system designed to achieve very high contrast
ratios using bright guide stars.  This system
would be suitable for exploring the planetary
systems of 13 bright stars (R < 3.8 mag.) in the
solar neighborhood (< 8 pc).  However, the
detection of a few planets, although dramatic,
would be insufficient scientific impetus given
the success of Doppler searches.  It is necessary
to show that any proposed instrument can
search a scientifically interesting range of
semimajor axes and accumulates a statistically
significant sample of exoplanets in reasonable
observing time.
Figure 4: The 1-σ, 1-hour speckle noise at H for GPI
with 18-cm subapertures and a maximum update rate of
2.5 kHz. The speckle noise is measured in units of the
guide star brightness. Therefore, these curves represent
the achievable contrast when speckle noise is dominant.
The system performance is a function of guide star
magnitude (thick vs. thin lines). The Fried parameter (at
500 nm) for Mauna Kea is 18.7 cm and 14.5 cm for
Cerro Pachon. The coronagraph occulter is opaque
within 0.09 arc seconds.
Overview of GPI
The Gemini Planet Imager (GPI) is a high
contrast or “extreme” AO (ExAO) system
(Macintosh et al. 2006). Basic R&D enabling
the construction of a practical system was
supported by the NSF’s Center for Adaptive
Optics. In 2004, Gemini supported two ExAO
conceptual design studies as part of its “Aspen
Process” next generation instrumentation
program. At the conclusion of those studies, the
Gemini Board recently selected GPI to proceed.
Work began in June 2006 and delivery of the
instrument is expected in late 2010.
Imaging a Jovian exoplanet requires a
contrast significantly better than that delivered
by existing astronomical adaptive optics (AO)
systems. Currently achievable contrast, about
10-5, is completely limited by quasi-static
wavefront errors, so that contrast does not
improve with integration times longer than
about 1 minute. Moreover, there are enough
slow drifts in these errors that PSF subtraction
does not increase contrast by more than a factor
of a few, except in the most ideal
circumstances. GPI will surpass the
performance of existing systems by two orders
of magnitude.
Table 1: Principal properties of GPI
Adaptive Optics Calibration WFS
Deformable
mirror
4096-actuator
Boston Micro-
machines
Type 1–2.4 µm
interferometer
Subaperture
18 cm (N = 44
subaps.)
Static
Accuracy
1 nm RMS
Limiting
I = 8 mag.
(goal: I = 9
mag.)
Science Instrument
Optics
quality
< 5 nm RMS
WFE per optic
Type Lenslet-based
integral field
Coronagraph Spatial
sampling
0.014 arc
seconds per
lenslet
Type Apodized-pupil
coronagraph
Field of
2.8 arc
second square
Inner
Working
Dist.
~ 3 λ/D Spectral
coverage
Y, J, H or K
Throughput 60% Spectral
resolution
λ/Δλ  ≈ 45
To achieve our science goals, GPI integrates
four key subsystems (Figure 3): 1) An AO
system that makes fast measurement of the
instantaneous wavefront, and provides
wavefront control via deformable mirrors; 2) A
calibration unit that provides precise and
accurate measurements of the time-averaged
wavefront at the science wavelength, so that the
final image is not dominated by persistent
speckles caused by quasi-static wavefront
errors; 3) A coronagraph that controls
diffraction and pinned speckles; and 4) An
integral field spectrograph unit (IFU) that
records the scientific data, providing low-
resolution spectroscopy and suppression of
residual speckle noise. The IFU incorporates a
dual-channel polarimeter for studies of
circumstellar dust.  An example of GPI contrast
performance based on detailed simulation of
this system is shown in Figure 4.
Technical aspects of GPI have been
presented in the recent literature, including:
apodized pupil Lyot coronagraphs (Soummer
2005, Sivaramakrishnan & Lloyd 2005);
optimal wavefront control for ExAO (Poyneer
et al. 2006); MEMS deformable mirrors
(Morzinski et al.  2006); simultaneous spectral
differential imaging and the effects of out-of-
pupil-plane optical aberrations (Marois et al.
2006); multiwavelength speckle noise
suppression (Marois et al. 2004); and the
wavefront calibration system (Wallace et al.
2006).
GPI’s ExoPlanet Parameter Space
The performance of GPI is characterized by the
detectable brightness ratio. The achievable
contrast will be a function of the brightness of
the wavefront reference, the angular separation,
and observing and wavefront-sensing
wavelengths. The detectability of planets in a
given sample of target stars can then be
estimated by comparing the distribution of
relative exoplanet brightness versus angular
separation with the expected performance.
This comparison also quantifies selection
effects that vary with properties of the planet
(age, mass, and orbital elements) and of the
primary star (spectral type and distance).
Our knowledge of the distribution of
planetary properties is incomplete, but a basic
premise is that sufficient information exists to
make a preliminary estimate of this
distribution.  Given these predictions it is
possible to estimate the scientific impact of
different design choices, e.g., precision and
accuracy of adaptive optics correction and
observing wavelength, and therefore make an
informed trade-off between cost and
performance.
Figure 5: Detectable companion contrast versus angular
separation for GPI, showing the direct detection of young
luminous planets in a hypothetical survey of field (< 50
pc) stars. The small dots represent the planet population:
those detected by GPI are drawn with a box, those
detectable in current Doppler surveys are shown with a
circle. The dashed line shows the GPI contrast threshold
(5 σ) for a 1-hour exposure at 1.65 µm. Within 100 λ/D
speckle noise dominates. In this example, which has no
speckle noise suppression and a fixed frame rate and
simple AO model, a universal contrast curve applies to
all targets.
The approach we adopt involves
making a Monte Carlo model for the population
of planets in the solar neighborhood (Graham et
al. 2002). This description includes the mass,
age and orbital elements of each accompanying
planet. When combined with cooling curves,
model atmospheres and the distance to the
primary star we can compute the brightness
ratio and angular separation at any epoch. Our
adopted planet properties are based on the
precision Doppler monitoring of about 2000 F,
G and K stars by eight exoplanets searches at
the AAT, Lick, Keck, ESO/Coralie,
Provence/Elodie, Whipple Observatory,
ESO/CES and McDonald. Approximately 200
exoplanets have been found with periods
between a few days and a few years and with M
sin i spanning 0.1–10 MJ. Since there is no
accepted theoretical model for the planet mass,
M, or semimajor axis, a, distributions, we adopt
a simple power law distribution: dN/dM ∝ Mα -
and dN/da ∝ aβ.
Figure 6: Exoplanets from the simulated GPI field star
survey. Heavy filled circles are GPI detected planets
from. Light dots are planets detected by a hypothetical 8-
year astrometric interferometer survey, with a limit of R
< 10 mag., and a precision of 30 µas. Exoplanets
detected in the Keck/Lick Doppler survey are shown as
stars. This illustrates how GPI explores a complementary
phase space to indirect searches.
Figure 5 shows the results of a Monte Carlo
experiment using GPI with ideal apodization,
Fried parameter, r0 = 100 cm at the observing
wavelength of 1.6 µm, operating at 2.5 kHz. In
this example the planet population is
parameterized by a planetary mass spectrum
with α = -1, a semi major axis distribution with
β = -1/2 between 0.1 < a/AU < 50, and a star
formation history such that the age of the disk
is 10 Gyr and the mean age of stars in solar
neighborhood is 5 Gyr. For this simple plot, we
assume no suppression of residual speckles,
and therefore speckle noise always dominates.
When this is true, it is a good approximation to
assume a universal contrast curve for all
targets, which means that the results can be
easily visualized in a contrast versus angular
separation plot.
Figure 7: Exoplanets from the simulated GPI field star
survey. Solid lines show the evolution of 1–20 MJ
exoplanets. Dotted lines are isochrones labeled in
log10(Gyr). Detected planets are filled circles. The
population straddles the H2O cloud condensation line at
about 400 K (dashed), and a few objects lie below the
NH3 condensation curve (dashed). The only confirmed
astronomical object that lies on this plot is Jupiter, with
Teff = 120 and log10g = 3.4.
This simple system can detect about 5% of the
planets in a survey of field stars. Preliminary
calculations of this type were first used to argue
for the scientific utility of GPI because the
exoplanet detection rate is comparable to that
delivered by Doppler searches.  The GPI IFU
will be used to suppress speckles, but the
results are not so easily visualized—in
subsequent simulations (e.g., Figure 7) all
significant noise sources, e.g., photon shot
noise, background, flat field noise, detector
read noise and dark current, are fully treated.
High fidelity calculations using the full
sensitivity of the system predict that in a survey
of approximately 3800 field stars with I < 8
mag. we would detect over 200 planets. Figure
7 shows two ways to visualize how GPI-
detected planets explore the potential discovery
phase space.  Figure 6 compares the catalog of
Doppler planets, a hypothetical astrometric
survey and the GPI exoplanets.
Figure 8: Exoplanet detection completeness contours for
an age-selected (< 2 Gyr) GPI survey of stars in the solar
neighborhood as a function of semimajor axis and
planetary mass. GPI completeness is good (>50%) for 5
MJ planets beyond 10 AU. Massive planets (12 MJ) are
found with similar completeness as close as 5 AU.
Approximately 500 such stars are accessible to GPI (I <
8 mag.) in the solar neighborhood and over 100
exoplanets should be detectable.
This comparison confirms that GPI is
complementary to indirect methods. GPI can
probe the outer regions of solar systems and
will answer our key science questions related to
planet formation and migration. Figure 7 plots
the GPI exoplanets atmospheric properties on
an effective temperature, log (g) diagram.  The
youngest detected exoplanets have ages of a
few hundred Myr, representing the youngest
systems in the field-star sample. The youngest
objects sample masses as low as 1 MJ. With
increasing age, the mean detected planet mass
increases.  The median exoplanet age is 1.3
Gyr, the oldest have ages of ~ 5 Gyr.  About
70% of the planets are cooler than that water
cloud condensation line, and three lie to the left
of the ammonia cloud condensation curve.  The
properties of young planets (< 100 Myr)
depend on initial conditions, and have been
excluded from these Monte Carlo simulations.
It is worth emphasizing that at GPI’s contrast
levels we do not require super-luminous “hot
start” planets but can detect planets formed
through low-entropy core-accretion.
Important complements to broad field-
star surveys are targeted surveys of young
clusters and associations or age-selected field
stars, where the mean GPI exoplanet detection
rate can approach 50%.  Figure 7 (right) shows
that the median detected exoplanet age in the
field survey is 1.3 Gyr.  Old stars can be
winnowed using a chromospheric activity
indicator such as Ca H & K. Employing a
selection criterion that eliminates stars older
than 2 Gyr boosts the planet detection rate by a
factor of four. The completeness of such a
survey is represented in Figure 8. About 500
stars in the solar neighborhood would be
selected by such a survey, which should yield
over 100 exoplanets. This suggests that a
preliminary estimate of the semimajor axis and
planetary mass distributions could be made in
15–30 nights of telescope time. The Gemini
Observatory plans to devote 100-200 nights to
GPI campaigns, therefore a number of
comprehensive exoplanet surveys will be
possible to accumulate statistics relevant to
planet formation and to study exoplanet
atmospheres.
GPI also enables a broad range of
adjunct science. Most relevant to the ExoPTF is
sensitive detection and characterization of
planetary debris disks. Unseen planets below
GPI’s detection threshold may gravitationally
sculpt debris disks, and therefore the ability to
detect disks with optical depths as small as 8 ×
10-5, or approximately one fiftieth of well-
known systems such as beta Pic or AU Mic, is
a key feature.  Other science missions include
high contrast imaging of minor planets and icy
moons in the solar system, studies of main
sequence binaries (GPI can see an M dwarf
next to an O star), and investigation of mass
loss from evolved stars.
Conclusions & Recommendations
to the ExoPTF
Outer planets (a > 10 AU) take more than thirty
years to complete one revolution. These planets
will remain undetected in the first-generation
Doppler surveys until about 2030. In contrast,
direct detection of exoplanets is feasible now
using advanced AO techniques. Within three
years the GPI instrument will begin to survey
for exoplanets beyond 10 AU. GPI will give
our first glimpse of the outer regions of
exoplanetary systems, where novel planet
formation pathways may operate, the
counterparts of inward planetary migration may
reside, and Kuiper belt analogs may be
discovered.  Around selected younger targets,
GPI will have significant sensitivity in the 5-10
AU range. From low-resolution spectra of the
atmospheres of exoplanets GPI will also yield
effective temperatures, gravity and
composition. As static wavefront errors set the
fundamental noise floor for direct detection,
whether on the ground or in space, GPI is a
unique pathfinder for future NASA missions.
The GPI program therefore represents the next
major step in direct planet detection and should
remain a priority of the Gemini Observatory.
The Gemini Observatory has
recognized that the full scientific potential of
GPI can only be realized if observing with this
instrument is coordinated into major
campaigns.  Substantial resources are needed
immediately if the US astronomical community
is to compete effectively to conduct these
hundred-night observing programs.  Foundation
science includes the assembly of catalogs of
adolescent (0.1–1 Gyr) stars, which are the
most fertile hunting grounds for self-luminous
exoplanets.  Efficient use of observing time
will require careful experimental design, and
resources will be needed to reduce and analyze
GPI data for reliable exoplanet discovery and
characterization. Support will also be necessary
to prepare for follow up observations of
candidates, including building a strong
theoretical basis for understanding exoplanets
atmospheres in the temperature range of
200–600 K.
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