Download Direct Imaging Detection of Planets

University of Tokyo – Princeton University Strategic Partnership Initiative
Direct Imaging Detection of Planets
Motohide Tamura, University of Tokyo, Astrobiology Institute
N. Jeremy Kasdin, Princeton University, Department of Mechanical and Aerospace Engineering
1. Abstract
We describe a project on the direct imaging of planets to be carried out over the next three years
with a new instrument nearing completion, constructed by a team at the University of Tokyo and
Princeton University. This joint program provides the opportunity for, and will greatly benefit
from, extensive in-person collaborations at the University of Tokyo, at the Japanese telescope
Subaru in Hawai‘i and at Princeton. In particular, such a collaboration will provide an entrée into
the very-fast-moving field of the discovery of planets around stars other than our Sun for young
scientists at both institutions.
2. Introduction
The Japanese astronomical community has a long history of working on major cosmological
projects with Princeton, beginning over 30 years ago with joint work on theoretical cosmology.
Through the years, this work has included the design, construction and operation of the Sloan
Digital Sky Survey, scientific participation by Princeton in the HyperSuprimeCam survey on the
Subaru telescope, and the current construction of the Prime Focus Spectrograph for Subaru. These
projects have led to, and greatly benefited from, extended visits between the two communities
by participants from both the Japanese community and from Princeton. At this point, several
generations of scientists have participated, often beginning their involvement as graduate students
and continuing into their faculty careers, where they have assumed leadership positions in the new
projects.
In the past six years or so, the scientific scope of the collaboration has broadened into a new
area, the detection and study of planets around stars other than our Sun. Beginning twenty years
ago with the discovery of a planet-mass body orbiting a pulsar and making its presence known
by a small periodic alteration of the pulse arrival time, this field has grown to be a fast-moving,
forefront area of astronomy which produces several discoveries a day and drives technological innovation in ever-more ambitious directions, with the eventual goal of imaging an Earth-size planet
around another star. The challenge is not the detection of the radiation or reflected light from
the planet per se, but doing so against the glare of the star, which outshines the planet by factors
of millions to 100 millions (think of everyday situations such as photographing a subject on a
sunny day). Most planet-detection techniques are therefore indirect: the planet is not directly
seen but its effect on the star which it orbits can be measured. For example, the enormously
successful Kepler satellite accurately monitors the light from stars and can detect the periodic
slight dimming as a planet crosses in front of the star during its orbit. The most challenging
technique is direct detection of the light from the planet, but this carries with it the possibility
of measuring the atmospheric composition and the planet’s colors and cloud structures. Doing
so requires a technique known as adaptive optics (AO), which greatly increases the brightness
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contrast in an image of a star, allowing faint objects to be detected near the star.
SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru) used adaptive optics instrumentation on the Subaru telescope to carry out the first strategic, multi-year program on
Subaru. Recently completed, it involved a large international collaboration whose primary partners were the National Astronomical Observatory of Japan and Princeton University. We propose
to build on the legacy of SEEDS by carrying out a several-year project using second-generation
AO technology currently in an advanced state of construction at the Subaru telescope and at
Princeton’s department of Mechanical and Aerospace Engineering. This project depends on international collaboration to optimize the instrumentation, carry out the observations and analyze
the results; further, it provides multiple opportunities for participation by both undergraduate
and graduate students and access to this fast-moving and important scientific field. The topic of
this work, planets around other stars and the possibility of life, has enormous public appeal and
visibility, enabling further opportunities for young scientists entering the field. We propose to
enhance this effort with multiple long-term professional visits, the development of collaborations
involving young scientists, student exchanges, and a series of meetings of the international collaboration partners. We expect the interactions and collaborations so fostered to last well beyond
the duration of the proposal period and to lead to many opportunities for future work.
3. Summary of SEEDS
Collaborations grow from an interest in common work, and in turn lead to new collaborations.
The far-flung SEEDS project is a case in point. SEEDS was proposed in 2008–9 as the first of
the Subaru telescope’s strategic projects - multi-year observing campaigns with dedicated observing time, designed to tackle large-scale investigations. Such investigations are open to the entire
Japanese astronomical community, and the original SEEDS proposal was submitted by a group of
41 scientists from several Japanese institutions, 3 from Max-Planck-Institute, Munich, one from
University of Hawai‘i and 6 from Princeton (postdoctoral fellow Amaya Moro-Martin, now at
Space Telescope Science Institute, and Professors Jeremy Kasdin, Gillian Knapp, David Spergel,
Edwin Turner and Robert Vanderbei).
SEEDS began observing in October 2009 and finished its formal observations in January 2015,
although follow-up observations and science analysis continue. The basic goals of SEEDS were
to characterize planetary systems around stars other than our own, to map the disks of gas and
dust which surround most young stars and in which planets form, to search for signs of planet
formation - massive bodies in rotating disks will open gaps and cause wakes in the otherwisesmooth distribution, and to measure the changes in such disks with time – these last, of course,
take place on timescales much longer than a human lifetime but trends can be investigated with
observations of stars of different ages. SEEDS was very successful, with more than 50 refereed
papers to date. These describe observations of structure in several tens of disks, inferences about
the masses and locations of the planets forming in these disks, investigations of mechanisms producing planetary orbital alignments, and most relevant for the topic of this proposal, the discovery
by imaging of several planets around nearby stars. The discovery of one such object is shown in
Figure 1; a “super-Jupiter” planet many times the mass of our solar system’s Jupiter. This planet,
one of only a handful of directly-imaged planets, was celebrated by giving its name to a craft beer.
Participation in SEEDS by Princeton scientists broadened considerably over the years. Professor
Adam Burrows, an expert on the complex atmospheres of gas-giant planets, and postdoctoral
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Figure 1. Discovery of a many-Jupiter-masses planet orbiting the nearby star κ
Andromedae. The adaptive-optics image taken as part of the SEEDS campaign is
shown on the left. The center of the image, at the location of the star, is blanked
out and some residual light remains. The planet is the faint object in the upper
left of the image. Right: Super-Jupiter beer, a grapefruit-flavored IPA, brewed in
honor of this planet by the Howe Sound Brewing Company, Squamish, BC.
fellow David Spiegel produced physical models to reproduce the data, and Professor Roman
Rafikov made several calculations of the dynamics of planet formation and its tell-tale signatures
in observated maps of disks. Postdoctoral fellow Michael McElwain and graduate student Timothy Brandt introduced new data-analysis techniques which greatly improved the detectability
of planets by improving the contrast between the starlight and the surroundings. Dr. Brandt
also analyzed the total sample of planets to gain a statistical understanding of the incidence of
massive planets at large distances from their host star, and wrote his doctoral dissertation on the
subject. Graduate student Ruobing Dong analyzed the observations of planet-forming disks and
constructed theoretical models both to show the effects of planet formation on disk structure (so
that one can use observations of structure to infer the presence of planets) and to create images
of his models as they would appear when imaged with instrumentation of Subaru’s capability. In
addition, Princeton astronomers participated in the observations on Mauna Kea, worked on the
choice of stars for which observations should be made, and participated in and hosted SEEDS
collaboration meetings. Over the course of this project, the next steps in planet imaging became
increasingly well-defined, and point both towards space-based observations and the next developments in ground-based observations.
4. CHARIS
Although SEEDS made major contributions to measuring the incidence of massive planets (Jupiter
mass and greater) it probed distances from the host stars of 10 AU and greater (an AU, astronomical unit, is the distance between the Earth and the Sun; Jupiter is at a distance of 5 AU).
To reach smaller scales, i.e. scales similar to that of the solar system, requires instrumentation
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with capabilities much enhanced over that of the SEEDS instrumentation. The second-generation
AO instrumentation which can access these scales is now in an advanced state of construction:
the extreme-AO instrument, built by Olivier Guyon and colleagues, is installed at the Subaru
telescope and is currently undergoing commissioning. The light beam produced by this apparatus will be fed into CHARIS, currently in an advanced stage of construction at Princeton’s
Department of Mechanical and Aerospace Engineering. CHARIS (Coronagraphic High-AngularResolution Imaging Spectrograph) will provide the close-in high contrast imaging of nearby stars
which will enable the discovery of Jupiter-mass planets at distances from the star of a few AU,
i.e. at solar system scales. It does this by greatly reducing the effect of “speckles” of starlight, as
seen in the central regions of the image in Figure 1. Further, CHARIS will allow for spectroscopic
analysis of the light from the planet, which can detect the presence of important atmospheric
constituents such as methane, carbon monoxide and water. CHARIS is expected to discover new
gas-giant planets at distances from their host stars comparable to those in the solar system, and
to provide a robust census of these planets for the nearby stars. Together with the results of
similar surveys being carried out by AO instrumentation operating on telescopes in the southern
hemisphere, these results will allow the incidence of giant planets on solar-system scales to be
measured, with major implications for our understanding of the processes which form planets.
These results are of great scientific importance, but the enormous public interest in planets and in
our origins will ensure a much more wide-ranging impact of the work across society. To quote the
proprietor of the Howe Sound Brewery (see above): “The great things in life should be celebrated”.
CHARIS is being built by a small team at Princeton with the financial support of the Japanese
science funding agencies. Professors Jeremy Kasdin (MAE, Princeton) and Masahiko Hayashi
(NAOJ) are the Principal Investigators, and the CHARIS construction team includes Tyler Groff,
Michael Galvin (MAE), MaryAnne Limbach (Limbach Optics), Timothy Brandt (Institute for
Advanced Study), Michael Carr, Craig Loomis (Astrophysics), Norman Jarosik (Physics), and
Jeffrey Chilcote (University of Toronto), with contributions from graduate student Johnny Greco,
Craig Loomis, Robert Lupton. Edwin Turner, James Gunn and Gillian Knapp (Astrophysics).
CHARIS is at present in an advanced stage of construction at Princeton. The current schedule
calls for it to be shipped to Hawai‘i in the summer of 2016, where it will be installed on the
Subaru telescope for commissioning and testing. Once its performance and stability are verified,
the CHARIS team will submit an observing request for of order 20 nights of observing over three
years (two observing trips per year) to carry out the initial observing program. It is for the support of collaborative work around this astronomical campaign that the present proposal is being
put forward.
There has been close collaboration among the CHARIS team members since the beginning, which
have included several extended visits to Tokyo, the Subaru site and Princeton since the initial
decision to pursue this project. This has been critical: the ab initio involvement of instrument
builders, scientists, and software builders has optimized the capabilities of the instrumentation.
It has also led to further enhancements to the long-term connections between the Princeton and
Japanese astronomical communities by bringing young people into the collaboration and, already,
beginning to look forward towards further joint work.
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5. The three-year observing program
As CHARIS moves towards its operational phase, the pace of interactions will pick up. There
will be a first collaboration meeting in late February 2016 in Japan at which the larger Japanese
community will be introduced to CHARIS and which will be attended by several of the US
CHARIS team members. This will be followed by two parallel activities for the next several
months: CHARIS completion, shipping and commissioning, including the development of the
software which will run the device and process the data; and intensive work on the list of stars
to be observed and preparation and submission of the observing proposal. Much of this will take
place in Japan and Hawai‘i and involve Princeton personnel at every stage. Once the observing
is underway, the team will send astronomers to Hawai‘i from both Japan and Princeton, meet
in person to finish the software (much of the work will be done beforehand, but much is needed
when actual data are encountered), and analyze and publish the science results as they come in.
All of this work will be supported by the CHARIS institutions.
6. Activities under the Princeton-Todai Partnership
This project provides ample opportunity for strengthening the international collaboration and
building upon that collaboration into the future. The specific activities for which we request the
support of the Princeton–Todai partnership are:
1. An annual meeting around the discussion of the CHARIS results. We propose to hold the
first in Japan in 2017, the second in Princeton in 2018 and the third in Hawai‘i in 2019. The
requested support is for local expenses for the host institution (facilities, catering etc); and for
travel by participants not at that institution. The participant lists will include young scientists
at the undergraduate, graduate and postdoctoral levels, and all interested scientists at the host
institution will be welcome. We are interested in opening the third annual meeting, in Hawai‘i,
by inviting astronomers from the Southern hemisphere telescopes carrying out planet imaging
projects, with the goals of assessing the new science obtained by this work and future directions.
This will be a particularly important event for the young astronomers.
2. Travel support for students to participate in the observing sessions at the Subaru telescope and,
on occasion, for their mentors. Working space constraints preclude taking parties of students to
the telescope, however attractive and desirable that is, and we expect one student per observing
trip.
3. Science analysis and publication activities. Although the internet has made far-flung collaborations in science almost ridiculously easy, with paper drafts sent back and forth instantly,
something often gets lost, particularly in collaborations of more than a handful of people. That
is the day-to-day participation in a science discovery; carrying out the science often defaults to
the whole project being done at one institution, where a small group works together and then
announces the result to a larger collaboration. Support for extended visits could bring together
the participants from different institutions to work together and in the process provide the framework for the development of further collaborative work, This could include the writing of the
technical papers, those describing the instrument and the software, as well as those describing the
astronomical results.
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Collaborative work is also likely to develop in additional directions: modeling the planets’ atmospheres to compare with the data; as-yet unanticipated improvements in the software, instrumentation and observing procedures: and the planning for future work that happens whenever
scientists sit down and work together.
7. Exchanges and Visits
7.1. Graduate Students. SEEDS involved an increasing number of graduate students as it
evolved, including the two from Princeton who did their dissertation research as mentioned above.
Astrophysics graduate student Johnny Greco has joined the CHARIS project for his dissertation
research, and has already spent a two-week period in Japan with one of his mentors, Tim Brandt.
Mr. Greco expects to spend extended periods at the University of Tokyo at least twice a year, and
there is a reasonable likelihood that additional graduate students from both Tokyo and Princeton
will become involved.
7.2. Undergraduate Students. Both MAE and Astrophysics have large summer undergraduate research programs, and several undergraduates have already worked on technical aspects of
CHARIS over the years. CHARIS offers the opportunity for undergraduate students to travel to
Japan with their mentors and work on astronomical or technical projects. Likewise, we expect at
least one undergraduate student from Tokyo.
Both departments are carrying out long-term efforts to increase the number of traditionally underrepresented groups in science, and these include the participation of minority undergraduates.
A necessary part of this effort is that it include the opportunity to make connections with the
international community.
7.3. Faculty, Staff and Postdoctoral Fellows. We request support for extended visits by
the faculty, staff, and postdocs involved in CHARIS between Tokyo and Princeton. As well
as working on the CHARIS results, these visits will support consultation and collaboration on
theoretical work, such as visits to Princeton by Dr. Satoro Sorahana, postdoctoral fellow at the
University of Tokyo, to work on atmospheric modeling.
8. Afterwards
As is the case with SEEDS, which led to joint work such as the CHARIS project, the collaborations
so formed will lead to further involvement and are indeed already doing so. As one example, there
is the possibility of installing a polarizer in the instrument, which will enable the measurement of
the detailed structure in the inner regions of planet-forming disks, of which there are many tens
accessible to CHARIS. There may well be additional investigations with the existing instrument
- following up observations of detected planets for several years so that their orbital motions can
be measured, for example. There will be detailed statistical analysis of the results and of their
implications for planet and star formation and planet migration (the movement of planets away
from the region where they were formed due to gravitational interactions).
And there are longer-term developments to which the young participants from both countries
will be introduced. These includes the upcoming NASA mission WFIRST (Wide Field InfraRed
Space Telescope), which is expected to image several thousand planets and in which Princeton
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is a key team member, and the science and technology definition teams for a future large space
telescope targeted at imaging Earth-like planets around other stars in the next 20 years.
9. Members of Collaboration (Princeton and Tokyo
Professor Motohide Tamura, Department of Astronomy and Chair, Program in Astrobiology, University of Tokyo
Professor N. Jeremy Kasdin, Department of Mechanical and Aerospace Engineering, Princeton
University
Mr. Michael Galvin, Department of Mechanical and Aerospace Engineering, Princeton University
Dr. Tyler D. Groff, Department of Mechanical and Aerospace Engineering, Princeton University
Mr. Johnny Greco, Department of Astrophysical Sciences, Princeton University
Professor Masahiko Hayashi, Department of Astronomy, University of Tokyo
Professor Emerita Gillian R. Knapp, Department of Astrophysical Sciences, Princeton University
Dr. Satoro Sorahana, Department of Astronomy, University of Tokyo