Professor Seager's Exoplanet Space Mission Research
Professor Seager aims to find Earth-like worlds. Space telescopes above the blurring effect of Earth's atmosphere give us our best capability to fin small rocky worlds. Professor Seager is involved in several space-based missions including as PI of ASTERIA (formerly ExoplanetSat), as Deputy Science Director of TESS, and as a scienficir leader of the Starshade project.
Professor Seager's research group also works and has worked with data from several space telescopes including Kepler, Spitzer, Hubble, and EPOXI. Above the blurring effects of Earth's atmosphere, space telescopes provide excellent observations of exoplanets and their host stars.
ASTERIA: Arcsecond Space Telescope Enabling Research in Astrophysics
Professor Seager's 6U CubeSat
ASTERIA is being implemented at JPL. ASTERIA, formerly called
ExoplanetSat, is motivated by the thought that no current mission has
the capability to survey the nearest sun-like stars for an Earth
analog. The best way to monitor the brightest sun-like stars (0 < V
< 6) for long-duration transiting exoplanets is by a targeted star
search; the brightest stars are too widely separated across the sky
for a single telescope to continuously monitor. A fleet of dozens of
identical ASTERIA spacecraft would be ideal to each monitor one star
at a time, instead of a single space telescope surveying thousands of
stars simultaneously. The detailed properties of each target star are
well known in advance.
ASTERIA is a prototype nanosatellite capable of monitoring a
single, bright, sun-like star. ASTERIA will have the precision
pointing control (8 arcsecond (three sigma) ) and precision thermal
stability (ten milli Kelvin) to reach high photometric precision. To
take advantage of CubeSat launch opportunities,
ASTERIA as a 6U CubeSat (roughly 10 x 20 x 30 cm, 12 kg) that will
operate in low-Earth orbit.
The ASTERIA project is a JPL-MIT collaboration. While the concept
was conceived of and brought through TRL 5 at MIT and Draper Lab, the
program is now funded at JPL through the Phaeton Program for training
early career employees. JPL is responsible for overall project
management, systems engineering, attitude determination and control,
flight software, spacecraft implementation, integration and test, and
mission operations. MIT will lead science operations. Flight hardware
delivery is scheduled for March 2017, with launch and deployment
targeted for summer 2017.
Conception of the ASTERIA prototype with deployed solar panels. Credit: Matt Smith.
TESS: Transiting Exoplanet Survey Satellite
The MIT-led NASA Mission TESS
is an all-sky survey for transiting exoplanets.
TESS will carry four identical specialized wide-field
CCD cameras, each covering 24 x 24 degress on the sky with a 100-mm
aperture. In a two-year all-sky survey of the solar neighborhood, TESS
will cover 400 times as much sky as Kepler did. In the process, TESS
will examine more than 0.5 million bright nearby stars and likely find
thousands of exoplanets with orbital periods (i.e., years) up to about
50 days. TESS will not be able to detect true Earth analogs (that is,
Earth-sized exoplanets in 365 day orbits about Sun-like stars), but it
will be capable of finding Earth-sized and super Earth-sized
exoplanets (defined as planets up to 1.75 times Earth's size) transiting M
stars, stars that are significantly smaller, cooler, and more common
than our Sun. The TESS search for habitable planets that may be
followed up for signs of life is, therefore, focused on M dwarf host
JWST: James Webb Space Telescope
The James Webb Space Telescope
is a 6.5 m effective diameter aperture infrared telescope, with a
launch scheduled for October 2018 on an Ariane 5 rocket from French
Guiana. JWST will be the premier space-telescope observatory of the
next decade, serving thousands of astronomers worldwide. It will study
every phase in the history of our Universe, ranging from the first
luminous glows after the Big Bang, to the formation of solar systems
capable of supporting life on planets like Earth, to the evolution of
our own Solar System. JWST is an international collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center is managing the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute will operate JWST after launch.
Professor Seager, and indeed most of the exoplanet atmosphere research community, is counting on the JWST for exquisite atmosphere observations of all kinds of transiting exoplanets. Of prime interest are the rocky worlds with thin atmospheres, transiting and in the habitable zones of "bright enough" M dwarf stars.
- Discovery and Characterization of Transiting Super Earths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope, D. Deming et al., PASP, 121, 952, 2009.
- Constraining Exoplanet Mass from Transmission Spectroscopy, J. de Wit, S. Seager, Sciences, 342, 6165, 2013.
is a carefully shaped screen with its own spacecraft, which,
when flown in formation with a space
telescope along the line of sight to a star, blocks enough of the
light from the star to reveal any orbiting planets. The starshade size
and shape, and the starshade-telescope separation, are chosen to
control diffraction around the screen and create a very deep and dark
shadow; the planet light is unaffected due to its angular separation
from the star. Creating shadows large enough for future space
telescopes and deep enough to image Earthlike planets requires
starshades that are tens of meters in diameter and separated from the
telescope by tens of thousands of kilometers.
Use of a starshade to suppress starlight has several key strengths for
discovering and characterizing nearby Earth analogs. First and
foremost, because starlight is blocked before entering the telescope,
no complicated optics are required, resulting in high throughput and
allowing the use of any relatively small commercially available space
telescope. This is in contrast to the lower throughput observatory and
extremely specialized and difficult wavefront sensing and control
system required for the internal coronagraph technique. Second, the
inner working angle (iwa) of the starshade/telescope combination (the
smallest angle on the sky at which an orbiting companion can be
detected) is independent of telescope aperture. Rather, it is
determined by the starshade size and separation from the telescope.
Similarly, a starshade based observing system has no limiting outer
working angle; the field of view is only limited by detector size.
A starshade-based starlight suppression system is, of course, not without its own challenges. The starshade must be manufactured to the required shape while meeting precise tolerances, the petal edges must be carefully designed to be small enough to minimize sunlight scatter into the telescope, the starshade must be deployed while meeting tight positioning requirements, the starshade must be precisely positioned and pointed during observation, and the basic optical response must be understood and proven. Nevertheless, while many of these challenges are new, the starshade structure has heritage from large radio antenna deployables and the JWST sunshield; and the fundamental engineering behind the structure and deployment is well understood.
Starshade at Northrop Grumman Corporation
Ongoing and Past Missions
NASA's Kepler Space Telescope
completed its prime mission and is living on as the
K2 Mission. During its prime mission, Kepler is
monitoring 150,000 stars for nearly four years to
search for a characteristic drop in brightness
indicative of an Earth-sized planet. A huge amount of
followup work confirmed and validated a large fraction of transit light curves as consistent with a planets.
Kepler will provided statistical frequency of different kinds of
planets orbityg different star types. Kepler fell short of its main mission goal of determining the frequency
of Earth-size planets in Earth-like orbits about
sun-like stars, and the exoplanet community continues to debate a value extrapolated from larger and shorter-period planets than the true Earth analog.
Professor Seager was a participating scientist member of the Kepler team and her group analzed Kepler data of Jupiter-size and Neptune-size planets, focusing on their radii, albedo constraints from secondary eclipse, statistical properties, and any possible interpretation of physical characteristics.
- The High Albedo of the Hot Jupiter Kepler-7 b,
B. Demory, S. Seager, et al. ApJ, 735, L12, 2011, 776, 25,
Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere, Demory et al. ApJ, 776, 25, 2013.
- Lack of Inflated Radii for Kepler Giant Planet Candidates Receiving Modest Stellar Irradiation, B. Demory and S. Seager, ApJS, 197, 12, 2011
Hubble Space Telescope
NASA's Hubble and Spitzer Space Telescope
provide observations of transiting planets,
planets that go in front of and behind their star
as seen from Earth. Observations of transiting
planets exploit separation of photons in time,
rather than in space. That is, observations are
made in the combined light of the planet-star
system. Primary and secondary eclipses enable
high-contrast measurements because the precise
on/off nature of the transit and secondary eclipse
events provide an intrinsic calibration reference.
This is one reason why the Hubble Space Telescope
and the Spitzer Space Telescope have been so
successful in measuring high-contrast transit
signals that were not considered in their
Members of Professor Seager's research group
analyze data from Hubble and spitzer and interpret
the data to understand the composition and
temperature of exoplanet atmospheres.
Image of NASA's Hubble Space Telescope
Hubble Space Telescope search for the transit of the Earth-mass exoplanet Centauri B b, B.-O. Demory et al., MNRAS, 450, 2043, 2015.
Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy, K. B. Stevenson et al., Science, 346, 838, 2014.
Clouds in the atmosphere of the super-Earth exoplanet GJ1214b, L. Kreidberg et al., Nature, 505, 69, 2014.
Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope, D. Deming et al., ApJ, 774, 95, 2013.
Spitzer Space Telescopes
NASA's Hubble and Spitzer Space Telescope provide observations of transiting planets, planets that go in front of and behind their star as seen from Earth. Spitzer operates at infrared wavelengths.
Members of Professor Seager's research group analyze data from Spitzer and interpret the data to understand the composition and temperature of exoplanet atmospheres.
Artist's rendering of NASA's Spitzer Space Telescope, against an infrared view of the sky. The band of light is the glowing dust emission from the Milky Way galaxy seen at 100 microns (as seen by the IRAS/COBE missions).
Infrared Radiation from an Extrasolar Planet, D. Deming,
S. Seager, L. J. Richardson, J. Harrington, Nature, 434, 7034, 2005.
Detection of Thermal Emission from a Super-Earth, B.-O. Demory et al., ApJ, 751, L28, 2012.
Detection of a Transit of the Super-Earth 55 Cancri e with Warm Spitzer, B.-O. Demory et al., A&A, 533, 114, 2011.
Strong Infrared Emission from the Extrasolar Planet HD 189733b, D. Deming, J. Harrington, S. Seager, L. J. Richardson, ApJ, 644, 560, 2006.
NASA's EPOXI is a 30 cm telescope now orbiting the sun millions of miles from Earth. EPOXI is a combination of the names for a space mission's two components: the Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). Professor Seager was a member of a team that used EPOXI to study stars with transiting planets and also to observe Earth from afar. Professor Seager's group is trying to understand what we would learn about Earth as an exoplanet from EPOXI's near infrared spectra of Earth. For a slightly different topic of inversion of EPOXI's Earth orbital light curves to determine Earth's physical characteristics, see the following paper by an EPOXI team member:
- Alien Maps of an Ocean-Bearing World, N. Cowan and the EPOXI Team, ApJ, 700, 915, 2009.
- Properties of an Earth-Like Planet Orbiting a Sun-Like Star: Earth Observed by the EPOXI Mission, T. Livengood and the EPOXI Team, AsBio, 11, 907, 2011.
- Rotational Variability of Earth's Polar Regions: Implications for Detecting Snowball Planets, N. Cowan and the EPOXI Team, ApJ, 731, 76, 2011.
EPOXI real video of Earth, with the moon transiting. When the images were acquired, the spacecraft was just outside the orbit of the Earth and ahead of Earth by 31 million miles, 1/3 AU, making it as far from Earth as Mercury is from the Sun. Credit: Don Lindler NASA/GSFC. For the video go to http://epoxi.umd.edu/3gallery/vid_Earth-Moon.shtml