Professor Seager's main research goal is to find and identify another Earth, including the search for signs of life by way of biosignature gases. First things, first, and so we start with an introduction.
Thousands of planets are known to orbit nearby, sun-like stars. These planets are called “exoplanets”. Professor Seager’s favorite exoplanet diagram is the mass-period diagram shown to the right. This diagram shows that exoplanets have all masses and semi-major axes possible, showcasing the random nature of planet formation and migration. The different planet detection techniques are shown in the diagram. Parts of the diagram with no planets are where technology can not yet reach exoplanets.
The figure below shows exoplanet discovery space as of 2014. Plotted as mass vs. orbital period (left) and not including Kepler discoveries. Plotted as radius vs. orbital period (right, and using a simplified mass-radius relationship to transform planet mass to radius) shows just how many exoplanets have been discovered, most by the Kepler Space Telescope. The paucity of planets of Earth's size or mass and orbit emphasizes the challenge exoEarth discovery with any planet-discovery technique.
|Credit N. Batalha, PNAS 2014
The goal in studying exoplanet atmospheres is to understand the atmospheric composition and temperature. We want to be able to recognize planet atmospheres like Earth’s: with water vapor, oxygen, ozone, and carbon dioxide. These strong absorbers would make the major contributions to the spectrum we could observe from afar. While the detection of true Earth twins is some time off, we are busy trying to understand hot Jupiter and hot Neptune atmospheres observed by primary and secondary eclipses for transiting exoplanets. Professor Seager’s group’s research focuses on computer models of exoplanet atmospheres and interpretation of data from space telescopes.
First Generation Exoplanet Atmospheres Highlights:
The first paper on hot Jupiter atmospheres
Seager, S., & Sasselov, D. D. 1998, “Extrasolar Giant Planets Under Strong Stellar Irradiation”, ApJ, 502, L157-161.View PDF
The first description of exoplanet transmission spectra, that led to the first detection of an exoplanet atmosphere
Seager, S., & Sasselov, D. D. 2000, “Theoretical Transmission Spectra During an Extrasolar Giant Planet Transit”, ApJ, 537, 916-921. View PDF
Co-authored one of the first two independent detections of photons from an exoplanet atmosphere
Deming, D., Seager, S., Richardson, L. J., & Harrington, J. 2005, “Detection of Infrared Radiation from an Extrasolar Planet”, Nature, 434, 740-743. View PDF
First general description of super Earth atmospheres, lead by Prof. Seager’s student Eliza Miller-Ricci
Miller-Ricci, E., Seager, S., & Sasselov, D. 2009 “The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres”, ApJ, 690, 1056-1067. View PDF
Second Generation Exoplanet Atmospheres Highlights:
The first paper on exoplanet atmosphere retrieval (led by then PhD student N. Madhusudhan).
Madhusudhan, N. & Seager, S. 2010, “A Temperature and Abundance Retrieval Method for Exoplanet Atmospheres”, ApJ, 707, 24-39.View PDF
One of the modern cornerstones of modern exoplanet atmospheric retrieval (led by then PhD student Bjoern Benneke).
Benneke, B., & Seager, S. 2012, “Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy”, ApJ, 753, 100-121. View PDF
A new planet mass estimation technique (led by then PhD student Julien de Wit).
de Wit, J. & Seager, S. 2012, “Constraining Exoplanet Mass from Transmission Spectroscopy”, Science, 342, 1473-1477. View PDF
A photochemistry code constructed from scratch for exoplanets, with several applications under either reduced or oxidized conditions. We hope to make it available in python in the future (led by then PhD student Renyu Hu).
Hu, R., Seager, S., & Bains, W. 2012 “Photochemistry in Terrestrial Exoplanet Atmospheres. I. Photochemistry Model and Benchmark Cases”, ApJ, 761, 166-195. View PDF
Exoplanet Interior Composition
The goal in studying exoplanet interiors is to learn what individual exoplanets are made of, and even what their internal structure might be. Professor Seager’s work has focused on the mass-radius relationships for exoplanets of a wide range of compositions and masses. Prof. Seager and colleagues. Exoplanet researchers have realized that there are fatal limitations to uncovering the interior composition of an exoplanet because only the mass and radius can be measured—no other information about the interior. This deadlock can be slightly aided in the future when enough exoplanets are found to do statistics, or possibly when molecules can be detected in super-Earth atmospheres in the future.
To download a MATLAB code to compute your own ternary diagrams for a super Earth of a given mass and radius: View Page
Selection of Prof. Seager’s most significant papers on exoplanet interiors.
Explanation of why the mass-radius relationships for planets of any composition are similar
Seager, S., Kuchner, M., Hier-Majumder, C. A., & Militzer, 2007, “Mass-Radius Relationship for Solid Exoplanets", ApJ, 669, 1279-1297. View PDF
Carbon planet prediction. This paper was rejected from the ApJ and Marc and I moved on to other things. We decided to let the paper live on astroph
Kuchner, M. & Seager, S., Extrasolar Carbon Planets, arXiv:astro-ph/0504214. View PDF
Exoplanet Habitable Zones
The habitable zone is a region around a star where a planet can have
surface temperatures consistent with the presence of liquid water. All
life on Earth requires liquid water, so the planetary
surfacetemperature requirement appears to be a natural one. The
climates of planets with thin atmospheres are dominated by external
energy input from the host star, so that a habitable is based on
distance from the host star. Small stars have a habitable zone much
closer to them as compared to Sun-like stars, owing to their lower
The surface temperature on an exoplanet is governed by the
greenhouse gases (or lack thereof ). Specifically, the greenhouse
gases absorb and reradiate energy from the host star, in the form of
upwelling infrared (IR) radiation from the s surface. Whereas on Earth
we are concerned with, e.g., parts-per-million rise in the greenhouse
gas CO2 concentrations, for potentially habitable exoplanets we do not
know a priori and cannot yet measure what gases are in the atmosphere
even to the tens of percent level. The atmospheric mass and
composition of any specific small exoplanet is not predictable.
The figure shows an extended habitable zone that captures some of the more modern thinking about habitable planets.
The light blue region depicts habitable zone for planets with
N2-CO2-H2O atmospheres. The yellow region shows the habitable zone as
extended inward for dry planets, with little surface water and little
water vapor in the atmosphere, as dry as 1% relative humidity. The
outer darker blue region shows the outer extension of the habitable
zone for hydrogen-rich atmospheres and can extend even out to
free-floating planets with no host star. The solar system planets
are shown with images. Known exoplanets are shown with symbols [here,
planets with a mass or minimum mass less than 10 Earth masses or a
radius less than 2.5 Earth radii.
Read more in Prof. Seager's review paper
Seager, 2013, Exoplanet Habitability, Science,
Exoplanet Biosignature Gases
A biosignature gas is defined as one that is produced by life and
accumulates in a s atmosphere to detectable levels. Any kind of ab
initio approach to predicting what biosignature gases might be is so
challenging that nearly all work done to date basically follows the
"We know what Earth life produces, so what might Earth s products look
like if transplanted to another, slightly different, Earth-like planet"
(Earth-like refers to a planet with about the same size and mass as
Earth, with oceans and continents, a thin N2-CO2-O2 atmosphere, and a
radiation environment similar to that of Earth's. Gases studied in
this context include oxygen, the otherwise unexplained simultaneous
presence of gases out of thermodynamic equilibrium (specifically
methane with oxygen), methyl halides, sulfur compounds, and some other
Prof. Seager's main exoplanet biosignature gas research aims to push
the frontiers to consider biosignature gases on planets very different
from Earth and also as many molecules as possible so we do not miss our
chance to identify gases that might be produced by life.
An excellent summary paper still relevant today summarizing Earth’s biosignatures (in which Prof. Seager played only a minor role)
Des Marais, D. J., Harwit, M., Jucks, K., Kasting, J. F., Lunine, J. I., Lin, D., Seager, S., Schneider, J., Traub, W., & Woolf, N. 2002, “Remote Sensing of Planetary Properties and Biosignatures on Extrasolar Terrestrial Planets”, Astrobiology, 2, 153-181. View PDF
The first paper showing biosignature
gases, if produced on rocky planets with hydrogen-rich atmospheres, could survive.
S., Bains, W., Hu, R. 2013, Biosignature Gases in H2-Dominated
Atmospheres on Rocky Exoplanets, ApJ, 777, 95. View PDF
theoretical experiment to investigate whether we can liberate
predictive atmosphere models from requiring fixed, Earth-like
biosignature gas source fluxes. New biosignature gases can be
considered with a check that the biomass estimate is physically
plausible. We have created a framework for linking biosignature gas
detectability to biomass estimates, including atmospheric
photochemistry and biological thermodynamics.
S., Bains, W., Hu, R. 2013, A Biomass-based Model to Estimate the Plausibility of Exoplanet Biosignature Gases, ApJ, 775, 104. View PDF
approach to biosignature gases will start with a long list of
potential biosignature gas molecules. To maximize our chances of
recognizing biosignature gases, we promote the concept that all stable
and potentially volatile molecules should initially be considered as
viable biosignature gases. We present a new approach to the subject of
biosignature gases by systematically constructing lists of volatile
molecules in different categories. An exhaustive list up to six non-H
atoms is presented, totaling about 14,000 molecules.
Seager, Bains, W., Petkowski, J. J. 2016, Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry, Astrobiology, 16, 465
Professor Seager has worked on a variety of other topics in exoplanets.
One of Prof. Seager's favorite papers is about how to extract information from a transit light curve, by recognizing that there are five equations and five unknowns for a circular orbit. Surprisingly, a star's density can be measured from the planet transit light curve. This work spawned became highly relevant after the Kepler space telescope was launched, providing the high-precision photometry needed for the applications described in this paper.
Seager, S., & Mallen-Ornelas, G. 2003, “On the Unique Solution of Planet and Star Parameters from an Extrasolar Planet Transit Light Curve”, ApJ, 585, 1038-1055.View PDF
She also published the first paper on constraining exoplanet rotation rates from oblateness measurements during transit
Seager, S., & Hui, L. 2002,“Constraining the Rotation Rate of Transiting Extrasolar Planets by an Oblateness Measurement”, ApJ, 574, 1004-1010.View PDF
An early paper on reflected light phase curves and polarization signatures
Seager, S., Whitney, B. A., & Sasselov, D. D. 2000, “Light Curves and Polarization of the Close-in Extrasolar Giant Planets”, ApJ, 540, 504-520.View PDF
And co-authored the first and to date only paper in exoplanet atmospheric refraction
Hui, L., & Seager, S. 2002, “Atmospheric Lensing and Oblateness Effect During an Extrasolar Planetary Transit”, ApJ, 572, 540-555.View PDF
Co-authored the first paper describing how to use an Earth-twin’s reflected light curve to determine the rotation rate and possibly infer the presence of weather and continents
Ford, E. B., Seager, S., & Turner, E. L. 2001, “Characterization of Extrasolar Terrestrial Planets from Diurnal Photometric Variability”, Nature, 412, 885-887. View PDF
Palle, E., Ford, E. B., Seager, S., Montanes-Rodriguez, P., & Vazquez, M. 2008, “Identifying the Rotation Rate and the Presence of Dynamic Weather on Extrasolar Earth-like Planets from Photometric Observations”, ApJ, 676, 1319-1329. View PDF
Exoplanet Space Telescopes
Prof. Seager is PI of ASTERIA (formerly a 3U CubeSat called ExoplanetSat). ASTERIA is a 6U CubeSat, a space telescope capable of high photometric precision on bright stars, whose program includes the search for transiting exoplanets, both new transits of known radial velocity planets as well as new discoveries. The prototype is being implemented at JPL and is a partnership with Draper Labs. The scientific goal is ultimately a search for transiting Earth-size planetse around the nearest, brightest, sun-like stars. The engineering goal is a fleet of small space telescopes, each observing its own host star.
Description of ExoplanetSat precision pointing and science case
Knapp, M., Smith, M., Pong, C., Nash, J., and Seager, S., ExoplanetSat: High Precision Photometry for Exoplanet Transit Detections in a 3U , IAC 16th Symposium on Small Satellite Missions, IAC-12-B4, 2, 7, 14450 CubeSat2012
Prof. Seager is a co-I on TESS, an MIT-led NASA mission to be launched in 2017 as an all-sky survey of bright nearby stars. TESS will find thousands of exoplaents, including hundreds of small planets transting M dwarf stars.
Ricker, G. et al. 2015, "Transiting Exoplanet Survey Satellite", JATIS SPIE, 1, 014003-1
In the past Prof. Seager was a Participating Scientist with Kepler, and her group studied hot Jupiters with Kepler data. Noteable papers are below.
Detection of a transit for a super Earth that was known from radial velocity surveys but not known to transit (independently found by the MOST space telescope)
Demory, B.-O., Gillon, M., Deming, D., Valencia, D., Seager, S., Benneke, B., Lovis, C., Cubillos, P., Harrington, J., Stevenson, K. B., and 4 coauthors, "Detection of a Transit of the Super-Earth 55 Cancri e with Warm Spitzer", A&A, 533, 114
First detection of thermal emission from a super Earth via secondary eclipse detection of 55 Cnc e
Demory, B.-O., Gillon, M., Seager, S., Benneke, B., Deming, D., & Jackson, B. 2012, "Detection of Thermal Emission from a Super-Earth", ApJ, L28-34
Observed cut off in incident flux for the inflated radii of hot Jupiters
Demory, B.-O., & Seager, S. 2011, "Lack of Inflated Radii for Kepler Giant Planet Candidates Receiving Modest Stellar Irradiation", ApJS, 197, 12-16
Recombination in the Early Universe
Professor Seager’s career started with recombination in the early Universe. This research field has seen a resurgence with the upcoming launch of ESA’s Planck Space Telescope. Prof. Seager is no longer actively working in this field of research. Prof. Seager’s major contribution to recombination in the early Universe is in the form of two papers:
A summary paper
Seager, S., Sasselov, D. D., & Scott, D. 1999, “A New Calculation of the Recombination Epoch”, 1999, ApJ, 523, L1-5.View PDF
and the full description
Seager, S., Sasselov, D. D., & Scott, D. 2000, “How Exactly Did the Universe Become Neutral?”, ApJS, 128, 407-430.View PDF