Lecture 2: Exoplanets andbrown dwarfs
Adam Showman
University of Arizonaon sabbatical at Peking University

Exoplanets: an exploding new field•

Over 3500 known extrasolar planets•

Nearly 700 planets have been detected with the “Doppler” method•

Nearly 2700 planets have been detected with “transit” method (plus many Kepler
candidates):

Together, these give the planetary mass, radius, and orbital properties.

•~50 planets discovered by direct imaging:Planet mass vs. year of discovery

Planet radius vs. year of discovery

Planet mass vs. semi-major axis

exoplanet.eu

Why study atmospheres of exoplanets?
•

Atmospheres of other planets exhibit a diverse array of behavior, including
composition, mass, history, temperature structure, weather, clouds, climate, anddynamics––

This diversity is inherently fascinating and deserves to be understood
Earth’s atmosphere is just one realization… no more or less interesting than any other•Studying other atmospheres puts Earth in context•

Helps us understand in general how atmospheres originate/evolve, and what
determines their composition, structure, dynamics, clouds and climate–

Such a general understanding cannot be obtained from studying just one planet alone•

For example, to deeply understand even an Earth-based phenomenon, like what sets
the structure, width, and climate of the Hadley cell, you need to understand how the
Hadley cell depends on planetary rotation rate, gravity, tropospheric structure,
tropospheric humidity, etc. This implies studying other planets.

•

Atmospheres/oceans are where life is mostly likely to evolve, so studying atmospheres
informs our understanding of habitability in the Universe

Fundamental motivation: to understand the atmosphere/interior
circulation and structure on exoplanets and brown dwarfs.
• What is the nature of the circulation (zonal jets, vortices, storms,
turbulence)? What are the wind speeds, temperature variations, key lengthscales, and time variability? How do they depend on parameters?

• How does the circulation work: what are the dynamical mechanisms
controlling it?

• What is the role of condensation and clouds? Coupling to atmosphericchemistry?

• Can we achieve a unified theory of giant planet atmospheric circulation that
explains observations of hot Jupiters, brown dwarfs, and solar systemplanets?

• Does this knowledge provide insights about the circulation and climate of(less easily observed) smaller planets?

Factors that affect atmospheric circulation and structure

External irradiation
Internal (convective) heat flux
Gravity (mass)
Rotation rate
Atmospheric composition
Clouds/chemistry
Interaction with interior
History

Factors that affect atmospheric circulation and structure

External irradiation
Internal (convective) heat flux
Gravity (mass)
Rotation rate
Atmospheric composition
Clouds/chemistry
Interaction with interior
History

~107
~106
~100
~100
~100

Observationally booming subfields yield constraints at theextreme ranges of key parameters

Hot Jupiters
Strong irradiation
Weak interior flux
Modest rotation

Solar System giants

Brown Dwarfs andDirectly Imaged Giants

Weak irradiation
Weak interior flux
Rapid rotation

Negligible irradiation
Strong interior flux
Very rapid rotation

This opens the possibility of synergy between subfields

External irradiation (W/m2)

Observational constraints at the corners of a wide parameter space

107

Young hot Jupiters and highly
irradiated brown dwarfs

Hot
Jupiters

106

105
104

Warm
Jupiters

103
102
10
1

Y dwarfs
1

10

102

103

T dwarfs
104

L dwarfs
105

106

Interior heat flux (W/m2)
Showman (2016)

Hot Jupiters: Spitzer light curves for HD 189733b

Knutson et al. (2007, 2009)

8 mm

24 mm

Lightcurves for hot Jupiters

WASP-19b (Wong et al. 2016)

WASP-43b (Stevenson et al. 2014)

HD209458b (Zellem et al. 2014)

WASP-18b (Maxted et al. 2013)

Dependence of day-night flux contrast on effective temperature

Komacek et al. (2017)

Figure courtesy of Tad Komacek

Eclipse mapping: obtaining 2D maps of the daysidestellar disk

Majeau et al. (2012),
de Wit et al. (2012)

Hot Jupiter circulation models typically predict several
broad, fast jets including equatorial superrotation

Knutson et al. (2007)

Showman et al. (2009)

Heng et al. (2011)

Rauscher & Menou (2012)

Showman & Polvani (2011), ApJ 738, 71

What causes the equatorial jet? The day-night thermal
forcing induces planetary-scale waves, which pump
momentum to the equator

Showman (2008)

Exoplanet characterization: Transit spectroscopy

Sing et al. (2008, 2009), Vidal-Madjar et al. (2011), Huitson et al. (2012),Gibson et al. (2012),
Barman (2007), Tinetti et al. (2007), Swain et al. (2008),…..

Hazes and composition on hot
Jupiters• Some hot Jupiters haveTransit spectra of 10 hot Jupiters

relatively featureless transit
spectra, indicating high-altitude,
spectrally grey hazes obscuringmolecular absorption bands.

•

In others, molecular bands (e.g.,water) are prominent, indicating
less hazes, and constraining theatmospheric composition.

•

In principle, spectra like thiscan be used to infer the waterabundance, but the degeneracy
with hazes makes it difficult toobtain absolute abundances.
Sing et al. (2016, Nature)Transmission spectra of smaller planets (super Earths / hot Neptunes)
GJ 1214b, very flat spectrum => requires
hazes, and probably also high MMW
Kreidberg et al. (2014)

GJ 436b, flat spectrum => hazes and/or
high MMW
Knutson et al. (2014)

HAT-P-11b, tentative detection of weak waterfeatures => suggests puffy H2 atmosphere
Fraine et al. (2014)

Do super Earths have H2dominated atmospheres like
Uranus and Neptune? Orhigh mean molecular weight(MMW) atmospheres ofwater, N2, CO2, etc?

Doppler detection of winds on HD 209458b?
• Snellen et al. (2010, Nature) obtained high-resolution 2 mm spectra of
HD 209458b during transit with the CRIRES spectrograph on the VLT• Tentative detection of ~2 km/sec blueshift in CO lines during transit of HD 209458b• Interpreted as winds flowing from day to night at high altitude (~0.01-0.1 mbar)

Doppler detection of winds during transit
Snellen et al. (2010)

HD 209458b: Residual 2 km/sec blueshift (2 mm, CRIRES)

Leading limb

Average during transit

Louden & Wheatley (2015)

HD 189733b: red (blue) shift on leading (trailing)limb detected using ingress/egress

Trailing limb

Leading limb

Average during transit

Trailing limb

Showman et al. (2013)

Equatorial jet recently observed on HD 189733b is similar to that
previously predicted by GCM simulations:

Louden & Wheatley (2015)

Doppler detection of equatorial jetHD 189733b

WASP-43b

HD 209458b

Showman et al. (2009),
Showman et al. (2009),
Kataria et al. (2015) Knutson et al. (2012)
Zellem et al. (2014)

Comparisons of data to GCMs

NASA Kepler mission

NASA Kepler mission
Detects planets using the transit technique—about 2300 planets discovered
Roughly half of stars have a “super Earth”… a planet 1-10 Earth masses
Kepler discovered several planets close to the classical habitable zone
Kepler 22b: a 2.4-Earth radiusplanet in the habitable zone

Kepler 20e: a (hot) planet
smaller than Earth!

Jenkins et al. (2015)

Kepler 452b: a 1.6-Earth radius planet in the
habitable zone of a sun-like star

Borucki et al. (2012)

Fressin et al. (2012)•••Kepler detected loads of “super Earths”

Borucki et al. (2011)

Dressing & Charbonneau (2013)

Kepler assessment of planet populations

Kepler showed that close-in super Earths are far more common than close-in giant
planets (but the issue is still unsettled for more distant planets).

Kepler homing in on habitable planets

Ballard et al. (2013)

Planets <3 Earth radii that receive comparable stellar flux to Earth

Kepler-452b

We can infer densities and therefore learn about composition!

Lopez et al. (2012)

Proxima Centauri b: a habitable-zone planet around the closest star•

Proxima Centauri is the closest star to the Sun—distance of 4.25 light years—in theconstellation Centaurus. It is a dim M dwarf with a luminosity only 0.2% of the Sun’s. It may
be a third (bound) component of the nearby Alpha Centauri binary star system•

Planet was discovered August 2016 by radial velocity—Msini of 1.3 Earth masses, orbital
period 11.2 days, orbital semi-major axis 0.05 AU, receiving 0.65 the stellar flux of Earth. This
would imply an effective temperature of ~230 K if the albedo is similar to Earth’s.
Stellar motion inferredfrom Doppler shift ofstellar lines, indicating aplanet.

•There is great interest in characterizing this planet to see if it
has an atmosphere and understand its climate.
It would probably be a synchronously rotating planet withpermanent day and nightsides.

Relative sizes ofsome stars…
Proxima Cen is notmuch bigger thanJupiter.

Relative sizes of Sun in our sky toProxima Cen in this planet’s sky

References1•••

Ballard S., Charbonneau, D., Fressin, F., Torres, G., Irwin, J., Désert, J.-M., Newton, E.,
Mann, A.W., Ciardi, D., Crepp, J.R., Henze, C.E., Brvson, S.T., Howell, S.B., Horch, E.P.,
Everett, M.E., Shporer, A., 2013: Exoplanet characterization by proxy: A transiting 2.15
R_Earth planet near the habitable zone of the late K dwarf Kepler-61, Astrophys. J., 773,
98-115.
Barman, T., 2007: Identification of absorption features in an extrasolar planet atmosphere,
Astrophys. J., 661, L191-L194.
Borucki, W.J., Koch, D.G., Basri, G., Batalha, N., Brown, T.M., Bryson, S.T., Caldwell,D., Christensen-Dalsgaard, J., Cochran, W.D., DeVore, E., Dunham, E.W., Gautier III,
T.N., Geary, J.C., Gilliland, R., Gould, A., Howell, S.B., Jenkins, J.M., Latham, D.W.,
Lissauer, J.J., Marcy, G.W., Rowe, J., Sasselov, D., Boss. A., Charbonneau, D., Ciardi, D.,
Doyle, L., Dupree, A.K., Ford, E.B., Fortney, J., Holman, M.J., Seager, S., Steffen, J.H.,
Tarter, J., Welsh, W.F., Allen, C., Buchhave, L.A., Christiansen, J.L., Clarke, B.D., Das,S., Désert, J.-M., Endl, M., Fabrycky, D., Fressin, F., Haas, M., Horch, E., Howard, A.,
Isaacson, H., Kjeldsen, H., Kolodziejczak, J., Kulesa, C., Li, J., Lucas, P.W., Machalek, P.,
McCarthy, D., MacQueen, P., Meibom, S., Miquel, T., Prsa, A., Quinn, S.N., Quintana,
E.V., Ragozzine, D., Sherry, W., Shporer, A., Tenenbaum, P., Torres, G., Twicken, J.D.,
Cleve, J.V., Walkowicz, L., Witteborn, F.C., Still, M., 2011: Characteristics of planetary
candidates observed by Kepler. II. analysis of the first four months of data, Astrophys. J.,
736, 19-40.

References2•••

Borucki, W.J., Koch, D.G., Batalha, N., Bryson, S.T., Rowe, J., Fressin, F., Torres, G.,
Caldwell, D.A., Christensen-Dalsgaard, J., Cochran, W.D., De Vore, E., Gautier III, T.N.,
Grary, J.C., Gilliland, R., Gould, A., Howell, S.B., Jenkins, J.M., Latham, D.W., Lissauer,
J.J., Marcy, G.W., Sasselov, D., Boss, A., Charbonneau, D., Ciardi, D., Kaltenegger, L.,
Doyle, L., Dupree, A.K., Ford, E.B., Foriney, J., Holman, M.J., Steffen, J.H., Mullally, F.,
Still, M., Tarter, J., Balland, S., Buchhave, L.A., Carter, J., Christiansen, J.L., Demory, B.O., Désert, J.-M., Dressing, C., Endl, M., Fabrycky, D., Fischer, D., Hass, M. R., Henze,
C., Horch, E., Howard, A.W., Isascson, H., Kjeldsen, H., Aher Johnson, J., Klaus, T.,
Kolodziejczak, J., Barclay, T., Li, J., Meibom, S., Prsa, A., Quinn, S.N., Quintana, E.V.,
Robertson, P., Sherry, W., Shporer, A., Tenenbaum, P., Thompson, S.E., Twicken, J.D.,
Van Cleve, J., Welsh, W.F., Basu, S., Chaplin, W., Migilio, A., Kawaler, S.D., Arentoft,
T., Stello, D., Metcalfe, T.S., Verner, G.A., Karoff, C., Lundkvist, M., Lund, M.N.,
Handberg, R., Elsworth, Y., Hekker, S., Huber, D., Bedding, T.R., Rapin, W., 2012:
Kepler-22b: A 2.4 Earth-radius planet in the habitable zone of a Sun-like star, Astrophys.
J., 745, 120-135.
de Wit, J., Gillon, M., Demory, B.-O., Seager, S., 2012: Towards consistent mapping of
distant worlds: secondary-eclipse scanning of the exoplanet HD189733b, Astron.
Astrophys., 548, A128.
Dressing, C.D., Charbonneau, D., 2013: The occurrence rate of small planets around small
stars, Astrophys. J., 767, 95-114.

References3
•
•

•
•
•

Fraine, J., Deming, D., Benneke, B., Knutson, H., Jordán, A., Espinoza, N., Madhusudhan,N., Wilkins, A., Todorov, K., 2014: Water vapour absorption in the clear atmosphere of a
Neptune-sized exoplanet, Nature, 513, 526-529.
Fressin, F., Torres, G., Rowe, J.F., Charbonneau, D., Rogers, L.A., Ballard, S., Batalha,
N.M., Borucki, W.J., Bryson, S.T., Buchhave, L.A., Ciardi, D.R., Désert, J.-M., Dressing,C.D., Fabrycky, D.C., Ford, E.B., Gautier III, T.N., Henze, C.E., Holman, M.J., Howard,
A., Howell, S.B., Jenkins, J.M., Koch, D.G., Katham, D.W., Lissauer, J.J., Marcy, G.W.,
Quinn, S.N., Ragozzine, D., Sasselov, D.D., Seager, S., Barclay, T., Mullally, F., Seader,
S.E., Still, M., Twicken, J.D., Thompson, S.E., Udiin, K., 2012: Two Earth-sized planets
orbiting Kepler-20, Nature, 482, 195-198.
Gibson, N. P., Aigrain, S., Pont, F., Sing, D. K., Désert, J.-M., Evans, T. M., Henry, G.,
Husnoo, N., Knutson, H., 2012a: Probing the haze in the atmosphere of HD189733b with
Hubble Space Telescope/WFC3 transmission spectroscopy, MNRAS, 422, 753-760.
Gibson, N. P., Aigrain, S., Roberts, S., Evans, T. M., Osborne, M., Pont, F., 2012b: A
Gaussian process framework for modelling instrumental systematics: application totransmission spectroscopy, MNRAS, 419, 2683-2694.
Heng, K., Menou, K., Phillipps, P.J., 2011: Atmospheric circulation of tidally locked
exoplanets: a suite of benchmark tests for dynamical solvers, MNRAS, 413, 2380-2402.

References4
•

•

•

•
•

Huitson, C.M., Sing, D.K., Vidal-Madjar, A., Ballester, G. E., Lecavelier des Etangs, A.,
Désert, J.-M., Pont, F., 2012: Temperature–pressure profile of the hot Jupiter HD 189733b
from HST sodium observations detection of upper atmospheric heating, MNRAS, 422,
2477-2488.
Jenkins, J.M., Twicken, J.D., Batalha, N.M., Caldwell, D. A., Cochran, W.D., Endl, M.,
Latham, D.W., Esquerdo, G.A., Seader, S., Bieryla, A., Petigura, E., Ciardi, D.R., Marcy,
G. W., Isaacson, H., Huber, D., Rowe, J.F., Torres, G., Bryson, S. T., Buchhave, L.,
Ramirez, I., Wolfgang, A., Li, J., Campbell, J. R., Tenenbaum, P., Sanderfer, D., Henze,
C.E., Catanzarite, J. H., Gilliland, R.L., Borucki, W.J., 2015: Discovery and validation of
Kepler-452b: A 1.6-Earth-radius super Earth exoplanet in the habitable zone of a G2 star,
Astron. J., 150, 56-74.
Kataria, T., Showman, A.P., Fortney, J.J., Stevenson, K.B., Line, M.R., Kreidberg, L.,
Bean, J.L., Désert, J.-M., 2015: The atmospheric circulation of the hot Jupiter WASP-43b:
Comparing three-dimensional models to spectrophotometric data, Astrophys. J., 801, 86.
Knutson, H.A., Benneke, B., Deming, D., Homeier, D., 2014: A featureless transmission
spectrum for the Neptune-mass exoplane.t GJ 436b, Nature, 505, 66-68.
Knutson, H.A., Charbonneau, D., Allen, L.E., Fortney, J.J., Agol, E., Cowan, N.B.,
Showman, A.P., Cooper, C.S., Megeath, S.T, 2007: A map of the day-night contrast of theextrasolar planet HD 189733b, Nature, 447, 183-186.

References5
•
•

•
•
•
•

•

Knutson, H.A., Charbonneau, D., Cowan, N.B., Agol, E., Showman, A.P., Fortney, J.J.,
Henry, G.W., Everett, M.E., Allen, L.E., 2009: Multi-wavelength constraints on the daynight circulation patterns of HD 189733b, Astrophys. J., 690, 822-836.
Knutson, H.A., Lewis, N., Fortney, J.J., Burrows, A., Showman, A.P., Cowan, N.B., Agol,
E., Aigrain, S., Charbonneau, D., Deming, D., Désert, J.-M., Henry, G.W., Langton, J.,
Laughlin, G. 2012: 3.6 and 4.5μm phase curves and evidence for non-equilibrium
chemistry in the atmosphere of extrasolar planet HD 189733b, Astrophys. J., 754, 22.
Komacek, T.D., Showman, A.P., Tan, X., 2017: Atmospheric circulation of hot Jupiters:
Dayside-nightside temperature differences. II. Comparison with observations, Astrophys.
J., 835, 198.
Kreidberg, L., Bean, J.L., Désert J.-M., Benneke, B., Deming, D., Stevenson, K.B.,
Seager, S., Berta-Thompson, Z., Seifahrt, A., Homeier, D., 2014, Clouds in the atmosphereof the super-Earth exoplanet GJ1214b, Nature, 505, 69-72.
Lopez, E.D., Forthey, J.J., Miller, N., 2012: How thermal evolution and mass-loss sculpt
populations of super-Earths and sub-Neptunes: Application to the Kepler-11 system andbeyond, Astrophys. J., 761, 59-71.
Louden, T., Wheatley, P.J., 2015: Spatially resolved eastward winds and rotation of
HD189733b, Astrophys. J. Lett., 2015, 814, L24.
Majeau, C., Agol, E., Cowan, N.B., 2012: A two-dimensional infrared map of theextrasolar planet HD189733b, Astrophys. J., 747, L20.

References6
•
•
•
•
•
•
•

•

Marois, C., Zuckerman, B., Konopacky, Q. M., Macintosh, B., Barman, T., 2010: Imagesof a fourth planet orbiting HR 8799, Nature, 468, 1080-1083.
Maxted, P.F.L., Anderson, D.R., Doyle, A.P., Gillon, M., Harrington, J., Iro, N., Jehin, E.,
Lafrenière, D., Smalley, B., Southworth, J., 2013: Spitzer 3.6 and 4.5μm full-orbit
lightcurves of WASP-18, MNRAS, 428, 2645-2660.
Rauscher, E., Menou, K., 2012: A general circulation model for gaseous exoplanets withdouble-gray radiative transfer, Astrophys. J., 750, 96.
Showman, A.P., 2008: Extrasolar planets: A whiff of methane, Nature, 452, 296-297.
Showman, A.P., 2016: Illuminating brown dwarfs, Nature, 533, 330-331.
Showman, A.P., Fortney, J.J., Lewis, N.K., Shabram, M., 2013: Doppler signatures of theatmospheric circulation on hot Jupiters, Astrophys. J., 762, 24.
Showman, A.P., Fortney, J.J., Lian, Y., Marley, M.S., Freedman, R.S., Knutson, H.A.,
Charbonneau, D., 2009: Atmospheric circulation of hot Jupiters: Coupled radiativedynamical general circulation model simulations of HD189733b and HD209458b,
Astrophys. J., 699, 564-584.
Showman, A.P., Polvani, L.M., 2011: Equatorial superrotation on tidally locked
exoplanets, Astrophys. J., 738, 71.

References7
•

•
•
•

•

Sing, D.K., Fortney, J.J., Nikolov, N., Wakeford, H.R., Kataria, T., Evans, T.M., Aigrain,S., Ballester, G.E., Burrows, A.S., Deming, D., Désert, J.-M., Gibson, N.P., Henry, G.W.,
Huitson, C.M., Knutson, H.A., Lecavelier des Etangs, A.L., Pont, F., Showman, A.P.,
Vidal-Madjar, A., Williamson, M.H., Wilson, P.A., 2016: A continuum from clear to
cloudy hot-Jupiter exoplanets without primordial water depletion, Nature, 529, 59-62.
Sing, D.K., Vidal-Madjar, A., Lecavelier des Etang, A., Désert, J.-M., Ballester, G.,
Ehrenreich, D., 2008: Determing atmospheric conditions at the terminator of the hot
Jupiter HD 209458b, Astrophys. J., 686, 667-673.
Snellen, I.A.G., de Kok, R.J., de Moij, E.J.W., Albrecht, S., 2010: The orbital motion,
absolute mass and high-altitude winds of exoplanet HD209458b, Nature, 465, 1049-1051.
Stevenson, K.B., Désert, J.-M., Line, M.R., Bean, J.L., Fortney, J.J., Showman, A.P.,
Kataria, T., Kreidberg, L., McCullough, P.R., Henry, G.W., Charbonneau, D., Burrows,
A., Seager, S., Madhusudhan, N., Williamson, M.H., Homeier, D., 2014: Thermal
structure of an exoplanet atmosphere from phase-resolved emission spectroscopy, Science,
346, 838-841.
Swain, M. R., Vasisht, G., Tinetti, G., 2008: The presence of methane in the atmosphere ofan extrasolar planet, Nature, 452, 329-331.

References8
•
•

•
•

•
•
•
•

Tinetti, G., Vidal-Madjar, A., Liang, M.-C., Beaulieu, J.-P., Yung, Y., Carey, S., Barber,
R.J., Tennyson, J., Ribas, I., Allard, N., Ballester, G.E., Sing, D.K., Selsis, F., 2007: Water
vapour in the atmosphere of a transiting extrasolar planet, Nature, 448, 169-171.
Vidal-Madjar, A., Sing, D.K., Lecavelier, A., Lecavelier des Etangs, A., Ferlert, R.,
Désert, J.-M., Hébrard, G., Boisse, I., Ehrenreich, D., Moutou, C., 2011: The upper
atmosphere of the exoplanet HD209458b revealed by the sodium D lines: Temperaturepressure profile, ionization layer, and thermosphere, Astron. Astrophys., 527, 1-10
Wong, I., Knutson, H.A., Kataria, T., Lewis, N.K., Burrows, A., Fortney, J.J., Schwartz, J.,
Shporer, A., Agol, E., Cowan, N.B., 2016: 3.6 and 4.5 μm spitzer phase curves of thehighly irradiated hot jupiters WASP-19b and HAT-P-7b, Astrophys. J., 823, 122.
Zellem, R.T., Lewis, N.K., Knutson, H.A., Griffith, C.A., Showman, A.P., Fortney, J.J.,
Cowan, N.B., Agol, E., Burrows, A., Charbonneau, D., Deming, D., Laughlin, G.,
Langton, J., 2014: The 4.5μm full-orbit phase curve of the hot Jupiter HD209458b,
Astrophys. J., 790, 53.
(p.2) http://www.esa.int/spaceinimages/Images/2003/06/Planet_transit(p.2) http://natgeo.nikkeibp.co.jp/nng/images/news/bigphotos/images/new-planet-infraredkeck-telescope_30112_big.jpg(p.3) http://exoplanet.eu/(p.10) http://planetquest-archive.jpl.nasa.gov/video/22

References9
•
•
•
•
•
•
•

(p.11) http://planetquest-archive.jpl.nasa.gov/video/26(p.25左上) https://kepler.nasa.gov/multimedia/videos/?ImageID=42(p.25右上) https://nasa3d.arc.nasa.gov/detail/kepler-db(p.25下) https://kepler.nasa.gov/multimedia/photos/?ImageID=9(p.31上) https://www.eso.org/public/images/eso1629d/(p.31左下) http://www.eso.org/public/images/eso0307b/(p.31右下) https://www.eso.org/public/images/eso1629k/