EXOPLANET ATMOSPHERES
Year 3 Term 2 Essay Project for the Degree of Bachelor of Science in Physics with Theoretical Physics

Ho Yin Desmond YUEN
Department of Physics, Blackett Laboratory, Imperial College London,
South Kensington Campus, London SW7 2AZ, United Kingdom
Submitted Version: Summer Term, 4th May 2015

ABSTRACT
The objective in studying exoplanet atmospheres is to understand their atmospheric composition and properties, thus, to deduce the planets’ habitability. Favoured by their close proximity, studying the atmospheres within our own Solar System and seeking for resemblance is a fundamental first step before we proceed off to extra-solar systems. While the ultimate goal of detecting a true Earth twin is some time off, we are busy trying to understand the atmospheres of hot Jupiters and hot Neptunes through observing primary and secondary transits of these exoplanets. During the past decade, conflicting observations between ground- and space-based facilities, different methods of data treatment, and resolving limitations of measuring instruments have been a source of debate in the astronomy community. Controversies over the atmospheres of two of the most extensively studied exoplanets, HD 189733b and GJ 436b, are discussed here.
Through a series of investigation and evaluation, the hot Jupiter, HD 189733b, is believed to possess a carbon-monoxide-rich atmosphere with a sodium-abundant troposphere, topped with high-altitude haze. The hot Neptune, GJ 436b, despite having a much lower surface temperature than HD 189733b, probably also has a carbon-monoxide-rich atmosphere. High atmospheric metallicity and exotic disequilibrium chemistry are both capable of explaining the dispute in GJ
436b’s atmospheric composition. Future work is also discussed in the light of recent discoveries of some super-Earth exoplanets.
Subject headings: transits, planetary systems, atmospheres
Methods: review, data analysis, comparison

Stars: HD 189733, GJ 436, Kepler-62

INTRODUCTION

The atmospheric composition of HD 189733b has been one of the hottest debates in the astronomy community ever since the hot Jupiter revealed itself by dimming the light of its parent star. Analyses of transit photometric data from the Infrared Array Camera
(IRAC) on Spitzer Space Telescope (SST) at the
4.5 band have led to conflicting conclusions of a carbon-monoxide-(CO) rich atmosphere by Désert et al. (2009) and a carbon-dioxide-(CO2) rich atmosphere by Fortney et al. (2010). Knutson et al. (2012) attempted to explain the discrepancy using vertical mixing theories of CO and the fact that the carbon bearing profile at the temperature regime of HD
189733b selects only CO and methane but no CO2.
Because of their similarities, the physical properties and circulation mechanisms of Venus and HD 189733b are also compared. The conclusion, indeed, favours a
CO-rich atmosphere. However, in order to judge solely on the 4.5 band, further investigation or new data has to be collected for HD 189733b because the band feature is still too wide that it incorporates both
CO’s 4.5 and CO2’s 4.4 band signatures. At visible wavelengths, strong lines of the alkali metal, sodium, were reported by Redfield et al. (2008), using observations from the High Resolution Spectrograph
(HRS) on the ground-based Hobby-Eberly Telescope
(HET). In contrast, through obtaining data from the
Space Telescope Imaging Spectrograph (STIS) on

Our current understanding in planetary atmospheres does not include a thorough explanation of their formation. However, presumably following a common origin, there exist multiple mechanisms, in which planetary atmospheres can evolve differently to the form we observe today. The most impactful ones are thermal escape, meteor impacts, and volcanic activities. A qualitative discussion of the examples within our Solar System, coupled with a quantitative approach to understand the physics behind planetary atmospheres, such as mixing ratios and the radiative transfer equation, will grant us an insight to the behaviour and composition of exoplanet atmospheres.
By far, either direct or indirect, there are in total six established methods of detecting exoplanets. After the launch of the new Kepler Space Telescope (KST) in
2009, transit photometry has surpassed the radial velocity method and become the most effective detection method in terms of number of exoplanets discovered. Transit photometry has also made studying the atmosphere of a transiting planet possible, too, through analysing the high resolution light curves and transmission spectra. Three of such transiting exoplanet atmospheres will be discussed: HD 189733b
(a hot Jupiter), GJ 436b (a hot Neptune), and Kepler62e (a super-Earth).
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Exoplanet Atmospheres – H. Y. D. Yuen
Hubble Space Telescope (HST), Sing et al. (2011) reported a featureless spectrum, which is an indication of high altitude haze in the atmosphere of HD 189733b.
However, featurelessness was later disproved due to inappropriate binning of the data. Huitson et al. (2012) collected a new set of data with much narrower binning and reported singlet sodium absorption.

distances from their parent stars will therefore have different raw materials to make up their bodies and atmospheres. (cf. “The Solar System”) After surviving the star’s T-Tauri phase, atmospheres can still evolve differently through three main processes.
All atmospheres are slowly boiling away into space due to the internal thermal velocities of the molecules near the top of the atmosphere. Any molecule with a thermal (kinetic) energy larger than the potential energy of the planet’s gravity will escape to space. We can compute the upward velocity needed for a molecule to thermally escape the planet’s gravity:

GJ 436b, categorized as a hot Neptune, is also an extensively studied and discussed object because of its unusual atmospheric chemistry. The first set of observations from the IRAC on Spitzer indicated that the planet possesses a CO-rich but methane- (CH4) poor atmosphere. This conclusion drawn by Stevenson et al. (2010) was primarily driven by a high
3.6 to 4.5 observed flux ratio in the emission spectrum. However, such an atmospheric composition puts the equilibrium chemistry, as suggested by
Beaulieu et al. (2011), at stake. Given the constraint of the planet’s temperature and the assumption of thermochemical equilibrium, one would expect a CH4rich atmosphere over a CO-rich one. Although chemical disequilibrium reactions like vertical mixing and polymerization might be responsible for the depletion of methane, a recent suggestion of a high metallicity atmosphere in GJ 436b provides a much more promising and simpler explanation that avoids complicated chemistry.

≈

1
2

>

(1)

where is assumed to be the molecule’s thermal energy, is the upward velocity of the molecule of mass , and is the radial distance from the centre of the planet of mass to the altitude of the molecule. As a result, we do not expect many light particles to be present in the atmospheres of terrestrial planets because they get thermally excited easily.
Terrestrial planets, albeit smaller in size than gas and ice giants, suffer from a high rate of meteor impacts because of their close proximities to the star and frequent bombardment by meteors attracted to the star’s gravity. Hence, meteor impacts is usually the largest contributor to the atmospheres of terrestrial planets due to vapourisation of surface rock during such impacts. The high abundance of sodium in
Mercury’s atmosphere is a great example of the contribution from meteor impacts.

Atmospheric bio-signatures like ozone and oxygen are signs of life in an atmosphere. We will try to look for and resemble any similarities between the atmospheres of our Earth and the super-Earth, Kepler62e, by exploiting their respective atmospheric biosignatures. Ultimately, such bio-signatures can help determine the habitability of these Earth-like and super-Earth exoplanets.

Furthermore, volcanic activities on terrestrial planets are also capable of altering climates and atmospheres. During major explosive eruptions, huge amounts of gas, aerosols, and ash are ejected into the lower atmosphere. Heavier ash particles will fall rapidly from the atmosphere and make little impact on climates but volcanic gases, such as sulphuric and carbonic compounds, will remain in the atmosphere and form clouds or haze. In the example of Venus, volcanoes, when they were still active in the past, constantly replenished the atmospheric sulphuric acids and carbon oxides, which left the atmosphere in its current form and chemical composition.

FORMATION & EVOLUTION
The present climates on all of the planets in our
Universe are the result of a multi-stage process, which began with a proto-solar body formed out of a concentration of giant and diffuse molecular cloud
(GMC) and collapsed under gravity to form a star. Most of the mass of the cloud ended up in the hot young star and the remainder, forming a flat disc of orbiting material in which most of the angular momentum of the system resided, aggregated to form planets. After the planets settled in their stable orbits, they cooled and might have acquired part of their atmospheres at some stage. However, it is still unknown whether the formation of planetary atmospheres was that early in the timescale or a bit later during the T-Tauri phase.

Examples of evolution of the terrestrial planetary atmospheres in the Solar System are immensely useful when probing exoplanet atmospheres because most transiting exoplanets are rocky, hot, and close to their parent stars unless the exoplanet possesses a history of migration.

Soon after a star begins to fuse hydrogen, it would have entered its T-Tauri phase, becoming two to three times more luminous and propelling dense and highspeed solar wind with its magnetic field. The principal effect of the T-Tauri solar wind was to sweep away the remaining gas and dust that had not yet formed planets out to the edge of the system. Planets with different

ATMOSPHERIC PHYSICS
Before conducting any in-depth studies on exoplanet atmospheres, we ought to first understand their chemical composition. In other words, how many different atmospheric species there is and how
2

Exoplanet Atmospheres – H. Y. D. Yuen light curve as the stellar light source propagates through the atmosphere of an exoplanet. (cf. “Transit”)

abundant each species is. The former can be found by transmission spectroscopy while the latter can be determined through a combination of transmission spectroscopy and mixing ratios. Some basic physics will be outlined in this section to allow us to understand the theories, methods, and their applications to the study of exoplanet atmospheres.

However, it is very common that the atmospheric particles are also emitting photons of frequency , mostly due to heating. Therefore, an extra term $ has to be added to the right hand side of equation (3) to represent the additional power, where the source intensity function is $. There is no surprise that the term differs with the fraction of photons attenuated by the slab solely in their intensity contributions because the extra light is emitted by the same material. Including both attenuation and emission, we arrive at the Schwarzschild-Milne equation (S-M Equation):
= − % − $&
(4)
If , , and $ are functions of thickness , a formal solution to the S-M equation can be obtained. First, introduce the optical path function ':

The basic operating principle behind transmission spectroscopy is the energy transfer between the atmospheric gases and the electromagnetic radiation from the host stars. The Lambert-Beer Law is handy for not just describing the way how radiative power from the host stars is affected by extinction and/or emission of radiation, but also for determining the optical thicknesses of exoplanet atmospheres.

'% & = (

*

*+

% ) & % ) &

)

(5)

where , is the incident position of the optical path. So, equation (4) can be rewritten as:
= %$ − & '.
(6)
Then, we multiply by the integrating factor - . and integrate to get:
% −

, &.

.

.

= ( $%') & - . ')
,

% & = ( $%') & - 01.0.
,

/

/2

') +

,-

(7)
0.

(8)

where spectral radiance equals , at the point , . A region of space is said to be optically thick at a frequency if the total optical path ' is much greater than 1, whereas the region is optically thin if ' is much less than 1.

Figure 1: A slab of photons travelling a distance of from a surface to a surface . The area of is slightly larger than , owing to the divergence of photons into a solid angle ∆ from each point of . However, for simplification, the volume of the slab is still given by × .

An important quantity that arises in the solution of Lambert-Beer Law and S-M Equation is the fraction of the spectral radiance transmitted or absorbed at a particular wavelength. This quantity is exactly what we measure and observe from the telescopes during events of transit. We are mostly interested in infrared and ultraviolet transmission because that wavelength region – also known as “atmospheric window” – is where the most significant atmospheric absorbers demonstrate their band signatures. Figures 2 and 3 are the absorption spectra of the most strongly absorbing species and ozone in the Earth’s atmosphere at infrared and ultraviolet wavelengths. The significance of studying these spectra will be explained later when super-Earths are discussed.

Consider a slab of photons of area and infinitesimally small thickness , with number density . (See Figure 1.) If the photons experience attenuation (either absorption or scattering) along the optical path through a gas or solution, the incident spectral radiance will be reduced by . Here we define the strength of attenuation, i.e. the attenuation coefficient , as the sum of an absorption coefficient and a scattering coefficient :
= +
(2)
The fraction of photons attenuated when passing through this slab would simply be the total opaque area of all the particles in the slab, , divided by the area of the slab, which yields . It follows that the reduction in spectral radiance, i.e. the number of photons attenuated by the slab , can be expressed as follows: =− (3)
Equation (3) serves as the skeleton of Lambert-Beer
Law, which grants us a rough idea of how much intensity of light would have been attenuated on the

In exoplanetary studies, the mass and volume mixing ratios are immensely useful in determining the atmospheric composition and abundance of different species. In agreement with transmission band depths, one can outline a scale height profile as a function of abundance of each species comprising the atmosphere.
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 3: Plot of absorption cross-section as a function of wavelength for ozone (O3). The ozone absorption profile is useful in examining Earth-like exoplanet atmospheres, see Figure 19. Plot credit to D. G. Andrews (2010).

Note that Dalton’s law of partial pressures and volumes hold:
' = 8 '4

(15)

3 = 8 34

(16)

4

Figure 2: Infrared absorption spectra for the six strongest absorbers in Earth’s atmosphere and for the six gases combined (bottom), in the absence of clouds. This spectrum is useful in examining Earthlike exoplanet atmospheres, see Figure 18. Plot credit to A. Dudhia and D. G. Andrews (2010).

4

Now, we can relate the mass mixing ratio with the partial pressures:
4

Consider a sample of air of uniform volume 3, temperature , and pressure ', containing a mixture of different gases 4 %5 = 1,2,3 … & .
If
there are 4 molecules of gas 4 in the sample and each gas molecule weighs 4 , the total number of molecules in the sample and the total mass of the sample would be:
=8
=8

4

4

4

4

4

=

4

(10)

4

3

34 =

4

'

=

34
=
3

4

=

'4
9
=
'
4

4

(18)

Exoplanetary atmospheres are complex and can possess very different optical thicknesses so astronomers have to be cautious when examining observational data. Such ambiguities often give rise to controversies over atmospheric compositions of many exoplanets, which a few will be discussed in this essay.
To allow us to better understand the exoplanet atmospheres, atmospheres within the Solar System are examples that are often being referred to as well.

(11)

To convert the mass mixing ratio into a more useful form, the ideal gas law is introduced:
'3 =
(12)
where is the Boltzmann’s constant. The partial pressure '4 of a particular gas is defined as the pressure that would be exerted by molecules of that gas if they were to occupy the sample alone. Similarly, the partial volume 34 of the gas is the volume that would be occupied by the molecules of that gas alone:
'4 =

(17)

For many purposes, mass and volume mixing ratios are convenient measures of concentration and composition since they are not affected by volume changes but only by chemical production or loss.

The mass mixing ratio 4 of a particular gas in the sample is defined as the ratio of the mass of the molecules of that particular gas to the total mass of the complete sample.
4

4 '4
4 '4
=
'
9 '

where the mean molecular mass 9 is ⁄ . As for the partial volumes, they can be used to define the volume mixing ratio as follows:

(9)

4

4

=

THE SOLAR SYSTEM
Most of the important progress on understanding planetary atmospheres and climates over the last few decades has been made by space missions within our very own Solar System. The significance of studying the Solar System is to grant us a rough idea of what to expect when we probe the exoplanet atmospheres.

(13)

(14)

The planets in our Solar System are categorized into three groups: terrestrial planets, gas giants, and
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 5: Hadley circulation of carbon monoxide on Venus. The mechanism is capable of explaining the gradient in the height against
CO mixing ratio plot in Figure 4 below the depletion zone. Diagram credit to F. W. Taylor (2010).
Figure 4: Carbon monoxide profile in the atmosphere of Venus, showing the reactions that produce CO in the upper dissociation zone and remove CO in the depletion zone, hot troposphere, and at the surface. Plot credit to F. W. Taylor (2010).

Jupiter contributing to the over-abundance of CO in its atmosphere but circulation and vertical mixing might be crucial. (cf. “An example of Hot Jupiter”)

ice giants. The terrestrial planets that are closest to the
Sun compose mainly of rocky and metallic materials because the temperature was and has been hot enough to vapourise the more volatile gaseous and icy materials, which were then swept away by the early TTauri solar wind. This explains why Mercury, Venus,
Earth, and Mars all possess “heavier” atmospheres that are non-hydrogen-dominated and abundant in carbon compounds, sodium, and nitrogen. In the outer Solar
System, temperature drops and the gas and ice giants, when they were still planetesimals, gravitationally attracted not only rock and ice but also the lighter gases that the terrestrial planets could not retain. The atmospheres of Jupiter, Saturn, Uranus, and Neptune are, therefore, “lighter” and dominated by hydrogen, helium, and methane. Putting forward this analogy to exoplanet atmospheres, we would expect hot Jupiters and Neptunes to possess “lighter” atmospheres whereas super-Earths and terrestrial exoplanets to possess more volatile atmospheres.

TRANSIT
Exoplanets are immensely hard to detect because they shine not by their own light, but by light reflected by their parent stars, which they orbit. In addition to this intrinsic difficulty of detecting such faint light sources, the light from their parent stars causes a glare that smears it out. As a consequence, astronomers have had to resort to indirect methods, such as transit photometry, in order to detect exoplanets orbiting close to their stars. The direct imaging method also yields success when detecting planets that are far from the glare of the star. Distances of transiting exoplanets from their parent stars range from 100< to a few AU while those of directly imaged exoplanets range from a few to 10= AU. As of today, we are able to identify 1206 transiting exoplanets and 53 directly imaged exoplanets and their orbital parameters, masses, and radii. The next objective would be to characterise their atmospheres, based on temperatures, chemical compositions and other physical properties, while the ultimate goal would be to determine the exoplanets’ habitability.

It is worthwhile to dig deeper into Venus because we will see resemblance between the atmosphere of
Venus and that of a hot Jupiter, HD 189733b. From
Figure 4, it is shown that carbon monoxide is concentrated only in the upper atmosphere due to photo-dissociation of carbon dioxide by solar ultraviolet (UV) radiation. CO is also quite abundant in the lower atmosphere. One might then assume volcanism should be the source of CO near the surface, however, one must also consider the age of Venus – roughly 4.5 million years old. Over such a long period of time, volcanic activities have died down and are insufficient to produce the observed latitudinal gradient of CO. The less stable CO has also depleted itself by reacting with sulphuric compounds to form the more stable carbon dioxide. So, in order to explain the CO abundance near the surface of Venus, a general circulation mechanism – the Hadley circulation model
– is required. (See Figure 5.) Similarly, for HD 189733b, there will be of course no volcanism on any typical hot

Transit is the event when an exoplanet passes across the face of (primary transit) or behind
(secondary transit) the parent star, a small amount of the combined observed brightness of the system is occulted. Through analytical studies of these transmission spectra and light-curves (see Figure 6), there exists unique solutions of the planet and star parameters, such as orbital periods, masses, and radii.
Also because of their periodic and special geometry, many follow-up observations of transiting planets are possible, including atmospheric transmission spectroscopy. During a primary transit, the transmission spectrum records the stellar flux that passes through the optically thin part of the planet’s atmosphere only.
The absorption depth %> ? & of the spectrum is, therefore, equivalent to the ratio of the cross-sectional
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 7: Plot of measured values of NO ⁄ ∗ ratios (planetary radius to parent stellar radius) against wavelength of light detected from
HD 189733b in microns. Green, dark blue, and pink points are from previous works. Light blue line gives the expected range of values if only water vapour was contributing to absorption. Grey dotted line gives the expected range of values if small particles contributed
Rayleigh scattering. Plot credit to Désert et al. (2009).

The exoplanet was discovered by F. Bouchy et al.
(2005), using primary transit. With a mass 13.8% higher than that of Jupiter, HD 189733b has an orbital period around its parent star, HD 189733 A, of 2.22 days. It also features an almost perfect edge-on orbit with an inclination of 85.5°, which largely facilitates detailed study of both its emission and absorption spectrum. One of the very first debates on the atmosphere of HD 189733b is the 4.5 transmission band. The band feature can be explained by either the bearing of carbon monoxide (CO), which is a strong absorber at exactly 4.5 , or carbon dioxide (CO2), which absorbs strongly at 4.4 .

Figure 6: Light curve of a transiting exoplanet. One full event of transit can be broken down into four points of contacts. First contact
(exterior ingress) is when the planet is entirely outside the star, moving inward. Second contact (interior ingress) is when the smaller body is entirely inside the star, moving further inward. Vice versa defines the third (interior egress) and fourth (exterior egress) contacts. Understanding the different points of contacts is extremely useful in reducing systematic errors of the transmission spectrum of an exoplanet’s atmosphere. (See Figure 10.) Measurable quantities that can be used to deduce physical properties of the exoplanet include the duration of transit @, the transit depth , the ingress and egress duration A, and the curvature B. Diagram credit to Brown et al. (2000).

area of the optically thick disk of the exoplanet % . & plus that of the atmospheric annulus % C & – which is a function of wavelength – to that of the parent star disk % ∗ ≡ F ∗ &.
>? =

.

+

∗

C %G&

.

=

+
F

C %G&

Summarising the previous works and comparing different sets of data, Désert et al. (2009) presented their new observations in Channel 2 of the Infrared
Array Camera (IRAC) on Spitzer at the 4.5 and 8 bands. In Figure 7, the bump at 4.5 goes 4 above the expected value with either sole contribution from water vapour or Rayleigh scattering of small particles present. Therefore, another absorbent is needed to explain this absorption signature. At the near-infrared wavelength, all main atmospheric constituents, such as hydrogen, water, and methane, have no strong spectral features, except
CO. Hence, Désert et al. (2009) concluded that CO could be a candidate for the 4.5 band feature.

(19)

∗

Half an orbital period later during a secondary transit, emission is directly detected from a planet’s atmosphere and the thermal emission spectrum can be acquired by taking the ratio % H & of the disk-averaged flux of the planet to that of the star.
H

=

=
. . %G&
=
∗ ∗

=

F

= J

.

F

.
∗

=
. %G&
=
∗ ∙ ∗

∙

K ∙L

. %G&
∗

M

=

(20)

In the contrary, Fortney et al. (2010) suggested that CO2 could play a much more important role than
CO because the mixing ratio of CO increases linearly with the atmospheric metallicity and that of CO 2 increases quadratically. Therefore, if the atmosphere of HD 189733b demonstrates high metallicity and is non-hydrogen-dominated, CO 2 will be much more abundant than CO. Figure 8 shows that the infrared data of Désert et al. (2009) matches better with the band-averaged absorption depth of the 30x solarmetallicity model, especially at 4.5 , implying that

where . , . , ∗ , and ∗ are the effective temperatures and radii of the exoplanet and the star. Absorption depths and flux ratios at multiple wavelengths in these two complementary spectra could then be directly interpreted as a transmission spectrum.

AN EXAMPLE OF HOT JUPITER – HD 189733b
HD 189733b is one of the most widely studied exoplanets and its atmosphere composition and structure have been a source of significant controversy.
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 8: Plot of absorption depth in percentage against wavelength of light detected from HD 189733b at four different metallicities from 1x to 30x solar-metallicity (black, orange, red, and purple lines).
Light blue points and error bars are the infrared data of Désert et al.
(2009). Band-averaged absorption depths of the solar-metallicity models at the wavelengths of the four infrared data points are shown as squares. Plot credit to Fortney et al. (2010).

CO2 might probably be the prominent species that contributes to the 4.5 band feature.

Figure 9: The Ellingham diagram, showing that the formation of CO is favoured over CO2 due to the negative gradient in a plot of Gibbs free energy against temperature. Plot credit to G. M. Palmgren
(2003).

Although both sides presented sound arguments, the 4.5 band feature would most likely be due to absorption by carbon monoxide because Fortney et al.’s (2010) approach was based on a theoretical prediction and lacked data support. Heading back to
Figure 8, it could be seen that the continuum of the
30x solar-metallicity model yields a better fit to the infrared data of Désert et al. only at 4.5 . The three other data points at 3.6
, 5.8
, and 8.0 all suggest a low atmospheric metallicity. Therefore, the quadratic contribution of CO2 due to high metallicity becomes questionable. A careful examination of the physical properties and orbital parameters of HD
189733b also yields bias towards the CO side. On one hand, the orbit of HD 189733b is only 0.031 AU away from its host star. The immense exposure to UV solar radiation for this hot Jupiter is capable of photodissociating all CO2 into CO, exactly alike the case in
Venus’s upper atmosphere. Its young age of just 60,000 also implies that the more reactive CO might not have reacted yet with sulphuric compounds to form the less reactive CO2. On the other hand, HD 189733b has a surface temperature of roughly 1100K, which is 1.5 times hotter than Venus. Thermodynamics shows that
CO formation is favoured over CO2 in such high temperatures because nature always chooses reactions that minimize Gibbs free energy, in other words, maximize entropy. (See Figure 9.)

suggest that approximately 20% of total atmospheric carbon should be borne by methane and the remainder primarily by CO. CO2 does not participate in any carbon bearing in this regime. Although the models of chemical equilibrium are successful in ruling out CO2’s contribution to the 4.5 band feature, the data of HD 189733b’s 4.5 night side flux from IRAC, Spitzer was found to be 3.2 smaller than the models’ prediction. This discrepancy, however, could be resolved by similar disequilibrium models like vertical mixing, which should lead to an enhanced absorption at 4.5 by CO.
To sum up the discussion so far, the atmosphere of
HD 189733b is believed to be CO-rich rather than CO2rich. But if we were to judge solely on the
4.5 transmission band, the debate remains open until a finer spectrum between 4 and 5 could be achieved. With current instruments and precision of
Spitzer/IRAC’s data, the 4.5 band is too wide that it encompasses both CO’s 4.5 and CO2’s 4.4 band features. Another controversy arises from the footprints of sodium (Na), an alkali metal, in the transmission spectrum. There were multiple reports of strong lines of sodium absorption in the visible wavelength region, however, some other observations returned a featureless spectrum, which is thought to be caused by thick atmospheric hazes.

The claim of carbon bearing of CO at the 4.5 transmission band was further consolidated by photometric analysis conducted by Knutson et al.
(2012). The approach used was to compare the day side and night side fluxes with models of chemical equilibrium, proposed by Williams et al. (2006). For planets in the temperature regime of HD 189733b and a hydrogen-dominated atmosphere, the models

Redfield et al. (2008) were the first ever to detect sodium absorption due to the atmosphere of an exoplanet with a ground-based technique – the High
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 10 (top): Spectrum of HD 189733 near the Na I doublet with normalized flux plotted against wavelength in angstroms Å.
Figure 10 (bottom): Plot of difference of the relative flux of the intransit and out-of-transit observations against wavelength of light detected in angstroms Å. An observation is considered in-transit if the entire exposure is obtained within first and fourth contact. (See
Figure 6.) Red line is the contribution from limb darkening of transits, which has been removed from the data. The right hand axis gives the effective planetary radius as a function of the wavelength of light detected. Plot credit to Redfield et al. (2008).

Figure 11: Plot of the usual planetary to stellar radii ratio (left hand axis) and the absorption scale heights (right hand axis) of the atmosphere of HD 189733b against wavelength of light detected in angstroms Å. Square data points from 5600Å red-ward are measurements from Pont et al. (2008), using the Advanced Camera for Surveys (ACS) on HST. Circle data points blue-ward of 5600Å are the STIS G430L measurements. A haze-free model atmosphere for
HD 189733b from Fortney et al. (2010) is also displayed (lilac spectrum) to address the discrepancy and lack of evidence of sodium absorption. Plot credit to Sing et al. (2011).

Resolution Spectrograph (HRS) on the 9.2 metre
Hobby-Eberly Telescope (HET). Figure 10 shows that the absorption of the Na I doublet was found to be %−67.2 ± 20.7& × 100T deeper than the adjacent bands, which is more than 3 away from the expected value of a flat featureless spectrum. This measurement of strong Na I absorption argues against the presence of clouds and haze high in the atmosphere of HD
189733b. Sing et al. (2011) combined his results with those of Pont et al. (2008) and claimed that a featureless slope, coupled with a lack of the expected
Na I doublet absorption in a wider spectrum, indicate optically thick atmospheric haze at high altitudes. (See
Figure 11.)

Figure 12: Same plot as Figure 11 but new data points from the
G750M band of the STIS on HST were added for comparison. Plot credit to Huitson et al. (2012).

After reviewing the data and evidence each side presented, it is believed that the atmosphere of HD
189733b at lower altitudes does consist of sodium, however, the Na I doublet absorption was narrowed and weakened due to high-altitude haze.

averaged over some 11 in-transit and 25 out-of-transit visits spread over a year, in order to minimize the random error.

Firstly, I hold strong faith in the presence of sodium in the atmosphere of HD 189733b because the planet’s close-in orbit implies that meteor impacts take place at a fairly high rate, which is analogous to
Mercury and its atmosphere. (cf. “Formation &
Evolution”) Recall that Mercury in our Solar System has a high sodium content (29%) in its atmosphere due to vapourisation of surface rock during events of meteor impacts, the hot Jupiter is probably in the same situation as Mercury is.

On top of that, given the weakness of any atmospheric absorption in a transmission spectrum, it is very common that the absorption footprints and features of a certain atmospheric species be hindered due to multiple factors and systematic errors, for example, atmospheric haze, limb darkening, and transits over active stellar regions. These systematic contributions are further amplified for this hot Jupiter because HD 189733 is a moderately active star. As a result, during most of the time, very little or no information about the observation target could be obtained, for example, in this case, the featureless spectrum presented by Sing et al. (2011). It has always been a challenge for the astronomy community to take these constraints into account and to improve on the precision and accuracy of observations. Redfield et al.
(2008) were well alert of any potential systematic errors and constructed an empirical “Monte Carlo”

Secondly, the HRS on HET is one of the finest ground-based telescope with a resolving power of >
60,000 whereas the STIS on HST (G750M band) reaches a resolving power of only = 5,000 . The result processing and analysis conducted by Redfield et al. (2008) were also exceptionally rigorous, in terms of error and uncertainty treatment. The results were
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 13: Pressure-temperature profile for HD 189733b. The two continua in black solid and dotted lines from a reference pressure of 100= V W upwards are believed to be due to high altitude haze, trapping heat in the thermosphere. Lower altitude regions with red dashed lines consist of the CO, CO2, and sodium layers. The pressuretemperature profile in these regions could tell us which temperature regime to use in our models when we have to determine the atmospheric composition of a particular planet, such as the CO and
CO2 controversy aforementioned. Plot credit to Huitson et al. (2012).

analysis, which is to randomly select test samples from the transit visits, run through the entire data analysis algorithm, and measure any systematic deviation.
Regardless of how evident the presence of sodium is in the atmosphere of HD 189733b, its presence does not rule out the possibility of the atmosphere being both sodium-rich and haze-filled. It was later found out by Huitson et al. (2013) that the atmosphere of HD 189733b is indeed sodiumabundant at lower altitudes while haze obscures and narrows the Na absorption at higher altitudes. Figure
12 shows the reason why Sing et al. (2011) did not observe any Na absorption – their data points were too widely binned (~500Å). Huitson et al. (2013) obtained observations that are of much narrower bin widths from the G750M band of the STIS on HST and confirmed a narrow singlet Na I feature. Also presented in their paper, Huitson et al. (2013) used the case of HD 189733b as reference and attempted to derive the pressure-temperature profile of any typical hot Jupiter. (See Figure 13.)

Figure 14 (top): Plot (absorption spectrum) of absorption depth in percentage against wavelength in microns. Observations from
Spitzer’s IRAC and HST’s NICMOS were plotted with error bars and compared with the simulated transmission spectra of GJ 436b, including the sole contribution of CH4, NH3, and water respectively.
Figure 14 (bottom): Plot of the same axes but the observations were compared with the simulated transmission spectra, including the sole contribution of carbon monoxide and carbon dioxide respectively. Plot credit to Beaulieu et al. (2011).

of its mass, radius, and orbital parameters. The planet’s close-in orbit also suggests that the planet has undergone a migration in the past from the outer regions towards the inner disk of the system. This, in turn, makes GJ 436b an unusual but interesting object to investigate because it displays characteristics of terrestrial planets (rocky contents), gas giants (a hydrogen-dominated atmosphere), and ice giants
(presence of carbon bearing molecules in the atmosphere). In a hydrogen-dominated atmosphere, the abundance of the two main carbon bearing species, carbon monoxide and methane, is governed by the chemical reaction:
CO + 3H ⇌ CH= + H O
(21)
where the favoured direction of reaction depends on the local temperature. At a temperature below 1000K and assuming a thermochemical equilibrium, the forward reaction rate is much faster than the backward reaction rate. Therefore, in both of the ice giants in our
Solar System, Uranus and Neptune, one would expect plenty of CH4 but traces of CO. Extending this to the hot
Neptune, GJ 436b, whose surface temperature is around 700K, one might also sensibly predict a methane-rich atmosphere, just like Beaulieu et al.
(2011) did.

AN EXAMPLE OF HOT NEPTUNE – GJ 436b
Thanks to the new Kepler Space Telescope (KST), transiting exoplanets that are much smaller in radius and mass become discoverable. Exoplanets that orbit
M-type stars are even easier to detect because high transit depths can be achieved, bringing us closer to the terrestrial and Earth-like regime. Gliese 436b, also known as GJ 436b, is amongst these smallest known transiting exoplanets that possess an atmosphere, too.
After being discovered by Butler et al. (2004), GJ
436b was reported to have diverse constituents, such as hydrogen, water vapour and carbon bearing species.
It was later confirmed by Figueira et al. (2009) that the hot Neptune is of a high rock content with a hydrogenand helium-dominated envelope, given the constraints
9

Exoplanet Atmospheres – H. Y. D. Yuen

Figure 15 (left): Normalized emission spectrum (normalized flux against orbital phase) of the secondary eclipses of GJ 436b at three of the Spitzer’s wavelengths, 3.6, 4.5, and 5.8 with 1 error bars.
Figure 15 (right): Emission spectrum of the same axes at the other three Spitzer’s wavelengths, 8.0, 16, and 24 with 1 error bars.
Plot credit to Stevenson et al. (2010).

Figure 16: Plot (absorption spectrum) of transit depths against wavelength. Red filled circles include previous published transit depths by Ballard et al. (2010), Alonso et al. (2008), Pont et al.
(2008), and Cáceres et al. (2009), along with the corrected data from
Beaulieu et al. (2011) at 3.6, 4.5, and 8.0 . Open circles are the statistically mishandled data by Beaulieu et al. (2011). Simulated transmission spectra that include a methane-rich atmosphere (blue), a CO-rich atmosphere (green), and a CO-rich atmosphere with an opaque cloud deck (grey) were also drawn for comparison. Coloured green, blue, and grey circles are the expected band-averaged values for these simulated models. Plot credit to Knutson et al. (2011).

Beaulieu et al. (2011) obtained the primary transit observations of GJ 436b from the IRAC on
Spitzer and combined theirs with Pont et al.’s (2009) data from the Near Infrared Camera and Multi-Object
Spectrometer (NICMOS) on HST. The full data set was then compared with the simulated transmission spectra, including sole contribution from methane, water, ammonia, carbon monoxide, and carbon dioxide respectively. In Figure 14, the data show consistent behaviour only with the spectrum of methane. In contrast with the conclusion drawn by Beaulieu et al.
(2011), Stevenson et al. (2010) analysed the emission spectrum of the secondary orbit of GJ 436b instead of the absorption spectrum of its primary orbit from the same measuring instrument (the IRAC on Spitzer) but arrived at conflicting findings, presented in Figure 15.
Methane absorbs strongly in the 3.6 band while carbon monoxide absorbs strongly in the 4.5 band, as aforementioned in the first controversy of HD
189733b. The presence of trough at 3.6 suggests very low absorption due to methane whereas the absence of decreased observed flux in the 4.5 band implies a large population of CO in the atmosphere.

IRAC on Spitzer tend to fluctuate a lot in general.
Despite the relatively old (~10 billion years old) and quiet nature of its parent star, the fluctuations could be due to occultation of star spots or other regions of nonuniform brightness on the star’s surface during the transit visits. Secondary eclipses are usually less prone to these problems because limb darkening and other potential systematic uncertainties could be ignored.
However, since secondary transit is the event when the planet hides itself behind its parent star, the observed intensity will be much weaker than that from a primary transit. The percentage contribution from random and systematic errors in secondary transit data, hence, will be much larger. In echo with the secondary transit data from Stevenson et al. (2010),
Knutson et al. (2011) also presented their own primary transit data from IRAC on Spitzer, in order to draw conclusions with a high confidence level.

Although there is qualitative agreement of the relative 3.6 to 4.5 flux ratio, the contradicting conclusions remain unresolved until Knutson et al.
(2011) re-analysed the data that is evident of a methane-rich atmosphere. Beaulieu et al. (2011) excluded the shallower 3.6 transit and kept the deeper visit in their analysis due to degenerate correction for the intra-pixel effect. However, Knutson et al. (2011) found that there is actually good overlap between the x and y positions spanned by the intransit and out-of-transit data and, hence, the shallower visit is not unreliable. Re-plotted and corrected data, thus, yields a better fit with a simulated transmission spectrum that includes a CH4-poor and
CO-rich atmosphere. (See Figure 16.)

The other problem we then have to deal with is where all these carbon monoxide come from and where the methane goes. It is obvious that the assumptions of the previous chemical reaction that coverts CO to CH4 – equation (21) – no longer hold.
Debate continues as there exist multiple explanations for the measured enhancement of CO over CH4.
In their paper, Stevenson et al. (2010) not just presented their findings on a CO-rich atmosphere of GJ
436b, but also suggested possible solutions to explain the awkward atmospheric composition. Recall the two assumptions for the chemical conversion of CO to CH4 to yield a faster forward reaction rate: the first assumption is that the gases are at a temperature below 1000K and the second is that a thermochemical equilibrium is maintained. At least one of these two assumptions have to be wrong in order to explain the

As it is also discussed extensively in Knutson et al.’s (2011) paper, the observations of GJ 436b by the
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Exoplanet Atmospheres – H. Y. D. Yuen

Figure 18: Transmission spectra of atmospheric models for a hot ocean-covered Kepler-62e (top), a cold ice-covered Kepler-62e
(middle), and Earth (bottom) for comparison. For details about the full atmospheric model for Earth, see Figure 2. Plot credit to
Kaltenegger et al. (2013).

Figure 17 (top): Mixing ratio profile for methane in the atmosphere of GJ 436b, for assumed atmospheric metallicities of 1 × (blue), 50 × (green), 1,000 × (orange), and 10,000 × (purple) solar.
Figure 17 (bottom): Mixing ratio profile for carbon monoxide in the atmosphere of GJ 436b, for same assumed atmospheric metallicities.
Plot credit to Moses et al. (2013).

atmosphere, which offers a much simpler explanation for the CO-dominated composition. As the metallicity of a hydrogen-dominated atmosphere is increased with its effective temperature, the overall hydrogen mole fraction, which includes both carbon-tohydrogen (C/H) and oxygen-to-hydrogen (O/H) mixing ratios, is decreased. Atmospheric species like
CO and CO2 that do not contain hydrogen become increasingly favoured over hydrogen-containing species like H2O and CH4, resulting in a progressively smaller CH4 to CO ratio in the atmosphere. (See Figure
17.) In fact, back in our own Solar System, high metallicity atmospheres are no surprises for Neptunemass planets. Neptune’s atmosphere, for example, possesses a C/H ratio of 40 − 120 × solar and an O/H ratio of > 400 × solar. Therefore, it is highly likely that the atmosphere of GJ 436b is metal-rich, too.

CO abundance. Secondary transit data suggest that the surface temperature of GJ 436b should be constrained to ~700]. Moreover, GJ 436b has a very short orbit of only 2 days and 15.5 hours and its rotation is likely misaligned with the star’s rotation. Therefore, it is safe to rule out any large temperature fluctuations on the day and night sides of the hot Neptune. (Note that the day and night sides of Mercury have a temperature difference of 600K!) The temperature assumption, hence, is fine but the equilibrium assumption would be in doubt. Stevenson et al. (2010) suggested two disequilibrium reactions that are allowed in the temperature regime of GJ 436b. One of the possibilities is vertical mixing. Because of the large binding energy of CO, the forward reaction proceeds much more slowly than expected when the temperature does go below 1000K. As vertical mixing dredges CO up from hotter parts to cooler parts of the atmosphere, CO does not convert to CH4 as quickly, resulting in a temporary over-abundance of CO. Alternatively, methane might be depleted by polymerization into chains of hydrocarbons, such as the oxidation coupling of methane to form ethene.
2CH= + O → C H= + 2H O
(22)

AN EXAMPLE OF SUPER-EARTH – KEPLER-62e
Kepler-62e is a super-Earth with a radius just 1.6 times that of Earth and stellar flux 1.2 times that of
Earth. Unlike the two previous exoplanets we have discussed about, the potentially habitable Kepler-62e has a 122 Earth-day orbit that lies in the inner part of the system’s habitable zone. With the planetary mass yet to be measured, the community is very excited about its atmosphere. The small planetary radius of
Kepler-62e implies that it should not have retained a primordial hydrogen-dominated atmosphere at its

Opposing to the need of exploiting extreme disequilibrium chemistry, Moses et al. (2013) has recently suggested that GJ 436b has a highly metallic
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Exoplanet Atmospheres – H. Y. D. Yuen

CONCLUSION
The field of exoplanatary science is rewarding yet daunting, in the sense that it consolidates the known knowns, finds answers to the known unknowns, and allows exploration of the unknown unknowns. The known knowns include our current understanding on general atmospheric physics and the Solar System atmospheres. The study of exoplanet atmospheres enable us to rigorously verify them. Meanwhile, there are still bits and pieces missing in our atmospheric model, for example, formation of atmospheres and some complicated disequilibrium chemistry concepts.
With access to a much wider pool of samples outside the Solar System, we look forward to obtain an answer for these known unknowns. The 1918 exoplanets discovered as of today is a collection of effort from astronomers all over the world. In this essay, we have discussed three examples of such and settled some of the controversies associated with these exoplanets.

Figure 19: Plot of absorption coefficients of the most common atmospheric absorbers (water vapour, oxygen molecule, ozone, and carbon dioxide) as a function of wavelengths. Dotted lines are the
Rayleigh scattering cross-sections for each species. For details about the ozone absorption spectrum, see Figure 3. Plot credit to
Ehrenreich et al. (2005).

estimated age of ~7 billion years. Also given the large amount of solid bodies already present in the inner orbits of the system, it strongly suggests that the super-Earth was formed outside of the ice-line and, thus, is now covered by a global ocean. Determining whether the super-Earth is ice-covered or oceancovered is critical because the resulting transmission spectrum will be completely different, as it is shown on
Figure 18. Until next time when Kepler-62e and its parent star align again with the Earth’s line-of-sight, some new data will be available and hopefully help resolve this issue.

The atmosphere of the hot Jupiter, HD 189733b, was found to be carbon-monoxide-rich because of strong absorption at the 4.5 band. Although contribution from carbon dioxide could be ruled out with low atmospheric metallicity, the resolution of the observations at 4.5 is not high enough for us to arrive at any robust conclusion. Sodium might also be a major constituent of the hot Jupiter’s atmosphere and the narrowing of sodium’s absorption hints that thick clouds or haze might be present at high altitudes as well. Turning to the hot Neptune, GJ 436b, both primary and secondary transit data support a carbonmonoxide-rich and methane-poor atmosphere.
Vertical mixing, polymerization of methane, and high atmospheric metallicity are all capable of explaining the over-abundance of CO. These possibilities mark the starting points for future theoretical work and modelling with this atmosphere. With the advent of the new exoplanet-hunting Kepler space telescope, our detection limit has stepped up to a whole new level – we are now able to detect much smaller Earth-like exoplanets that lie in the habitable zone of their respective systems. Kepler-62e is one of the recently discovered super-Earths and is probably covered by a global ocean. Collecting new data from the next transit will definitely give us more clues while exploiting atmospheric bio-signatures will practically help determine the planet’s habitability.

Extending to other super-Earths and Earth-like planets in general, the next possible step for us to do is to look for atmospheric bio-signatures and, more importantly, to investigate their habitability. Our
Earth’s most distinctive atmospheric bio-signatures are ozone (O3), nitrous oxide (N2O), oxygen (O2), and water vapour. Ozone is continually regenerated in the stratosphere by ultraviolet photo-dissociation of oxygen. The Hartley ( 200 − 300 & and Chappuis
( 375 − 650
) bands of O3 seem to be the best indicators of an Earth-like atmosphere. One might, however, doubt that the O3 bands might potentially be contaminated by O2 and H2O band signatures at similar wavelengths, as it could be seen on Figure 19.
However, both transitions of H2O and O2 are strong, narrow, and could be easily separated. Nitrous oxide in
Earth’s atmosphere is produced naturally in the soil by micro-organisms and is a good sign of life. However, its absorption bands in the infrared region are so weak that it could hardly be detected should there be any in exoplanet atmospheres. Oxygen in Earth’s atmosphere is produced in large quantities by plants and photosynthetic bacteria. Unluckily, at its transmission spectrum, bands of molecular oxygen are almost completely masked by its own Rayleigh scattering cross-section. (See dotted lines on Figure 19.) So there is no distinctive band that features O2’s presence.
However, since the presence of ozone indirectly indicates that of oxygen, future work in improving ozone detection could be value-adding.

As technology advances, we ought not just to expand the size of library of the exoplanets, but also to understand every single exoplanet better. A combination of transit photometry and transmission spectroscopy is probably inadequate to achieve this aim. However, detection method like direct imaging paints a much more promising picture for the future, allowing us to measure the true mass, orbital inclination, and albedo of the exoplanets. These observed properties can then be used to infer the chemical composition of both the planet’s surface and atmosphere. 12

Exoplanet Atmospheres – H. Y. D. Yuen
Ehrenreich, D. et al. (2005), “The Transmission Spectrum of Earth-size
Transiting Planets”, Astronomy and Astrophysics 448, P. 379 – 393
Figueira, P. et al. (2009), “Bulk composition of the transiting hot
Neptune around GJ 436”, Astronomy and Astrophysics 493, P. 671 –
676
Fortney, J. J. et al. (2009), “Transmission Spectra of Three-dimensional
Hot Jupiter Model Atmospheres”, Astrophysical Journal 709, P. 1396
– 1406
Huitson, C. M. et al. (2012), “Temperature Pressure Profile of the Hot
Jupiter HD 189733b from HST Sodium Observations: Detection of
Upper Atmospheric Heating”, Monthly Notices of the Royal
Astronomical Society (accepted)
Kaltenegger, L. et al. (2013), “Water Planets in the Habitable Zone:
Atmospheric Chemistry, Observable Features, and the Case of
Kepler-62e and -62f”, Astrophysical Journal 775, P. L47
Knutson, H. A. et al. (2011), “A Spitzer Transmission Spectrum for the
Exoplanet GJ 436b, Evidence for Stellar Variability, and Constraints on Day-side Flux Variations”, Astrophysical Journal (accepted)
Knutson, H. A. et al. (2012), “3.6 and 4.5 Phase Curves and Evidence for Non-equilibrium Chemistry in the Atmosphere of Extrasolar
Planet HD 189733b”, Astrophysical Journal (in press)
Kok, R. J. de et al. (2013), “Detection of Carbon Monoxide in the Highresolution Day-side Spectrum of the Exoplanet HD 189733b”,
Astronomy and Astrophysics (accepted)
Line, M. R. et al. (2014), “A Systematic Retrieval Analysis of Secondary
Eclipse Spectra II: A Uniform Analysis of Nine Planets and their C to
O Ratios”, Astrophysical Journal (accepted)
Moses, J. I. et al. (2013), “Compositional Diversity in the Atmospheres of
Hot Neptunes, with Application to GJ 436b”, Astrophysical Journal
(submitted)
Observatory of Paris, Extrasolar Planets Encyclopaedia “exoplanet.eu”
Palmgren, G. M. (2003), “Reducing metals as a brazing flux”, United
States Patent, US 6,575,353 B2
Pont, F. et al. (2013), “The Prevalence of Dust on the Exoplanet HD
189733b from Hubble and Spitzer Observations”, Monthly Notices of the Royal Astronomical Society (accepted)
Redfield, S. et al. (2007), “Sodium Absorption from the Exoplanetary
Atmosphere of HD 189733b Detected in the Optical Transmission
Spectrum”, Astrophysical Journal (accepted)
Saumon, D. et al. (2006), “Ammonia as a Tracer of Chemical Equilibrium in the T7.5 Dwarf Gliese 570D”, Astrophysical Journal 647, P. 552 –
557
Seager, S. & Mallen-Ornelas, G. (2003), “A Unique Solution of Planet and
Star Parameters from an Extrasolar Planet Transit Light-curve”,
Astrophysical Journal 585, P. 1038 – 1055
Sing, D. K. et al. (2011), “Hubble Space Telescope Transmission
Spectroscopy of the Exoplanet HD 189733b: High-altitude
Atmospheric Haze in the Optical and near-UV with STIS”, Monthly
Notices of the Royal Astronomical Society (accepted)
Sing, D. K. et al. (2012), “GTC OSIRIS Transiting Exoplanet Atmospheric
Survey: Detection of Sodium in XO-2b from Differential Long-Slit
Spectroscopy”, Monthly Notices of the Royal Astronomical Society
(accepted)
Stevenson, K. B. et al. (2010), “Possible Thermochemical Disequilibrium in the Atmosphere of the Exoplanet GJ 436b”, Nature 464, P. 1161 –
1164
Taylor, F. W. (2010), “Planetary Atmospheres”, Oxford University Press,
First Edition, ISBN: 978-0-19-954741-8
Williams, P. K. G. et al. (2006), “Resolving the Surfaces of Extra-solar
Planets with Secondary Eclipse Light Curves”, Astrophysical Journal
649, P. 1020 – 1027
Zahnle, K. et al. (2009), “Atmospheric Sulphur Photochemistry on Hot
Jupiters”, Astrophysical Journal 701, P. L20 – L24

EPILOGUE
“We are an impossibility in an impossible Universe.” –
Ray Bradbury
We, as we exist, are obviously not an impossibility but it is inarguable that the human race will face extinction one day. A short-term threat would be the depletion of resources on Earth while, as we project ahead, the Earth will face its fate of being engulfed by the Sun in its red giant phase. Will we be able to survive and prove Ray Bradbury’s statement wrong? The study of exoplanet atmospheres will hopefully pave us the way to the solution in the future.

ACKNOWLEDGEMENT
I would like to thank Dr. David Clements for his continual guidance and detailed feedback throughout.
Gratitude is also extended to Dr. Yvonne Unruh for her helpful comments during the viva presentation. This work is based on various techniques and observations with multiple space telescopes and ground-based telescopes. They include:
1.
2.
3.
4.
5.

Hobby-Eberly Telescope (HET), operated by McDonald
Observatory;
Hubble Space Telescope (HST), operated by NASA, ESA, and Space Telescope Science Institute (STScl);
Kepler Space Telescope (KST), operated by NASA and
Laboratory for Atmospheric and Space Physics (LASP);
Spitzer Space Telescope (SST), operated by NASA, Jet
Propulsion Laboratory (JPL), and Caltech; and
Very Large Telescope (VLT), operated by European
Southern Observatory (ESO).

REFERENCES
Andrews, D. G. (2010), “An Introduction to Atmospheric Physics”,
Cambridge University Press, Second Edition, ISBN: 978-0-52169318-9
Beaulieu, J. P. et al. (2011), “Methane in the Atmosphere of the
Transiting Hot Neptune GJ 436b”, Astrophysical Journal (in press)
Bouchy, F. et al. (2005), “ELODIE Metallicity-biased Search for Transiting
Hot Jupiters”, Astronomy and Astrophysics 444, P. L15 – L19
Brown, T. M. et al. (2000), “Hubble Space Telescope Time-series
Photometry of the Transiting Planet HD 209548”, Astrophysical
Journal 552, P. 699 – 709
Butler, R. P. et al. (2004), “A Neptune-mass Planet Orbiting the Nearby M
Dwarf GJ 436”, Astrophysical Journal 617, P. 580 – 588
Charbonneau, D. et al. (2002), “Detection of an Extrasolar Planet
Atmosphere”, Astrophysical Journal 568, P. 377 – 384
Désert, J. M. et al. (2009), “Search for Carbon Monoxide in the
Atmosphere of the Transiting Exoplanet HD 189733b”, Astrophysical
Journal 699, P. 478 – 485

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