Exoplanet Spectroscopy and Photometry withthe Twinkle Space Telescope

Billy Edwards1, Malena Rice2, Tiziano Zingales1,3, MarcellTessenyi4,1, Ingo Waldmann1, Giovanna Tinetti1,4, Enzo Pascale5,6,

and Subhajit Sarkar6

1Department of Physics and Astronomy, University CollegeLondon, Gower Street, London WC1E 6BT, UK. E-mail:

billy.edwards.16@ucl.ac.uk2Yale University, Department of Astronomy, Steinbach Hall, New

Haven, CT 06511, USA3INAF, Osservatorio Astronomico di Palermo, 90134 Palermo,

Italy4Blue Skies Space Ltd., 69 Wilson Street, London, UK

5Dipartimento di Fisica, La Sapienza Universita di Roma, PiazzaleAldo Moro 2, 00185 Roma, Italy

6Cardiff University, School of Physics and Astronomy, QueensBuildings, The Parade, Cardiff, CF24 3AA, UK

Abstract

The Twinkle space telescope has been designed for the characterisationof exoplanets and Solar System objects. Operating in a low Earth, Sun-synchronous orbit, Twinkle is equipped with a 45 cm telescope and visible(0.4 - 1µm) and infrared (1.3 - 4.5µm) spectrometers which can be oper-ated simultaneously. Twinkle is a general observatory which will provideon-demand observations of a wide variety of targets within wavelengthranges that are currently not accessible using other space telescopes oraccessible only to oversubscribed observatories in the short-term future.

Here we explore the ability of Twinkle’s spectrometers to characterisethe currently-known exoplanets. We study the spectral resolution achiev-able by combining multiple observations for various planetary and stellartypes. We also simulate spectral retrievals for some well-known planets(HD 209458 b, GJ 3470 b and 55 Cnc e).

From the exoplanets known today, we find that with a single transitor eclipse, Twinkle could probe 89 planets at low spectral resolution (R<20) as well as 12 planets at higher resolution (R >20) in channel 1 (1.3- 4.5µm). With 10 observations, the atmospheres of 144 planets could becharacterised with R <20 and 81 at higher resolutions.

Upcoming surveys will reveal thousands of new exoplanets, many ofwhich will be located within Twinkle’s field of regard. TESS in particular

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is predicted to discover many targets around bright stars which will besuitable for follow-up observations. We include these anticipated planetsand find that the number of planets Twinkle could observe in the nearinfrared in a single transit or eclipse increases to 558 for R >20 and 41at lower resolutions. By stacking 10 transits or eclipses, there are 1185potential targets for study at R <20 as well as 388 planets at higherresolutions.

The majority of targets are found to be large gaseous planets althoughby stacking multiple observations smaller planets around bright stars (e.g.55 Cnc e) could be observed with Twinkle. Photometry and low resolu-tion spectroscopy with Twinkle will be useful to refine planetary, stellarand orbital parameters, monitor stellar activity through time and searchfor transit time and duration variations (TTVs and TDVs). Refinementof these parameters could be used to in the planning of observations withlarger space-based observatories such as JWST and ARIEL. For planetsorbiting very bright stars, Twinkle observations at higher spectral reso-lution will enable us to probe the chemical and thermal properties of anatmosphere. Simultaneous coverage across a wide wavelength range willreduce the degeneracies seen with Hubble and provide access to detectionsof a wide range molecules. There is the potential to revisit them manytimes over the mission lifetime to detect variations in cloud cover.

Acknowledgements

This research has made use of the NASA Exoplanet Archive, which is operatedby the California Institute of Technology, under contract with the NationalAeronautics and Space Administration under the Exoplanet Exploration Pro-gram. Additionally, the Open Exoplanet Catalogue, TEPCat and exoplanet.euhave been utilised as supplementary data sources.

This work has been funded through the ERC Consolidator grant ExoLights (GA617119) and the STFC grant ST/S002634/1.

1 Introduction

As of July 2018, over 3700 exoplanets have been discovered (nearly 3000 ofwhich transit their stars) as well as 2700 Kepler candidates yet to be confirmedas planets. On top of this, in the next few years Gaia is anticipated to discover upto ten thousand Jupiter-sized planets [56, 42] whilst TESS is expected to detectthousands of transiting planets of Earth size or larger [48, 58, 5]. Additionally,space surveys CHEOPS [9] and K2 [30], along with ground-based surveys likeNGTS [71], ESPRESSO [41], WASP [43], HATNet [3], HATSouth [4], MEarth[39], TRAPPIST [32], CARMENES [46], SPIROU [2] and HARPS [40], will leadto many more transiting exoplanet detections as well as further characterisationof planetary parameters.

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1.1 Characterisation of Exoplanet Atmospheres from Space

Whilst many planets have been detected and it is thought that planets are com-mon in our galaxy (e.g. [29, 6, 12, 18, 73]), our current knowledge of their at-mospheric, thermal and compositional characteristics is still very limited. Spacetelescopes such as Hubble and Spitzer, as well as some ground-based observa-tories, have provided constraints on these properties for a limited number oftargets and, in some cases, have identified the key molecules present in their at-mospheres whilst also detecting the presence of clouds and probing the thermalstructure (e.g. [10, 38, 57, 54, 21, 65, 76]). However, the breadth and quality ofcurrently available data is limited by the absence of a dedicated space-based ex-oplanet spectroscopy mission and therefore progress in this area has been slowerthan desired.

Spitzer is, along with Hubble, part of NASA’s Great Observatories Program.Launched in 2003, Spitzer carried an Infrared Array Camera (IRAC), an InfraredSpectrograph (IRS) and a Multiband Imaging Photometer (MIPS). The IRSwas split over four sub-modules with operational wavelengths of 5.3 - 40µm[28] and has not been operational since Spitzer’s helium coolant was depletedin 2009. Since the cool phase of Spitzer’s mission ended, only the IRAC hasremained operational, though with reduced capabilities. The Hubble WFC3currently delivers spectroscopic data at wavelengths shorter than 1.7µm. Thus,at the time of writing no space telescope capable of infrared spectroscopy beyond1.7µm is operational.

The James Webb Space Telescope (JWST) is expected to be launched in March2021. A Near-Infrared Spectrometer (NIRSpec) and camera (NIRCam) areincluded within the instrument suite [17] and thus will provide the infraredcapability that is currently missing (0.6 - 5.3µm and 0.6 - 5.0µm respectively).Additionally, the Mid-Infrared Instrument (MIRI) covers the wavelength range5 - 28µm and is capable of medium resolution spectroscopy whilst the Near-Infrared Imager and Slitless Spectrograph (NIRISS) will cover visible and nearinfrared wavelengths from 0.6 - 5.0µm [72]. JWST has the capability to view awide range of exoplanet targets [8, 26, 36]. Although a good fraction of JWSTobservation time is expected to be allocated for exoplanet science (e.g. [16, 7]),for a space observatory of this scale, over-subscription is likely to be an issueand not all interesting science cases will necessarily require the sensitivity andaccuracy of JWST.

Further into the future, the ESA ARIEL space mission will conduct a popu-lation survey of the atmospheres of ∼1000 transiting exoplanets over a widewavelength range (0.5 - 7.8µm) followed by a detailed study of a few hundredselected planets. ARIEL aims to deliver a comprehensive catalogue of plan-etary spectra, which will yield molecular abundances, chemical gradients andatmospheric structures [62, 44]. ARIEL is expected to be launched in 2028.

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1.2 Twinkle

The Twinkle Space Mission is a new, fast-track satellite designed for launch inearly 2022. It has been conceived for providing faster access to spectroscopicdata from exoplanet atmospheres and Solar System bodies, but it is also capableof providing spectra of brown dwarfs, bright stars, nebulae and galaxies. Twinkleis equipped with a visible (0.4 - 1µm) and infrared (1.3 - 4.5µm) spectrometer(split into two channels at 2.42µm). Twinkle has been designed with a telescopeaperture of 45cm and will operate in a low Earth, Sun-synchronous orbit [51, 31].

Twinkle is a general observatory which is being managed by Blue Skies SpaceLtd (BSSL). Scientists will be able to purchase telescope time and Twinkle willprovide on-demand observations of a wide variety of targets within wavelengthranges that are currently not accessible using other space telescopes or acces-sible only to oversubscribed observatories in the short-term future. Whilst itis thought that Twinkle will have considerable capabilities for observing SolarSystem Objects [19], the focus of this paper is to explore its spectrophotom-etry capabilities to observe a variety of exoplanets in the optical and infraredwavelength regimes.

The Exoplanet Light Visible Spectrometer (ELVIS) incorporated into the Twin-kle instrumentation is an adapted version of the Ultraviolet and Visible Spec-trometer (UVIS) flown on the ExoMars Trace Gas Orbiter. By modifying thegrating and its coatings, the optimum spectral range of the instrument will bechanged from 0.2 - 0.6µm to 0.4 - 1µm [51] with a resolving power R∼250.Twinkle’s infrared spectrometers will provide R∼250 for λ <2.42µm and R∼60for λ <2.42µm [70].

The light entering Twinkle’s telescope will be directed through three entranceslits as it is focused onto the instrumentation. Figure 1 shows the expectedangular sizes of Twinkle’s slits for each spectral band. Lenses produce severalspectra on the detector, with the spectrum from the star slit in the centre andwith three spectra from the background slits on either side [70]. The backgroundslits are utilised to provide spectra of the sky close to the star, permittingsubtraction of background light from the planet/star spectra.

Figure 1: Angular sizes of Twinkle’s star and background slits. Whilst slitsizes are each to scale, the separation between them is not

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For exoplanet observations the cadence will vary depending upon the brightnessof the host star. For bright targets, such as 55 Cnc, exposure times of ∼1 secondare required to avoid detector saturation whilst for fainter targets (e.g. GJ 1214)the cadence of Twinkle observations would be of the order of several minutes.

2 Methodology

Here we take the Twinkle design as discussed in [51], [31], and [70]. Twinkle iscurrently entering a Phase B design review and thus the technical specificationsmay change. An updated analysis will be published when the design is finalised.

2.1 Twinkle Field of Regard

Twinkle’s field of regard, the region of sky in which it can be pointed, is a cone,centred on the anti-sun vector and extending to ±40◦ from the ecliptic plane.This field may be extended to up to 60◦ for a limited number of targets. Twin-kle’s sky visibility is therefore similar to that of CHEOPS [13] but is in contrastto TESS [48], JWST [22] and ARIEL [62], all of which have continuous cover-age around the ecliptic poles and a partial visibility of the whole sky at lowerlatitudes. Here, we compare Twinkle’s 40◦ field of regard to the celestial coordi-nates of planet-hosting stars to determine the number of known and predictedtargets within Twinkle’s view.

Due to Twinkle’s low Earth orbit it will not always be possible to view the tran-sits (or eclipses) in their entirety; instead, partial transits (or eclipses) will oftenbe observed. For instance, Figure 2 shows modelled light curves for observationsof all observable transits of WASP-127b during the year 2021; this includes 12complete transits along with 6 partial transits. The radiometric model usedhere calculates the SNR achieved based upon observing a full transit or occul-tation. A follow-up paper will assess the impact of Earth obscuration and thesubsequent effect on the number of potentially observable targets. It should benoted that exoplanet observations with Hubble are similarly restricted by Earthobscuration and that these partial transits will also occur for observations withCHEOPS.

2.2 Observing techniques

Twinkle will enable the study of exoplanets simultaneously at multiple wave-lengths through transit, eclipse and phase-curve observations (see e.g. [61] foran overview of the information content of these techniques for atmospheric char-acterisation). During transit, stellar light can be observed passing through theterminator region of the planet (transmission spectroscopy). Similarly, when thestar eclipses the planet (i.e. the planet passes behind its host star in our line ofsight) the flux difference resulting from the planet’s dayside emission or reflec-tion can be measured (emission or reflection spectroscopy). Phase curves are

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Figure 2: Modelled observations of partial and full transits of WASP-127 bin channel 1 at R = 250 for 2021. Of the 18 available transits in 2021, 12 ofthem can be observed by Twinkle in their entirety. The assumed stellar andplanetary parameters are: RA = 160.56◦, Dec = -3.84◦, R∗ = 1.39 R�, T∗ =5650 K, Stellar K Mag = 8.64, RP = 15.03 R⊕, TP = 1533 K, Period = 4.18days

created by studying photometry of the star-planet system over a large portionof the planet’s orbit. Here, we consider transit and eclipse observations which,via spectroscopy, can allow for the atmospheric composition of the planet to beobtained.

Additionally, photometric or low resolution observations in the optical and in-frared can be utilised to refine planetary and orbital parameters, monitor stellaractivity over time (e.g. [11]) or search for transit time variations (TTVs, [1]) andtransit duration variations (TDVs, [33]). TTVs have been extensively used as avaluable technique for finding additional planets within systems and constrain-ing planetary masses and orbits (e.g. [27, 35, 23, 74, 34]). Combined TTV andTDV signals are paramount to search for exomoons [59]. The temporal binningrequired for Twinkle’s observations will depend upon the brightness of the hoststar but for brighter targets will be very short (<30 seconds) and therefore maybe utilised to provide precise measurements of TTV and TDV due to low massobjects present in some planetary systems. For fainter stars, a temporal binningof a few minutes may be required, depending on the wavelength range consid-ered. Whilst other future space facilities may also be able to detect TTV andTDV signals, Twinkle can obtain NIR light curves which exhibit highly reduceddistortion from limb darkening and stellar activity. Additionally, multi-colour

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Table 1: Assumed molecular weight by planetary classification

Planet Class Radius Bounds Mean Molecular Weight (a.m.u.)Earth Rp ≤ 1 R⊕ 18

Super-Earth 1 R⊕ <Rp ≤ 1.7 R⊕ 181.7 R⊕ <Rp ≤ 2 R⊕ 2.6

Sub-Neptune 2 R⊕ <Rp ≤ 3.5 R⊕ 2.6Neptune 3.5 R⊕ <Rp ≤ 6 R⊕ 2.6Jupiter Rp >6 R⊕ 2.3

light curves significantly attenuate degeneracy of fitted limb darkening param-eters across all wavelengths. Therefore Twinkle’s capabilities for both high andlow resolution transit (and eclipse) observations are considered here.

2.3 Acquisition of Exoplanet Data and Missing Informa-tion

Planetary data was downloaded from NASA’s Exoplanet Archive in order toaccount for all confirmed planets before being filtered such that only transitingplanets were considered. The database was last accessed on 31st July 2018.However, the major exoplanet catalogues are sometimes incomplete and thusan effort has been made here to combine them (for a review of the current stateof exoplanet catalogues see [15]).

Hence, the data was verified, and in some cases gaps filled, utilising the OpenExoplanet Catalogue [47], exoplanet.eu [52] and TEPCat [55]. If parametervalues differed between sources, an average was taken. Planets not included inthe NASA Exoplanet Archive were not added to the analysis to ensure that onlyconfirmed planets were utilised.

Unknown parameters were inferred based on the following assumptions:

• If the inclination is known, the impact parameter is calculated from:

b = acos(i)R∗

(1)

• Else, b = 0.5 (i.e. the midpoint of the equator and limb of the star)

• Planetary effective temperature (Tp) is estimated from:

Tp = T∗

(√1−AR∗

2aε

)1/2

(2)

where a greenhouse effect of ε = 0.8 and a planetary albedo of A = 0.3(TP < 700K) or A = 0.1 (Tp > 700K) are assumed [60, 53]

• Planetary mass (Mp) was estimated utilising Forecaster [14]

• Atmospheric molecular mass was assumed from planetary class as statedin Table 1

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2.4 Twinkle Radiometric Model and Assessing Target Suit-ability

The Twinkle Radiometric Model has been created using the methods describedfor calculating the signal and noise contributions for other exoplanet infraredspectroscopic missions in [45, 50]. The radiometric model simulates obser-vational and instrumentation effects, utilising target characteristics to assesswhether emission or transmission spectroscopy is preferable and to estimate therequired number of observations to achieve a desired resolving power and signalto noise ratio (SNR).

The downloaded target information was inserted into the model and the aver-age SNR across each spectrometer for one transit or eclipse was obtained forthe maximum resolving power. The resolving power can be decreased from itsnative value in order to increase the SNR per wavelength bin. Stacking multi-ple observations increases the SNR of the final dataset and for this exercise theincrease in SNR has been calculated from:

SNRN =√N × SNR1 (3)

where N is the number of observations.

2.5 Future Planet Discoveries

TESS and other surveys are predicted to discover thousands of planets aroundbright stars. In the first two years of operation, TESS is anticipated to detectover 4500 planets around bright stars and more than 10,000 giant planets aroundfainter stars [5]. Here, these predicted TESS planets around brighter stars areincorporated into the analysis to highlight Twinkle’s capabilities to study futurediscoveries. The MAST archive has been utilised to obtain stellar parametersfor the host stars of these predicted planets.

2.6 Spectral Retrievals

Determining the composition of exoplanetary atmospheres, and thus gaining anunderstanding of the atmospheric properties, provides insight about the pro-cesses occurring on these planets, as well as the presence or absence of clouds –an important constraint to understand atmospheric dynamics. Many moleculeshave absorption lines within Twinkle’s spectral bands including H2O, carbon-bearing molecules (CO2, CO, CH4, C2H2, C2H4, C2H6), exotic metallic com-pounds (TiO, VO, SiO, TiH) and nitrogen/sulphur-bearing species (H2S, SO2,NH3 and HCN).

We have performed several spectral retrieval simulations to assess the infor-mation content of the spectra obtainable through Twinkle observations. The

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known population of exoplanets is diverse and thus to better assess Twinkle’scapabilities we focus here on three well known, but very different, planets: HD209458 b, GJ 3470 b and 55 Cnc e. These planets all orbit very bright starswith K magnitudes of 6.31, 7.99 and 4.02 respectively.

To simulate emission and transmission forward models and atmospheric re-trievals we use the open-source exoplanet atmospheric retrieval framework Tau-REx [68, 69] https://github.com/ucl-exoplanets/TauREx_public). We as-sume plane parallel atmospheres with 100 layers and include the contributionsof collision-induced absorption (cia) of H2-H2 and H2-He, Rayleigh scatteringand grey-clouds. We use cross-section opacities calculated from the ExoMoldatabase [75] where available and from HITEMP [49] and HITRAN [25] other-wise. We have adopted Tau-REx to simulate retrievals at various resolutions.The assumed planetary characteristics are contained in Table 2. For HD 209458b we take the atmospheric composition retrieved from observations with Hub-ble WFC3 [64, 37]. We note that, whilst the spectral range for WFC3 is idealfor detecting water, constraining other molecules is more difficult with manymodels fitting the data. These degeneracies cause the retrieved abundances todiffer from those expected from chemistry models [66].

Table 2: Planetary characteristics utilised for simulating atmospheric retrievals

Planet EquilibriumTemperature [K] Clouds? Mean Molecular

Weight Molecular Abundances (Source)

55 Cnc e 2000 No 2.33COC2H2HCN

1x10−3

1x10−5

1x10−5[63]

GJ 3470 b 700 Yes 2.54

H20CH4CONH3CO2

1x10−2

4x10−3

1x10−3

1x10−4

1x10−5

[67]

HD 209458 b 1000 Yes 2.34

H20HCNNH3CH4

1x10−5

1x10−6

1x10−6

1x10−8

[64][37]

3 Results

3.1 Targets within Twinkle’s Field of Regard

NASA’s Exoplanet Archive contains 2974 known transiting planets (as of July2018) and Figure 3 displays the locations of these exoplanets and highlightsthose within the sweep of Twinkle’s field of regard. We note that the originalKepler field (which accounts for 2289 transiting planets) lies far from the eclipticand thus cannot be observed with a 40◦ field of regard centred on the anti-sun vector. An extension of the field of regard to 60◦ from the ecliptic mayallow some of these targets to be observed. However, Twinkle is best-suited forobserving planets around bright stars and thus is not impeded by being unableto observe planets in the Kepler field which are generally hosted by fainter stars.

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Of the 685 transiting planets not discovered by the original Kepler mission, 548lie within Twinkle’s field of regard.

The predicted TESS planets are shown in Figure 3 and we find that of the 4376targets plotted, 1815 lie within the 40◦ field of regard of Twinkle. TESS is alsopredicted to discover more than 10,000 planets around fainter stars and whilstsome of these will be within Twinkle’s field of regard, many are unlikely tobe suitable targets for observation with Twinkle. However, some larger, hotterplanets may be observable.

We require that all targets in our study must, as a minimum, have the followingknown parameters:

• Stellar data:

– Right ascension and declination– Mass– Radius– K Mag– 3070 K < T∗ < 7200 K

• Planet data:

– Radius– Period

Of the 548 currently known planets that Twinkle could observe, 530 were suf-ficiently characterised for inclusion in this analysis. For the simulated TESSplanets within Twinkle’s field of regard, 1608 could be analysed. Figure 4 showsstellar magnitudes that could currently be probed by Twinkle. Planets are foundto be most commonly orbiting stars of with K magnitudes between 9 and 12.The magnitudes of the simulated TESS target stars are also plotted here, andit is clear that many more planets are expected to be found around bright starswithin Twinkle’s field of regard.

3.2 Potentially Observable Targets

Defining a requirement for SNR ≥ 7, Figures 5, 6 and 7 display the numberof planets that could be observed in channels 0, 1 and 2, at a given resolution,for a given number of transit or eclipse observations. Assuming the same SNRrequirement, the resolution achieved over a given number of transits or eclipsesis displayed in Figures 8, 9 and 10 showing the distribution of the radius ofobservable planets along with the planet’s temperature class. The SNR require-ment will depend upon the user’s preferences and the observations undertakenand therefore should be taken only as an indicative value.

We find that in one observation Twinkle could study 89 known planets and 469predicted TESS planets in channel 1 with R < 20 and 12 known planets and

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29 predicted TESS planets at higher resolutions. With ten transits or eclipses,spectroscopy is possible for 81 known and 307 TESS planets at R>20, and for144 known and 1041 TESS planets at lower resolution. In each case the majorityof targets are large gaseous planets (e.g. 84.5% Jupiters and 15% Neptunes/Sub-Neptunes for R >20 in 10 observations). With a larger number of observations,the atmospheres of 46 smaller, potentially rocky planets (R<1.7R⊕) are char-acterisable using low resolution spectroscopy (R<20) in channel 1 with 20 ob-servations. The relation between stellar magnitude, planetary radius and theachievable resolving power is shown in Appendix A.

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Figure 3: Top: Currently-known transiting exoplanets, their planetary classi-fication and the sweep of Twinkle’s field of regard. The initial Kepler field isthe densely populated region at RA -70◦, Dec 45◦ and the locations of somewell-known exoplanets are noted. Middle: Predicted TESS planet detections.Bottom: Currently-known planets and predicted TESS detections

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4 6 8 10 12 14Star K Magnitude

0

100

200

300

400

500

600

700

Num

ber w

ith E

xopl

anet

s in

FOR

Known PlanetsPredicted TESS Planets

Figure 4: Stellar brightness in K mag for known planets and predicted TESSdetections within Twinkle’s field of regard

Figure 5: Number of known and predicted TESS planets with Twinkle’s fieldof regard for which SNR ≥ 7 is achievable at a given resolving power in channel0 (0.4 - 1.0µm)

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Figure 6: Number of known and predicted TESS planets with Twinkle’s fieldof regard for which SNR ≥ 7 is achievable at a given resolving power in channel1 (1.3 - 2.42µm)

Figure 7: Number of known and predicted TESS planets with Twinkle’s fieldof regard for which SNR ≥ 7 is achievable at a given resolving power in channel2 (2.42 - 4.5µm)

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Figure 8: Achievable resolving power in infrared channel 0 (0.4 - 1.0µm) forcurrently known (coloured) and expected TESS planets (grey) assuming a re-quirement of SNR ≥ 7 for a given number of transit or eclipse observations.The shape of the points indicates the stellar type of the host star and the greenline represents Twinkle’s maximum resolving power in the channel

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Figure 9: Achievable resolving power in infrared channel 1 (1.3 - 2.42µm)for currently known (coloured) and expected TESS planets (grey) assuming arequirement of SNR ≥ 7 for a given number of transit or eclipse observations.The shape of the points indicates the stellar type of the host star and the greenline represents Twinkle’s maximum resolving power in the channel

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Figure 10: Achievable resolving power in infrared channel 2 (2.42 - 4.5µm)for currently known (coloured) and expected TESS planets (grey) assuming arequirement of SNR ≥ 7 for a given number of transit or eclipse observations.The shape of the points indicates the stellar type of the host star and the greenline represents Twinkle’s maximum resolving power in the channel

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3.3 Spectral Retrievals

For the three chosen planets, we calculate the resolution achievable to reachan SNR > 7 for each channel with a given number of transits/eclipses and runTau-REx spectral retrievals with the observation parameters contained in Table3.

Table 3: Parameters of the simulated atmospheric retrievals with Tau-REx

Planet Number of Observations R (λ <2.42µm) R (λ >2.42µm) Observation TypeHD 209458 b 1 250 60 TransitGJ 3470 b 10 65 20 Transit55 Cnc e 10 10 20 Eclipse

HD 209458 b could be observed, in a single transit, at the highest resolutionpossible with Twinkle’s instrumentation and the subsequent retrieval correctlyidentifies the abundances and cloud pressure as shown in Figure 11 and Table4. Figure 12 displays the retrieval for GJ 3470 b and it can be seen thatafter 10 transits, at a resolution in channel 1 similar to the max resolutionof Hubble WFC3, the molecular abundance of H2O, CH4, NH3 and CO2 havebeen correctly recovered, even with a cloudy atmosphere (assuming a grey cloudwith a cloud top pressure of 10 mbar). However, as shown in Table 4 the COabundance is not recovered as the strongest CO band accessible to Twinkle isat 4.7µm which is at the edge of its observable wavelength range and oftentimesmasked by other molecular constituents.

Due to its small size, 55 Cnc e is challenging to observe and thus 10 eclipse ob-servations are required to obtain low resolution spectra. Despite this difficulty,Tau-REx retrievals of 55 Cnc e resulted in the main constituents of the simu-lated atmosphere (CO and HCN) being identified well (Figure 13 and Table 4).Increasing the number of visits to 20 results in the accurate recovery of the sim-ulated abundances of CO and HCN as well as more accurately retrieving C2H2.The retrievability of C2H2 highlights the benefit of higher resolution spectrawhen measuring abundances of complex carbon molecules. The posterior plotfor each of these retrievals is contained Appendix B.

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Table 4: The original and retrieved abundances for each planet from the sim-ulated retrievals with Tau-REx. We note that we correctly retrieve the abun-dances for HD 209458 b as well as most of the abundances for GJ 3470 b and55 Cnc e. However, the CO abundance is not recovered for GJ 3470 b and for55 Cnc e C2H2 is also not constrained. This is due to other molecules obscuringthe absorption features as can be seen in the contributions plots in Figures 12and 13

Planet Molecule Input Abundance (log10) Retrieved Abundance (log10) Retrieval Abundance Uncertainty (log10)

HD 209458 b

H20 -5.00 -5.02 +0.17-0.17

HCN -6.00 -6.09 +0.33-0.44

NH3 -6.00 -5.98 +0.14-0.14

CH4 -8.00 -9.61 +1.54-1.63

GJ 3460 b

H20 -2.00 -2.03 +0.36-0.51

CH4 -2.40 -2.45 +0.37-0.46

CO -3.00 -7.49 +3.39-3.01

NH3 -4.00 -4.15 +0.38-0.44

CO2 -5.00 -5.13 +0.44-0.52

55 Cnc e

CO -3.00 -3.10 +0.42-0.41

HCN -5.00 -5.02 +0.17-0.17

C2H2 -5.00 -7.47 +2.60-2.98

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Figure 11: Spectral retrieval of HD 209458 b (input values: PClouds = 1x104

Pa, H20 = 1x10−5, HCN = 1x10−6, NH3 = 1x10−6, CH4 = 1x10−8) at R = 250(λ < 2.42µm) and R = 60 (λ > 2.42µm) with 1 complete transit observationwhich recovers the main molecular composition but does not constrain CH4due to the very low abundance of the molecule. The bottom panel shows theindividual contributions of each molecule and of Rayleigh scattering, collisioninduced absorption (cia) and clouds. Note that, as shown in Figure 2, morethan one visit may be required to observe a full transit light curve and thatthis is the raw performance of the instrument and the plot may not show anoptimised setting

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Figure 12: Spectral retrieval for GJ 3470 b (input values: PClouds = 1x103 Pa,H20 = 1x10−2, CH4 = 4x10−3, CO = 1x10−3, NH3 = 1x10−4, CO2 = 1x10−5)at R = 65 (λ < 2.42µm) and R = 20 (λ > 2.42µm) with 10 complete tran-sit observations which correctly recovers the major molecular abundances andcloud pressure but does not detect CO. The bottom panel shows the individualcontributions of each molecule and of Rayleigh scattering, collision induced ab-sorption (cia) and clouds. Note that, as shown in Figure 2, more than ten visitsmay be required to observe ten full transit light curves

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Figure 13: Spectral retrieval for 55 Cnc e (input values: cloud free, CO =1x10−3 Pa, C2H2 = 1x10−5, HCN = 1x10−5) at R = 10 (λ < 2.42µm) and R =20 (λ > 2.42µm) with 10 complete eclipse observations which correctly recoversthe HCN and CO abundances but does not constrain the C2H2 abundance.The bottom panel shows the individual contributions of each molecule and ofRayleigh scattering, collision induced absorption (cia) and clouds. Note that,as shown in Figure 2, more than ten visits may be required to observe ten fulleclipse light curves

22

4 Discussion

This first iteration of assessing Twinkle’s performance for exoplanetary sciencehas shown that many planets are potentially observable with Twinkle. Twinkle iscurrently entering a Phase B design review and thus the technical specificationsmay change. An updated analysis will be published when the design is finalised.

We find that a large number of targets could be studied with multi-band pho-tometry (i.e. photons binned into a limited number of broad photometric bands)or low resolution spectroscopy in a single observation. Simultaneous photomet-ric measurements in the optical and infrared would allow for rigorous constraintson the planetary, stellar and orbital parameters of a system. Eclipse photom-etry in the visible and infrared provide the bulk temperature and albedo ofthe planet, thereby allowing an estimate of the planetary energy balance andinsight into whether the planet has an additional energy input, such as an inter-nal heat source. For the planets discovered by TESS and other transit surveys,constraining planetary ephemerides will be a key requirement to allow for spec-troscopic observations further into the future. The planets which have deepertransits could be observed from the ground but fainter and shallower transitswill require space-based facilities. Twinkle may also be able to detect TTVs andTDVs but this capability will depend upon the cadence of the observations aswell as the impact of Earth obscuration and therefore requires further analysison specific targets. The temporal binning for brighter targets will be very short(<30 seconds) whilst for fainter stars, a temporal binning of a few minutes maybe required, depending on the wavelength range considered. Such a cadenceis suitable for TTV and TDV analysis and Twinkle’s ability to obtain IR lightcurves will be useful for studying limb darkening and stellar activity which areexpected to exhibit highly reduced distortion over this spectral range comparedto visible wavelengths. Additionally, multi-colour light curves significantly at-tenuate degeneracy of fitted limb darkening parameters across all wavelengths.

Simulations shown here indicate that, for very bright targets, Twinkle will becapable of spectroscopy with resolutions R >20 and sufficient precision to probethe major species present in their atmospheres and constrain lower abundancemolecules. For some planets, such as HD 209458 b, this will be achievable in onetransit or eclipse whilst others may require between 10 to 20 observations. Theachievable resolution is plotted as a function of stellar k magnitude (Figures 14 -15, Appendix A) and show a cut-off limit of K mag∼12 for spectroscopy (R>20).Hence many Kepler planets orbit stars too faint for spectroscopic follow-up withTwinkle but TESS is expected to provide a multitude of suitable targets. Themajority of planets that can be observed spectroscopically with Twinkle are hot,giant planets or Neptunes although some smaller or cooler planets around verybright stars might be feasible.

By simultaneously providing spectroscopic data over a large wavelength range,Twinkle will be able to reduce the degeneracies that affect current observa-tions with Hubble and Spitzer. Whilst the spectral range of WFC3 is ideal fordetecting water vapour and characterising clouds, Twinkle’s wavelength rangecontains absorption features from a wide variety of molecules (e.g. H2O, CH4,

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CO2, CO, NH3, HCN, TiO, ViO). The instrument spectral resolving power al-lows in principle the detection of many trace gases, the only limiting factorbeing the precision achieved which depends upon the integration time (i.e. thenumber of transit or eclipse events). Twinkle’s observations will be demand-based and thus the requirements in terms of spectral resolution and SNR willdepend on the user’s preferences. The values here utilised for retrievals havebeen selected to provide an overview of the capabilities of the satellite and toallow a potential user to assess Twinkle’s suitability for the observations theydesire. From the spectral retrievals conducted here it is found that the majorconstituents of an atmosphere could potentially be recovered with low resolutionspectroscopy (R∼20). However, for weaker molecular transitions or to retrievetrace gas abundances more accurately, longer integration time may be required(e.g. Figure 13). In the case of Figure 13, the underlying issue may be thecross-section utilised for C2H2 which has been determined at Earth-like tem-peratures, not the ∼2000K assumed here for 55 Cnc e. This highlights the needfor accurate line lists over a wide temperature range.

The visible part of the spectrum can be utilised to measure the planetary albedo,Rayleigh scattering and detect/characterise clouds. Star spots and faculae mayaffect the observed transit depth at wavelengths shorter than 2µm and Twinkle’sspectral coverage should allow stellar activity to be monitored and to remove, ormitigate, its impact on the observations. For hotter planets, metallic resonanceline (e.g. Na and K) dominate the opacities over visible wavelengths [20]. Thespectral resolving power considered for Twinkle in the optical (R∼250) willallow for such detections on planets orbiting very bright stars. A wide spectralrange will also be advantageous in the search for condensates or hazes withmany species expected to condense in exoplanetary atmospheres, as suggestedby current observations [65, 54].

In the thermal regime, usually probed in the infrared through eclipse observa-tions, redundancy in molecular detection is also necessary to allow for the re-trieval of the vertical thermal structure and molecular abundances (e.g. [24, 61]).The capability to observe multiple absorption bands of the same molecule pro-vides some redundancy and significantly improves the reliability of a detection.Additionally, insights into the vertical distribution of species can be gained byobserving bands of different intensity which probe different atmospheric levels.

For a more refined list of targets and performances, several factors not studiedin this paper must also be accounted for. These are detailed in Sections 4.2 -4.4 below.

4.1 Complimentary with Other Facilities

Twinkle could be utilised to provide preliminary observations of targets for otherobservatories such as JWST and ARIEL, refining transit times and reducing therisk of missing transits due to poor ephemeris data. Given its capabilities, JWSTtime will be extremely precious [16], with over-subscription likely to be an issue,and thus any insight about the atmospheres of recently discovered planets will behighly advantageous to guide the selection of the very best targets. ARIEL will

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be launched in 2028 and aims at observing a very large population of exoplanetatmospheres [62]. A key decision for ARIEL is the selection of optimal anddiverse targets before its launch and Twinkle could be utilised to inform thesedecisions as well as provide initial insights into the mission’s science objectives.ARIEL will require a robust and efficient schedule to observe a large populationof exoplanets. Hence providing constraints on the planetary, stellar and orbitalparameters with Twinkle would enhance the mission’s scientific yield.

Additionally, Twinkle could enhance ground-based observations. Ground-basedsurveys are capable of extremely high resolution spectroscopy over narrow wave-bands, but the spectral continuum is unknown. By observing the same targetover a broader wavelength range from space, at a lower resolution, Twinkle willbe able to provide the missing, highly complementary information.

4.2 Earth Obscuration

The radiometric model used calculates the SNR achieved based upon observinga full transit or occultation. However, due to Twinkle’s low Earth orbit, itwill not always be possible to view the transits (or eclipses) in their entirety;instead, partial transits (or eclipses) will often be observed. Observing partialtransits/eclipses will reduce the SNR achieved for a given number of observationsand thus the number of transits (or eclipses) predicted by the radiometric modelis, in these cases, an underestimate of the number that will need to be observed.A subsequent paper will assess the impact of Earth obscuration and the effecton the number of potentially observable targets.

4.3 Scheduling

The observability of a target has been determined by assuming that a givennumber of complete transits or eclipses could be viewed during the missionlifetime. In addition to Earth obscuration, scheduling constraints, such as tele-scope housekeeping, slewing between targets and observations of other targets,will impact the number of transits or eclipses that are observable in a given timeperiod. Such constraints have not been included in this exercise and the devel-opment of an optimised schedule is dependent upon an improved understandingof the additional observations required due to Earth obscuration.

The expectation is that the main issue may be observation overlaps (i.e. twoplanets transiting at the same time) rather than an insufficient number of po-tential observations. Our analysis finds that, for currently-known planets withinTwinkle’s field of regard, around 85% of targets have at least 5 transits/eclipseswhich could be viewed in a year whilst over 60% of targets have 10 potential ob-servations or more per year. Therefore, during the mission lifetime, most planetswill have many transits and eclipses which could be observed. Further targetselection studies will occur in due course and will incorporate these constraints.

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4.4 Future Planet Discoveries

The main focus of this work is on Twinkle’s capability to observe currently-known planets whilst also considering the predicted TESS yield. Additionally toTESS, by the launch date, other space-based missions such as Gaia, CHEOPSand K2, as well as ground-based surveys including NGTS, ESPRESSO, andWASP, are expected to have discovered hundreds of new planets around brightstars within Twinkle’s field of regard. This will provide an expanded list ofplanets including many that are anticipated to be observable by Twinkle.

Predicted TESS detections were included in this study to provide an indicationof the number, and type, of planets projected to be discovered which will besuitable for observations with Twinkle. We find that discoveries from the TESSsurvey could more than double the number of exoplanet atmospheres that Twin-kle is capable of observing. As further planets are discovered they will be addedto this analysis to produce a comprehensive Twinkle target list.

5 Conclusion

In this study we present an initial survey of Twinkle’s capability for optical andinfrared observations of exoplanets. For the several hundred currently-knownplanets which lie within Twinkle’s field of regard and whose parameters areknown, we have estimated the spectral resolution which could be obtained fora given number of transit or eclipse events.

Within a single transit or eclipse observation, it is predicted that 82 existingtargets could be observed in at R <20 in channel 1 (1.3 - 2.42µm) whilst 68planets could be observed spectroscopically (R >20) with 10 transits or eclipses.The planets observable spectroscopically are found to be generally hot, withplanetary radii greater than 10 R⊕, and orbiting bright stars (K magnitude<11).

Spectral retrieval simulations of HD 209458 b, GJ 3470 b and 55 Cnc e highlightthe expected capability of Twinkle for atmospheric characterisation in case ofplanets around very bright stars. We find that most abundant molecular species,cloud and atmospheric key parameters can be retrieved reasonably well at thespectral resolution obtainable with Twinkle.

Future surveys will reveal thousands of new exoplanets, some of which will belocated within Twinkle’s field of regard. Analysis of the predicted detectionssuggests the number of exoplanets, and exoplanet atmospheres, Twinkle is capa-ble of characterising will dramatically increase from planets found with TESS.

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Appendix A: K Magnitude Limits

Figure 14: Achievable resolving power in the visible channel (0.4 - 1.0µm) forcurrently-known exoplanets and predicted TESS planets within Twinkle’s fieldof regard assuming a requirement of SNR ≥ 7 for a given number of transit oreclipse observations. The colour of the point indicates the planetary radius, theshape indicates the stellar type of the host star and the green line representsTwinkle’s maximum resolving power in the channel

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Figure 15: Achievable resolving power in the first infrared channel (1.3 -2.42µm) for currently-known exoplanets and predicted TESS planets withinTwinkle’s field of regard assuming a requirement of SNR ≥ 7 for a given numberof transit or eclipse observations. The colour of the point indicates the planetaryradius, the shape indicates the stellar type of the host star and the green linerepresents Twinkle’s maximum resolving power in the channel

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Figure 16: Achievable resolving power in the second infrared channel (2.42- 4.5µm) for currently-known exoplanets and predicted TESS planets withinTwinkle’s field of regard assuming a requirement of SNR ≥ 7 for a given numberof transit or eclipse observations. The colour of the point indicates the planetaryradius, the shape indicates the stellar type of the host star and the green linerepresents Twinkle’s maximum resolving power in the channel

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Appendix B: Retrieval Posteriors

Figure 17: Posteriors for spectral retrieval of HD 209458 b (PClouds = 1x104,H20 = 1x10−5, HCN = 1x10−6, NH3 = 1x10−6, CH4 = 1x10−8) at R = 250(λ < 2.42µm) and R = 60 (λ > 2.42µm) with 1 transit observation whichcorrectly recovers the major molecular abundances and cloud pressure but doesnot constrain CH4

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Figure 18: Posteriors for spectral retrieval for GJ 3470 b (PClouds = 1x103,H20 = 1x10−2, CH4 = 4x10−3, CO = 1x10−3, NH3 = 1x10−4, CO2 = 1x10−5)at R = 65 (λ < 2.42µm) and R = 20 (λ > 2.42µm) with 10 transit observationswhich correctly recovers the major molecular abundances and cloud pressurebut does not constrain CO

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Figure 19: Posteriors for spectral retrieval for 55 Cnc e (cloud free, CO =1x10−3, C2H2 = 1x10−5, HCN = 1x10−5) at R = 10 (λ < 2.42µm) and R =20 (λ > 2.42µm) with 10 eclipse observations which correctly recovers the HCNand CO abundances but does not obtain the correct C2H2 abundance

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