The 0.6-4.5𝜇m broadband transmission spectra of TRAPPIST-1 planetsE. Ducrot1, M. Sestovic2, B. M. Morris3, M. Gillon1, A. H. M. J. Triaud4, J. de Wit5, D. Thimmarayappa1, E. Agol3,

Y. Almleaky6,7, A. Burdanov1, A. J. Burgasser8, L. Delrez9, B-O. Demory2,10, E. Jehin1, J. Leconte11, J. McCormac12, C. Murray9, D. Queloz9, F. Selsis11, S. Thompson9, and V. Van Grootel1

Credit: NASA/JPL-Caltech

Abstract: The TRAPPIST-1 planetary system is an exceptional opportunity for the atmospheric characterization of temperate terrestrial exoplanets with the upcoming James Webb Space Telescope (JWST).Understanding the impact of stellar contamination on the planets' transit transmission spectra is an essential primordial step to this characterization. Planetary transits themselves can be used to scan the stellarphotosphere and to constrain its heterogeneity through transit depth variations in time and wavelength. In this context, we present here our analysis of 169 transits observed in the optical from space with K2 and fromthe ground with SPECULOOS (SSO) and Liverpool robotic telescopes. Combining our measured transit depths with literature results gathered in the mid-IR with Spitzer/IRAC and in the near-IR with HST/WFC3, weconstruct the broadband transmission spectra of the TRAPPIST-1 planets over the 0.6-4.5 𝜇m spectral range.

Context

The nearby (~12 pc) TRAPPIST-1 system is composed of an M8-type dwarf star orbited byseven nearly Earth-sized, temperate, planets [1]. Recent works outlined that an inhomogeneousstellar photosphere - as anticipated for red a dwarf like TRAPPIST-1 - could stronglycomplicate the information content of exoplanets' transmission spectra, limiting thedeciphering of their atmospheric properties [2, 3].

From TRAPPIST-1’s K2 variability, Rackham et al. [2] estimated TRAPPIST-1’s coverage to be8#$%&'% of cold spots and 54#+,%&,% of hot faculae, assuming Solar-type spots. Such a strongheterogeneous photosphere could alter the transit depth of the planets by up to 15 times thestrength of planetary features. More recently, Zhang et al. 2018 [4] analysed the near-IR dataobtained with HST/WFC3 for several TRAPPIST-1 planets, and fit their resulting transitspectra with the Rackham et al. [2] stellar contamination model. They concluded that the starshould be almost entirely covered by spots (∼30%) and faculae (∼63%) and predicted dramatic(a few dozens of %) chromatic variations of the transit depths, especially in the optical.

To verify those predictions, we analysed 169 transit light curves to construct the broadbandtransmission spectra of the TRAPPIST-1 planets. We confront these spectra with stellarcontamination models to assess the impact of the heterogeneity of the star's photosphere onthe atmospheric characterization of its planets.

Context

• 169 transits analysed, from observations made by K2, SPECULOOS (SSO) [5, 6] andLiverpool telescope (LT) [7] (see Table.1)

Table 1. Number of transitsobserved by K2, SSO, and LTanalysed in this work for eachTRAPPIST-1 planet

Planet K2 SPECULOOS 𝐋𝐓

TRAPPIST-1b 42 20 4

TRAPPIST-1c 29 11 5

TRAPPIST-1d 15 5 1

TRAPPIST-1e 8 8 2

TRAPPIST-1f 6 2 /

TRAPPIST-1g 3 3 /

TRAPPIST-1h 1 3 1

• We used different approaches in our data analysis toensure the robustness of our results

• First, we analysed each transit individually to extract theirindividual properties to, notably, search for signs ofvariability

• Then, we proceeded to a global analysis of all transit lightcurves for each planet to determine precisely the averagetransit depths in K2, SSO, and LT bandpasses

• Finally, we performed an additional global analysis, thistime enabling all transits to have different depths in orderto assess their variability

• Analyses were performed with an adaptive Markov ChainMonte-Carlo (MCMC) code [8]

Data analysis

The four domes of the SpeculoosSouthern Observatory (credit: E.Jehin)

Figure 2. Left: Period-folded photometric measurements obtained by K2 near the transits of the seven planets,corrected for the measured TTVs. Coloured dots show the unbinned measurements; open circles depict the 5minute-binned measurements for visual clarity. The best-fit transit models are shown as dark blue lines. The numbers oftransits that were observed to produce these combined curves are written on the plot. Right: Similarly but for SSO.

Changes in the transit depths measured for a planet in a given bandpass could result from theevolution of stellar heterogeneities on or outside the chord transited.Figure. 1 shows the evolution of the transit depths derived from our global analyses of K2and SSO light curves:• For all planets the depths are consistent from a transit to another, with no discrepancy

larger than 3𝜎.• The standard deviation is consistent with the mean of the measurements errors for most

of the planets/instruments associations. Exceptions for planet c (SSO) and planet d (K2),these mild discrepancies could be genuine, but could also originate from small-numberstatistics.

Figure. 2 shows the detrended period-folded photometry measured for each planet by K2 andSPECULOOS, as well as the corresponding best-fit transit model:• Measurements obtained in/out-of-transit do not reveal any transit shape asymmetry that

could be related to the crossing of spot/faculae.• Inspection of all individual transit light curves did not reveal crossing events neither.

Observations

[0.9 µm]

T1b 0.760 0.737

0.785

0.714

0.810

T1c 0.735 0.708

0.764

0.681

0.793

T1d 0.353 0.326

0.380

0.299

0.407

T1e 0.458 0.435

0.483

0.412

0.508

T1f 0.671 0.622

0.723

0.573

0.775

T1g 0.754 0.720

0.789

0.686

0.824

T1h 0.321 0.286

0.357

0.251

0.393

Epoch [arbitrary]

Tran

sit D

epth

[%]

. .

. .

[0.6 µm]

T1b 0.721 0.703 0.744

0.685

0.767

T1c 0.684 0.666

0.703

0.648

0.722

T1d 0.411 0.385

0.439

0.359

0.467

T1e 0.452 0.420

0.486

0.388

0.520

T1f 0.526 0.492 0.564

0.458

0.602

T1g 0.667 0.606 0.737

0.545

0.807

T1h 0.346

Epoch [arbitrary]

Tran

sit D

epth

[%]

. .

. .

Figure 1. Left: Evolution of the measured transit depths from the global analysis of transit light curves gathered by K2.Horizontal black lines show the medians of the global MCMC posterior probability distribution functions, PDFs, (withtheir 1 and 2𝜎 confidence intervals, in shades of grey), dotted lines show the medians of the global MCMC posteriorsPDFs for all transits of the same planet observed by Spitzer, as reported in [9]. Events are ranked in order of capture, leftto right (but not linearly in time). Right: Similarly, but for transit observed with SSO.

Temporal evolution of the transit depths

K2 SPECULOOS

T1b 0.721 0.716

0.726

T1c 0.696 0.690

0.702

T1d 0.359 0.352

0.365

T1e 0.475 0.467

0.484

T1f 0.627 0.618

0.637

T1g 0.760 0.750

0.771

T1h 0.332 0.320

0.344

1.0 2.0.5 5.

Wavelength [µm]

Tran

sit D

epth

[%]

. .

. .

Figure 3. Spectra of the seven TRAPPIST-1 planets. The continuous line is theweighted mean of all non-HST measurements for each planet (with its 1𝜎confidence, in shades of grey). Each point stands for the median of the globalMCMC posterior PDF with error bars at the effective wavelength of theinstrument (13 points (14 for T1b) per planet: one for K2, one for SSO, one for LT,9 for HST/WFC3 [10] and one (two for T1b, 3.6µm and 4,5 µm) for Spitzer [9].

&

On Figure 3. we construct the broadband 0.6-4.5 µm transit transmission spectra ofTRAPPIST-1 planets:• Our analysis reveals chromatic structures at the level of only 200-300ppm in the transit

transmission spectra of planets b, d, and f• Measurements obtained with the HST data are significantly deeper than those obtained at

other wavelengths for planets b and d. Given the large systematic effects affecting HSTspectrophotometry, an independent confirmation will be required.

• Transit depth measured for planet f at 0.6𝜇m (K2) is ∼3𝜎 shallower than the mean of theother measurements. Could be due to its low statistical significance, but also because of thedetrending of strong K2 systematic effects and significant stellar variability applied to thelight curve before its modelling

Transmission spectra of the TRAPPIST-1 planets

• A possible argument against a stellar contamination origin ofthe structure visible in the transit spectra of planets b, d, and f,is the absence of similar structures for planets with similartransit impact parameters, i.e. transiting nearly the same chordsof the stellar disk.

1.0 2.0.5 5.0.0

0.5

1.0

1.5

1.2 1.4 1.61.3

1.4

1.5

1.6

1.7

. .

. . Wavelength [µm]

Tran

sit D

epth

[%]

TRAPPIST−1 b+c

Discussion and conclusionFigure 4. confronts our results to the predictions made in Zhang et al. 2018 [4]:

• The dramatic drop of the transit depth in the visible is not observed. Prediction for K2 bandpass are discrepant by ∼ 20σ from the observations, ∼ 7σ for SSO, and ∼ 4σ for Spitzer.

Although our measurements do not confirm the conclusions of Zhang et al. 2018 [4], significant stellar contamination ofTRAPPIST-1 planets’ transmission spectra remains a possibility. The star’s photosphere is definitely heterogeneous, as its K2photometry shows a quasi-periodic variability of a couple % with a dominant period of 3.3d that is consistent with therotation of an evolving inhomogeneous photosphere (Luger et al. 2017) [11], or with the characteristic timescale betweenflares followed by spot brightening (Morris et al. 2018) [12].

The recent announcement of the delayed launch of JWST gives us the opportunity to investigate further the photosphericstructure of TRAPPIST-1 - notably through photometric monitoring at different wavelengths - and its impact on theplanets’ transmission spectra. Furthermore, the JWST delay offers more time for the development of new strategies tooptimally disentangle the stellar contamination from the planetary transmission effects.

Figure 4. Comparison of the stellar contamination spectrum inferred by [4] for TRAPPIST-1, b+ctransits at two different resolutions (continuous black line and grey line) with the K2, SSO and LTmeasurements presented in this work, and the Spitzer and HST/WFC3 presented in [9] and [10],respectively (red points). The green line represents the weighted mean of all measurements exceptHST Finally, the grey horizontal bars are the band-integrated value for Zhang et al. [4] model.

Affiliations:1Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Universite de Liège, Allée du 6 Aout 19C, B-4000 Liége, Belgium 2University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, CH-3012, Bern, Switzerland 3Astronomy Department, University of Washington, Seattle, WA 98195 USA4School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmimgham B15 2TT, UK 5Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

6Space and Astronomy Department, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saui Arabia7King Abdullah Centre for Crescent Observations and Astronomy (KACCOA), Makkah Clock, Saudia Arabia8Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA, 92093, USA 9Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0H3, UK10University of Copenhagen, Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum, DK-1350, Copenhagen, Denmark11Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, F-33615 Pessac, France 12Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL

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