# ExoTETHyS: Tools for Exoplanetary Transits around host stars

Giuseppe Morello, Antonio Claret, Marine Martin-Lagarde, Christophe

Cossou, Angelos Tsiara, Pierre-Olivier Lagage

► To cite this version:

Giuseppe Morello, Antonio Claret, Marine Martin-Lagarde, Christophe Cossou, Angelos Tsiara, et al. ExoTETHyS: Tools for Exoplanetary Transits around host stars. Journal of Open Source Software, 2020, 5 (46), pp.1834. 10.21105/joss.01834. insu-03744536

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Distributed under a Creative Commons Attribution 4.0 International License
# ExoTETHyS: Tools for Exoplanetary Transits around host stars

Giuseppe Morello¹, ², ³, Antonio Claret⁴, ⁵, Marine Martin-Lagarde¹, Christophe Cossou⁶, Angelos Tsiara³, and Pierre-Olivier Lagage¹

1 AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France 2 INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy 3 Dept. of Physics & Astronomy, University College London, Gower Street, WC1E 6BT London, UK 4 Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain 5 Dept. Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva s/n, 10871, Granada, Spain 6 Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud, Université Paris-Saclay, bâtiment 121, Université Paris-Sud, 91405, Orsay Cedex, France

DOI: 10.21105/joss.01834

**Software**

- Review ⚙️

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- Repository ⚙️

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- Archive ⚙️

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Editor: Arfon Smith 👤

**Reviewers:**

- @steven-murray

- @williamjameshandley

Submitted: 08 October 2019

Published: 11 February 2020

**License**

Authors of papers retain copyright and release the work under a Creative Commons Attribution 4.0 International License (CC-BY).

**In partnership with**

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AMERICAN
ASTRONOMICAL
SOCIETY

This article and software are linked with research article DOI 10.3847/1538-3881/ab63dc, published in the The Astronomical Journal.

## Summary

ExoTETHyS is a Python2/3 package which aims to provide a stand-alone set of tools for modeling spectro-photometric observations of transiting exoplanets. The package is not a mere collection of pre-existing software, but contains algorithms with new features as well as enhancements of existing codes. The first release contains the following subpackages:

1. SAIL (Stellar Atmosphere Intensity Limb), i.e., a calculator of stellar limb-darkening coefficients that outperforms the existing software by one order of magnitude in terms of light-curve model accuracy, i.e., down to <10 parts per million (ppm);

2. TRIP (Transit Ring-Integrated Profile), which can compute an exact transit light-curve by direct integration of the occulted stellar flux from the model intensities, without using a parameterization (limb-darkening law) to approximate the stellar intensity profile. Following requests by users, we are adding a function to model eclipsing binaries.

Scientific details of the algorithms and a comprehensive comparison to other relevant codes can be found in a companion paper (Morello et al. (2019), accepted by AJ). Technical usage details can be found on GitHub.

ExoTETHyS is currently being used with synthetic data in order to assess the overall uncertainty in exoplanet spectra obtained with JWST and ARIEL, including the uncertainties in the measured stellar parameters and in the stellar models (e.g., Sarkar, Madhusudhan, & Papageorgiou (2020)). It is also being used for the analysis of real exoplanetary transits (e.g., Tsiaras, Waldmann, Tinetti, Tennyson, & Yurchenko (2019)). Another project is the empirical validation of stellar limb-darkening models through the analysis of high-precision photometric observations obtained, in particular, with Kepler, K2, TESS, CHEOPS and PLATO. We intend to include more stellar databases for this purpose. New tools are under development to estimate and correct for the exoplanet nightside pollution (significantly different treatment and formulas than discussed by Kipping & Tinetti (2010)) and the stellar activity effects. Future work will include rotational oblateness and gravity darkening, tidal deformations, the possible presence of exomoons and exorings.
## Acknowledgments

G. M., M. M.-L. and P.-O. L. were supported by the LabEx P2IO, the French ANR contract 05-BLANNT09-573739 and the European Unions Horizon 2020 Research and Innovation Programme, under Grant Agreement N 776403. G.M. also acknowledges the contribution from INAF through the “Progetto Premiale: A Way to Other Worlds” of the Italian Ministry of Education, University, and Research.

A.C. acknowledges financial support from the Spanish MEC (AYA2015-71718-R and ESP2017-87676-C5-2-R), the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

## References

* Kipping, D. M., & Tinetti, G. (2010). Nightside pollution of exoplanet transit depths. *MN-RAS*, 407(4), 2589–2598. doi:10.1111/j.1365-2966.2010.17094.x

* Morello, G., Claret, A., Martin-Lagarde, M., Cossou, C., Tsiaras, A., & Lagage, P.-O. (2019). The ExoTETHyS package: Tools for Exoplanetary Transits around Host Stars. *arXiv e-prints*, arXiv:1908.09599. Retrieved from http://arxiv.org/abs/1908.09599

* Sarkar, S., Madhusudhan, N., & Papageorgiou, A. (2020). JexoSim: a time-domain simulator of exoplanet transit spectroscopy with JWST. *MNRAS*, 491(1), 378–397. doi:10.1093/mnras/stz2958

* Tsiaras, A., Waldmann, I. P., Tinetti, G., Tennyson, J., & Yurchenko, S. N. (2019). Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b. *Nature Astronomy*, 3, 1086–1091. doi:10.1038/s41550-019-0878-9