Understanding Stellar Contamination in Exoplanet Transmission Spectra as an Essential Step in Small Planet Characterization

Apai, Dániel; Rackham, Benjamin V.; Giampapa, Mark S.; Angerhausen, Daniel; Teske, Johanna; Barstow, Joanna; Carone, Ludmila; Cegla, Heather; Domagal-Goldman, Shawn D.; Espinoza, Néstor; Giles, Helen; Gully-Santiago, Michael; Haywood, Raphaelle; Hu, Renyu; Jordan, Andres; Kreidberg, Laura; Line, Michael; Llama, Joe; López-Morales, Mercedes; Marley, Mark S.; de Wit, Julien

doi:10.48550/arxiv.1803.08708

Cited by 28 publications

(34 citation statements)

References 39 publications

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“…Before combining the transmission spectra from each night, we first considered the impact of stellar photospheric heterogeneity, which can have an observable effect on transmission spectra (Pont et al 2008(Pont et al , 2013Sing et al 2011;Oshagh et al 2014;Zhang et al 2018), even if magnetically active regions are not occulted by the transiting exoplanet (McCullough et al 2014;Rackham et al 2018;Rackham et al 2019;Apai et al 2018). Qualitatively, global variations in stellar activity could manifest themselves as an overall dimming or brightening of the star, which could lead to significant variations in transit depths.…”

Section: Stellar Activitymentioning

confidence: 99%

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K

Weaver¹,

López‐Morales²,

Espinoza³

et al. 2019

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We present a new ground-based visual transmission spectrum of the hot Jupiter WASP-43b, obtained as part of the ACCESS Survey. The spectrum was derived from four transits observed between 2015 and 2018, with combined wavelength coverage between 5,300 Å-9,000 Å and an average photometric precision of 708 ppm in 230 Å bins. We perform an atmospheric retrieval of our transmission spectrum combined with literature HST/WFC3 observations to search for the presence of clouds/hazes as well as Na, K, Hα, and H 2 O planetary absorption and stellar spot contamination over a combined spectral range of 5,318 Å-16,420 Å. We do not detect a statistically significant presence of Na I or K I alkali lines, or Hα in the atmosphere of WASP-43b. We find that the observed transmission spectrum can be best explained by a combination of heterogeneities on the photosphere of the host star and a clear planetary atmosphere with H 2 O. This model yields a log-evidence of 8.26 ± 0.42 higher than a flat (featureless) spectrum. In particular, the observations marginally favor the presence of large, lowcontrast spots over the four ACCESS transit epochs with an average covering fraction f het = 0.27 +0.42 −0.16 , and temperature contrast ∆T = 132 K ± 132 K. Within the planet's atmosphere, we recover a log H 2 O volume mixing ratio of −2.78 +1.38 −1.47 , which is consistent with previous H 2 O abundance determinations for this planet.

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Section: Stellar Activitymentioning

confidence: 99%

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K

Weaver¹,

López‐Morales²,

Espinoza³

et al. 2019

Self Cite

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“…A possible obstacle on the way for a proper and robust characterization of the atmospheres of the TRAPPIST-1 planets was presented by Apai et al (2018) and Rackham et al (2018), who argue that the photospheric heterogeneity of the host star, and specifically spots and faculae, could alter, hide, or even mimic planetary signatures in the transit transmission spectra. Taking into account this possible stellar contamination, and basing their re-analysis on existing HST near-IR data, Zhang et al (2018) predicted considerable changes of the transit depths of TRAPPIST-1 planets with wavelength.…”

mentioning

confidence: 99%

Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared

Burdanov

Lederer

Gillon

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The TRAPPIST-1 planetary system is a favorable target for the atmospheric characterization of temperate earth-sized exoplanets by means of transmission spectroscopy with the forthcoming James Webb Space Telescope (JWST). A possible obstacle to this technique could come from the photospheric heterogeneity of the host star that could affect planetary signatures in the transit transmission spectra. To constrain further this possibility, we gathered an extensive photometric data set of 25 TRAPPIST-1 transits observed in the near-IR J band (1.2 µm) with the UKIRT and the AAT, and in the NB2090 band (2.1 µm) with the VLT during the period 2015-2018. In our analysis of these data, we used a special strategy aiming to ensure uniformity in our measurements and robustness in our conclusions. We reach a photometric precision of ∼ 0.003 (RMS of the residuals), and we detect no significant temporal variations of transit depths of TRAPPIST-1 b, c, e, and g over the period of three years. The few transit depths measured for planets d and f hint towards some level of variability, but more measurements will be required for confirmation. Our depth measurements for planets b and c disagree with the stellar contamination spectra originating from the possible existence of bright spots of temperature 4500 K. We report updated transmission spectra for the six inner planets of the system which are globally flat for planets b and g and some structures are seen for planets c, d, e, and f.

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“…While the higher precision of JWST may help, we will be probing ever smaller signals and so the issue of stellar contamination of the transmission spectrum is likely to persist. Fortunately, there are various interdisciplinary efforts underway to study the impact of stellar contamination on exoplanet transmission spectra and develop tools and methods to mitigate its impact (Apai et al 2018;Rackham et al 2019a) 3 . New facilities and observations are also being developed with primary goals of quantifying and correcting for stellar contamination, which can be used to reevaluate all existing, and prepare for upcoming, exoplanet transmission spectroscopic observations.…”

Section: Discussionmentioning

confidence: 99%

Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum

Barclay,

Kostov,

Colón

et al. 2021

Preprint

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Transmission spectroscopy of transiting exoplanets is a proven technique that can yield information on the composition and structure of a planet's atmosphere. However, transmission spectra may be compromised by inhomogeneities in the stellar photosphere. The sub-Neptune-sized habitable zone planet K2-18 b has water absorption detected in its atmosphere using data from the Hubble Space Telescope (HST). Herein, we examine whether the reported planetary atmospheric signal seen from HST transmission spectroscopy of K2-18 b could instead be induced by time-varying star spots. We built a time-variable spectral model of K2-18 that is designed to match the variability amplitude seen in K2 photometric data, and used this model to simulate 1000 HST data-sets that follow the K2-18 b observation strategy. More than 1% of these provide a better fit to the data than the bestfitting exoplanet atmosphere model. After resampling our simulations to generate synthetic HST observations, we find that 40% of random draws would produce an atmospheric detection at a level at least as significant as that seen in the actual HST data of K2-18 b. This work illustrates that the inferred detection of an atmosphere on K2-18 b may alternatively be explained by stellar spectral contamination due to the inhomogeneous photosphere of K2-18. We do not rule out a detection of water in the planet's atmosphere, but provide a plausible alternative that should be considered, and conclude that more observations are needed to fully rule out stellar contamination.

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Understanding Stellar Contamination in Exoplanet Transmission Spectra as an Essential Step in Small Planet Characterization

Cited by 28 publications

References 39 publications

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K

Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared

Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum

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Understanding Stellar Contamination in Exoplanet Transmission Spectra as an Essential Step in Small Planet Characterization

Cited by 28 publications

References 39 publications

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H2O, but No Evidence of Na or K

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H2O, but No Evidence of Na or K

Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared

Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum

Contact Info

Product

Resources

About

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K

ACCESS: A Visual to Near-infrared Spectrum of the Hot Jupiter WASP-43b with Evidence of H₂O, but No Evidence of Na or K