The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets (1711.05691v3)

Abstract: Transmission spectra are differential measurements that utilize stellar illumination to probe transiting exoplanet atmospheres. Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to starspots and faculae will be imprinted in the observed transmission spectrum. However, few constraints exist for the extent of photospheric heterogeneities in M dwarfs. Here, we model spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and the associated 0.3-5.5 $\mu$m stellar contamination spectra. We find that large ranges of spot and faculae covering fractions are consistent with observations and corrections assuming a linear relation between variability amplitude and covering fractions generally underestimate the stellar contamination. Using realistic estimates for spot and faculae covering fractions, we find stellar contamination can be more than $10 \times$ larger than transit depth changes expected for atmospheric features in rocky exoplanets. We also find that stellar spectral contamination can lead to systematic errors in radius and therefore the derived density of small planets. In the case of the TRAPPIST-1 system, we show that TRAPPIST-1's rotational variability is consistent with spot covering fractions $f_{spot} = 8^{+18}_{-7}\%$ and faculae covering fractions $f_{fac} = 54^{+16}_{-46}\%$. The associated stellar contamination signals alter transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1-15 $\times$ the strength of planetary features, significantly complicating $JWST$ follow-up observations of this system. Similarly, we find stellar contamination can lead to underestimates of bulk densities of the TRAPPIST-1 planets of $\Delta(\rho) = -3^{+3}_{-8} \%$, thus leading to overestimates of their volatile contents.

• The paper reveals that stellar photospheric heterogeneity induces spurious spectral features and erroneous exoplanet density estimations.
• It employs a forward modeling approach to assess the impact of starspots and faculae on the transmission spectra of transiting exoplanets, particularly in the TRAPPIST-1 system.
• The study warns that neglecting detailed stellar contamination models can lead to significant biases in the characterization of rocky exoplanet atmospheres.

Summary of "The Transit Light Source Effect" by Rackham et al.

This paper addresses the critical issue of false spectral features in transit spectroscopy and the consequential misestimation of exoplanet densities due to stellar heterogeneity, particularly in M dwarf stars. Such biases have emerged as significant challenges in the precise characterization of transiting exoplanets. The authors propose that stellar photospheric heterogeneity, encompassing starspots and faculae, can dramatically influence the observed transmission spectra of exoplanets. This influence not only leads to apparent absorption features that are not present in the planet's atmosphere but can also result in significant errors in the derived physical properties of the planet, such as radius and density.

The paper provides a forward modeling approach to understand the effect of these stellar heterogeneities. The model applied considers the impact of starspots and faculae on the contamination of observed spectra. Crucially, the authors find that correcting stellar contamination through simple models assuming linear correlations with observed variability underestimates the extent of contamination. When observing the TRAPPIST-1 system, a well-studied system with seven transiting exoplanets, the variability observed is consistent with spot covering fractions significantly larger than previously estimated using simplistic models.

Among the key numerical results, the authors highlight that stellar contamination can exceed the transit depth changes expected from atmospheric features by more than an order of magnitude. For instance, the stellar contamination signal might surpass $10 \times$ the expected atmospheric signal strength, especially for rocky exoplanets. This finding underscores the necessity for rigorous models to discern actual exoplanetary features from stellar-induced artifacts.

The implications of these results are manifold. Practically, the misinterpretation of spectral features due to stellar contamination can lead to flawed conclusions about atmospheric and bulk planetary compositions, affecting not only the paper of individual exoplanets but the broader power-law distributions derived from such samples. Theoretically, this research calls for a re-evaluation of planetary atmosphere modeling techniques and the assumptions underpinning those models, emphasizing the need for detailed stellar characterization.

In light of these findings, the paper suggests a potential reevaluation of exoplanet density distributions, particularly those derived from Kepler data or upcoming TESS detections in the face of significant stellar heterogeneity. The emphasis on the TRAPPIST-1 system serves as a sobering reminder that future observatories like the James Webb Space Telescope might confront the non-negligible contamination effects of stellar heterogeneity on small exoplanetary observations.

Speculating on further advancements, this work paves the way for future research to address the transit light source effect in high-precision spectroscopy strategically. A detailed understanding of these stellar heterogeneities would be pivotal to unraveling the genuine signatures of exoplanetary atmospheres. As exoplanet science progresses, this paper stresses the essential marriage of stellar astrophysics with exoplanetary science to ensure the reliability of atmospheric studies and the constraints they place on habitability and the potential for life beyond our solar system.