Abstract

Space-based transit surveys such as TESS and Kepler have identified thousands of exoplanet candidates by detecting small decreases in stellar flux during planetary transits. However, many of these signals are false positives and are, in reality, eclipsing binaries, background stars, and stellar variability. Confirming the nature of these candidates requires high-precision ground-based follow-up to validate the transit signal and rule out false positives. Small observatories face significant challenges in achieving adequate photometric precision due to instrumental constraints, atmospheric variability, and reliability. This study assesses whether well-configured small-aperture ground-based observatories can produce robust transit light-curve analyses and basic atmospheric characterizations suitable for professional archives and exoplanet validation. High-precision transit photometry and low-resolution optical spectroscopy of the benchmark hot Jupiter HD 209458b were performed using two small-aperture telescopes equipped with modern CMOS detectors. Transit depth and timing were measured through differential photometry to evaluate precision and reliability. Low-resolution wide-field spectroscopy (R ≈ 200) was used to examine wavelength-dependent transit behavior. A transit depth of 17.89 parts per thousand (ppt) was measured, closely matching the predicted 17.8 ppt from TESS ephemerides. Spectroscopic analysis showed qualitative evidence of absorption in the Na I D region, consistent with previous atmospheric detections from larger observatories. The precision and stability of the photometric data demonstrate the ability to distinguish genuine planetary transits from common false positives. These results show that, with rigorous observational and data-reduction techniques, small observatories can produce reliable and reproducible exoplanet photometric and spectroscopic measurements. This work highlights the growing role of smaller research and educational facilities in confirming planetary candidates and reducing false positives in support of major space-based survey missions.

Introduction

In the past two decades, the discovery of thousands of exoplanets, planets outside of our solar system, has greatly improved our understanding of planetary system formation and evolution. Space-based transit surveys, such as NASA’s Transiting Exoplanet Survey Satellite (TESS) [1] and the Kepler Space Telescope [2], have advanced this field by detecting small decreases in stellar brightness during periods when bodies cross a star’s face, a phenomenon known as a transit. These missions deliver continuous photometry across large areas of the sky [3] and have identified thousands of exoplanet candidates, significantly expanding the catalog of known planetary systems. [4] These exoplanet candidates may have organized, regular orbits around a star and require further investigation to be classified.

Not all transit-like signals indicate genuine exoplanets. Many detections are later classified as false positives, including eclipsing binaries, blended stellar systems, and normal stellar variability. Confirming planetary status requires follow-up observations to refine orbit parameters, validate transit depths, and rule out alternative explanations. [5] The main limitation of this confirmation process is the limited access to large-aperture, high-resolution facilities required for detailed ground-based imaging.

Ground-based transit photometry has become a robust and standardized method over the past decade. [6] Techniques such as differential photometry, ensemble comparison-star selection, detrending, and calibration now allow small-aperture telescopes to achieve sub-millimagnitude precision under favorable conditions. Professional–amateur networks, including the American Association of Variable Star Observers (AAVSO) related groups, have established observing protocols [7] and submission requirements that enable well-equipped small observatories to contribute to the confirmation process. Submission requirements include 3 observations of a complete transit and baseline buffer of 30 minutes on either side, and image quality that allows for less than one minute of angular error versus expected transit. These efforts are valuable for monitoring transit timing variation (TTV), clarifying orbital patterns, and extending observational baselines for suspected planetary systems. Collectively, these data types are called ephemeridies.

Atmospheric characterization of these exoplanets, particularly via high-resolution transmission spectroscopy (HRTS), remains a focus of major professional institutions [8]. HRTS studies require large collection areas, stable throughput, and sufficient spectral resolving power to isolate narrow atomic and molecular absorption bands. Ground-based observations are challenged by interference from the Earth’s atmosphere. [9] While complete atmospheric retrieval requires telescopes with significant resolving power (~10,00) and larger apertures (greater than 10”), some broader transmission signatures can be studied at lower spectral resolution when acquisition and reduction procedures are carefully managed.

Expanding the community of capable observers by using commercially available small-aperture telescopes, rigorous acquisition standards, and robust reduction pipelines is a practical way to increase global follow-up capacity. Amateur-led and small-observatory programs have already contributed measurably [10] by refining transit ephemerides, validating candidate events, and providing rapid-response follow-up observations.

AAVSO provides quantitative reporting standards that define acceptable uncertainty thresholds for observations. [11] Essential factors for high-quality ground-based transit observations include standardized calibration frames (bias, dark, flat), accurate stellar magnitude selection, precise time stamping, correction for atmospheric interference, and seeing-induced systematics, and accurate stellar magnitude selection. Analysis requires appropriate comparison-star selection and rigorous statistical treatment of uncertainties. When these procedures are properly implemented, the effects of Earth’s atmosphere can be substantially mitigated [12], thereby enabling reliable transit-depth measurements and reproducible spectroscopic trends even with modest apertures and limited spectral resolution.

This study examines whether well-configured small-aperture ground-based observatories can produce reliable transit light curves and basic atmospheric measurements suitable for professional archives and exoplanet validation. By evaluating their ability to measure precise transit depths, refine orbital timing, and detect broad wavelength-dependent atmospheric features, this research explores how smaller research and educational observatories can help confirm planetary candidates, reduce false positives, and support large space-based survey missions.

Methodology

A. Observatory Site and Infrastructure

Observations were conducted on the rooftop observatory platform at Thomas Clement Hall (TCH) in Columbia, Maryland. This elevated site provided improved sky access and a clearer horizon than ground level, though it introduced challenges, including increased wind and heat-induced turbulence from the building. Both systems were mounted on harmonic-drive equatorial mounts for accurate tracking. Before each session, polar alignment was completed using plate-solving routines in the ZWO ASIAIR Plus control platform.

B. Target Selection and Observing Preparation

Targets were chosen based on predicted transit visibility, stellar brightness, and observation conditions from Columbia, Maryland. Transit events were verified with published ephemerides several days in advance to ensure full coverage of the observation window. Observations were confirmed at least 24 hours before each event to allow optical systems and detectors to reach thermal equilibrium. One hour before observations, initial system checks were performed. These included automated focus adjustment, image-scale verification, detector orientation confirmation, and guiding calibration. These procedures ensured the stability of both photometric and spectroscopic systems prior to data acquisition.

C. Environmental and Operational Conditions
Observations occurred in the fall (September to November), with nighttime temperatures in Columbia, Maryland, ranging from 2°C to 21°C (35°F to 70°F). Cameras were cooled by 10–20°C relative to ambient temperature using thermoelectric coolers. Dew-heater straps were used to prevent condensation during periods of high humidity. Typical seeing conditions ranged from 1.5 to 2.5 arcseconds. Wind conditions were evaluated based on the rooftop location, and observations were suspended if vibration exceeded the guiding tolerances. Targets were observed at low airmass when possible. Stellar full-width at half maximum (FWHM) values were continuously monitored to assess atmospheric stability and minimize drift during extended observations. Automated meridian flips maintained continuous tracking as targets crossed the local meridian. After each meridian transition, automated plate-solving, recentering, autofocus, and guiding recalibration ensured the consistency of photometric and spectroscopic data.

Image and spectroscopic data acquisition commenced approximately 30 minutes prior to the predicted first contact to establish a stable out-of-transit baseline. Observations continued throughout the entire transit and for at least 30 minutes following the fourth contact to ensure reliable normalization. Exposure times were optimized to maximize signal-to-noise ratio, prevent saturation, and maintain adequate cadence to resolve the ingress and egress phases. All exposures were taken at fixed intervals to maintain consistent timing. Instrument settings, including filter, gain, and binning, remained constant throughout each session to preserve dispersion geometry and minimize drift.

D. Observational Setup 1 – V-Band Transit Photometry

The photometric configuration utilized a 122 mm apochromatic refractor operating at f/5.6 with a focal reducer. Imaging was conducted with a Sony IMX585 monochrome CMOS detector. All transit observations employed a Baader V broadband photometric filter to ensure stable transmission and an appropriate signal-to-noise ratio. Autoguiding was implemented with a 50 mm guide scope and a non-cooled monochrome guide camera. Multi-star guiding algorithms maintain root-mean-square (RMS) tracking errors below 0.50 arcseconds under stable conditions.

E. Observational Setup 2 – Low-Resolution Optical Spectroscopy

The primary telescope was a 203 mm Ritchey–Chrétien at f/6 with a focal reducer, and the primary sensor was a Panasonic MN34230ALJ monochrome CMOS detector. A Sony IMX571 APS-C monochrome CMOS detector was used for supplementary imaging and calibration verification. Spectral dispersion was achieved with Star Analyser SA-100 and SA-200 transmission gratings configured for wide-field spectroscopy. The system’s resolving power was approximately 200 across the visible spectrum. Autoguiding used a 32 mm guide scope and a non-cooled monochrome guide camera, maintaining root-mean-square (RMS) tracking stability below 0.50 arcseconds.

F. Data Reduction and Quality Control

Raw images were visually inspected before software analysis. Frames with tracking errors, focus issues, satellite trails, cosmic-ray or saturation distortion, or pronounced background gradients were excluded. Calibration included dark subtraction and flat-field correction for the specific detector and filter.

Photometric data were processed with AstroImageJ. Differential photometry uses carefully chosen comparison stars in the same field of view. Comparison stars are selected for stable brightness and flux to reduce atmospheric extinction. Aperture sizes were set in the software based on measured FWHM to balance signal-to-noise ratio. Normalized differential light curves were generated to determine transit depth and mid-transit timing. Remaining trends were examined to reduce correlated noise and systematic drift.

Spectroscopic data were processed using RSpec software. Procedures included background subtraction, extraction of one-dimensional spectra along the dispersion axis, wavelength calibration using known stellar absorption features, and continuum normalization to correct for instrument variations. The analysis focused on comparing relative spectra rather than performing absolute flux calibration. Relative in-transit spectra were directly examined next to out-of-transit baseline spectra to assess deviations by wavelength. Absolute atmospheric retrieval modeling was not performed due to the low spectral resolution and the limitations imposed by telluric contamination. The analysis focused on repeatability, environmental monitoring, and the control of systematic errors, in accordance with the standards used in professional exoplanet observation programs.

Results 

1. Photometric Precision and Statistical Significance

With the precision of the transit established, the accuracy of the image data was assessed by calculating the root-mean-square (RMS) scatter of the post-fit residuals in the normalized light curve (Figure 2). With the application of the transit model, the residual at each time step (=r_i ) is defined as the difference between the observed light (F_obs,i ) and the model prediction F_model,i ):

The RMS of the residuals is calculated as:

This expression Σ r_i² denotes the squared residuals at each time step are summed and then divided by the total number of images ( yielding the average squared residual. Taking the square root returns the value in the original flux units. The RMS quantifies the typical scatter of data points around the best-fit transit model (Figure 2, dark blue line) and directly measures the noise in the light curve. For this observation, the measured residual RMS was . This value corresponds to 0.05807% of observations being distorted. The low RMS value indicates that systematic effects, such as variations in atmospheric transparency, guiding drift, flat-field imperfections, and detector noise, were effectively minimized.

Figure 2: Complete transit of HD 209458b with a stable comparison star light curve, based on single-image measurements acquired over four hours on 10/5/2025.

The combination of a transit depth of 17.89 ppt and an RMS of 0.58 ppt confirms that the detected signal is astrophysical in origin and that the ground-based observations accurately reproduced the known transit parameters of HD 209458b. For multi-hour ground-based broadband photometry, this level of precision demonstrates exceptional observational stability. The recovered transit depth and residual RMS are consistent with values reported from TESS photometry, within accepted error margins. This agreement demonstrates that the observations satisfy the photometric quality standards required for AAVSO reporting and NASA-compatible parameter validation.

Figure 3: Stellar profile of HD 209458b showing precise alignment, with HWHM = 4.52 px (1.75″)

C. Low-resolution optical spectroscopy

To investigate the atmosphere of HD 209458b low-resolution transmission spectroscopy (R ≈ 200) was performed to track changes in the star’s brightness. Various wavelengths were measured during the planet’s transit. The time-averaged spectrum was compiled separately for in-transit and out-of-transit intervals. Variations at specific wavelengths reveal whether the planet’s atmosphere absorbs light, which correlates with potential chemical compounds.

The raw two-dimensional spectral frame (Figure 4) was obtained directly from the detector. The horizontal axis represents the dispersion of light by wavelength, and the vertical axis displays the star’s profile before the extraction of the one-dimensional spectrum.

Figure 4: Raw 2D spectral frame of HD 209458b showing stellar spectra along the dispersion axis

D. Transmission Computation

The wavelength-dependent transit depth is:

where F_in (λ) the flux measured during transit, and F_out (λ) is the baseline flux measured outside of transit. Physically, the transit depth directly relates to the apparent planetary radius. If atmospheric opacity increases at a given wavelength, the effective planetary radius increases slightly, thereby increasing the transit depth. The spectrum was continuum-normalized using the out-of-transit baseline to suppress large-scale instrumental trends and emphasize small wavelength-dependent atmospheric variations.

E. Identified Spectral Features and Atmospheric Modeling

The spectrum shows the expected absorption features of the F8V host star. Intensity, expressed as normalized flux, is plotted as a function of wavelength, with prominent stellar absorption lines manifesting as downward dips in the continuum. Notable features include the Ca II H and K lines in the blue region, the CH G-band near 4300 Å, Fe I blends in the mid-optical range, and the Mg b triplet near 5170 Å, all of which align with their established positions. The overall continuum shape and consistent placement of these features confirm the accuracy of the wavelength calibration and the stability of spectral extraction achieved with RSpec.
Beyond stellar features, Figure 5 delineates wavelength regions pertinent to planetary atmospheric analysis, particularly the sodium (Na I) D doublet near 5890–5896 Å, the hydrogen Balmer line Hα at 6563 Å, and the potassium (K I) doublet near 7665–7699 Å. In this region, a measurable increase in flux relative to adjacent wavelength bins is observed, consistent with a wavelength-dependent transit response, indicating enhanced absorption near the Na I doublet. In to show that these broadband atmospheric features were successfully detected and qualitatively matched to the mathematically modeled transmission spectra in Figure 6. Demonstrating this capability supports the scientific value of small-aperture facilities for ground-based follow-up programs supported by amateur enthusiasts. Although the low-resolution spectra cannot resolve individual line cores, the broadband structure in the sodium region is consistent with theoretical expectations for hot-Jupiter atmospheres, where sodium is a primary optical absorber. This broad structure also enables a qualitative comparison with modeled atmospheric absorption. Consequently, Figure 5 supports the recovery of the stellar spectrum and provides visual evidence of valid atmospheric signatures in the transmission spectrum, rather than instrumental noise.

Figure 5: Continuum-normalized low-resolution (R ≈ 200) optical spectrum of HD 209458 showing major stellar features and red telluric absorption, confirming stable calibration and adequate signal-to-noise.

To evaluate the (Na I) D feature in the low-resolution data, a simplified atmospheric transmission model was developed in MATLAB to simulate how wavelength-dependent opacity affects the apparent planetary radius and transit depth. The model establishes that increased opacity at specific wavelengths raises the effective atmospheric altitude, resulting in a larger observed planetary radius, as described by the following relation:

Where R_p,0 is the base planetary radius, H is the atmospheric scale height, and κ(λ) is the wavelength-dependent atmospheric opacity. The MATLAB simulation (Figure 6) produced a broadened sodium absorption feature that aligns with results from low-resolution spectroscopy.

The simulated sodium enhancement qualitatively corresponds to the observed broadband increase near 5890–5896 Å, thereby supporting the interpretation of atmospheric sodium absorption.

Figure 6: Simulated low-resolution transmission spectrum of HD 209458b (4000–8000 Å) generated in MATLAB

F. Resolution Constraints and Conservative Interpretation

At a spectral resolution of R ≈ 200, wavelength bins are much broader than the intrinsic widths of atomic transitions. As a result, narrow spectral lines blend, appearing as broadened absorption features. The measured transit depth at each wavelength reflects an averaged atmospheric response over a finite interval Δλ, rather than a fully articulated atomic line profile.

The resolving power of any system is defined as:

With R≈200 and at λ=5890 Å the smallest wavelength separation we can distinguish is 30 Å. Since the Na I D doublet lines are separated by only 6 Å, they cannot be resolved individually. Instead, our measurement represents a blended feature averaged across a wavelength bin of roughly 30 Å. This instrumental smoothing reduces sensitivity to detailed line shapes and subtle velocity effects. However, a resolution of 200 does not prevent the detection of strong sources of atmospheric opacity. Broad absorbers, such as sodium in hot-Jupiter atmospheres, cause large enough changes in effective planetary radius to produce measurable increases in transit depth, even when individual line cores are unresolved. These signals are well-suited to identifying broadband atmospheric trends with sufficient signal-to-noise, making it appropriate and effective for ground-based transmission spectroscopy.

Discussion
HD 209458b is considered a benchmark in exoplanet science. It was the first exoplanet on which atmospheric sodium absorption was detected using the Hubble Space Telescope, establishing transmission spectroscopy as a key method for studying exoplanet atmospheres. The host star is bright and well characterized, and the planet’s orbital parameters, including period, inclination, and radius ratio, are precisely known from earlier photometric and spectroscopic studies. Because of these characteristics, HD 209458b represents an ideal target for testing observational techniques. The primary objective of this study was to evaluate whether a carefully calibrated small-aperture ground-based system operating at low spectral resolution could reproduce wavelength-dependent transit behavior consistent with professional space-based observations.

The combined photometric and spectroscopic results demonstrate that well-controlled ground-based observations using modest-aperture instruments can reliably reproduce the primary observational signatures of a benchmark planetary system. HD 209458b, recognized as one of the most extensively studied transiting exoplanets, serves as an optimal target for validating photometric precision, systematic control, and low-resolution spectroscopic sensitivity.

The measured transit depth of 17.89 parts per thousand closely aligns with the predicted value of 17.8 parts per thousand based on ExoFOP-TESS ephemerides and established system parameters. The minimal difference of 0.09 ppt, representing less than 0.5% of the total signal, confirms accurate timing, stable observing conditions, and a robust data reduction process.

The post-fit residual root-mean-square (RMS) scatter of 0.58 parts per thousand demonstrates sub-ppt photometric precision per exposure. This sustained precision over several hours indicates strong observational stability and suggests that atmospheric variations, guiding drift, detector noise, and calibration uncertainties were effectively minimized.

Image-quality measurements further corroborate these findings. The full width at half maximum (FWHM) of 4.49 pixels, equivalent to 1.75 arcseconds, is consistent with moderate atmospheric seeing and appropriate image sampling for precision photometry. Independent half-width measurements confirmed that the stellar point-spread function (PSF) remained symmetric and stable throughout the observation. The absence of elongation or time-dependent broadening indicates accurate guiding, stable focus, and minimal systematic drift.

The combination of low RMS scatter and PSF stability confirms that the light curve is dominated by genuine astrophysical variability rather than instrumental or atmospheric artifacts. The agreement with TESS-derived parameters demonstrates that these observations meet the accepted standards for ground-based reporting of the NASA TESS Follow-up Observing Program (TFOP) and the ExoFOP archives.

Low-resolution wide transmission spectroscopy (R ≈ 200) recovered several stellar absorption features characteristic of an F8V host star, including Ca II H and K, the Mg b triplet, and Fe I blends. The most significant atmospheric finding is the enhanced differential response observed in the sodium (Na I D) region near 5890–5896 Å. These narrow atomic absorption features are broadened and averaged across wavelength bins, due to the lower resolving power. Their presence still serves as internal consistency checks, confirming stable wavelength calibration and reliable spectral extraction.

High-resolution transmission spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) [16] modeled through MATLAB establish the benchmark optical atmospheric profile for HD 209458b. Space-based spectrograph findings reveal fully differentiated sodium absorption and evidence for an extended atmosphere. The present ground-based study employs lower spectral resolution, resulting in broad telluric contamination in the red optical region, particularly from water vapor and molecular oxygen, constrains interpretation. However, the qualitative agreement in the sodium region is consistent with expectations. The similarity between the observed spectrum and the expected band-averaged structure reinforces confidence that the measured differential response reflects genuine atmospheric opacity rather than instrumental effects. Consequently, these data are not suitable for detailed line-profile analysis, absolute atmospheric retrieval, or quantitative abundance determination, but rather for confirmation of broad atmospheric trends.

The photometric and spectroscopic consistency demonstrated in this study indicates that small-aperture ground-based systems, when operated with rigorous observational protocols, can generate scientifically valuable exoplanet follow-up data. Although these systems do not replace high-resolution spectrographs on large telescopes, they can validate transit timing, refine transit depths, and detect broad trends in atmospheric opacity in bright, well-characterized systems.

As the number of exoplanet candidates identified by space-based surveys such as TESS continues to increase, distributed networks of trained ground-based observers can significantly accelerate follow-up capacity. While incapable of replacing large-scale observatories, this study demonstrates that high-precision transit photometry combined with low-resolution spectroscopy serves as a viable and valuable complement to professional observatory programs, reproducing both the photometric depth and the qualitative atmospheric signature of HD 209458 b.

Conclusion

This study shows that carefully controlled ground-based observations using commercially available telescopes and detectors can successfully reproduce the key photometric and spectroscopic signatures of the benchmark hot Jupiter HD 209458b. Internal validation testing through statistical image analysis and MATLAB modeling showed the validity of small-aperture collection.

Image-quality analysis confirmed stable, well-sampled stellar imaging throughout the session, showcasing minimal systematic errors. The resulting light curve shows residual scatter consistent with expected astrophysical and atmospheric noise rather than instrumental instability, supporting the accuracy of the derived transit parameters. Low-resolution spectroscopy recovered the anticipated prominent stellar absorption features and identified a wavelength-dependent response in the sodium region, consistent with modeled Hubble Space Telescope observations. While the spectral resolution is insufficient for detailed atmospheric retrieval, the agreement with established measurements indicates that small-aperture ground-based systems remain sensitive to strong sources of atmospheric opacity in bright exoplanet systems.

Overall, these results highlight the valuable scientific contributions of small-aperture facilities when rigorous observing and data-reduction methods are used. They offer a practical, scalable approach for transit validation, orbital refinement, and preliminary atmospheric studies. In the continuing exoplanet discovery effort, well-executed ground-based research can complement professional observatory programs and enhance our understanding of planetary systems beyond the Solar System.

Acknowledgements

Thank you to Dr. James Kaiser for his mentorship throughout this process.

Contact: maryelizabeth.hommel@howardcc.edu, kdaizeguigure@howardcc.edu

References

• [1] Ricker, G. R., “Transiting Exoplanet Survey Satellite (TESS)”, <i>Journal of Astronomical Telescopes, Instruments, and Systems</i>, vol. 1, Art. no. 014003, 2015. doi:10.1117/1.JATIS.1.1.014003.
• [2] Borucki, W. J., “Kepler Planet-Detection Mission: Introduction and First Results”, <i>Science</i>, vol. 327, no. 5968, p. 977, 2010. doi:10.1126/science.1185402.
• [3] MIT TESS Science Office, “TESS Observations,” Massachusetts Institute of Technology, Mar. 2025. [Online]. Available: https://tess.mit.edu/observations/. [Accessed: Mar. 16, 2026]
• [4] Stassun, K. G., “The TESS Input Catalog and Candidate Target List”, <i>The Astronomical Journal</i>, vol. 156, no. 3, Art. no. 102, IOP, 2018. doi:10.3847/1538-3881/aad050.
• [5] NASA Exoplanet Archive, “Exoplanet Criteria for Inclusion in the Exoplanet Archive,” California Institute of Technology, Dec. 6, 2024. [Online]. Available: https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html. [Accessed: Mar. 16, 2026].
• [6] American Association of Variable Star Observers (AAVSO), “Exoplanet Section,” D. Conti, Ed. [Online]. Available: https://www.aavso.org/exoplanet-section. [Accessed: Mar. 16, 2026].
• [7] M. Conti, “A practical guide to exoplanet observation,” 2024. [Online]. Available: https://www.astrodennis.com/Guide.pdf. [Accessed: Feb. 5, 2026].
• [8] Exoplanet transit imaging and analysis process,” British Astronomical Association. [Online]. Available: https://britastro.org/section_information_/exoplanets-section-overview/exoplanet-transit-imaging-and-analysis-process. [Accessed: Feb. 5, 2026].
• [9] Stassun, K. G., “The Revised TESS Input Catalog and Candidate Target List”, The Astronomical Journal, vol. 158, no. 4, Art. no. 138, IOP, 2019. doi:10.3847/1538-3881/ab3467.
• [10] Kokori, A., “ExoClock Project. III. 450 New Exoplanet Ephemerides from Ground and Space Observations”, The Astrophysical Journal Supplement Series, vol. 265, no. 1, Art. no. 4, IOP, 2023. doi:10.3847/1538-4365/ac9da4.
• [11] Kane, S. R., Mahadevan, S., von Braun, K., Laughlin, G., and Ciardi, D. R., “Refining Exoplanet Ephemerides and Transit Observing Strategies”, Publications of the Astronomical Society of the Pacific, vol. 121, no. 886, IOP, p. 1386, 2009. doi:10.1086/648564.
• [12] Kimeswenger, S., Rainer, M., Przybilla, N., and Kausch, W., “Refined telluric absorption correction for low-resolution ground-based spectroscopy: Resolution and radial velocity effects in the O₂ A-band for exoplanets and K I emission lines,” J., vol. 161, no. 2, Art. no. 66, 2021, doi: 10.3847/1538-3881/abccd5.
• [13] Barman, “Identification of Absorption Features in an Extrasolar Planet Atmosphere,” The Astrophysical Journal, vol. 661, no. 2, p. L191, May 2007, doi: 10.1086/518736.
• [14] Vidal-Madjar et al., “An extended upper atmosphere around the extrasolar planet HD 209458b,” Nature, vol. 422, pp. 143–146, 2003.
• [15] K. Sing et al., “A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion,” Nature, vol. 529, pp. 59–62, 2016.
• [16] Deming et al., “Infrared transmission spectroscopy of the exoplanets HD 209458b and XO-1b using the Hubble Space Telescope,” The Astrophysical Journal, vol. 774, 95, 2013.