Precise Transit Photometric Studies of Exoplanets and Exomoons

Abstract

The most challenging limitation in the transit photometry method arises from the noises in the photometric signal. In particular, the ground-based telescopes are heavily affected by the noise due to the perturbation in Earth’s atmosphere. Use of telescopes with larger apertures can improve the photometric signal-to-noise ratio (S/N) to a great extent. However, detecting a transit signal out of a noisy light curve of the host star and precisely estimating the transit parameters call for various noise reduction techniques. In our first project, we have presented multi-band transit photometric follow-up studies of five hot-Jupiters e.g., HAT-P-30 b, HATP- 54 b, WASP-43 b, TrES-3 b and XO-2 N b, using the 2m Himalayan Chandra Telescope (HCT) at the Indian Astronomical Observatory, Hanle and the 1.3m J. C. Bhattacharya Telescope (JCBT) at the Vainu Bappu Observatory, Kavalur. In order to reduce the noise components present in the observational data, we have used a critical noise treatment approach using sophisticated techniques, such as the wavelet denoising and Gaussian process regression, which effectively reduce both time-correlated and time-uncorrelated noise components from the transit light curves. In addition to these techniques, use of our state-of-the-art model algorithm have allowed us to estimate the physical properties of the target exoplanets with a better accuracy and precision compared to the previous studies. Unlike the ground-based telescopes, the observations from the space-based telescopes are free from any noise component due to the interference of Earth’s atmosphere. This is the reason why most of the sophisticated telescopes used in exoplanetary science are space-based. However, the observations from these spacebased telescopes still contain noise components due to various instrumental effects and the stellar activity and pulsations. In our second project, we have presented the critical analysis of space-based transit photometric observations from the Transiting Exoplanet Survey Satellite (TESS). We have developed an optimized noise treatment and modeling algorithm based on the algorithm used in our previous project, which also implements the techniques like the wavelet denoising and Gaussian process regression. We have demonstrated the effectiveness of our algorithm by implementing it to the TESS transit photometric observations for four hot Jupiters: KELT-7 b, HAT-P-14 b, WASP-29 b, WASP-95 b, and a hot Neptune: WASP-156 b. The better quality of photometric data from TESS, combined with our state-of-the-art noise reduction and analysis technique, has resulted into much more accurate and precise values of the physical properties for the target exoplanets than that reported in earlier works. The effectiveness of the transit photometry method to detect and characterize exoplanets has already been demonstrated by the discovery of thousands of exoplanets using several ground-based as well as space-based survey missions. With the advent of the upcoming next generation large telescopes, the detection of exomoons in a few of these exoplanetary systems is very plausible. In our third project, we present a comprehensive analytical formalism in order to model the transit light curves for such moon hosting exoplanets. In order to achieve analytical formalism, we have considered circular orbit of the exomoon around the host planet, which is indeed the case for tidally locked moons. The formalism uses the radius and orbital properties of both the host planet and its moon as model parameters. The coalignment or non-coalignment of the orbits of the planet and the moon is parameterized using two angular parameters and thus can be used to model all the possible orbital alignments

for a star-planet-moon system. This formalism also provides unique and direct solutions to every possible star-planet-moon three circular body alignments. Using the formula derived, a few representative light curves are also presented. Rocky exomoons around the giant exoplanets in habitable zones hold special significance as they can harbor life. Although the detection of exomoons has yet remained elusive, mainly due to their smaller expected size, the next generation large telescopes can provide unique opportunities for their detection and characterization. In our fourth project, we have studied the capability of the large space based James Webb Space Telescope (JWST) to detect the smaller sub-Earth sized exomoons in the habitable zones of G- and K-type stars. We have consider three different sizes of the moon, i.e. similar in size to the mars, the titan and the luna, and estimated the minimum photometric precision required to detect them. By comparing them to the expected obtainable photometric precision using the NIRCAM instrument of JWST and using different near-infrared filters, we have concluded that exomoons as small as the titan would be detectable around a G2 type star and that as small as the luna would be detectable around a K2 type star.