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Register a new userA transit timing variation observed for the long-period extremely low density exoplanet HIP 41378f
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HIP 41378 f is a temperate $9.2pm0.1 R_{oplus}$ planet with period of 542.08 days and an extremely low density of $0.09pm0.02$ g cm$^{-3}$. It transits the bright star HIP 41378 (V=8.93), making it an exciting target for atmospheric characterization including transmission spectroscopy. HIP 41378 was monitored photometrically between the dates of 2019 November 19 and November 28. We detected a transit of HIP 41378 f with NGTS, just the third transit ever detected for this planet, which confirms the orbital period. This is also the first ground-based detection of a transit of HIP 41378 f. Additional ground-based photometry was also obtained and used to constrain the time of the transit. The transit was measured to occur 1.50 hours earlier than predicted. We use an analytic transit timing variation (TTV) model to show the observed TTV can be explained by interactions between HIP 41378 e and HIP 41378 f. Using our TTV model, we predict the epochs of future transits of HIP 41378 f, with derived transit centres of T$_{C,4} = 2459355.087^{+0.031}_{-0.022}$ (May 2021) and T$_{C,5} = 2459897.078^{+0.114}_{-0.060}$ (Nov 2022).
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The presence of rings around a transiting planet can cause its radius to be overestimated and lead to an underestimation of its density if the mass is known. We employ a Bayesian framework to show that the anomalously low density ($sim$0.09 g cm${^{-3}}$) of the transiting long-period planet HIP$,$41378$,f$ might be due to the presence of opaque circum-planetary rings. Given our adopted model priors and data from the K2 mission, we find the statistical evidence for the ringed planet scenario to be comparable to that of the planet-only scenario. The ringed planet solution suggests a larger planetary density of $sim$1.23$,$g$,$cm$^{-3}$ similar to Uranus. The associated ring extends from 1.05 to 2.59 times the planetary radius and is inclined away from the sky-plane by $sim$25$^mathrm{o}$. Future high-precision transit observations of HIP$,$41378$,f$ would be necessary to confirm/dismiss the presence of planetary rings.
Photometric follow-ups of transiting exoplanets (TEPs) may lead to discoveries of additional, less massive bodies in extrasolar systems. This is possible by detecting and then analysing variations in transit timing of transiting exoplanets. In 2009 we launched an international observing campaign, the aim of which is to detect and characterise signals of transit timing variation (TTV) in selected TEPs. The programme is realised by collecting data from 0.6--2.2-m telescopes spread worldwide at different longitudes. We present our observing strategy and summarise first results for WASP-3b with evidence for a 15 Earth-mass perturber in an outer 2:1 orbital resonance.
In 2015, K2 observations of the bright (V = 8.9, K = 7.7) star HIP 41378 revealed a rich system of at least five transiting exoplanets, ranging in size from super-Earths to gas giants. The 2015 K2 observations only spanned 74.8 days, and the outer three long-period planets in the system were only detected with a single transit, so their orbital periods and transit ephemerides could not be determined at that time. Here, we report on 50.8 days of new K2 observations of HIP 41378 from summer 2018. These data reveal additional transits of the long-period planets HIP 41378 d and HIP 41378 f, yielding a set of discrete possible orbital periods for these two planets. We identify the most probable orbital periods for these two planets using our knowledge of the planets transit durations, the host stars properties, the systems dynamics, and data from the ground-based HATNet, KELT, and WASP transit surveys. Targeted photometric follow-up during the most probable future transit times will be able to determine the planets orbital periods, and will enable future observations with facilities like the James Webb Space Telescope. The methods developed herein to determine the most probable orbital periods will be important for long-period planets detected by the Transiting Exoplanet Survey Satellite, where similar period ambiguities will frequently arise due to the telescopes survey strategy.
Transiting extrasolar planets are key objects in the study of the formation, migration, and evolution of planetary systems. In particular, the exploration of the atmospheres of giant planets, through transmission spectroscopy or direct imaging, has revealed a large diversity in their chemical composition and physical properties. Studying these giant planets allows one to test the global climate models that are used for the Earth and other solar system planets. However, these studies are mostly limited either to highly-irradiated transiting giant planets or directly-imaged giant planets at large separations. Here we report the physical characterisation of the planets in a bright multi-planetary system (HIP41378) in which the outer planet, HIP41378 f is a Saturn-sized planet (9.2 $pm$ 0.1 R$_oplus$) with an anomalously low density of 0.09 $pm$ 0.02 g cm$^{-3}$ that is not yet understood. Its equilibrium temperature is about 300 K. Therefore, it represents a planet with a mild temperature, in between the hot Jupiters and the colder giant planets of the Solar System. It opens a new window for atmospheric characterisation of giant exoplanets with a moderate irradiation, with the next-generation space telescopes such as JWST and ARIEL as well as the extremely-large ground-based telescopes. HIP41378 f is thus an important laboratory to understand the effect of the irradiation on the physical properties and chemical composition of the atmosphere of planets.
We observed Kepler-421 during the anticipated third transit of the snow-line exoplanet Kepler-421b in order to constrain the existence and extent of transit timing variations (TTVs). Previously, the Kepler Spacecraft only observed two transits of Kepler-421b leaving the planets transit ephemeris unconstrained. Our visible light, time-series observations from the 4.3-meter Discovery Channel Telescope were designed to capture pre-transit baseline and the partial transit of Kepler-421b barring significant TTVs. We use the light curves to assess the probabilities of various transit models using both the posterior odds ratio and the Bayesian Information Criterion (BIC) and find that a transit model with no TTVs is favored to 3.6-sigma confidence. These observations suggest that Kepler-421b is either alone in its system or is only experiencing minor dynamic interactions with an unseen companion. With the Kepler-421b ephemeris constrained, we calculate future transit times and discuss the opportunity to characterize the atmosphere of this cold, long-period exoplanet via transmission spectroscopy. Our investigation emphasizes the difficulties associated with observing long-period exoplanet transits and the consequences that arise from failing to refine transit ephemerides.
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