Possible Case of Exoplanetary Rings Around HIP 41378 F

Possible Case of Exoplanetary Rings around HIP 41378 f

Babatunde Akinsanmi1,2,6, N.C. Santos1,2, J.P. Faria1, M. Oshagh1,3, S.C.C. Barros1, A. Santerne4, S. Charnoz5

Motivation ● Planetary rings are yet to be detected around exoplanets despite their prevalence around the giant planets and other rocky bodies of the solar system. However, the transit method provides a useful way to search for rings around them.

● Rings around a transiting planet can cause its radius (R) to appear larger which leads to an underestimation of its density (흆) if combined with the mass (M) since 흆 ∝ M/R3.

● We therefore investigate the possibility that the extremely low density of HIP 41378f can be due to the presence of planetary rings.

Afﬁliations: 1 Institute of Astrophysics and Space Science, Porto, Portugal. 2 Department of Physics and Astronomy, University of Porto, Portugal. 3 Institut für Astrophysik, Georg-August-Universität Göttingen, Germany. 4 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France. 5 Institut de Physique du Globe de Paris, France. 6 National Space Research and Development Agency, Airport Road, Abuja, Nigeria. Background

Ring signature in Light curve HIP 41378 f ● Rings around a transiting planet can cause ● Belongs to a unique class of planets referred to as light curve anomalies during start/end of “super-puffs”, owing to their extremely low transit making it a useful probe for the densities (<0.1g/cm3). presence of the rings. ● Observed in Campaigns C5 & C18 of the Kepler K2 mission with parameters[1]: Deeper transit of a ringed planet (Akinsanmi et al. 2018) ○ R = 9.2 ± 0.1 R⊕

○ M = 12 ± 3 M⊕ ○ 흆 = 0.09 g/cm3 ○ a = ∼1.4 AU ○ P = 542 days ● The large distance from the star is favorable for ring stability. Therefore, a good target to probe for rings, and explain observed low density.

Babatunde Akinsanmi: Possible case of exoplanetary rings around HIP 41378 f 2 Back to Title Method Models ● Planet-only model: parameters- radius, semi-major axis, inclination & limb darkening coeﬃcients.

● Ringed planet model: Ring is opaque. The model has extra 4 parameters to deﬁne rings ○ Inner and outer ring radii: Rin, Rout ○ Orientation angles: ir & θ from the sky-plane and orbital plane respectively.

Model Comparison

● Use Bayes factor BRP to compare the evidence between the ringed planet model (R) and planet-only model (P) to determine which scenario is most probable - given the K2 data. Planet-only

○ BRP > 3.2 signiﬁcantly favors presence of rings model P [2] Ringed model otherwise non-signiﬁcant or comparable if BRP ≈ 1. R

Babatunde Akinsanmi: Possible case of exoplanetary rings around HIP 41378 f 3 Back to Title Results & Discussion

● Comparing the evidence for both models, we obtain

BRP = 1.51 implying that the ringed model only provides comparable (but slightly higher) evidence to the planet-only model.

● Thus, the ringed model explains the data only slightly better than a planet-only model. However, it is interesting that it performs just as well despite being penalized for having 4 extra ring parameters and no obvious anomalies is seen during transit start/end.

● Both model ﬁts to the data are quite similar, indicating that any ring around this planet must mimic well a planet-only scenario (e.g by starting close to the planet surface).

● At longer observation wavelength, ring might be transparent and we might notice shallower transit corresponding to the smaller planet.

Babatunde Akinsanmi: Possible case of exoplanetary rings around HIP 41378 f 4 Back to Title Results & Discussion

● The ringed planet solution suggests a smaller planetary radius of 3.7R⊕ which increases its density to 1.2g/cm3 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 ~25o.

● The nature of this possible ring (opaque and close to planet surface) is what makes it difficult to distinguish between both models from the lightcurve

★ The full results of this work was published in Akinsanmi et al. 2020 [3]

Babatunde Akinsanmi: Possible case of exoplanetary rings around HIP 41378 f 5 Back to Title Conclusions

● A smaller planet with opaque rings provides a good ﬁt to the K2 light-curve of HIP 41378 f but we are only able to say that the ring hypothesis presents one plausible explanation for the inferred low density of the planet.

● Other phenomena can be responsible for the low density such as extended atmosphere or inﬂation from stellar irradiation. However, they do not seem as likely given the parameters of the planet.

● Further observations will be necessary to Credit: Tania Cunha / Planetário do Porto – confirm/characterise the ring scenario. Animate Centro Ciência Viva) / IA

Acknowledgements This work was supported by Fundação para a Ciência e a Tecnologia (FCT, Portugal)/MCTES by FEDER through COMPETE2020 - POCI by these grants: UID/FIS/04434/2019, PTDC/FIS-AST/28953/2017 & POCI-01-0145-FEDER-028953, and PTDC/FIS-AST/32113/2017 & POCI-01-0145-FEDER-032113. References 1. Santerne, A., Malavolta, L., Kosiarek, M. R., et al. 2019, arXiv e-prints,arXiv:1911.07355 2. Kass, R. E. & Raftery, A. E. 1995, Journal of the American Stat. Assoc., 90, 773. 3. Akinsanmi B., Santos N. C., Faria J. P.,et al. 2020, A&A, 635, L8

Babatunde Akinsanmi: Possible case of exoplanetary rings around HIP 41378 f 6 Back to Title