First comparative exoplanetology within a transiting multi-planet system: Comparing the atmospheres of V1298 Tau b and c

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² School of Earth and Planetary Sciences (SEPS), National Institute of Science Education and Research (NISER), Jatani, India
³ Astrophysics Research Center (ARCO), The Open University of Israel, Ra’anana 43107, Israel
⁴ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 6-3 Aramaki aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁵ Department of Geophysics, Graduate School of Science, Tohoku University, 6-3 Aramaki aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁶ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
⁷ Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁸ Department of Astronomy, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁹ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁰ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA
¹¹ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
¹² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹³ Centre for Exoplanets and Habitability, University of Warwick, Coventry CV4 7AL, UK
¹⁴ Lowell Observatory, 1400 W Mars Hill Road, Flagstaff, AZ, 86001, USA
¹⁵ Center for Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA
¹⁶ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁸ Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^★ Corresponding author; s.barat@uva.nl, j.m.l.b.desert@uva.nl

Received: 15 June 2024
Accepted: 30 October 2024

Abstract

The V1298 Tau system is a multi-planet system that provides the opportunity to perform comparative exoplanetology between planets orbiting the same star. Because of its young age (20–30 Myr), this system also provides the opportunity to compare the planet’s early evolutionary properties, right after their formation. We present the first atmospheric comparison between two transiting exoplanets within the same multiple planet system: V1298 Tau b and V1298 Tau c. We observed one primary transit for each planet with the Hubble Space Telescope (HST), using Grism 141 (G141) of Wide Field Camera 3 (WFC3). We fit the spectroscopic light curves using state-of-the-art techniques to derive the transmission spectrum for planet c and adopted the transmission spectrum of planet b obtained with the same observing configuration and data analysis methods from previous studies. We measured the mass of planet b and c (8₋₂⁺⁴, 17₋₆⁺¹³ M_⊕ ; respectively) from the transmission spectrum and found the two planets to have masses in the Neptune or sub-Neptune regime. Using atmospheric retrievals, we measured and compared the atmospheric metallicities of planet b and c (logZ/Z_⊙=−2.04_−0.59^0.69, logZ/Z_⊙ = −0.16_−0.94^1.15, respectively), and found them to be consistent with the solar or sub-solar, which is low (at least one order of magnitude) compared to known mature Neptune and sub-Neptune planets. This discrepancy could be explained by ongoing early evolutionary mechanisms, which are expected to enrich the atmospheres of such young planets as they mature. Alternatively, the observed spectrum of planet c can be explained by atmospheric hazes, which is in contrast to planet b, where efficient haze formation can be ruled out. Higher haze formation efficiency in planet c could be due to differences in atmospheric composition, temperature and/or higher UV flux compared to planet b. In addition, planet c is likely to experience a higher fraction of mass loss compared to planet b, given its proximity to the host star.