The TOI-1117 Multi-planetary System: 3 Sub-Neptunes, 1 In Both The Neptunian Desert And Radius Valley

We present the discovery of three sub-Neptune planets around TOI-1117, a Sun-like star with mass 0.97±0.02M_⊙, radius 1.05±0.03R_⊙, age 4.42±1.50 Gyr and effective temperature 5635±62 K. Light curves from TESS and LCOGT show a transiting sub-Neptune with a 2.23 day period, mass M_b=8.90+0.95−0.96M_⊙ and radius R_b=2.46+0.13−0.12R_⊙.

This is a rare ‘hot Neptune’ that falls within the parameter spaces known as the ‘Neptunian Desert’ and the ‘Radius Valley’. Two more planetary signals are detected in HARPS radial velocities, revealing two non-transiting planets with minimum masses M_c=7.46+1.43−1.62M_⊙ and M_d=9.06+2.07−1.78M_⊙, and periods of 4.579±0.004 and 8.67±0.01 days. The eccentricities were poorly constrained by the HARPS data, with upper limits e_b=0.11, e_c=0.29, and ed=0.24. However, dynamical simulations of the TOI-1117 system, suggest that the orbits must be nearly circular to be stable.

The simulations also show that TOI-1117b and c are likely to be in a near 2:1 resonance. The multi-planet nature of TOI-1117 makes it a more complex case for formation theories of the Neptunian Desert and Radius Valley, as current theories such as high-eccentricity migration are too turbulent to produce a stable, non-eccentric, multi-planet system. Moreover, analysis of TOI-1117b’s photoevaporation history found rocky core and H/He atmosphere models to be inconsistent with observations, whilst water-rich scenarios were favoured.

Isobel S. Lockley, David J. Armstrong, Jorge Fernández Fernández, Sarah Millholland, Henrik Knierim, Elisa Delgado Mena, Sergio Sousa, Karen A. Collins, Cristilyn N. Watkins, Steve B. Howell, Vardan Adibekyan, Ravit Helled, Carl Ziegler, Daniel Bayliss, César Briceño, Amadeo Castro-González, Catherine A. Clark, Kevin I. Collins, Jessie L. Christiansen, Kaiming Cui, Rodrigo Diaz, Jon M. Jenkins, Marcelo A. F. Keniger, Michelle Kunimoto, Nicholas Law, Jorge Lillo-Box, Colin Littlefield, Andrew W. Mann, Morgan A. Mitchell, Louise D. Nielsen, José Rodrigues, Pam Rowden, Nuno C. Santos, Sara Seager, Peter J. Wheatley, Joshua Winn

Comments: To be published in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.05521 [astro-ph.EP] (or arXiv:2506.05521v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.05521
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From: Isobel Lockely Miss
[v1] Thu, 5 Jun 2025 19:06:08 UTC (2,358 KB)
https://arxiv.org/abs/2506.05521
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