Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna

Castro-González, A.; Bouchy, F.; Correia, A. C.M.; Sozzetti, A.; Lillo-Box, J.; Figueira, P.; Lavie, B.; Lovis, C.; Hobson, M. J.; Sousa, S. G.; Adibekyan, V.; Standing, M. R.; Hara, N. C.; Barrado, D.; Silva, A. M.; Bourrier, V.; Korth, J.; Santos, N. C.; Damasso, M.; Zapatero Osorio, M. R.; Rodrigues, J.; Alibert, Y.; Barros, S. C.C.; Cristiani, S.; Di Marcantonio, P.; González Hernández, J. I.; Lo Curto, G.; Martins, C. J.A.P.; Nunes, N. J.; Palle, E.; Pepe, F.; Suárez Mascareño, A.; Tabernero, H. M.

Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna

Mark

Castro-González, A. ; Bouchy, F. ; Correia, A. C.M. ; Sozzetti, A. ; Lillo-Box, J. ; Figueira, P. ; Lavie, B. ; Lovis, C. ; Hobson, M. J. and Sousa, S. G. , et al. (2025) In Astronomy and Astrophysics 699.

Abstract: Context. The formation and evolution of ultra-short-period (USP) rocky planets is poorly understood. However, it is widely thought that these planets could not have formed at their present-day close-in orbits, but instead migrated inwards through interactions with outer neighbours. Aims. We aim to confirm and characterise the USP Earth-sized validated planet K2-157 b (P_orb = 8.8 h) and constrain the presence of additional companions in the system through radial velocity (RV) measurements. Methods. We measured 49 RVs with the ESPRESSO spectrograph and tested different planetary and non-planetary configurations to infer the model that best represents our data set. We derived the orbital and physical properties of the system... (More); Context. The formation and evolution of ultra-short-period (USP) rocky planets is poorly understood. However, it is widely thought that these planets could not have formed at their present-day close-in orbits, but instead migrated inwards through interactions with outer neighbours. Aims. We aim to confirm and characterise the USP Earth-sized validated planet K2-157 b (P_orb = 8.8 h) and constrain the presence of additional companions in the system through radial velocity (RV) measurements. Methods. We measured 49 RVs with the ESPRESSO spectrograph and tested different planetary and non-planetary configurations to infer the model that best represents our data set. We derived the orbital and physical properties of the system through a global RV and transit model. Results. We detected two additional super-Neptune-mass planets located within the warm Neptunian savanna, K2-157 c (P_orb,c = 25.942⁺₋⁰₀⁰⁴⁵₀₄₄ d, M_p,c sin i = 30.8 ± 1.9 M_⨁) and K2-157 d (P_orb,d = 66.50⁺₋⁰₀⁷¹₅₉ d, M_p,d sin i = 23.3 ± 2.5 M⨁). The joint analysis constrains the mass of K2-157 b at the 2.7σ level, M_p,b = 1.14⁺₋⁰₀⁴¹₄₂ M_⨁ (<2.4 M⨁ at 3σ), which, together with the inferred radius, R_p,b = 0.935 ± 0.090 R⨁, make the planet compatible with a rocky composition with a likely (68% confidence) higher iron-to-silicate mass fraction than Earth. K2 data discard non-grazing transit configurations for K2-157 c (i_c < 88.4^◦ at 3σ), and ESPRESSO data constrain the eccentricities of K2-157 c and K2-157 d to e_c < 0.2 and e_d < 0.5 at 3σ. Our dynamical analysis indicates that the system is stable for eccentricities up to e_c, e_d ∼ 0.3 and mutual inclinations up to ∼60^◦. At a population level, we find that the trend that the closest USP planets tend to orbit late-type stars does not hold when scaling the orbital separation to the Roche limit, which suggests that the orbital distribution of the closest planets across spectral types is primarily determined by tidal disruption. Conclusions. The orbital architecture of K2-157 is unusual in the known exoplanet plethora, with only one similar case reported to date: 55 Cnc. The USP planets of these systems, being accompanied by massive, long-period, relatively spaced, and possibly misaligned neighbours, could have migrated inwards through eccentricity-based mechanisms triggered by secular interactions.
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Please use this url to cite or link to this publication: https://lup.lub.lu.se/record/fb5a8e13-741c-4c80-ae41-d89744ba5d00

author

Castro-González, A. ; Bouchy, F. ; Correia, A. C.M. ; Sozzetti, A. ; Lillo-Box, J. ; Figueira, P. ; Lavie, B. ; Lovis, C. ; Hobson, M. J. and Sousa, S. G. , et al. (More)

Castro-González, A. ; Bouchy, F. ; Correia, A. C.M. ; Sozzetti, A. ; Lillo-Box, J. ; Figueira, P. ; Lavie, B. ; Lovis, C. ; Hobson, M. J. ; Sousa, S. G. ; Adibekyan, V. ; Standing, M. R. ; Hara, N. C. ; Barrado, D. ; Silva, A. M. ; Bourrier, V. ; Korth, J. ^LU ; Santos, N. C. ; Damasso, M. ; Zapatero Osorio, M. R. ; Rodrigues, J. ; Alibert, Y. ; Barros, S. C.C. ; Cristiani, S. ; Di Marcantonio, P. ; González Hernández, J. I. ; Lo Curto, G. ; Martins, C. J.A.P. ; Nunes, N. J. ; Palle, E. ; Pepe, F. ; Suárez Mascareño, A. and Tabernero, H. M. (Less)

organization

Astrophysics

publishing date

2025-07-01

type

Contribution to journal

publication status

published

subject

Astronomy, Astrophysics and Cosmology

keywords

planets and satellites: detection, planets and satellites: dynamical evolution and stability, planets and satellites: individual: K2-157 b, stars: individual: K2-157 (EPIC 201130233), techniques: photometric, techniques: radial velocities

in

Astronomy and Astrophysics

volume

699

article number

A344

publisher

EDP Sciences

external identifiers

scopus:105011741483

ISSN

0004-6361

DOI

10.1051/0004-6361/202554736

language

English

LU publication?

yes

additional info

id

fb5a8e13-741c-4c80-ae41-d89744ba5d00

date added to LUP

2025-12-15 14:11:51

date last changed

2025-12-15 14:32:44

@article{fb5a8e13-741c-4c80-ae41-d89744ba5d00,
  abstract     = {{<p>Context. The formation and evolution of ultra-short-period (USP) rocky planets is poorly understood. However, it is widely thought that these planets could not have formed at their present-day close-in orbits, but instead migrated inwards through interactions with outer neighbours. Aims. We aim to confirm and characterise the USP Earth-sized validated planet K2-157 b (P<sub>orb</sub> = 8.8 h) and constrain the presence of additional companions in the system through radial velocity (RV) measurements. Methods. We measured 49 RVs with the ESPRESSO spectrograph and tested different planetary and non-planetary configurations to infer the model that best represents our data set. We derived the orbital and physical properties of the system through a global RV and transit model. Results. We detected two additional super-Neptune-mass planets located within the warm Neptunian savanna, K2-157 c (P<sub>orb,c</sub> = 25.942<sup>+</sup><sub>−</sub><sup>0</sup><sub>0</sub><sup>045</sup><sub>044</sub> d, M<sub>p,c</sub> sin i = 30.8 ± 1.9 M<sub>⨁</sub>) and K2-157 d (P<sub>orb,d</sub> = 66.50<sup>+</sup><sub>−</sub><sup>0</sup><sub>0</sub><sup>71</sup><sub>59</sub> d, M<sub>p,d</sub> sin i = 23.3 ± 2.5 M⨁). The joint analysis constrains the mass of K2-157 b at the 2.7σ level, M<sub>p,b</sub> = 1.14<sup>+</sup><sub>−</sub><sup>0</sup><sub>0</sub><sup>41</sup><sub>42</sub> M<sub>⨁</sub> (&lt;2.4 M⨁ at 3σ), which, together with the inferred radius, R<sub>p,b</sub> = 0.935 ± 0.090 R⨁, make the planet compatible with a rocky composition with a likely (68% confidence) higher iron-to-silicate mass fraction than Earth. K2 data discard non-grazing transit configurations for K2-157 c (i<sub>c</sub> &lt; 88.4<sup>◦</sup> at 3σ), and ESPRESSO data constrain the eccentricities of K2-157 c and K2-157 d to e<sub>c</sub> &lt; 0.2 and e<sub>d</sub> &lt; 0.5 at 3σ. Our dynamical analysis indicates that the system is stable for eccentricities up to e<sub>c</sub>, e<sub>d</sub> ∼ 0.3 and mutual inclinations up to ∼60<sup>◦</sup>. At a population level, we find that the trend that the closest USP planets tend to orbit late-type stars does not hold when scaling the orbital separation to the Roche limit, which suggests that the orbital distribution of the closest planets across spectral types is primarily determined by tidal disruption. Conclusions. The orbital architecture of K2-157 is unusual in the known exoplanet plethora, with only one similar case reported to date: 55 Cnc. The USP planets of these systems, being accompanied by massive, long-period, relatively spaced, and possibly misaligned neighbours, could have migrated inwards through eccentricity-based mechanisms triggered by secular interactions.</p>}},
  author       = {{Castro-González, A. and Bouchy, F. and Correia, A. C.M. and Sozzetti, A. and Lillo-Box, J. and Figueira, P. and Lavie, B. and Lovis, C. and Hobson, M. J. and Sousa, S. G. and Adibekyan, V. and Standing, M. R. and Hara, N. C. and Barrado, D. and Silva, A. M. and Bourrier, V. and Korth, J. and Santos, N. C. and Damasso, M. and Zapatero Osorio, M. R. and Rodrigues, J. and Alibert, Y. and Barros, S. C.C. and Cristiani, S. and Di Marcantonio, P. and González Hernández, J. I. and Lo Curto, G. and Martins, C. J.A.P. and Nunes, N. J. and Palle, E. and Pepe, F. and Suárez Mascareño, A. and Tabernero, H. M.}},
  issn         = {{0004-6361}},
  keywords     = {{planets and satellites: detection; planets and satellites: dynamical evolution and stability; planets and satellites: individual: K2-157 b; stars: individual: K2-157 (EPIC 201130233); techniques: photometric; techniques: radial velocities}},
  language     = {{eng}},
  month        = {{07}},
  publisher    = {{EDP Sciences}},
  series       = {{Astronomy and Astrophysics}},
  title        = {{Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna}},
  url          = {{http://dx.doi.org/10.1051/0004-6361/202554736}},
  doi          = {{10.1051/0004-6361/202554736}},
  volume       = {{699}},
  year         = {{2025}},
}

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Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna