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Discovery of two warm mini-Neptunes with contrasting densities orbiting the young K3V star TOI-815

Psaridi, A. ; Davies, M.B. LU and West, R.G. (2024) In Astronomy and Astrophysics 685.
Abstract
We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K–M binary system. Analysis of its spectra and rotation period reveal the star to be young, with an age of 200+−400200 Myr. TOI-815b has a 11.2-day period and a radius of 2.94 ± 0.05 R+ with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62 ± 0.10 R+, based on observations of three nonconsecutive transits with TESS; targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6 ± 1.5 M+ (ρP = 1.64+−003331 g cm−3) and 23.5 ± 2.4 M+ (ρP = 7.2+−1110 g... (More)
We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K–M binary system. Analysis of its spectra and rotation period reveal the star to be young, with an age of 200+−400200 Myr. TOI-815b has a 11.2-day period and a radius of 2.94 ± 0.05 R+ with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62 ± 0.10 R+, based on observations of three nonconsecutive transits with TESS; targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6 ± 1.5 M+ (ρP = 1.64+−003331 g cm−3) and 23.5 ± 2.4 M+ (ρP = 7.2+−1110 g cm−3), respectively. Thus, the planets have very different masses, which is unusual for compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those undergoing strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere that constitutes a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3σ level. This emphasizes the peculiarity of the system’s orbital architecture, and probably hints at an eventful dynamical history. © The Authors 2024. (Less)
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keywords
planets and satellites: composition, planets and satellites: detection, planets and satellites: formation, stars: individual: TOI-815, techniques: photometric, techniques: radial velocities, Orbits, Photometry, Satellites, Stars, Binary systems, Neptune, Planets and satellites: compositions, Planets and satellites: detections, Planets and satellites: formation, Spectra's, Star: individual: TOI-815, Stars: individual: proxima Centauri, Techniques: photometric, Techniques: radial velocities, Planets
in
Astronomy and Astrophysics
volume
685
article number
A5
publisher
EDP Sciences
external identifiers
  • scopus:85192389557
ISSN
0004-6361
DOI
10.1051/0004-6361/202348678
language
English
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yes
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7a1d2a60-aed5-4ea9-adf3-da123229f85c
date added to LUP
2025-12-11 14:59:37
date last changed
2025-12-11 14:59:37
@article{7a1d2a60-aed5-4ea9-adf3-da123229f85c,
  abstract     = {{We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K–M binary system. Analysis of its spectra and rotation period reveal the star to be young, with an age of 200+−400200 Myr. TOI-815b has a 11.2-day period and a radius of 2.94 ± 0.05 R+ with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62 ± 0.10 R+, based on observations of three nonconsecutive transits with TESS; targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6 ± 1.5 M+ (ρP = 1.64+−003331 g cm−3) and 23.5 ± 2.4 M+ (ρP = 7.2+−1110 g cm−3), respectively. Thus, the planets have very different masses, which is unusual for compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those undergoing strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere that constitutes a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3σ level. This emphasizes the peculiarity of the system’s orbital architecture, and probably hints at an eventful dynamical history. © The Authors 2024.}},
  author       = {{Psaridi, A. and Davies, M.B. and West, R.G.}},
  issn         = {{0004-6361}},
  keywords     = {{planets and satellites: composition; planets and satellites: detection; planets and satellites: formation; stars: individual: TOI-815; techniques: photometric; techniques: radial velocities; Orbits; Photometry; Satellites; Stars; Binary systems; Neptune; Planets and satellites: compositions; Planets and satellites: detections; Planets and satellites: formation; Spectra's; Star: individual: TOI-815; Stars: individual: proxima Centauri; Techniques: photometric; Techniques: radial velocities; Planets}},
  language     = {{eng}},
  publisher    = {{EDP Sciences}},
  series       = {{Astronomy and Astrophysics}},
  title        = {{Discovery of two warm mini-Neptunes with contrasting densities orbiting the young K3V star TOI-815}},
  url          = {{http://dx.doi.org/10.1051/0004-6361/202348678}},
  doi          = {{10.1051/0004-6361/202348678}},
  volume       = {{685}},
  year         = {{2024}},
}