Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N, and TESS

Abstract

We present a precise characterization of the TOI-561 planetary system obtained by combining previously published data with TESS and CHEOPS photometry, and a new set of 62 HARPS-N radial velocities (RVs). Our joint analysis confirms the presence of four transiting planets, namely TOI-561 b (P = 0.45 d, R = 1.42 R⊕, M = 2.0 M⊕), c (P = 10.78 d, R = 2.91 R⊕, M = 5.4 M⊕), d (P = 25.7 d, R = 2.82 R⊕, M = 13.2 M⊕) and e (P = 77 d, R = 2.55 R⊕, M = 12.6 R⊕). Moreover, we identify an additional, long-period signal (>450 d) in the RVs, which could be due to either an external planetary companion or to stellar magnetic activity. The precise masses and radii obtained for the four planets allowed us to conduct interior structure and atmospheric escape modelling. TOI-561 b is confirmed to be the lowest density (ρb = 3.8 ± 0.5 g cm−3) ultra-short period (USP) planet known to date, and the low metallicity of the host star makes it consistent with the general bulk density-stellar metallicity trend. According to our interior structure modelling, planet b has basically no gas envelope, and it could host a certain amount of water. In contrast, TOI-561 c, d, and e likely retained an H/He envelope, in addition to a possibly large water layer. The inferred planetary compositions suggest different atmospheric evolutionary paths, with planets b and c having experienced significant gas loss, and planets d and e showing an atmospheric content consistent with the original one. The uniqueness of the USP planet, the presence of the long-period planet TOI-561 e, and the complex architecture make this system an appealing target for follow-up studies.

Generalized Peas in a Pod: Extending Intra-system Mass Uniformity to Non-TTV Systems via the Gini Index

“…Most commonly, such long-period planets are often gas giants that form beyond the snowline of their system, and therefore represent an entirely different regime of both size and composition when compared to their close-in companions. For systems that instead contain ultra-short period (USP) members, such as TOI-561 (Weiss et al 2021;Lacedelli et al 2022), it has been shown that the USP planets are often markedly smaller than other members of the same system, and may have experienced a much more dynamically volatile formation history. (Winn et al 2018).…”

TOI-561 b: A Low-density Ultra-short-period “Rocky” Planet around a Metal-poor Star

“…Sector 8 photometry featured two transits that Weiss et al (2021) interpreted as two transits of the same planet at 16.287 ± 0.005 days (giving a total of three transiting planets for the system), while Lacedelli et al (2021) interpreted it as single transits of two different planets (giving four transiting planets). Follow-up photometry from CHEOPS lifts this degeneracy in favor of the four-planet model, with planet d at an orbital period of 25.7124 ± 0.0002 days and planet e at a period of 77.03 ± 0.25 days (Lacedelli et al 2022). The additional TESS observations of Sectors 45 and 46 further support the four-planet model as described in Lacedelli et al (2022).…”

“…21 TOI-561 is 10 ± 3 Gyr old (Weiss et al 2021), making TOI-561 b one of the oldest known super-Earth-sized planets, indicating that rocky planets have been forming for nearly the age of the universe, even in metal-poor environments. Previous studies of this system suggest an even more unique aspect of this planet: it is the lowest-density super-Earth discovered to date, and potentially inconsistent with having a rocky planet composition (Lacedelli et al 2021(Lacedelli et al , 2022.…”

“…The TESS spacecraft has observed TOI-561 in Sectors 8, 35, 45, and 46. Weiss et al (2021) and Lacedelli et al (2021) used Sector 8, while Lacedelli et al (2022) used Sectors 8 and 35. We used all four available sectors, analyzing both TESS's typical 2 minute cadence and 20 s cadence light curves for Sectors 45 and 46.…”

“…Lacedelli et al (2021) used RVs from the TNG/HARPS-N spectrograph and found an even lower mass of M b = 1.59 ± 0.36 M ⊕ , a radius of R b = 1.43 ± 0.11 R ⊕ , and a corresponding density of ρ b = 3.0 ± 0.8 g cm −3 . More recently, Lacedelli et al (2022) combined the literature RVs from HIRES and HARPS-N with new HARPS-N RVs, finding a mass of M b = 2.00 ± 0.23 and a density of ρ b = 3.8 ± 0.5 g cm −3 . Based on their analysis, TOI-561 b is inconsistent with a rocky composition and was suggested to have a water steam envelope.…”