Two warm Neptunes transiting HIP 9618 revealed by TESS and Cheops

Abstract

HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright (G = 9.0 mag) solar analogue. TESS photometry revealed the star to have two candidate planets with radii of 3.9 ± 0.044 R⊕ (HIP 9618 b) and 3.343 ± 0.039 R⊕ (HIP 9618 c). While the 20.77291 d period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-d gap in the time series, leaving many possibilities for the period. To solve this issue, CHEOPS performed targeted photometry of period aliases to attempt to recover the true period of planet c, and successfully determined the true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE, and CAFE revealed a mass of 10.0 ± 3.1M⊕ for HIP 9618 b, which, according to our interior structure models, corresponds to a $6.8\pm 1.4~{{\ \rm per\ cent}}$ gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of <18M⊕. Follow-up and archival RV measurements also reveal a clear long-term trend which, when combined with imaging and astrometric information, reveal a low-mass companion ($0.08^{+0.12}_{-0.05} M_\odot$) orbiting at $26.0^{+19.0}_{-11.0}$ au. This detection makes HIP 9618 one of only five bright (K < 8 mag) transiting multiplanet systems known to host a planet with P > 50 d, opening the door for the atmospheric characterization of warm (Teq < 750 K) sub-Neptunes.

Confirming long-period transiting exoplanets with TESS and CHEOPS. The case of HD 22946 d

“…We demonstrated how CHEOPS can be used to followup TESS duotransits to expand the sample of long-period planets. Through the CHEOPS GTO Duos program, we have contributed to the discovery of 6 planets with orbital periods longer than 20 d, radii smaller than 5 R ⊕ , and host stars brighter than G = 12 mag (Osborn et al, 2022(Osborn et al, , 2023Tuson et al, 2023;Ulmer-Moll et al, 2023;Garai et al, 2023). There are only ∼ 18 other planets confirmed by TESS in this parameter space, illustrating the power of the TESS and CHEOPS synergy for the discovery of small, long-period planets transiting bright stars.…”

The TESS-Keck Survey. XVI. Mass Measurements for 12 Planets in Eight Systems

“…However, in two cases, we measure only an upper limit on the planet mass. The first, HIP 9618 c, was externally validated and confirmed by Osborn et al (2023) using a combination of TESS and CHEOPS photometry and CAFE, HARPS-N, and SOPHIE RVs (see Section 10.6.2 for details). The second, TOI-554.02 (HD 25463 c) has not yet been confirmed, so we take additional measures to statistically validate this planet.…”

“…The TOI-669 Keck-NIRC2 observation was taken on 2019 June 9 using the K filter (λ 0 = 2.196 μm and Δλ = 0.336 μm) and an integration time of 10 s. The star appears single, and the Keck-NIRC2 observation rules out companions of Δ7.6 mag at 5σ confidence at a separation of 0 5. Osborn et al (2023) reported the discovery and confirmation of HIP 9618 b and c using space-based photometry from TESS and CHEOPS (Benz et al 2021) along with radial velocity observations from CAFE, HARPS-N, and SOPHIE (Perruchot et al 2008;Cosentino et al 2012;Aceituno et al 2013). To rule out dilution from nearby sources, the authors observed HIP 9618 with a variety of optical speckle and near-infrared (NIR) adaptive optics (AO) instruments, including Keck-NIRC2.…”

“…It was not until later that TFOP follow-up revealed these two transits were actually of different depth and duration. To include the 1788 BTJD transit of HIP 9618 b in their analysis, Osborn et al (2023), hereafter O23, re-extracted aperture photometry for HIP 9618 starting from the 2 minute cadence target pixel files. In place of the PDC algorithm, they then use a custom light curve detrending method similar to Vanderburg et al (2019) in order to remove spacecraft systematics.…”

The “Drake Equation” of Exomoons—A Cascade of Formation, Stability and Detection

“…Somewhat surprisingly, the number of promising targets in our search area has increased very significantly in the past few years, and 6 out of 9 systems were discovered in 2021 or later. This rapidly increasing rate of discoveries can be devoted to the long observation time basis for planet search (e.g., repeated TESS visits) and the evolution of the observation strategies (e.g., follow-up long-period TESS planet candidates to determine the accurate period, (e.g., [122,[131][132][133]), which help the precise characterization of longperiod transiting planets. The chances of obtaining more and more appropriate candidates within the exomoon search region are increased due to the longer time coverage of the observations and also the better resolution of the full sky with appropriately long and precise observations (as indicated by the arrows in Figure 5.…”