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A&A
Volume 699, July 2025
Article Number A38
Number of page(s) 15
Section Planets, planetary systems, and small bodies
DOI https://doi.org/10.1051/0004-6361/202554137
Published online 01 July 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1 Introduction

The number of exoplanet detections has risen from a few dozen 25 years ago to more than 5000 today. This rise in discoveries has provided the community with a wide range of new insights into planetary diversity. Planets are now found on all kinds of orbits, from close-in, hot terrestrial or Jupiter-sized planets to planets on orbits of thousands of days. Our knowledge of the demographics of exoplanets and their statistical properties has greatly increased in the past 30 years (Udry & Santos 2007; Winn & Fabrycky 2015, Zhu & Dong 2021), and we have a much broader view today than we did 20 years ago. However, while the number of known exoplanets around stars with masses similar to or lower than that of the Sun has drastically grown, especially thanks to the transit method, detections around intermediate-mass stars, in the range 1.5–5 M⊙, remain rather rare.

Studying a wide range of stellar parameters, like mass and metallicity, for this population is crucial to improving our understanding of planet formation mechanisms. Depending on their mass, stars are expected to dissipate their disks at different rates (Kennedy & Kenyon 2009), which could favor or hinder the formation of particular types of planets. For example, higher-mass stars are expected to disperse their disks more rapidly, limiting the duration of the planet formation process. The predicted time available for planet formation differs between the two main planet formation paradigms: core accretion (Pollack et al. 1996; Alibert et al. 2005) and gravitational instability (Boss 1997; Durisen et al. 2007). Testing these hypotheses with observational data is thus essential to improving our current models.

The CORALIE radial-velocity search for companions around evolved stars (CASCADES) survey was launched almost 20 years ago with the aim of filling this gap by focusing on intermediate-mass stars in their post-main-sequence phase (Ottoni et al. 2022). When on the main sequence, intermediate-mass stars (1.5–5 M⊙, spectral type A to F) have high effective temperatures and rotational velocities, which result in a low number of spectral lines as well as a broadening of the lines. These characteristics represent a real challenge (see, e.g., Bouchy et al. 2001 on the impact of line broadening), limiting our ability to achieve the radial-velocity (RV) precision required to detect low-mass companions (Galland et al. 2005). When they evolve off the main sequence to the red giant branch, stars expand, which slows their rotation and decreases their surface temperature. This reduction in temperature and rotation rate facilitates more precise RV measurements, as a larger number of spectral lines become detectable via high-resolution spectroscopy and the effect of line broadening is reduced. Studying intermediate-mass stars in their evolved state, which we refer to here as giant stars, is thus a way to overcome the difficulties linked to them when they are still on the main sequence.

Additionally, studying the population of exoplanets around giant stars provides insights into the dynamical evolution of the planet-star connection. Planets with short orbital periods are expected to be engulfed by their host star during stellar evolution (Kunitomo et al. 2011), which should also be reflected in the demographics through a lack of close-in planets. While hot Jupiters have been found around lower-mass evolved stars (M* < 1.5 M⊙; Lillo-Box et al. 2014; Pereira et al. 2024; Grunblatt et al. 2019; Temmink & Snellen 2023), they have yet to be found around more massive giant stars. Studying these targets is thus a way to understand if this phenomenon really happens, and how (Hon et al. 2023; Lin et al. 2024).

Following the first detection of a planet in orbit around a giant star in 2002 by Frink et al. (2002), approximately 200 exoplanets1 have so far been detected in orbit around evolved hosts (as of January 2025; see Fig. 2). As this number remains modest, our understanding of the demographics of this population of exoplanets can still be improved. Giant stars, being bright and easy to observe, act as excellent proxies for studying exoplanets around intermediate-mass stars. Consequently, several RV surveys have been conducted to study these stars (Frink et al. 2001; Sato et al. 2005; Wittenmyer et al. 2011, and Jones et al. 2011, among others).

However, studying the RV time series of giant stars can also be troublesome. While evolved stars offer certain observational advantages over their main-sequence counterparts, they are also known to be intrinsically variable. Signals of stellar origin, for instance caused by oscillations or convective motion, can produce periodic shifts in the RV time series of giant stars, which, if not accounted for, could be mistaken for a signal of planetary origin (Hekker et al. 2006a; Hekker et al. 2006b; Delgado Mena et al. 2023). These activity-induced variations can occur over a wide range of periods, from several days to hundreds of days, with amplitudes exceeding 100 m s−1. Disentangling true planetary signals from stellar activity is thus particularly challenging, requiring a careful analysis of the spectroscopic data to avoid false positives.

In this paper we present the detection of five new exoplanets orbiting giant stars: two planets around HD 87816, two planets around HD 94890, and one planet around HD 102888. These targets have been monitored for over ten years as part of the CASCADES survey, allowing us to confirm their planetary nature and characterize their orbits. We also confirm the presence of the inner companion of HD 121056 with a period of 89 days, previously reported by Feng et al. (2022), Wittenmyer et al. (2015) and Jones et al. (2015b), and propose an update to the period and orbital parameters of HD 121056 c, for which previous values have varied across different studies. As we have access to a longer baseline of data than the previous studies of this star, we are confident that our revised period reflects the true orbital period of HD 121056 c. With fewer than 30 known multi-planet systems around giant stars, these discoveries provide valuable insights that help us understand the architecture of planetary systems in evolved stellar environments.

Section 2 provides a summary of the CASCADES survey, including our sample selection and the instrumental setup. In Sect. 3, we present the observational data used for this work. Section 4 outlines our methods for planet detection and the process used to obtain their orbital parameters and posterior distributions; each system is also discussed individually. We conclude in Sect. 5.

2 The CASCADES survey

2.1 Sample description and history

The CASCADES survey was launched in 2006 with the aim of performing a long-term, volume-limited survey of giant stars. It is conducted with the CORALIE spectrograph on the Leonhard Euler 1.2m Swiss telescope at La Silla, Chile. The 641 G- and K-type stars of the survey were selected from the Hipparcos catalog (ESA 1997). Only stars located at less than 300 pc and with a precision on the parallax better than 14% were considered (this limit was 10% at first before the extension of the sample in 2011). Visual binaries (separated by less than 6″) were discarded from the sample to prevent contamination by the secondary star. To isolate the giant stars in the Hertzsprung-Russell (HR) diagram, a MV cutoff was introduced, only keeping stars with MV < 2.5. Then, only stars with a B–V color index in the range 0.78 < B–V < 1.06 were kept, as can be seen in Fig. 1. The magnitude upper limit and the color index lower bound ensure that stars have left the main sequence, while the higher bound of B–V was introduced to avoid later-type stars because of their higher activity level.

Estimates of stellar parameters, including masses and radii, were computed by Ottoni et al. (2022) for all of the 641 stars of the survey. Effective temperatures (Teff), the surface gravity (log g), and the metallicity ratio ([Fe/H]) were obtained from the analysis of high-resolution spectra. Ottoni et al. (2022) used the CORALIE spectra obtained after 2014 (after an instrument upgrade), which they stacked to create a high signal-to-noise master spectrum. The effective temperature Teff, log g and [Fe/H] were obtained from these master spectra using the line list of Tsantaki et al. (2013) for the stars with Teff < 5200 K and that of Sousa et al. (2008) for the others. They followed the method presented in Sousa (2014), using the ARES (Automatic Routine for line Equivalent widths in stellar Spectra; Sousa et al. 2007; Sousa et al. 2015) + MOOG (a widely used radiative transfer code for stellar abundance analysis; Sneden 1973) methodology.

The luminosities were estimated by Ottoni et al. (2022) using the corrected parallaxes from Bailer-Jones et al. (2018, obtained from the Gaia DR2 parallaxes), the V-band magnitudes from Høg et al. (2000) and the bolometric correction relation of Alonso et al. (1999). Then, the radii were derived by using the Stefan-Boltzmann relation with the values of luminosity and effective temperatures obtained from the previous steps. Finally, the masses were estimated using the SPInS software (Lebreton & Reese 2020), using the position of the star in the HR diagram, log g, and [Fe/H].

It is important to note that, for giant stars, it is hard to have a precise estimate of the mass from the evolutionary tracks. This is because, in this region of the HR diagram, evolutionary tracks of stars of different masses pile up in a very dense area. It is possible to obtain a better estimate of the masses by using more advanced techniques like asteroseismology, for instance. Ottoni et al. (2022) and Buldgen et al. (2022) computed the masses of four of the targets of the sample using asteroseismology and found that the masses are, on average, overestimated by the approach that uses SPInS. However, the masses obtained for the full sample by Ottoni et al. (2022) can still be used as a first reference for these stars. Fully detailed information on the survey can be found in Ottoni et al. (2022), where it was originally presented.

As of January 2025, the CASCADES survey has detected four exoplanets around giant hosts (Ottoni et al. 2022; Pezzotti et al. 2022). More than 20 000 RV measurements have been made for the entire sample, with a median exposure time of 300 seconds and a mean precision of around 6 m s−1. With more than 15 years of data, this survey is particularly well suited to identify long-period companions on periods of several thousand days.

thumbnail Fig. 1

CASCADES sample. Gray dots show all the stars of the HIPPARCOS sample with a precision on the parallax better than 14%. The green circles show the 641 targets of the CASCADES survey. The blue and red vertical lines represent the two B-V cutoffs that were introduced to select the targets, and the horizontal gray line shows the absolute magnitude threshold that was used. The four targets that are discussed in this work are represented by the four red stars.
thumbnail Fig. 2

Exoplanet detections around giant stellar hosts (log g < 3.5 cm s−2) from the NASA Exoplanet archive. Circles are color-coded as a function of the detection method (blue for RV and brown for transit). Yellow stars represent the targets published by Ottoni et al. (2022) and Pezzotti et al. (2022), and red stars the targets of this study.

2.2 Instrument description

The CORALIE spectrograph (Queloz et al. 2000) was installed in 1998 on the 1.2 m Leonhard Euler Swiss telescope located at the La Silla observatory in Chile. CORALIE is a two-fiber-fed echelle spectrograph working in the visible (3880–6810 Å). The instrument received two major hardware upgrades in 2007 and in 2014, which resulted in increases in overall throughput and resolution (see, e.g., Ségransan et al. 2010). With these updates, the instrument’s optomechanical configuration changed, which led us to treat data coming from these different periods as if they came from different instruments. For this reason, we refer to the pre-upgrade data as CORALIE98, the ones after the 2007 upgrade as CORALIE07, and the latest ones as CORALIE14. For the analysis, we thus used different instrument offsets and precision for these three versions of CORALIE. From the analysis of quiet stars in the survey, which we consider to be constant, we have determined that the instrumental stability for each version of the instrument is typically σCOR98 = 5 m s−1, σCOR07 = 8 m s−1 and σCOR14 = 3 m s−1. Temperature and air pressure variations during the night can lead to a spectral drift, which induces systematic shifts in the wavelength calibration of the instrument. If not accounted for, the drift can introduce errors in the RV measurements. As CORALIE is not kept in a vacuum vessel, changes in the weather conditions typically introduce a drift that is tracked throughout the night. When the drift exceeds 110 m s−1, the CORALIE data reduction software can no longer reliably correct for it, and the quality control (QC) flag is set to “Failed.”

One of the targets of this work, HD 121056, was also observed with the FEROS (Kaufer et al. 1999) and CHIRON (Schwab et al. 2012 and Spronck et al. 2011) spectrographs. We present here a combined solution including CORALIE, CHIRON, and FEROS, based on a longer time span of observations.

3 Observations and stellar characteristics

All the stellar parameters of the four stars discussed in this work can be found in Table 1. A summary of the observational data available for all targets is presented in Table 2. The CORALIE data reduction software versions used for this work are 3.3 for CORALIE98, 3.4 for CORALIE07, and 3.8 for CORALIE14.

Table 1

Stellar parameters for HD 87816, HD 94890, HD 102888, and HD 121056.

Table 2

Summary of observations.

4 Analysis of the radial velocity time series

The monitoring of the targets of the sample and the identification of possible planetary signals were conducted using the Data & Analysis Center for Exoplanets (DACE) web tool2, which allows us to visualize the time series of RVs and line profile indicators, as well as their periodograms (Zechmeister & Kürster 2009). A more in-depth analysis of the RV time series was then performed using the Kepmodel python package (Delisle & Ségransan 2022).

We only considered the observations that passed the CORALIE QC, and discarded all measurements with an uncertainty on the RV exceeding 20 m s−1 (which were obtained in poor weather conditions). We corrected all the RV time series for secular acceleration using the latest values of parallax and proper motion in right ascension and declination available from the SIMBAD database (Wenger et al. 2000), based on Gaia Early Data Release 3 (Riello et al. 2021). Although included for completeness, this effect is negligible compared to the intrinsic variability of the stars at these distances. Then, we fit an RV offset for each dataset, treating different versions of the CORALIE spectrograph as independent instruments. We also included in the fit our assumptions on the stability of the different versions of CORALIE (5 m s−1, 8 m s−1, and 3 m s−1 for CORALIE98, 07, and 14, respectively). To account for additional noise from stellar activity-related effects, we also included a global stellar jitter term.

To identify significant periodic signals, we conducted an iterative periodogram analysis. A Keplerian model was fit to the dominant peak in the periodogram if its false alarm probability (FAP; using that provided by Kepmodel) was below 10−2. This process was repeated iteratively on the residuals until no significant peak could be detected. Each detected signal was sub-sequently refined using a full Keplerian fit. As highlighted in studies of stable giant stars by Frink et al. (2001) and Hekker et al. (2006c), the intrinsic RV scatter for quiet giant stars can typically reach 20 m s−1. Consequently, we consider any residual scatter of this magnitude to be consistent with the expected stellar jitter rather than indicative of an additional astrophysical signal.

The final orbital parameters for each system, presented in Tables 4 to 8, were derived using the Markov chain Monte Carlo (MCMC) algorithm implemented in SAMSAM (Delisle 2022). The MCMC was run for 300 000 iterations and modeled the following orbital parameters: orbital period (P), RV semi-amplitude (K), mean longitude (λ0), and the parameters e⋅cos⁡ω$\[\sqrt{e} ~\cdot ~\cos~ \omega\]$ and e⋅sin⁡ω$\[\sqrt{e} ~\cdot ~\sin~ \omega\]$, where e and ω are the orbital eccentricity and the argument of periastron, respectively. The different offsets γi, and instrumental precisions σi (for CHIRON and FEROS), where i corresponds to the instrument name, were also sampled with the MCMC.

The mean longitude, e⋅cos⁡ω$\[\sqrt{e} \cdot ~\cos~ \omega\]$ and e⋅sin⁡ω$\[\sqrt{e} ~\cdot ~\sin~ \omega\]$ are sampled with uniform priors, while the orbital period and the RV amplitude are sampled with a log-uniform prior. We also used a uniform prior for the COR14 offset of reference and for the relative offsets between COR98/COR07 and COR14: ΔγCOR98/07 − COR14 = 𝒰(−50, 50), as larger offset values between the different versions of the instrument are not expected. The stellar jitter term σjit and the CHIRON and FEROS instrument stability are sampled with uniform priors. The full list of priors used for the MCMC can be found in Table 3.

4.1 The HD 87816 planetary system: An eccentric inner planet and a massive, long-period outer one

HD 87816 has been observed with CORALIE for approximately 18 years, starting in 2007, with a total of 91 individual observations. Via analysis of its RV time series, we identified two candidates (Figs. 3 to 5), one of which is on a very eccentric orbit.

The periodograms obtained from the RV time series can be seen in Fig. 4. After fitting for the two Keplerian orbits, one can see that the residuals do not show any remaining significant periodic signal. The full orbital parameters obtained from the MCMC analysis can be found in Table 4.

HD 87816 b, a planet with a minimum mass mb sin ib = 6.7 MJup, takes 484 days to orbit its host star on an orbit with an eccentricity of e = 0.78, making it one of the most eccentric RV-detected planets around a giant star3. As the periastron passage of HD 87816 b was expected to occur in December 2024, we increased our observing cadence of this star during this period to obtain a good sampling of the rapid rise and fall of the RVs at this passage of the orbit. This can be observed in Fig. 3.

HD 87816 c, the second planet of the system with a minimum mass of mc sin ic = 12.2 MJup, orbits its host star with a period of approximately 7600 days and an eccentricity of e = 0.19. As the period of HD 87816 c exceeds the time span of our observations, its value cannot be easily constrained from our analysis. This difficulty arises from the combination of the long orbital period and the need to account for different offset values associated with the three versions of CORALIE. This can be seen in Fig. B.2, where clear correlations between the period of HD 87816 c and the instrumental offsets can be seen. Consequently, this explains the large uncertainties that we obtain from our MCMC analysis. We also note that the imprecision of mass estimation models for giant stars adds to the uncertainty on the mass of the outer companion.

Periodograms of the time series of the full width at half maximum (FWHM) of the cross-correlation function, bisector inverse slope (BIS), and the H-alpha can be seen in Fig. A.1a. Following recent observations to complete the orbital coverage of HD 87816 b, a significant peak emerged in the periodogram of the FWHM time series at 519 days. We checked for a correlation between the FWHM and the RVs but did not find any. Thanks to the long baseline of the observations, the two peaks, corresponding respectively to the 484-day orbital period of HD 87816 b and the 519-day signal for the FWHM variation, are clearly separated in the frequency domain. This is supporting our confidence in the planetary nature of the 484-day signal. However, the origin of the 519-day signal still remains unclear. A possible explanation could be that it results from a long-term variation in the FWHM time series (potentially due to, e.g., a long-period double-lined spectroscopic binary or a long-timescale magnetic cycle) convolved with our sampling window. This interpretation is supported by the fact that the signal completely vanishes if we discard the last 26 high-cadence observations. No significant peak can be seen for any of the other activity-index time series, reinforcing the interpretation of a planetary origin of the periodic signals observed in the RVs.

Table 3

Priors used for the MCMC sampling.

thumbnail Fig. 3

Top: RV time series of HD 87816 obtained between 2007 and 2024. The dots are color-coded by instrument (CORALIE98 in purple, CORALIE07 in red, and CORALIE14 in olive). Our two-planet model is overplotted in black, with one very eccentric planet orbiting with a period of 484 days and a longer-period planet at 7600 days. Bottom: residuals after subtracting the two-planet model.
thumbnail Fig. 4

Top: periodogram of the RV time series of HD 87816. Middle: periodogram obtained after fitting a first Keplerian model with a period of 480 days. Bottom: Periodogram of the residuals of the RV time series after subtraction of the two-planet model. The horizontal lines show the different FAP levels: 1% (dashed orange line) and 0.1% (dashed-dotted blue line). The highest peak is marked with a red dot, and the corresponding period is indicated next to it.
thumbnail Fig. 5

Top: phase-folded RVs for the best model obtained for HD 87816 b. Bottom: same but for HD 87816 c.
Table 4

Orbital parameters obtained for HD 87816 b and c.

4.2 Two massive planets around HD 94890

We have 76 RV measurements of HD 94890, conducted with the three different versions of CORALIE. This star was observed between 2007 and 2025, and we report here the detection of a two-planet system around it.

The RV time series of HD 94890 can be seen in Fig. 6, where a clear pattern is noticeable without the need for an advanced analysis. We followed the same periodogram approach as we did for the other stars in this work. All the periodograms of each step of the iterative fitting can be found in Fig. 7. Assuming a stellar mass M* = 1.74 M⊙ for HD 94890, we find planetary masses of 2.1 MJup and 8.9 MJup for HD 94890 b and c, respectively. Both planets are on relatively low-eccentricity orbits, with e = 0.22 for HD 94890 b and e = 0.05 for HD 94890 c. After the subtraction of our best two-planet model, no significant peak remains in the periodogram of the RV time series. The residuals (Fig. 6, bottom) do not show any clear periodicity and seem compatible with the RV scatter expected for giant stars. HD 94890 b and HD 94890 c orbit their host star with periods of 824 days and 2492 days, respectively, resulting in a period ratio notably close to 3. This value suggests a potential 3:1 mean-motion resonance, wherein the inner planet completes three orbits for every orbit of the outer one. Such a resonant configuration can lead to significant gravitational interactions between the planets, potentially affecting the observed RV of the host star and making it differ from that expected for two noninteracting Keplerian orbits. Moreover, these interactions depend on the true masses and orbital inclinations of the two planets in resonance. A detailed dynamical analysis could therefore constrain the orbital inclination and yield more accurate estimates of the planetary masses. While such an investigation is beyond the scope of this work, it represents a promising direction for future research and could significantly enhance our understanding of this system.

The periodograms of the time series of FWHM, H-alpha, and BIS (Fig. A.1b) do not show any significant peak. We also checked for correlations between these quantities and the RVs, without finding any. The phase-folded RVs of HD 94890 b and HD 94890 c can be found in Fig. 8. Our data fully cover both orbits, and thus we do not need additional points to fill missing phases.

thumbnail Fig. 6

Same as Fig. 3 but for HD 94890.
thumbnail Fig. 7

Same as Fig. 4 but for HD 94890.
thumbnail Fig. 8

Top: phase-folded orbit of HD 94890 b obtained with CORALIE. Bottom: same but for HD 94890 c.
Table 5

Orbital parameters obtained for HD 94890 b and c.

4.3 A planet and an unseen companion around HD 102888

HD 102888 has been observed for 12 years with the CORALIE spectrograph between 2012 and 2024. By looking at the 36 CORALIE RV measurements from Fig. 9, a 200 m s−1 peak-to-peak periodic variation of the RV can be seen quite easily.

After fitting a Keplerian model to this periodic signal, we noticed a remaining long-term trend in the RV data, which we modeled with a linear drift. Both the drift and the model for the Keplerian orbit can be seen alongside the data in Fig. 9. The period of the variation was identified using a periodogram approach. The periodogram of the RV time series can be found in Fig. 10. The peak at 252 days is easily identifiable and well above the FAP threshold for significant detection (0.01). After fitting for the planet and the drift, no significant peak can be seen on the periodogram of the residuals. A phase-folding of the data with the best Keplerian model is shown in Fig. 11. Knowing that activity-induced RV variations could be at the origin of periodic signals, we looked for hints of stellar activity-related periodic signals in the FWHM, BIS, and H-alpha index time series of HD 102888. The resulting periodograms can be found in Fig. A.1c, where we see that no periodic variation of any of these quantities is found.

From our analysis, HD 102888 b is a planet with minimum mass of mb sin ib = 5.7 MJup orbiting its star in 252 days on an orbit with an amplitude K = 102 m s−1 and eccentricity of e = 0.1. The remaining linear trend after subtraction of the Keplerian model from our data corresponds to an acceleration of γ˙$\[\dot{\gamma}\]$ = 6.4 ± 1 m s−1 yr−1. This hints at the presence of an additional unseen companion on a much larger orbit that far exceeds our observation baseline. Following Winn et al. (2010), and assuming that this companion is on a circular orbit and that its mass is much smaller than that of HD 102888, we can approximate γ˙∼GMcsin⁡ic/ac2$\[\dot{\gamma} \sim G M_{\mathrm{c}} ~\sin~ i_{\mathrm{c}} / a_{\mathrm{c}}^{2}\]$ to have an estimate on the order of magnitude of the mass of this companion. In our case, this yields (Mcsin⁡icMJup)(ac10AU)−2∼3.6,$\[\left(\frac{M_{\mathrm{c}} ~\sin~ i_{\mathrm{c}}}{M_{\mathrm{Jup}}}\right)\left(\frac{a_{\mathrm{c}}}{10 ~\mathrm{AU}}\right)^{-2} \sim 3.6,\]$(1)

where ac is the semimajor axis of the companion and Mc is its mass. Using this estimation, HD 102888 c could, for instance, be a 14 MJup brown dwarf on a 20 AU orbit, a 30 MJup one on a 30 AU orbit or a 57 MJup one on a 40 AU orbit. While this is only a rough estimate, this gives good hope that HD 102888 c could be a companion of HD 102888 of substellar nature. The full orbital parameters that we derived for this system can be found in Table 6.

thumbnail Fig. 9

Same as Fig. 3 but for HD 102888.
thumbnail Fig. 10

Same as Fig. 4 but for HD 102888 after subtracting the linear drift.
thumbnail Fig. 11

Phase-folded RV data of HD 102888 with the period of HD 102888 b at 252 days. Overplotted in black is the best-fit Keplerian model.
Table 6

Orbital parameters obtained for the system around HD 102888.

4.4 Update on the planetary system around HD 121056

The presence of a planetary system around HD 121056 has already been reported by several teams in the past (Jones et al. 2015a; Jones et al. 2015b; Wittenmyer et al. 2015; Feng et al. 2022). The detection of multi-planet systems around evolved stars remains rare, with fewer than 30 systems known to date4. The system around HD 121056 is of particular interest, as it also features a short period planet, another rare discovery for giant stars5. While all the previous studies on HD 121056 agree on the period of the inner planet at approximately 89 days for a mass of about 1 MJup, different values for the period of the outer companion have been reported, as one can see in Table 7.

Using the same approach as for the other targets studied in this work, we analyzed the RV time series of HD 121056 (Fig. 12) using a periodogram approach by fitting Keplerian models iteratively to the highest significant peak on the residuals (Fig. 13). After subtracting our best two-planet model, the periodogram of the residuals does not show any significant peak. Our best-fit two-planet model can be seen in Fig. 12, where it is overplotted on the RV time series. The phase-folded RVs of HD 121056 b and c are shown in Fig. 14. We looked for hints or stellar activity-related effects on the time series of FWHM, BIS, and H-alpha (Fig. A.1d) but did not find any significant periodicities or correlation with the RVs.

We confirm the presence of the inner planet with a period of 89 days and mass mb sin ib = 1.2 MJup, and propose an update to the period of the outer companion. We find that HD 121056 c is a massive planet with minimum mass mc sin ic = 5 MJup orbiting its host star in approximately 3130 days. Both planets are on relatively low-eccentricity orbits, with e = 0.13 for HD 121056 b and e = 0.3 for HD 121056 c. As our baseline of observation is much longer than that of previous studies, our data span a full orbit of HD 121056 c, allowing us to find a more accurate period of this second companion. All the orbital parameters of this system can be found in Table 8. A comparison with the results of other studies is given in Table 7.

thumbnail Fig. 12

Same as Fig. 3 but for HD 121056. FEROS and CHIRON data are shown in dark green and dark blue, respectively.
thumbnail Fig. 13

Same as Fig. 4 but for HD 121056.
thumbnail Fig. 14

Top: phase-folded RVs of HD 121056 b obtained with CORALIE, FEROS, and CHIRON. Bottom: same but for HD 121056 c.
Table 7

Comparison of the orbital parameters HD 121056 b and c from different studies.

Table 8

Orbital parameters obtained for HD 121056 b and c.

5 Conclusion

We have reported the detection of five new exoplanets orbiting evolved intermediate-mass stars:

  • HD 87816 b and HD 87816 c are two massive planets with minimum masses mbsin ib = 6.7 MJup and mcsin ic = 12.2 MJup. The inner planet is on a very eccentric orbit, with an eccentricity e = 0.78 and a period of 484 days, while the outer planet is on a less eccentric orbit with e = 0.19 and a period of approximately 7600 days;

  • HD 94890 b and HD 94890 c are two massive planets from the same system, with minimum masses mb sin ib = 2.1 MJup and mc sin ic = 8.9 MJup. They both have relatively long periods, 824 days for the inner planet and 2490 days for the outer. Both planets are on low-eccentricity orbits, eb = 0.22 and ec = 0.05;

  • HD 102888 b is a 5.7 MJup planet orbiting its giant host star in 252 days on a low-eccentricity orbit with e = 0.11. After subtracting our best-fit model, we identified a remaining linear trend in the data, which is compatible with the presence of an unseen companion with a much longer period. Our analysis indicates that this companion is likely substellar and could be a more or less massive brown dwarf, depending on the distance at which it is orbiting its host star.

In addition to these three new planetary systems, we also confirmed the detection of HD 121056 b, which had already been reported by other teams, and provide an update on the orbital parameters of the second planet of the system HD 121056 c. We find an orbital period of 3128 days for this outer companion, which has a minimum mass mc sin ic = 5 MJup on a moderately eccentric orbit with e = 0.3. Regarding the inner companion, the orbital period that we find, approximately 89 days, is compatible with that of previous studies, though our estimate of the minimum mass is slightly lower, at mb sin ib = 1.2 MJup.

For all the reported new detections, we looked for hints of stellar activity by studying the time series of FWHM, BIS, and H-alpha. No correlation could be found between any of these quantities and the RV modulation that we observed, giving us confidence about the planetary-origin nature of our observations. After subtracting our best-fit models, the residuals of all systems exhibit weighted root mean squared values consistent with expectations for giant stars.

Data availability

The RV data for all the targets discussed in this work are available in electronic form at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/699/A38. The data can also be downloaded and visualized on the DACE platform https://doi.org/10.82180/dace-smvbg35u.

Acknowledgements

We thank all the observers of the Swiss telescope at La Silla Observatory who contributed to the observations of this program over the past 19 years. This work was supported by the Swiss National Science Foundation (SNSF) under grant number 205010. This publication makes use of The Data & Analysis Center for Exoplanets (DACE), which is a facility based at the University of Geneva (CH) dedicated to extrasolar planets data visualisation, exchange and analysis. DACE is a platform of the Swiss National Centre of Competence in Research (NCCR) PlanetS, federating the Swiss expertise in Exoplanet research. The DACE platform is available at https://dace.unige.ch. A.De. acknowledges the financial support of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation under grants 51NF40_182901 and 51NF40_205606. NCS acknowledges funding by the European Union (ERC, FIERCE, 101052347). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. This work was supported by FCT – Fundação para a Ciência e a Tecnologia through national funds by these grants: UIDB/04434/2020 DOI: 10.54499/UIDB/04434/2020, UIDP/04434/2020 DOI: 10.54499/UIDP/04434/2020. A.N. acknowledges support from the Swiss National Science Foundation (SNSF) under grant PZ00P2_208945.

Appendix A Periodograms of FWHM, BIS, and H-alpha

thumbnail Fig. A.1

Periodograms of the time series of the FWHM, BIS, and H-alpha index for the four targets of this work: HD 87816 (panel a), HD 94890 (panel b), HD 102888 (panel c), and HD 121056 (panel d). The horizontal lines show the different FAP levels: 1% (dashed orange line) and 0.1% (dashed-dotted blue line). The vertical red lines indicate the orbital periods of the detected exoplanets around each star.

Appendix B Corner plot distribution of parameters

thumbnail Fig. B.1

Posterior distributions of the orbital parameters of HD 102888 b obtained from the MCMC analysis.
thumbnail Fig. B.2

Same as Fig. B.1 but for HD 87816 b and c.
thumbnail Fig. B.3

Same as Fig. B.1 but for HD 94890 b and c.
thumbnail Fig. B.4

Same as Fig. B.1 but for HD 121056 b and c.

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1

This number was estimated with the help of the NASA Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu), by considering host stars with log g ≤ 3.5 cm s−2.

3

As of January 2025, only KIC 3526061 b (e = 0.85, Karjalainen et al. 2022) and HD 76920 b (e = 0.86, Wittenmyer et al. 2017) are known to be on more eccentric orbits.

4

This number was obtained using the NASA exoplanet archive (https://exoplanetarchive.ipac.caltech.edu/) by restricting the detections to host stars with surface gravity log g ≤ 3.5 cm s−2 and looking at systems with two or more planets.

5

HD 121056 is one of two known planetary systems around a giant star with a period under 100 days and discovered by RV, alongside HD 33142 d (89.9 days, Trifonov et al. 2022).

All Tables

Table 1

Stellar parameters for HD 87816, HD 94890, HD 102888, and HD 121056.

In the text
Table 2

Summary of observations.

In the text
Table 3

Priors used for the MCMC sampling.

In the text
Table 4

Orbital parameters obtained for HD 87816 b and c.

In the text
Table 5

Orbital parameters obtained for HD 94890 b and c.

In the text
Table 6

Orbital parameters obtained for the system around HD 102888.

In the text
Table 7

Comparison of the orbital parameters HD 121056 b and c from different studies.

In the text
Table 8

Orbital parameters obtained for HD 121056 b and c.

In the text

All Figures

thumbnail Fig. 1

CASCADES sample. Gray dots show all the stars of the HIPPARCOS sample with a precision on the parallax better than 14%. The green circles show the 641 targets of the CASCADES survey. The blue and red vertical lines represent the two B-V cutoffs that were introduced to select the targets, and the horizontal gray line shows the absolute magnitude threshold that was used. The four targets that are discussed in this work are represented by the four red stars.
In the text
thumbnail Fig. 2

Exoplanet detections around giant stellar hosts (log g < 3.5 cm s−2) from the NASA Exoplanet archive. Circles are color-coded as a function of the detection method (blue for RV and brown for transit). Yellow stars represent the targets published by Ottoni et al. (2022) and Pezzotti et al. (2022), and red stars the targets of this study.
In the text
thumbnail Fig. 3

Top: RV time series of HD 87816 obtained between 2007 and 2024. The dots are color-coded by instrument (CORALIE98 in purple, CORALIE07 in red, and CORALIE14 in olive). Our two-planet model is overplotted in black, with one very eccentric planet orbiting with a period of 484 days and a longer-period planet at 7600 days. Bottom: residuals after subtracting the two-planet model.
In the text
thumbnail Fig. 4

Top: periodogram of the RV time series of HD 87816. Middle: periodogram obtained after fitting a first Keplerian model with a period of 480 days. Bottom: Periodogram of the residuals of the RV time series after subtraction of the two-planet model. The horizontal lines show the different FAP levels: 1% (dashed orange line) and 0.1% (dashed-dotted blue line). The highest peak is marked with a red dot, and the corresponding period is indicated next to it.
In the text
thumbnail Fig. 5

Top: phase-folded RVs for the best model obtained for HD 87816 b. Bottom: same but for HD 87816 c.
In the text
thumbnail Fig. 6

Same as Fig. 3 but for HD 94890.
In the text
thumbnail Fig. 7

Same as Fig. 4 but for HD 94890.
In the text
thumbnail Fig. 8

Top: phase-folded orbit of HD 94890 b obtained with CORALIE. Bottom: same but for HD 94890 c.
In the text
thumbnail Fig. 9

Same as Fig. 3 but for HD 102888.
In the text
thumbnail Fig. 10

Same as Fig. 4 but for HD 102888 after subtracting the linear drift.
In the text
thumbnail Fig. 11

Phase-folded RV data of HD 102888 with the period of HD 102888 b at 252 days. Overplotted in black is the best-fit Keplerian model.
In the text
thumbnail Fig. 12

Same as Fig. 3 but for HD 121056. FEROS and CHIRON data are shown in dark green and dark blue, respectively.
In the text
thumbnail Fig. 13

Same as Fig. 4 but for HD 121056.
In the text
thumbnail Fig. 14

Top: phase-folded RVs of HD 121056 b obtained with CORALIE, FEROS, and CHIRON. Bottom: same but for HD 121056 c.
In the text
thumbnail Fig. A.1

Periodograms of the time series of the FWHM, BIS, and H-alpha index for the four targets of this work: HD 87816 (panel a), HD 94890 (panel b), HD 102888 (panel c), and HD 121056 (panel d). The horizontal lines show the different FAP levels: 1% (dashed orange line) and 0.1% (dashed-dotted blue line). The vertical red lines indicate the orbital periods of the detected exoplanets around each star.
In the text
thumbnail Fig. B.1

Posterior distributions of the orbital parameters of HD 102888 b obtained from the MCMC analysis.
In the text
thumbnail Fig. B.2

Same as Fig. B.1 but for HD 87816 b and c.
In the text
thumbnail Fig. B.3

Same as Fig. B.1 but for HD 94890 b and c.
In the text
thumbnail Fig. B.4

Same as Fig. B.1 but for HD 121056 b and c.
In the text

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