Cluster planet IC 4651 9122 b

On the incidence of planet candidates in open clusters and a planet confirmation
https://arxiv.org/abs/1807.03196

Quote :

Detecting exoplanets in clusters of different ages is a powerful tool for understanding a number of open questions, such as how the occurrence rate of planets depends on stellar metallicity, on mass, or on stellar environment. We present the first results of our HARPS long-term radial velocity (RV) survey which aims to discover exoplanets around intermediate-mass (between ~ 2 and 6 Msun) evolved stars in open clusters. We selected 826 bona fide HARPS observations of 114 giants from an initial list of 29 open clusters and computed the half peak-to-peak variability of the HARPS RV measurements, namely DeltaRV/2, for each target, to search for the best planet-host candidates. We also performed time series analysis for a few targets with enough observations to search for orbital solutions. Although we attempted to rule out the presence of binaries on the basis of previous surveys, we detected 14 new binary candidates in our sample, most of them identified from a comparison between HARPS and CORAVEL data. We also suggest 11 new planet-host candidates based on a relation between the stellar surface gravity and DeltaRV/2. Ten of the candidates have less than 3 Msun, showing evidence of a low planet occurrence rate for massive stars. One of the planet-host candidates and one of the binary candidates show very clear RV periodic variations, allowing us to confirm the discovery of a new planet and to compute the orbital solution for the binary. The planet is IC 4651 9122b, with a minimum mass of msini = 6.3 MJ and a semi-major axis a = 2.0 AU. The binary companion is NGC 5822 201B, with a very low minimum mass of msini = 0.11 Msun and a semi-major axis a = 6.5 AU, which is comparable to the Jupiter distance to the Sun.

SIMBAD link for star:
http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Cl*+IC+4651+MMU+9122&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

To find the star in SIMBAD, use "Cl* IC 4651 MMU 9122"

Thanks! I've thrown in a SIMBAD link in the original post.

I had to browse through Vizier catalogs to get some information about stellar parameters (not provided in discovery paper, neither the mass). So we're dealing with a 2.4 Gyrs-old, 1.83 M_Sun K-giant 58-60 times more luminous than Sun. Another evolved exoplanet host, then.

An average metallicity of 0.21 is estimated, no clue about distance from Sun, but likely within 890-900 pc.

The paper notes that they are looking for planets around evolved host stars. They are assuming all the target stars are at the main sequence turnoff for the cluster, details of which are given in table 4. For IC 4651, this is 2.1 solar masses, with a metallicity of +0.12 dex and an age of about 1.1 Gyr.

The Gaia DR2 parallax on SIMBAD page is 1.1372±0.0429 milliarcseconds. Naïve conversion of parallax to distance using 1/parallax gives 879±33 parsecs, though the naïve conversion leads to biased estimates.

Not only does this planet probbly not exist, several of the planets that have been found in clusters so far may not.

Planets around evolved intermediate-mass stars in open clusters II. Are there really planets around IC4651No9122, NGC2423No3 and NGC4349No127?
https://arxiv.org/abs/1807.09608

Quote :

(shorter version)The aim of this work is to search for planets around intermediate-mass stars in open clusters by using RV data obtained with HARPS from an extensive survey with more than 15 years of observations for a sample of 142 giant stars in 17 open clusters. We present the discovery of a periodic RV signal compatible with the presence of a planet candidate in the 1.15 Gyr open cluster IC4651 orbiting the 2.06 M⊙ star No. 9122. If confirmed, the planet candidate would have a minimum mass of 7.2 MJ and a period of 747 days. However, we also find that the FWHM of the CCF varies with a period close to the RV, casting doubts on the planetary nature of the signal. We also provide refined parameters for the previously discovered planet around NGC2423 No. 3 but show evidence that the BIS of the CCF is correlated with the RV during some of the observing periods. This fact advises us that this might not be a real planet and that the RV variations could be caused by stellar activity and/or pulsations. Finally, we show that the previously reported signal by a brown dwarf around NGC4349 No. 127 is presumably produced by stellar activity modulation. The long-term monitoring of several red giants in open clusters has allowed us to find periodic RV variations in several stars. However, we also show that the follow-up of this kind of stars should last more than one orbital period to detect long-term signals of stellar origin. This work warns that although it is possible to detect planets around red giants, large-amplitude, long-period RV modulations do exist in such stars that can mimic the presence of an orbiting planetary body. Therefore, we need to better understand how such RV modulations behave as stars evolve along the RGB and perform a detailed study of all the possible stellar-induced signals (e.g. spots, pulsations, granulation) to comprehend the origin of RV variations.

Probably applies to planets around red giant stars in general, not just the cluster ones.

Quote :

Long-term period signals have been observed in many K giants either produced by stellar activity or by oscillations. The amplitudes of these signals can also reach hundreds of m s⁻¹, mimicking the presence of giant planets. The three cases presented in this work are a clear example of the need to perform detailed analysis of RV modulations in giant stars before the real nature of a planetary signal can be assumed. Moreover, the analysis of the FWHM of the CCF has revealed as essential in order to discard the presence of a companion but we note that this indicator is not widely used in the validation of planets around evolved stars. Finally, more theoretical and observational study is needed to further understand the different nature of stellar oscillations in evolved stars.