New Jovian analog and activity cycle characterized by HARPS-N in the metal-rich binary star XO-2

An Italian team used more than 10 years of observational data from the HARPS-N high-resolution spectrograph mounted at the Telescopio Nazionale Galileo to discover a new Jupiter-like planet around XO-2S and to characterize the Solar-like activity cycle of its binary companion XO-2N.

So far, only four binary stars in which both components host planets have been discovered. These are all large-separation binaries because this kind of analysis is more challenging for those at close separations. Since about one-third of all the stars in our Galaxy are binaries, understanding the formation and architecture of these systems isof great scientific interest.

Among the few known systems of this kind, XO-2 represents a unique case. First of all, the two planetary systems appear to be different from each other. In particular, XO-2N hosts a hot Jupiter with a true mass of 0.6 Jovian masses and a period of 2.6 days, while XO-2S has a warm Saturn-like planet (m = Jupiter mass, P = 18 days) and a temperate Jupiter-like one (m ~ 1.38 Jupiter mass, Period = 120 days). In addition, both stars have the same mass and radius as the Sun and both are metal-rich (Z > 2Z_sun) but their metallicities differ by 4σ. This latter fact is peculiar as visual binaries are expected to form in the same cloud and should have the same chemical composition.

After the discovery of the first two planets of XO-2S by Desidera et al. (2014), in the framework of the Global Architecture of Planetary Systems (GAPS) project, the team continued to monitor with HARPS-N both stars of the XO-2 binary system, collecting a total of 39 spectra for XO-2N and 106 spectra of XO-2S. These spectra have been combined with literature data taken with HIRES and HDS, and TESS light curves have been used to search for transits of the innermost planets of XO-2S. They found evidence for an additional long-period planet around XO-2S that is an example of a Jovian analog with mass ∼3.7 Jovian masses, orbital radius ∼5.5 au, and eccentricity = 0.09. No new planet was found around XO-2N in addition to the hot Jupiter with a mass of 0.6 Jovian masses discovered by Burke et al. (2007), but the correlation between RV residuals and the chromospheric activity indicator shows that this star displays a Sun-like activity cycle. An analysis of the stability and detection limits of XO-2S revealed a large region of dynamic stability between 1 and 3 astronomical units, where an additional planet with mass < 32 Earth masses could potentially be present.

These results show that the planetary system of XO-2S is at least one order of magnitude more massive than that of XO-2N.

Link to the paper.

Artistic image of the XO-2S planetary system. Credits: ESO/L. Calcada

Best-fit Keplerian model for XO-2S d, after removing the RV signal of the other two planets. Credits: A. Ruggieri et al. A&A 2024