A Comprehensive Statistical Assessment of Star-Planet Interaction

Abstract

We investigate whether magnetic interaction between close-in giant planets and their host stars produce observable statistical enhancements in stellar coronal or chromospheric activity. New Chandra observations of 12 nearby (d < 60 pc) planet-hosting solar analogs are combined with archival Chandra, XMM-Newton, and ROSAT coverage of 11 similar stars to construct a sample inoculated against inherent stellar class and planet-detection biases. Survival analysis and Bayesian regression methods (incorporating both measurements errors and X-ray upper limits; 13/23 stars have secure detections) are used to test whether "hot Jupiter" hosts are systematically more X-ray luminous than comparable stars with more distant or smaller planets. No significant correlations are present between common proxies for interaction strength (M P /a 2 or 1/a) versus coronal activity (L X or L X /L bol ). In contrast, a sample of 198 FGK main-sequence stars does show a significant (∼ 99% confidence) increase in X-ray luminosity with M P /a 2 . While selection biases are incontrovertibly present within the main-sequence sample, we demonstrate that the effect is primarily driven by a handful of extreme hot-Jupiter systems with M P /a 2 > 450 M Jup AU −2 , which here are all X-ray luminous but to a degree commensurate with their Ca II H and K activity, in contrast to presented magnetic star-planet interaction scenarios that predict enhancements relatively larger in L X . We discuss these results in the context of cumulative tidal spin-up of stars hosting close-in gas giants (potentially followed by planetary infall and destruction). We also test our main-sequence sample for correlations between planetary properties and UV luminosity or Ca II H and K emission, and find no significant dependence.

Magnetism and activity of planet hosting stars

“…Chandra data are red, XMM-Newton data are blue, and ROSAT data are green. Miller et al (2015) found no significant increase in X-ray emission among the stars most likely to show elevated activity due to star-planet interactions. Light green points were excluded from their analysis as being "atypically active."…”

“…They found no significant elevation in a sample of solar analogs (Figure 3), but did find a significant correlation driven by a few very massive, very close-in exoplanets orbiting X-ray bright Figure 3. Figure 4 of Miller et al (2015), showing the distribution of coronal activity vs. plausible scalings for planet-star interaction strength for solar analogs in their sample. Open symbols represent upper limits.…”

“…Such action is instructive for determining the factors contributing to tidal coupling between stars and planets, but dampens hopes for using stellar magnetic fields to probe exoplanetary magnetic fields. Figure 7 of Miller et al (2015), showing the distribution of coronal activity vs. plausible scalings for planet-star interaction strength for their entire sample, with the same color scheme as Figure 3 for the Solar analogs, and other stars in black. The double diamond symbols (which have a fit shown in cyan) are "extreme" systems which drive the weak correlation shown by the black dashed line.…”

Tidal effects on stellar activity

“…While initial studies observed a trend of stars with close-in and massive planets to be more active than stars with small and far-away planets (Kashyap et al 2008), these trends have simultaneously a large scatter over the whole sample ) and can in part be traced back to selection effects from the efficiency of planet-detection methods for active and inactive stars . Some effects, especially for extremely close and massive planets, seem to be still present when strictly controlling for the spectral type of the sample stars (Miller et al 2015); however, not all Hot Jupiters necessarily have an active host star (Poppenhäger et al 2009;Miller et al 2012;Pillitteri et al 2014b). One important point is that even if planets may increase the stellar activity through some form of star-planet interaction, the magnetic braking of the star due to its stellar wind is going on at the same time (Penev et al 2012).…”

Is the activity level of HD 80606 influenced by its eccentric planet?

“…The idea initially gathered some support, but was later refuted by more detailed statistical analysis, showing that the correlation was in all likelihood created by selection effects (Poppenhaeger et al 2010;Poppenhaeger & Wolk 2014). Miller et al (2015) recently studied the possible correlation between activity and the most common proxies for interaction derived from orbital parameters, namely M p /a 2 or 1/a, and found no evidence for a correlation. Given the difficulty in characterizing the different biases at work, several studies chose to focus on individual planet-hosting stars and searched for a correlation A&A 592, A143 (2016) between the host activity level and the orbital phase of a close-in massive planet.…”