Magnetic field strengths of hot Jupiters from signals of star–planet interactions

Article metrics


Evidence of star–planet interactions in the form of planet-modulated chromospheric emission has been noted for a number of hot Jupiters. Magnetic star–planet interactions involve the release of energy stored in the stellar and planetary magnetic fields. These signals thus offer indirect detections of exoplanetary magnetic fields. Here, we report the derivation of the magnetic field strengths of four hot Jupiter systems, using the power observed in calcium ii K emission modulated by magnetic star–planet interactions. By approximating the fractional energy released in the calcium ii K line, we find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, around 10–100 times larger than the values predicted by dynamo scaling laws for planets with rotation periods of around 2–4 days. However, these values are in agreement with scaling laws relating the magnetic field strength to the internal heat flux in giant planets. Large planetary magnetic field strengths may produce observable electron cyclotron maser radio emission by preventing the maser from being quenched by the planet’s ionosphere. Intensive radio monitoring of hot Jupiter systems will help to confirm these field values and inform the generation mechanism of magnetic fields in this important class of exoplanets.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Ca ii K residual spectra, and summed residual power as a function of time, stellar rotational phase and planetary orbital phase.
Fig. 2: Observed powers in the Ca ii K line residuals as a function of relevant magnetic SPI parameters.
Fig. 3: Magnetic field strengths.

Data availability

This work made use of archived data from the PolarBase archive ( and the Canada–France–Hawaii Telescope data archive ( The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. The reduced spectra used here are also publicly available via the PolarBase archive and the Canada–France–Hawaii Telescope data archive.


  1. 1.

    Wood, B. E., Müller, H. R., Zank, G. P., Linsky, J. L. & Redfield, S. New mass-loss measurements from astrospheric Lyα absorption. Astrophys. J. 628, L143–L146 (2005).

  2. 2.

    Cuntz, M., Saar, S. & Zdzislaw, E. On stellar activity enhancement due to interactions with extrasolar giant planets. Astrophys. J. 533, L151–L154 (2000).

  3. 3.

    Stevens, I. R. Magnetospheric radio emission from extrasolar giant planets: the role of the host stars. Mon. Not. R. Astron. Soc. 356, 1053–1063 (2005).

  4. 4.

    Vidotto, A. A., Jardine, M. & Helling, Ch. Transit variability in bow shock-hosting planets. Mon. Not. R. Astron. Soc. 414, 1573–1582 (2011).

  5. 5.

    Strugarek, A. Assessing magnetic torques and energy fluxes in close-in star–planet systems. Astrophys. J. 833, 140–152 (2016).

  6. 6.

    Rogers, T. M. & McElwaine, J. N. The hottest hot Jupiters may host atmospheric dynamos. Astrophys. J. Lett. 841, L26–L32 (2017).

  7. 7.

    Cohen, O. et al. The interaction of Venus-like, M-dwarf planets with the stellar wind of their host star. Astrophys. J. 806, 41–51 (2015).

  8. 8.

    Jakosky, B. M. et al. Mars’ atmospheric history derived from upper-atmosphere measurements of 38Ar/36Ar. Science 355, 1408–1410 (2017).

  9. 9.

    Blackman, E. G. & Tarduno, J. A. Mass, energy, and momentum capture from stellar winds by magnetized and unmagnetized planets: implications for atmospheric erosion and habitability. Mon. Not. R. Astron. Soc. 481, 5146–5155 (2018).

  10. 10.

    Lazio, T. J. W. et al. Planetary Magnetic Fields: Planetary Interiors and Habitability (Keck Institute for Space Studies, 2016).

  11. 11.

    Shkolnik, E. L. & Llama, J. in Handbook of Exoplanets (eds Deeg, H. J. & Belmonte, J. A.) 1737–1753 (Springer, 2017).

  12. 12.

    Shkolnik, E., Walker, G. A. H. & Bohlender, D. A. Evidence for planet-induced chromospheric activity on HD 179949. Astrophys. J. 597, 1092–1096 (2003).

  13. 13.

    Shkolnik, E., Walker, G. A. H., Bohlender, D. A., Gu, P.-G. & Kürster, M. Hot Jupiters and hot spots: the short- and long-term chromospheric activity on stars with giant planets. Astrophys. J. 622, 1075–1090 (2005).

  14. 14.

    Gurdemir, L., Redfield, S. & Cuntz, M. Planet-induced emission enhancements in HD 179949: results from McDonald observations. Publ. Astron. Soc. Aust. 29, 141–149 (2012).

  15. 15.

    Shkolnik, E., Bohlender, D. A., Walker, G. A. H. & Collier Cameron, A. The on/off nature of star–planet interactions. Astrophys. J. 676, 628–638 (2008).

  16. 16.

    Cauley, P. W., Shkolnik, E. S., Llama, J., Bourrier, V. & Moutou, C. Evidence of magnetic star–planet interactions in the HD 189733 system from orbitally phased Ca ii K variations. Astron. J. 156, 262–273 (2018).

  17. 17.

    Walker, G. A. H. et al. MOST detects variability on τ Bootis A possibly induced by its planetary companion. Astron. Astrophys. 482, 691–697 (2008).

  18. 18.

    Pagano, I. et al. CoRoT-2a magnetic activity: hints for possible star–planet interaction. Earth Moon Planets 105, 373–378 (2009).

  19. 19.

    Scandariato, G. et al. A coordinated optical and X-ray spectroscopic campaign on HD 179949: searching for planet-induced chromospheric and coronal activity. Astron. Astrophys. 552, 7–20 (2013).

  20. 20.

    Maggio, A. et al. Coordinated X-ray and optical observations of star–planet interaction in HD 17156. Astro. J. 811, L2–L7 (2015).

  21. 21.

    Pillitteri, I. et al. FUV variability of HD 189733. Is the star accreting material from its hot Jupiter? Astrophys. J. 805, 52–70 (2015).

  22. 22.

    Cranmer, S. R. & Saar, S. H. Exoplanet-induced chromospheric activity: realistic light curves from solar-type magnetic fields. Preprint at (2007).

  23. 23.

    Llama, J. et al. Exoplanet transit variability: bow shocks and winds around HD 189733 b. Mon. Not. R. Astron. Soc. 436, 2179–2187 (2013).

  24. 24.

    Fares, R. et al. A small survey of the magnetic fields of planet-host stars. Mon. Not. R. Astron. Soc. 435, 1451–1462 (2013).

  25. 25.

    Lanza, A. F. Stellar coronal magnetic fields and star–planet interaction. Astron. Astrophys. 505, 339–350 (2009).

  26. 26.

    Lanza, A. F. Star–planet magnetic interaction and activity in late-type stars with close-in planets. Astron. Astrophys. 544, 23–39 (2012).

  27. 27.

    Cohen, O. et al. The dynamics of stellar coronae harboring hot Jupiters. I. A time-dependent magnetohydrodynamic simulation of the interplanetary environment in the HD 189733 planetary system. Astrophys. J. 733, 67–79 (2011).

  28. 28.

    Scharf, C. A. Possible constraints on exoplanet magnetic field strengths from planet–star interaction. Astrophys. J. 722, 1547–1555 (2010).

  29. 29.

    van Haarlem, M. P. et al. LOFAR: The LOw-Frequency ARray. Astron. Astrophys. 556, 2–55 (2013).

  30. 30.

    Zaghoo, M. & Collins, G. W. Size and strength of self-excited dynamos in Jupiter-like extrasolar planets. Astrophys. J. 862, 19–29 (2018).

  31. 31.

    Sánchez-Lavega, A. The magnetic field in giant extrasolar planets. Astrophys. J. 609, L87–L90 (2004).

  32. 32.

    Saur, J., Grambusch, T., Duling, S., Neubauer, F. M. & Simon, S. Magnetic energy fluxes in sub-Alfvénic planet–star and moon–planet interactions. Astron. Astrophys. 552, 119–139 (2013).

  33. 33.

    Lanza, A. F. Star–planet magnetic interactions and evaporation of planetary atmospheres. Astron. Astrophys. 557, 31–44 (2013).

  34. 34.

    Aschwanden, M. J., Xu, Y. & Jing, J. Global energetics of solar flares. I. Magnetic energies. Astrophys. J. 797, 50–85 (2014).

  35. 35.

    Veronig, A., Temmer, M., Hanslmeier, A., Otruba, W. & Messerotti, M. Temporal aspects and frequency distributions of solar soft X-ray flares. Astron. Astrophys. 382, 1070–1080 (2002).

  36. 36.

    Johns-Krull, C. M., Hawley, S. L., Basri, G. & Valenti, J. A. Hamilton echelle spectroscopy of the 1993 March 6 solar flare. Astrophys. J. Supp. 112, 221–243 (1997).

  37. 37.

    Klocová, T., Czesla, S., Khalafinejad, S., Wolter, U. & Schmitt, J. H. M. M. Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit. Astron. Astrophys. 607, 66–78 (2017).

  38. 38.

    Yadav, R. K. & Thorngren, D. P. Estimating the magnetic field strengths in hot Jupiters. Astrophys. J. Lett. 849, L12–L16 (2017).

  39. 39.

    Christensen, U. R., Holzwarth, V. & Reiners, A. Energy flux determines magnetic field strength of planets and stars. Nature 457, 167–169 (2009).

  40. 40.

    Reiners, A. & Christensen, U. R. A magnetic field evolution scenario for brown dwarfs and giant planets. Astron. Astrophys. 522, 13–20 (2010).

  41. 41.

    Thorngren, D. P. & Fortney, J. J. Bayesian analysis of hot-Jupiter radius anomalies: evidence for Ohmic dissipation? Astron. J. 155, 214–224 (2018).

  42. 42.

    Kislyakova, K. G., Holmström, M., Lammer, H., Odert, P. & Khodachenko, M. L. Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations. Science 346, 981–984 (2014).

  43. 43.

    Bourrier, V., Lecavelier des Etangs, A., Ehrenreich, D., Tanaka, Y. A. & Vidotto, A. A. An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b. Astron. Astrophys. 591, 121–135 (2016).

  44. 44.

    Weber, C. et al. How expanded ionospheres of hot Jupiters can prevent escape of radio emission generated by the cyclotron maser instability. Mon. Not. R. Astron. Soc. 469, 3505–3517 (2017).

  45. 45.

    Daley-Yates, S. & Stevens, I. R. Inhibition of the electron cyclotron maser instability in the dense magnetosphere of a hot Jupiter. Mon. Not. R. Astron. Soc. 479, 1194–1209 (2018).

  46. 46.

    Weber, C. et al. Supermassive hot Jupiters provide more favourable conditions for the generation of radio emission via the cyclotron maser instability—a case study based on Tau Bootis b. Mon. Not. R. Astron. Soc. 480, 3680–3688 (2018).

  47. 47.

    Chen, J. & Kipping, D. Probabilistic forecasting of the masses and radii of other worlds. Astrophys. J. 834, 17–30 (2017).

  48. 48.

    Mittag, M., Schmitt, J. H. M. M. & Schröder, K.-P. Ca ii H + K fluxes from S-indices of large samples: a reliable and consistent conversion based on PHOENIX model atmospheres. Astron. Astrophys. 549, 117–129 (2013).

  49. 49.

    Scandariato, G. et al. HADES RV programme with HARPS-N at TNG. IV. Time resolved analysis of the Ca ii H&K and Hα chromospheric emission of low-activity early-type M dwarfs. Astron. Astrophys. 598, 28–42 (2017).

  50. 50.

    Husser, T.-O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, 6–15 (2013).

  51. 51.

    Soubiran, C., Le Campion, J.-F., Brouillet, N. & Chemin, L. The PASTEL catalogue: 2016 version. Astron. Astrophys. 591, 118–125 (2016).

  52. 52.

    Eker, Z. et al. Main-sequence effective temperatures from a revised mass–luminosity relation based on accurate properties. Astron. J. 149, 131–147 (2015).

  53. 53.

    Goodman, J. & Weare, J. Ensemble samplers with affine invariance. Comm. Appl. Math. Comp. Sci. 5, 65–80 (2010).

  54. 54.

    Foreman-Mackey, D., Hogg, D. W., Lang, D., & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pac. 125, 306–312 (2013).

  55. 55.

    Butler, R. P. et al. Catalog of nearby exoplanets. Astrophys. J. 646, 505–522 (2006).

  56. 56.

    Fares, R. et al. MOVES. I. The evolving magnetic field of the planet-hosting star HD 189733. Mon. Not. R. Astron. Soc. 471, 1246–1257 (2017).

  57. 57.

    Bouchy, F. et al. ELODIE metallicity-biased search for transiting hot Jupiters. II. A very hot Jupiter transiting the bright K star HD 189733. Astron. Astrophys. 444, L15–L19 (2005).

  58. 58.

    Winn, J. et al. The Transit Light Curve Project. V. System parameters and stellar rotation period of HD 189733. Astron. J. 133, 1828–1835 (2007).

  59. 59.

    Boisse, I. et al. Stellar activity of planetary host star HD 189733. Astron. Astrophys. 495, 959–966 (2009).

  60. 60.

    Catala, C., Donati, J.-F., Shkolnik, E., Bohlender, D. & Alecian, E. The magnetic field of the planet-hosting star τ Bootis. Mon. Not. R. Astron. Soc. 374, L42–L46 (2007).

  61. 61.

    Jeffers, S. V. et al. The relation between stellar magnetic field geometry and chromospheric activity cycles. II. The rapid 120-day magnetic cycle of τ Bootis. Mon. Not. R. Astron. Soc. 479, 5266–5271 (2018).

  62. 62.

    Marsden, S. C. et al. A BCool magnetic snapshot survey of solar-type stars. Mon. Not. R. Astron. Soc. 444, 3517–3536 (2014).

  63. 63.

    Fares, R. et al. Magnetic field, differential rotation and activity of the hot-Jupiter-hosting star HD 179949. Mon. Not. R. Astron. Soc. 423, 1006–1017 (2012).

Download references


We thank T. Barman for discussions concerning details of the PHOENIX models. P.W.C. and E.L.S. acknowledge support from NASA Origins of the Solar System grant no. NNX13AH79G (PI: E.L.S.). This work has made use of NASA’s Astrophysics Data System and used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.

Author information

E.L.S. was responsible for most of the original observing proposals and data collection. P.W.C. was responsible for the flux-calibration and SPI signal analysis, as well as the manuscript preparation. J.L. was responsible for some original SPI signal analysis and also contributed to the manuscript. A.F.L. provided interpretation of the SPI theories and oversight of the theory application. All authors contributed material to the manuscript.

Correspondence to P. Wilson Cauley.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information: Nature Astronomy thanks Scott Wolk and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Table 1; Supplementary Figs. 1–3.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark