Two massive rocky planets transiting a K-dwarf 6.5 parsecs away



HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered1,2. The Spitzer Space Telescope detected a transit of the innermost of these planets, HD 219134 b, whose mass and radius (4.5 M and 1.6 R respectively) are consistent with a rocky composition1. Here, we report new high-precision time-series photometry of the star acquired with Spitzer revealing that the second innermost planet of the system, HD 219134c, is also transiting. A global analysis of the Spitzer transit light curves and the most up-to-date HARPS-N velocity data set yields mass and radius estimations of 4.74 ± 0.19 M and 1.602 ± 0.055 R for HD 219134 b, and of 4.36 ± 0.22 M and 1.511 ± 0.047 R for HD 219134 c. These values suggest rocky compositions for both planets. Thanks to the proximity and the small size of their host star (0.778 ± 0.005 R)3, these two transiting exoplanets — the nearest to the Earth yet found — are well suited for a detailed characterization (for example, precision of a few per cent on mass and radius, and constraints on the atmospheric properties) that could give important constraints on the nature and formation mechanism of the ubiquitous short-period planets of a few Earth masses.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Spitzer transit photometry of the planets HD 219134 b and c.
Figure 2: Mass–radius relationship for small planets with precisions on the masses better than 20%.


  1. 1

    Motalebi, F. et al. The HARPS-N rocky planet search. I. HD 219134 b: a transiting rocky planet in a multi-planet system at 6.5 pc from the Sun. Astron. Astrophys. 584, A72 (2015).

    Article  Google Scholar 

  2. 2

    Vogt, S. S. et al. Six planets orbiting HD 219134. Astrophys. J. 814, 12 (2015).

    Article  ADS  Google Scholar 

  3. 3

    Boyajian, T. S. et al. Stellar diameters and temperatures. II. Main-sequence K- and M-stars. Astrophys. J. 757, 112 (2012).

    Article  ADS  Google Scholar 

  4. 4

    Gillon, M. et al. An educated search for transiting habitable planets: targetting M dwarfs with known transiting planets. Astron. Astrophys. 525, A32 (2011).

    Article  Google Scholar 

  5. 5

    Cosentino, R. et al. HARPS-N: the new planet hunter at TNG. Proc. SPIE (2012).

    Google Scholar 

  6. 6

    Fazio, G. G. et al. The Infrared Array Camera (IRAC) for the Spitzer Space Telescope. Astrophys. J. Suppl. Ser. 154, 10–17 (2004).

    Article  ADS  Google Scholar 

  7. 7

    Ingalls, J. G. et al. Intra-pixel gain variations and high-precision photometry with the Infrared Array Camera (IRAC). Proc. SPIE (2012).

    Google Scholar 

  8. 8

    Knutson, H. A., Charbonneau, D., Allen, L. E., Burrows, A. & Megeath, S. T. The 3.6-8.0 μm broadband emission spectrum of HD 209458b: evidence for an atmospheric temperature inversion. Astrophys. J. 673, 526–531 (2008).

    Article  ADS  Google Scholar 

  9. 9

    Gillon, M. et al. The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b. Astron. Astrophys. 542, A4 (2012).

    Article  ADS  Google Scholar 

  10. 10

    Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, 171–175 (2002).

    Article  ADS  Google Scholar 

  11. 11

    Schwarz, G. E. Estimating the dimension of a model. Ann. Stat. 6, 461–464 (1978).

    MathSciNet  Article  Google Scholar 

  12. 12

    Jeffreys, H. The Theory of Probability 3rd edn (Oxford Univ. Press, 1998).

    Google Scholar 

  13. 13

    Scuflaire, R. et al. The Liège oscillation code. Astrophys. Space Sci. 316, 149–154 (2008).

    Article  ADS  Google Scholar 

  14. 14

    Johnson, M. C. et al. A 12-year activity cycle for the nearby planet host star HD 219134. Astrophys. J. 821, 74 (2016).

    Article  ADS  Google Scholar 

  15. 15

    Ramírez, I. et al. Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, galactic chemical evolution, and exoplanets. Astrophys. J. 756, 46 (2012).

    Article  ADS  Google Scholar 

  16. 16

    Ramírez, I., Allende Prieto, C. & Lambert, D. L. Oxygen abundances in nearby FGK stars and the galactic chemical evolution of the local disk and halo. Astrophys. J. 764, 78 (2013).

    Article  ADS  Google Scholar 

  17. 17

    Pace, G. Chromospheric activity as age indicator. An L-shaped chromospheric-activity versus age diagram. Astron. Astrophys. 551, L8 (2013).

    Article  ADS  Google Scholar 

  18. 18

    Goldreich, P. & Soter, S. Q in the Solar System. Icarus 5, 375–389 (1966).

    Article  ADS  Google Scholar 

  19. 19

    Murray, C. D. & Dermott, S. F. Solar System Dynamics (Cambridge Univ. Press, 2001).

    Google Scholar 

  20. 20

    Zeng, L. & Sasselov, D. A detailed model grid for solid planets from 0.1 through 100 Earth masses. Publ. Astron. Soc. Pacif. 125, 227–239 (2013).

    Article  ADS  Google Scholar 

  21. 21

    Zeng, L., Sasselov, D. D. & Jacobsen, S. B. Mass–radius relation for rocky planets based on PREM. Astrophys. J. 819, 127 (2016).

    Article  ADS  Google Scholar 

  22. 22

    Miller-Ricci, E., Seager, S. & Sasselov, D. The atmospheric signatures of super-Earths: how to distinguish between hydrogen-rich and hydrogen-poor atmospheres. Astrophys. J. 690, 1056–1067 (2009).

    Article  ADS  Google Scholar 

  23. 23

    Kley, W & Nelson, R. P. Planet–disk interaction and orbital evolution. Annu. Rev. Astron. Astrophys. 50, 211–249 (2012).

    Article  ADS  Google Scholar 

  24. 24

    Hansen, B. M. S. & Murray, N. Migration then assembly: formation of Neptune-mass planets inside 1 AU, Astrophys. J. 751, 158 (2012).

    Article  ADS  Google Scholar 

  25. 25

    Chatterjee, S. & Tan, J. C. Inside-out planet formation, Astrophys. J. 780, 53 (2014).

    Article  ADS  Google Scholar 

  26. 26

    Hansen, B. M. S. & Murray, N. Testing in situ assembly with the Kepler planet candidate sample. Astrophys. J. 775, 53 (2013).

    Article  ADS  Google Scholar 

  27. 27

    Chatterjee, S. & Tan, J. C. Vulcan planets: inside-out formation of the innermost super-Earths. Astrophys. J. 798, L32 (2015).

    Article  ADS  Google Scholar 

  28. 28

    Martin, R. G. & Livio, M. On the formation of super-Earths with implications for the Solar System. Astrophys. J. 822, 90 (2016).

    Article  ADS  Google Scholar 

  29. 29

    Ricker, G. R. et al. The transiting exoplanet survey satellite (TESS). Proc. SPIE (2014).

    Google Scholar 

  30. 30

    Fortier, A. et al. CHEOPS: a space telescope for ultra-high precision photometry of exoplanet transits. Proc. SPIE (2014).

    Google Scholar 

  31. 31

    Meschiari, S. et al. Systemic: a testbed for characterizing the detection of extrasolar planets. I. The Systemic Console package. Publ. Astron. Soc. Pacif. 121, 1016–1027 (2009).

    Article  ADS  Google Scholar 

  32. 32

    Baranne, A. et al. ELODIE: a spectrograph for accurate radial velocity measurements. Astron. Astrophys. Suppl. 119, 373–390 (1996).

    Article  ADS  Google Scholar 

  33. 33

    Noyes, R. W. et al. Rotation, convection, and magnetic activity in lower main-sequence stars. Astrophys. J. 279, 763–777 (1984).

    Article  ADS  Google Scholar 

  34. 34

    Wright, J. T. Radial velocity jitter in stars from the California and Carnegie Planet Search at Keck Observatory. Publ. Astron. Soc. Pacif. 117, 657–664 (2005).

    Article  ADS  Google Scholar 

  35. 35

    Maldonado, J. & Villaver, E. Evolved stars and the origin of abundance trends in planet hosts. Astron. Astrophys. 588, A98 (2016)

    Article  ADS  Google Scholar 

  36. 36

    Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–452 (2009).

    Article  ADS  Google Scholar 

  37. 37

    Thoul, A., Bahcall, J. N. & Loeb, A. Element diffusion in the solar interior. Astrophys. J. 421, 828–842 (1994).

    Article  ADS  Google Scholar 

  38. 38

    Claret, A. A new non-linear limb-darkening law for LTE stellar atmosphere models. Calculations for –5.0 ≤ log[M/H] ≤ +1, 2000K ≤ Teff ≤ 50000 K at several surface gravities. Astron. Astrophys. 363, 1081–1190 (2000).

    ADS  Google Scholar 

  39. 39

    Claret, A. & Bloemen, S. Gravity and limb-darkening coefficients for the Kepler, CoRoT, Spitzer, uvby, UBVRIJHK, and Sloan photometric systems. Astron. Astrophys. 529, A75 (2011).

    Article  ADS  Google Scholar 

  40. 40

    Stevenson, K. B. et al. Transit and eclipse analyses of the exoplanet HD 149026b using BLISS mapping. Astrophys. J. 754, 136 (2012).

    Article  ADS  Google Scholar 

  41. 41

    Gelman, A. & Rubin, D. B. Inference from iterative simulation using multiple sequences. Stat. Sci. 7, 457–472 (1992).

    Article  Google Scholar 

  42. 42

    Winn, J. N. Exoplanets transits and occultations. Exoplanets (ed. Seager, S. ) 55–77 (Univ. Arizona Press, 2010).

  43. 43

    Lovis, C. & Fischer, D. A. Radial velocity technique for exoplanets, in Exoplanets (ed. Seager S., ) 27–53 (Univ. Arizona Press, 2010).

  44. 44

    Kasting, J. F., Whitmire, D. P. & Reynolds, R. T. Habitable zones around main-sequence stars. Icarus 101, 108–128 (1993).

    Article  ADS  Google Scholar 

  45. 45

    de Wit, J. & Seager, S. Constraining exoplanet mass from transmission spectroscopy. Science 342, 1473–1477 (2013).

    Article  ADS  Google Scholar 

  46. 46

    Tsiaras, A. et al. Detection of an atmosphere around the super-Earth 55 Cancri e. Astrophys. J. 820, 99 (2016).

    Article  ADS  Google Scholar 

  47. 47

    Clampin, M. The James Webb Space Telescope: capabilities for transiting exoplanets observations. Proc. Pathways Towards Habitable Planets (16 July 2015);

  48. 48

    Schlawin, E. et al. Two NIRCAM channels are better than one: how JWST can do more science with NIRCAM’s short-wavelength dispersed Hartmann sensor. Publ. Astron. Soc. Pacif., 129, 015001 (2017).

    Article  ADS  Google Scholar 

  49. 49

    Ridden-Harper, A. A. et al. Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e. Astron. Astrophys. 593, A219 (2016).

    Article  Google Scholar 

  50. 50

    Ehrenreich, D. et al. A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436 b. Nature 522, 459–461 (2015).

    Article  ADS  Google Scholar 

  51. 51

Download references


This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Support for this work was provided by NASA. M.G. is grateful to NASA and SSC Director for having supported his searches for RV planets with Spitzer. M.G. and V.V.G. are Research Associates at the Belgian Scientific Research Fund (F.R.S.-FNRS). The research leading to these results has received funding from the ARC grant for Concerted Research Actions, financed by the Wallonia–Brussels Federation. The authors thank N. Lewis for information on the potential for atmospheric characterization of HD 219134 b and c with JWST.

Author information




M.G. led the HD 219134 b+c transit search with Spitzer, planned and analysed the Spitzer observations, performed the global analysis of the Spitzer and HARPS-N data, and wrote most of the manuscript. B.-O.D. performed an independent analysis of the Spitzer data to verify M.G.’s results. V.V.G. performed the stellar evolutionary modelling of the host star. A.C.C., D.C., D.L., C.L., E.M., F.M., G.M., M.M., F.A.P., G.P., D.Sa., D.Sé., A.S. and S.U. form the HARPS-N science team which managed the RV monitoring of the system. All authors contributed to the writing of the manuscript.

Corresponding author

Correspondence to Michaël Gillon.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Figures 1–2 and Supplementary Tables 1–3. (PDF 707 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Gillon, M., Demory, B., Van Grootel, V. et al. Two massive rocky planets transiting a K-dwarf 6.5 parsecs away. Nat Astron 1, 0056 (2017).

Download citation

Further reading


Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing