A rocky composition for an earth-sized exoplanet

ABSTRACT Planets with sizes between that of Earth (with radius ) and Neptune (about 4) are now known to be common around Sun-like stars1,2,3. Most such planets have been discovered through

the transit technique, by which the planet’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet’s mass—and

hence its density, which is a clue to its composition—is more difficult. Planets of size 2–4 have proved to have a wide range of densities, implying a diversity of compositions4,5, but these

measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star

every 8.5 hours (ref. 6). Given a radius of 1.20 ± 0.09 and a mass of 1.69 ± 0.41, the planet’s mean density of 5.3 ± 1.8 g cm−3 is similar to Earth’s, suggesting a composition of rock and

iron. Access through your institution Buy or subscribe This is a preview of subscription content, access via your institution ACCESS OPTIONS Access through your institution Subscribe to this

journal Receive 51 print issues and online access $199.00 per year only $3.90 per issue Learn more Buy this article * Purchase on SpringerLink * Instant access to full article PDF Buy now

Prices may be subject to local taxes which are calculated during checkout ADDITIONAL ACCESS OPTIONS: * Log in * Learn about institutional subscriptions * Read our FAQs * Contact customer

support SIMILAR CONTENT BEING VIEWED BY OTHERS A SUPER-MASSIVE NEPTUNE-SIZED PLANET Article 30 August 2023 A SECONDARY ATMOSPHERE ON THE ROCKY EXOPLANET 55 CANCRI E Article 08 May 2024

EVIDENCE FOR THE VOLATILE-RICH COMPOSITION OF A 1.5-EARTH-RADIUS PLANET Article 15 December 2022 REFERENCES * Howard, A. W. et al. Planet occurrence within 0.25 AU of solar-type stars from

Kepler. _Astrophys. J._ 201 (Suppl.). 15 (2012) Article  Google Scholar  * Petigura, E., Marcy, G. W. & Howard, A. A plateau in the planet population below twice the size of Earth.

_Astrophys. J._ 770, 69 (2013) Article  ADS  Google Scholar  * Fressin, F. et al. The false positive rate of Kepler and the occurrence of planets. _Astrophys. J._ 766, 81 (2013) Article  ADS

  Google Scholar  * Valencia, D., Guillot, T., Parmentier, V. & Freedman, R. S. Bulk composition of GJ 1214b and other sub-Neptune exoplanets. _Astrophys. J._ 775, 10 (2013) Article  ADS

  Google Scholar  * Fortney, J. J. et al. A framework for characterizing the atmospheres of low-mass low-density transiting planets. _Astrophys. J._ 775, 80 (2013) Article  ADS  Google

Scholar  * Sanchis-Ojeda, R. et al. Transits and occultations of an Earth-sized planet in an 8.5-hour orbit. _Astrophys. J._ 774, 54 (2013) Article  ADS  Google Scholar  * Borucki, W. J. et

al. Kepler planet-detection mission: introduction and first results. _Science_ 327, 977–980 (2010) Article  ADS  CAS  Google Scholar  * Vogt, S. et al. HIRES: the high-resolution echelle

spectrometer on the Keck 10-m telescope. _Proc. SPIE_ 2198, 362–375 (1994) Article  ADS  CAS  Google Scholar  * Butler, R. P. Attaining Doppler precision of 3 m s−1. _Publ. Astron. Soc.

Pacif._ 108, 500–509 (1996) Article  ADS  Google Scholar  * Pepe, F. et al. An Earth-sized planet with an Earth-like density. _Nature_ http://dx.doi.org/10.1038/nature12768 (this issue) *

Fortney, J. J. et al. Planetary radii across five orders of magnitude in mass and stellar insolation: application to transits. _Astrophys. J._ 659, 1661–1672 (2007) Article  ADS  CAS  Google

Scholar  * Hauck, S. A., II et al. The curious case of Mercury’s internal structure. _J. Geophys. Res._ 118, 1204–1220 (2013) Article  CAS  Google Scholar  * Lopez, E. D., Fortney, J. J.

& Miller, N. How thermal evolution and mass-loss sculpt populations of super-Earths and sub-Neptunes: application to the Kepler-11 system and beyond. _Astrophys. J._ 761, 59 (2012)

Article  ADS  Google Scholar  * Perez-Becker, D. & Chiang, E. Catastrophic evaporation of rocky planets. _Mon. Not. R. Astron. Soc._ 433, 2294–2309 (2013) Article  ADS  Google Scholar  *

Rappaport, S. et al. Possible disintegrating short-period super-Mercury orbiting KIC 12557548. _Astrophys. J._ 752, 1 (2012) Article  ADS  Google Scholar  * Jackson, B., Stark, C. C.,

Adams, E. R., Chambers, J. & Deming, D. A survey for very short-period planets in the Kepler data. _Astrophys. J_. (submitted); preprint at http://arxiv.org/abs/1308.1379 (2013) *

Muirhead, P. S. et al. Characterizing the cool KOIs. III. KOI 961: a small star with large proper motion and three small planets. _Astrophys. J._ 747, 144 (2012) Article  ADS  Google Scholar

  * Ofir, A. & Dreizler, S. An independent planet search in the Kepler dataset. I. One hundred new candidates and revised Kepler objects of interest. _Astron. Astrophys._ 555, A58 (2013)

Article  ADS  Google Scholar  * Rappaport, S. et al. The Roche limit for close-orbiting planets: minimum density, composition, and application to the 4.2 hr planet KOI 1843.01. _Astrophys.

J._ 773, L15 (2013) Article  ADS  Google Scholar  * Lissauer, J. J. et al. All six planets known to orbit Kepler-11 have low densities. _Astrophys. J._ 770, 131 (2013) Article  ADS  Google

Scholar  * Valenti, J. A. & Fischer, D. A. Spectroscopic properties of cool stars (SPOCS). I. 1040 F, G, and K dwarfs from Keck, Lick, and AAT planet search programs. _Astrophys. J._ 159

(Suppl.). 141–166 (2005) Article  ADS  CAS  Google Scholar  * Dotter, A. et al. The Dartmouth stellar evolution database. _Astrophys. J._ 178 (Suppl.). 89–101 (2008) Article  CAS  Google

Scholar  * Torres, G., Andersen, J. & Giménez, A. Accurate masses and radii of normal stars: modern results and applications. _Astron. Astrophys. Rev._ 18, 67–126 (2010) Article  ADS 

Google Scholar  * Seager, S. et al. Mass-radius relationships for solid exoplanets. _Astrophys. J._ 669, 1279–1297 (2007) Article  ADS  CAS  Google Scholar  * Wright, J. T. et al. The

exoplanet orbit database. _Publ. Astron. Soc. Pacif._ 123, 412–422 (2011) Article  ADS  Google Scholar  * Valenti, J. & Piskunov, N. Spectroscopy made easy: a new tool for fitting

observations with synthetic spectra. _Astron. Astrophys._ 118 (Suppl.). 595–603 (1996) ADS  Google Scholar  * 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  * Torres, G., Winn, J. N. & Holman, M. J. Improved parameters for extrasolar transiting planets. _Astrophys.

J._ 667, 1324 (2012) Google Scholar  * Henry, G. W. & Winn, J. N. The rotation period of the planet-hosting star HD 189733. _Astron. J._ 135, 68–71 (2008) Article  ADS  CAS  Google

Scholar  * 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) Article  ADS  CAS  Google Scholar  * Mamajek, E. E. & Hillenbrand, L. A. Improved age estimation for solar-type dwarfs using activity-rotation diagnostics. _Astrophys. J._ 687,

1264–1293 (2008) Article  ADS  CAS  Google Scholar  * Jones, B. F., Fischer, D. & Shetrone, M. The evolution of the lithium abundances of solar-type stars. VII. M34 (MGC 1039) and the

role of rotation in lithium depletion. _Astron. J._ 114, 352–362 (1997) Article  ADS  CAS  Google Scholar  * Seager, S. & Mallén-Ornelas, G. A unique solution of planet and star

parameters from an extrasolar planet transit light curve. _Astrophys. J._ 585, 1038–1055 (2003) Article  ADS  Google Scholar  * Mandel, K. & Agol, E. Analytic light curves for planetary

transit searches. _Astrophys. J._ 580, L171–L175 (2002) Article  ADS  Google Scholar  * Batalha, N. et al. Kepler’s first rocky planet: Kepler-10b. _Astrophys. J._ 729, 27 (2011) Article 

ADS  Google Scholar  * Marcy, G. W. & Butler, R. P. Precision radial velocities with an iodine absorption cell. _Publ. Astron. Soc. Pacif._ 104, 270–277 (1992) Article  ADS  Google

Scholar  * Valenti, J. A., Butler, R. P. & Marcy, G. W. 1995, Determining spectrometer instrumental profiles using FTS reference spectra. _Publ. Astron. Soc. Pacif._ 107, 966–976 (1995)

Article  ADS  Google Scholar  * Isaacson, H. & Fischer, D. Chromospheric activity and jitter measurements for 2630 stars on the California planet search. _Astrophys. J._ 725, 875–885

(2010) Article  ADS  CAS  Google Scholar  * Chubak, C. et al. Precise radial velocities of 2046 nearby FGKM stars and 131 standards. _Astrophys. J._. (submitted); available at

http://arxiv.org/abs/1207.6212 (2012) * Dumusque, X. et al. An Earth-mass planet orbiting α Centauri B. _Nature_ 491, 207–211 (2012) Article  ADS  CAS  Google Scholar  * Johnson, J. A. et

al. Retired A stars and their companions. VII. 18 new jovian planets. _Astrophys. J._ 197 (Suppl.). 26 (2012) Article  Google Scholar  * Hatzes, A. P. et al. The Mass of CoRoT-7b.

_Astrophys. J._ 743, 75 (2011) Article  ADS  Google Scholar  * Pecaut, M. J., Mamajek, E. E. & Bubar, E. J. A revised age for upper Scorpius and the star formation history among the

F-type members of the Scorpius-Centaurus OB association. _Astrophys. J._ 746, 154 (2012) Article  ADS  Google Scholar  Download references ACKNOWLEDGEMENTS This Letter and another10 were

submitted simultaneously and are the result of coordinated, independent radial-velocity observations and analyses of Kepler-78. We thank the HARPS-N team for their collegiality. We also

thank E. Chiang, I. Crossfield, R. Kolbl, E. Petigura, and D. Huber for discussions, S. Howard for support, C. Dressing for a convenient packaging of stellar models, and A. Hatzes for a

thorough review. This work was based on observations at the W. M. Keck Observatory granted by the University of Hawaii, the University of California, and the California Institute of

Technology. We thank the observers who contributed to the measurements reported here and acknowledge the efforts of the Keck Observatory staff. We thank those of Hawaiian ancestry on whose

sacred mountain of Mauna Kea we are guests. Kepler was competitively selected as the tenth Discovery mission with funding provided by NASA’s Science Mission Directorate. J.N.W. and R.S.-O.

acknowledge support from the Kepler Participating Scientist programme. A.W.H. acknowledges funding from NASA grant NNX12AJ23G. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Institute for

Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA, Andrew W. Howard, Benjamin J. Fulton & Evan Sinukoff * Department of Physics, and Kavli

Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA Roberto Sanchis-Ojeda & Joshua N. Winn * Astronomy Department,

University of California, Berkeley, 94720, California, USA Geoffrey W. Marcy & Howard Isaacson * Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, 02138,

Massachusetts, USA John Asher Johnson * Department of Astronomy, Yale University, New Haven, 06510, Connecticut, USA Debra A. Fischer * Department of Astronomy and Astrophysics, University

of California, Santa Cruz, 95064, California, USA Jonathan J. Fortney Authors * Andrew W. Howard View author publications You can also search for this author inPubMed Google Scholar *

Roberto Sanchis-Ojeda View author publications You can also search for this author inPubMed Google Scholar * Geoffrey W. Marcy View author publications You can also search for this author

inPubMed Google Scholar * John Asher Johnson View author publications You can also search for this author inPubMed Google Scholar * Joshua N. Winn View author publications You can also

search for this author inPubMed Google Scholar * Howard Isaacson View author publications You can also search for this author inPubMed Google Scholar * Debra A. Fischer View author

publications You can also search for this author inPubMed Google Scholar * Benjamin J. Fulton View author publications You can also search for this author inPubMed Google Scholar * Evan

Sinukoff View author publications You can also search for this author inPubMed Google Scholar * Jonathan J. Fortney View author publications You can also search for this author inPubMed 

Google Scholar CONTRIBUTIONS This measurement was conceived and planned by A.W.H., G.W.M., J.A.J., J.N.W. and R.S.-O. HIRES observations were conducted by A.W.H., G.W.M., H.I, B.J.F. and

E.S. The HIRES spectra were reduced and Doppler-analysed by A.W.H., G.W.M., H.I. and J.A.J. Data modelling was done primarily by A.W.H. and R.S.-O. A.W.H. was the primary author of the

manuscript, with important contributions from J.N.W., R.S.-O. and J.J.F. Figures were generated by A.W.H., R.S.-O, B.J.F. and E.S. All authors discussed the results, commented on the

manuscript and contributed to the interpretation. CORRESPONDING AUTHORS Correspondence to Andrew W. Howard or Roberto Sanchis-Ojeda. ETHICS DECLARATIONS COMPETING INTERESTS The authors

declare no competing financial interests. EXTENDED DATA FIGURES AND TABLES EXTENDED DATA FIGURE 1 WAVELENGTH-CALIBRATED SPECTRA OF THREE STARS NEAR THE AGE-SENSITIVE LI I LINE (6,708 Å).

This line is not detected in the Kepler-78 spectrum, suggesting that lithium has been depleted, consistent with an age exceeding half a billion years for this K0 star. The lithium line is

also not detected in the 4.6-billion-year-old Sun. (Gyr, billion years; Myr, million years.) It is clearly seen in the rotationally broadened spectrum of [PZ99] J161618.0 − 233947, a star

whose spectral type (G8) is similar to that of Kepler-78, but that is much younger (about 11 million years)43. Additional iron and calcium lines are labelled. EXTENDED DATA FIGURE 2

CORRELATIONS BETWEEN MODEL PARAMETERS IN THE TRANSIT ANALYSIS. Greyscale contours denote confidence levels, with thick black lines highlighting the 1_σ_, 2_σ_ and 3_σ_ contour levels. The

strongest correlations are between transit depth, scaled semi-major axis and impact parameter. EXTENDED DATA FIGURE 3 APPARENT RADIAL-VELOCITY VARIATIONS OF KEPLER-78 FOR THE OFFSET-SLOPE

MODEL. The top panel shows the complete 45-day time series of relative radial velocities (red filled circles). Eight grey boxes highlight nights of intensive observations. The measurements

from these nights are shown in the eight subpanels. In each subpanel, the radial velocities (red filled circles) and best-fit offset-slope model (solid black line) are shown. The

radial-velocity curves for 100 randomly selected models from the MCMC chain are underplotted in grey, showing the range of variation within the model distribution. SUPPLEMENTARY INFORMATION

SUPPLEMENTARY TABLES This file contains Supplementary Table 1, which shows time series radial velocity measurements and associated activity measurements. (PDF 145 kb) POWERPOINT SLIDES

POWERPOINT SLIDE FOR FIG. 1 POWERPOINT SLIDE FOR FIG. 2 RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Howard, A., Sanchis-Ojeda, R., Marcy, G. _et al._

A rocky composition for an Earth-sized exoplanet. _Nature_ 503, 381–384 (2013). https://doi.org/10.1038/nature12767 Download citation * Received: 25 September 2013 * Accepted: 11 October

2013 * Published: 30 October 2013 * Issue Date: 21 November 2013 * DOI: https://doi.org/10.1038/nature12767 SHARE THIS ARTICLE Anyone you share the following link with will be able to read

this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative