Images of a Fourth Planet Orbiting HR 8799

LETTER doi:10.1038/nature09684

Images of a fourth planet orbiting HR 8799

Christian Marois1, B. Zuckerman2, Quinn M. Konopacky3, Bruce Macintosh3 & Travis Barman4

z100 High-contrast near-infrared imaging of the nearby star HR 8799 range between 30 and 160 Myr (here we represent this as 60{30 Myr), has shown three giant planets1. Such images were possible because consistent with an earlier estimate of 20–150 Myr (ref. 5). Two recent of the wide orbits (.25 astronomical units, where 1 AU is the Earth– analyses (R. Doyon et al., and B. Zuckerman et al., manuscripts in Sun distance) and youth (,100 Myr) of the imaged planets, which preparation) independently deduce that HR 8799 is very likely to be are still hot and bright as they radiate away gravitational energy a member of the 30 Myr Columba association6. This conclusion is acquired during their formation. An important area of contention based on common Galactic space motions and age indicators for stars in the exoplanet community is whether outer planets (.10 AU)more located between the previously-known Columba members and HR massive than Jupiter form by way of one-step gravitational instabil- 8799. The younger age suggests smaller planet masses, but to be con- 2 z20 ities or, rather, through a two-step process involving accretion of a servative, we use both age ranges (30{10 Myr (Columba association) z100 1 core followed by accumulation of a massive outer envelope com- and 60{30 Myr ) to derive the physical properties of planet e. posed primarily of hydrogen and helium3. Here we report the pres- ence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a ab21 July 2010, L′ band 13 July 2010, Ks band N challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young E giant planets and known cold/warm debris belts4, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (.10 AU) separations. New near-infrared observations of HR 8799, optimized for detecting close-in planets, were made at the Keck II telescope in 2009 and 2010. (See Table 1 for a summary.) A subset of the images is presented in Fig. 1. A fourth planet, designated HR 8799e, is detected at six different 0.5′′ epochs at an averaged projected separation of 0.3680 6 0.003’’ (14.5 6 0.4 AU). Planet e is bound to the star and is orbiting anticlock- b wise (see Fig. 2), as are the three other known planets in the system. The c measured orbital motion, 46 6 10 mas yr21, is consistent with a roughly circular orbit of semimajor axis (a)14.5AU with a ,50-year period. Knowledge of the age and luminosity of the planets is critical for deriving their fundamental properties, including mass. In 2008 we used various techniques to estimate an age of 60 Myr with a plausible e

20 AU d Table 1 | HR 8799e astrometry, photometry and physical 0.5′′ characteristics c 1 November 2009, L′ band Epoch, band, wavelength Separation [E, N] from the host star 2009 Jul. 31, Kp band 2.124 mm(60.0190)[20.2990, 20.2170] Figure 1 | HR 8799e discovery images. Images of HR 8799 (a star at 2009 Aug. 1, L9 band 3.776 mm(60.0130)[20.3030, 20.2090] 39.4 6 1.0 pc and located in the Pegasus constellation) were acquired at the 22 2009 Nov. 1, L9 band 3.776 mm(60.0100)[20.3040, 20.1960] Keck II telescope with the Angular Differential Imaging technique (ADI) to 2010 Jul. 13, Ks band 2.146 mm(60.0080)[20.3250, 20.1730] allow a stable quasi-static point spread function (PSF) while leaving the field-of- 2010 Jul. 21, L9 band 3.776 mm(60.0110)[20.3240, 20.1750] view to rotate with time while tracking the star in the sky. The ADI/LOCI22,23 2010 Oct. 30, L9 band 3.776 mm(60.0100)[20.3340, 20.1620] SOSIE software24 was used to subtract the stellar flux, and to combine and flux- 24 Parameter Value calibrate the images. Our SOSIE software iteratively fits the planet PSF to derive relative astrometry and photometry (the star position and its Projected separation, avg. from all epochs* (AU) 14.5 6 0.4 21 photometry were obtained from unsaturated data or from its PSF core that was Orbital motion (arcsec yr ) 0.046 6 0.010 detectable through a flux-calibrated focal plane mask). a,AnL9-band image Period for a face-on circular orbit (yr) ,50 DKs 2.146 mm{ (mag) 10.67 6 0.22 acquired on 21 July 2010; b, a Ks-band image acquired on 13 July 2010 (arrows DL9 3.776 mm{ (mag) 9.37 6 0.12 in a and b point towards planet e); c,anL9-band image acquired on 1 November Absolute magnitude at 2.146 mm, MKs (mag) 12.93 6 0.22 2009. All three sequences were ,1 h long. No coronagraphic focal plane mask Absolute magnitude at 3.776 mm, ML’ (mag) 11.61 6 0.12 was used on 1 November 2009, but a 400-mas-diameter mask was used on 13 Luminosity (log L[) 24.7 6 0.2 July and 21 July 2010. HR 8799e is located southwest of the star. Planets b, c and z20 z3 Mass for 30{10 Myr (MJup)7{2 d are seen at respective projected separations of 68, 38 and 24 AU from the z100 z3 , Mass for 60{30 Myr (MJup)10{3 central star, consistent with roughly circular orbits at inclinations of 40u (refs 1 11–13). Their masses (7, 10 and 10 MJup for b, c and d for 60 Myr age ; 5, 7 and * The projected separation error (in AU) also accounts for the uncertainty in the distance to the star. { Planet-to-star flux ratios, expressed as difference of magnitude. No reliable photometry was derived 7 MJup for 30 Myr age) were estimated from their luminosities using age- for the Kp-band 2009 Jul. 31 data. dependent evolutionary models25. North is up and east is left.

1National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia V9E 2E7, Canada. 2Physics & Astronomy Department, University of California, Los Angeles, California 90095, USA. 3Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA. 4Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, Arizona 86001, USA.

Projected separation (AU) 60 40 20 0 –20 –40 –60 –80 1.5 e –0.15 1.0 40 Projected separation ( –0.20 b c 0.5 –0.25 1 s.d. error bars 20 –0.30 –0.35 –0.40

0.0 1: 31 July 2009 0 Δ declination ( ′′ )

2: 1 August 2009 AU

e 3: 1 November 2009 ) 4: 13 July 2010 –0.5 –20 5: 21 July 2010 d 6: 30 October 2010

–1.0 1 0 –1 –2 Δ right ascension (′′) Figure 2 | HR 8799e 2009–10 astrometry. Main figure, the 2009–10 orbital Planet e is confirmed as bound to HR 8799, and it is moving at motions of the four planets—b, c, d, e. Crosses denote the positions for 2009 and 46 6 10 mas yr21 anticlockwise. In the main figure, the orbits of the giant 2010 first an last epochs for b, c and d, and for all six epochs for e. A square is planets of our Solar System (Jupiter, Saturn, Uranus and Neptune) are drawn to drawn over the cross symbol of each planet’s first epoch. Inset, a zoomed scale (light grey circles). With a period of ,50 years, the orbit of HR 8799e will version of planet e’s astrometry, including the expected motion (curved solid be rapidly constrained by future observations; at our current measurement line) if it is an unrelated background object; each epoch is labelled by a number accuracy, it will be possible to measure orbital curvature after only 2 years. 1–6; a dashed line connects the star to each epoch data point; error bars, 61s.d.

HR 8799e is located very near planets c and d in a Ks versus Ks 2 L’ for young planets with distinct spectra and colours. Using the two colour–magnitude diagram, suggesting that all three planets have sim- overlapping age ranges outlined above and the evolutionary models ilar spectral shapes and bolometric luminosities. We, therefore, adopt described in the HR 8799bcd discovery article1, we estimate the mass of z3 z3 the same luminosity for these three planets; however, given the larger planet e to be 7{2 MJup (30 Myr) and 10{3 MJup (60 Myr), where MJup photometric error-bars and sparse wavelength coverage associated is the mass of Jupiter; see Fig. 3. The broadband photometry of planets with planet e, we have conservatively assigned to it a luminosity error b, c, and d provide strong evidence for significant atmospheric cloud (Table 1) twice as large as those for planets b, c and d1. This luminosity coverage, while recent spectroscopy of planets b and c show evidence 8–10 estimate is consistent with empirically calibrated bolometric correc- for non-equilibrium CO/CH4 chemistry . Given the limited wave- tions for brown dwarfs7, although such corrections may be ill-suited length coverage of the discovery images for planet e, it is too early to

–1 Figure 3 | The mass of HR 8799e from the age–luminosity relationship. 25 Stars Solid lines are luminosity-versus-age tracks for planet evolution models (luminosities are normalized to the solar luminosity, L[). Objects above 13 –2 MJup are typically considered to be outside the planet-mass regime; however, the tail end of the planet distribution found by radial velocity surveys extends above this IAU-defined mass limit26. Boxed areas show adopted luminosity ranges (61s.d.) and estimated age ranges for the four HR 8799 planets: cross- z20 –3 hatched boxes show age range 30{10 Myr; grey boxes show age range z100 3 60{30 Myr; planets c, d and e have similar luminosities, but the luminosity 2 uncertainty for e is larger and indicated by the darker box/opposite hatch. For 4 Brown comparison, the ages and luminosities of four recently imaged planet-mass

) dwarfs companions near other stars are indicated (numbered 1–4; see key on figure) ( –4 showing 1s.d. error bars for the luminosity and estimated age ranges). An HR 8799 asteroseismology study suggested that the HR 8799 system might be as old as log( L / 1 cde ,1 Gyr (ref. 27), but it is highly unlikely that such an old star would have very 21,28 M massive debris belts ; such an age would also require planetary masses far too –5 13 b high for long-term stability13. The older age also requires an inclination of the Jup stellar pole relative to the line of sight of ,50u, inconsistent with the nearly face- 10 M on planetary system and the ,25u inclination upper limit measured from 4 Jup Spitzer images of the outer dust halo . Mass estimates based on any existing M M –6 1: 2m 1207 b 3 5 evolutionary model at ages as young as 20–30 Myr suffer from unconstrained Jup Jup 2: AB Pic b initial formation conditions; the masses presented here could be 3: 1RXS 1609 b 4: β Pic b underestimated if the planets formed by core-accretion, though ‘cold start’ core-accretion models29 do not reproduce the observed luminosity for any –7 combination of mass and age. While this additional uncertainty can lead temporarily to ambiguity about the planets’ masses and formation history (core-accretion or gravitation instability), it does highlight the importance of 6.0 6.5 7.0 7.5 8.0 8.5 discovering and following in orbit planet-mass companions at ages when log[Age (yr)] formation processes are important.

Separation (AU) 10 20 30 40 50

1 2 Jupiter Saturn Uranus Neptune 14 32 Solar System

Stellar flux density, normalized to Solar System value at 50AU 25 6.2 2.8 1.6 1

12 ed c b 14 32 HR 8799

20 40 60 80 100 Separation (AU) Figure 4 | Comparison of HR 8799 and our Solar System. Top, Solar System; four planets of HR 8799 and the preferred locations of the inner warm debris bottom, the HR 8799 system. HR 8799 infrared data indicate the existence of an disk and the inner edge of the outer cold disk (90 AU)4, then (1) the indicated 4:1 asteroid belt analogue located at 6–15 AU (we have moved the estimated outer and 2:1 period resonances between the inner/outer edge of the warm debris belt edge of this belt to 10 AU because of planet e’s estimated chaotic region30), a and planet e, and (2) a 3:2 mean motion resonance of b with the inner edge of Edgeworth-Kuiper-belt-like debris disk at .90 AU and a small particle halo the outer cold disk, are both consistent with the observations. By analogy, the extending up to 1,000 AU(ref. 4). The red shaded regions represent the locations inner and outer edges of the main asteroid belt of our Solar System are, of the inner and outer debris belts in both systems (the Solar System Oort comet respectively, in 4:1 and 2:1 mean motion resonances with Jupiter. Many cloud and the HR 8799 halo are not shown). The horizontal axis of the HR 8799 members of the Edgeworth–Kuiper belt, including Pluto, are in a 3:2 mean plot is compressed by the square root of the ratio of the luminosity of HR 8799 motion orbital resonance with Neptune. Solar System planet images are from (4.92 6 0.41 L[) to that of the Sun to show both systems over the same NASA; HR 8799 artwork is from Gemini Observatory and L. Cook. Planet equilibrium temperature range. Given the current apparent separations of the diameters are not to scale. say much about the atmospheric properties of this particular planet; formation of planet e. In addition, disk instability mechanisms pref- however, given that its near-infrared colour is similar to those of the erentially form objects more massive than these planets16,17. If the HR other three planets, we can anticipate similar cloud structure and 8799 system represented low-mass examples of such a population, chemistry for planet e. brown-dwarf companions to young massive stars would be plentiful. Stability analyses11–13 have shown that the original three-planet sys- Nearby young star surveys18–20 and our nearly complete survey of 80 temmaybeinameanmotionperiodresonancewithanupperlimiton stars with similar masses and ages to HR 8799 have discovered no such planetary masses of ,20 MJup assuming an age of up to 100 Myr. With population of brown dwarf companions. HR 8799e and possibly d are the discovery of a fourth planet, we revisit the stability of this system. We close enough to the primary star to have formed by bottom-up accre- searched for stable orbital configurations with the HYBRID/Mercury tion in situ15, but planets b and c are located where the collisional 14 package using the 30-Myr (5, 7, 7 and 7 MJup for b, c, d and e respec- timescale is conventionally thought to be too low for core accretion tively) and 60-Myr (7, 10, 10 and 10 MJup) masses. In our preliminary to form giant planets before the system’s gas is depleted. A hybrid search, we held the parameters for b, c and d fixed to those matching process with different planets forming through different mechanisms either the single resonance (1:2 resonance between planets c and d only) cannot be ruled out, but seems unlikely with the similar masses and or double resonance (1:2:4 resonance between planets b, c, and d) stable dynamical properties of the four planets. It is possible that one mech- solutions found to date13, but allowed the parameters for e to vary within anism dominated the other and the planets later migrated to their the regime allowed by our observations. On the basis of the single- current positions. The HR 8799 debris disk is especially massive for resonance configuration and using the 30-Myr masses, in 100,000 trials a star of its age (or for any older main sequence star21), which could seven solutions for e were found that are stable for at least 160 Myr (the indicate an extremely dense protoplanetary disk. Such a disk could maximum estimated age of the system), and an additional five solutions have induced significant migration, moving planets formed by disk- were found that are stable for over 100 Myr. All maximally stable solu- instability inward, or the disk could have damped the residual eccent- tions have a semimajor axis of ,14.5 AU, with planets c, d and e in a 1:2:4 ricity from multi-planet gravitational interactions that moved core- resonance (planet b not in resonance). A set of 100,000 trials was also accretion planets outward. The massive debris disk and the lack of performed using the 60-Myr masses, but only two solutions were found higher-mass analogues to this system do suggest that HR 8799 repre- that are stable for over 100 Myr, each of which requires a semimajor axis sents the high-mass end of planet formation. of ,12.5 AU,4s away from our astrometry. This is suggestive that a The HR 8799 system does show interesting similarities with our younger system age and lower planet masses are preferred, although a Solar System; all giant planets are located past the estimated snow line much more thorough search of parameter space is required (see the of each system (,2.7 AU for the Solar System, ,6 AU for HR 8799), and Supplementary Information for tables of stable solutions). the debris belts of each system are located at similar equilibrium tem- The mechanism for the formation of this system is unclear. It is peratures (Fig. 4). With its four massive planets, massive debris belts challenging for gravitational-instability fragmentation to occur at and large scale, the HR 8799 planetary system is an amazing example a , 20–40 AU (refs 15,16)—ruling that mechanism out for in situ of extreme systems that can form around stars.

Received 5 November; accepted 18 November 2010. 20. Chauvin, G. et al. Deep imaging survey of young, nearby austral stars. VLT/NACO near-infrared Lyot-coronagraphic observations. Astron. Astrophys. 509, A52–A68 Published online 8 December 2010. (2010). 21. Rhee, J. H., Song, I., Zuckerman, B. & McElwain, M. Characterization of dusty debris 1. Marois, C. et al. Direct imaging of multiple planets orbiting the star HR 8799. disks: the IRAS and Hipparcos catalogs. Astrophys. J. 660, 1556–1571 (2007). Science 322, 1348–1352 (2008). 22. Marois, C., Lafrenie`re, D., Doyon, R., Macintosh, B. & Nadeau, D. Angular differential 2. Kuiper, G. P. On the origin of the Solar System. Proc. Natl Acad. Sci. USA 37, 1–14 imaging: a powerful high-contrast imaging technique. Astrophys. J. 641, 556–564 (1951). (2006). 3. Mizuno, H. Formation of the giant planets. Prog. Theor. Phys. 64, 544–557 (1980). 23. Lafrenie`re, D., Marois, C., Doyon, R., Nadeau, D. & Artigau, E´. A new algorithm for 4. Su, K. Y. L. et al. The debris disk around HR 8799. Astrophys. J. 705, 314–327 point-spread function subtraction in high-contrast imaging: a demonstration with (2009). angular differential imaging. Astrophys. J. 660, 770–780 (2007). 5. Moo´r, A. et al. Nearby debris disk systems with high fractional luminosity 24. Marois, C., Macintosh, B. & Ve´ran, J.-P. Exoplanet imaging with LOCI processing: reconsidered. Astrophys. J. 644, 525–542 (2006). photometry and astrometry with the new SOSIE pipeline. Proc. SPIE 7736, 6. Torres, C. A. O., Quast, G. R., Melo, C. H. F. & Sterzik, M. F. in Handbook of Star Forming 77361J–77361J-12 (2010). Regions Vol. II, The Southern Sky (ed. Reipurth, B.) 757–812 (ASP Monograph 25. Baraffe, I., Chabrier, G., Barman, T. S., Allard, F. & Hauschildt, P. H. Evolutionary Publications, MP 005, 2008). models for cool brown dwarfs and extrasolar giant planets. The case of HD 7. Golimowsky, D. A. et al. L’ and M’ photometry of ultracool dwarfs. Astrophys. J. 127, 209458. Astron. Astrophys. 402, 701–712 (2003). 3516–3536 (2004). 26. Eggenberger, A. & Udry, S. Detection and characterization of extrasolar planets 8. Hinz, P. M. et al. Thermal infrared MMTAO observations of the HR 8799 planetary through Doppler spectroscopy. EAS Publ. Ser. 41, 27–75 (2010). system. Astrophys. J. 716, 417–426 (2010). 27. Moya, A. et al. Age determination of the HR8799 planetary system using 9. Janson, M., Bergfors, C., Goto, M., Brandner, W. & Lafrenie`re, D. Spatially resolved asteroseismology. Mon. Not. R. Astron. Soc. 405, L81–L85 (2010). spectroscopy of the exoplanet HR 8799 c. Astrophys. J. 710, L35–L38 (2010). 28. Ga´spa´r, A. et al. The low level of debris disk activity at the time of the late heavy 10. Bowler, B. P., Liu, M. C., Dupuy, T. J. & Cushing, M. C. Near-infrared spectroscopy of bombardment: a Spitzer study of Praesepe. Astrophys. J. 697, 1578–1596 (2009). the extrasolar planet HR 8799 b. Astrophys. J. 723, 850–868 (2010). 29. Marley, M. S., Fortney, J., Hubickyj, O., Bodenheimer, P. & Lissauer, J. J. On the 11. Goz´dziewski, K. & Migaszewski, C. Is the HR 8799 extrasolar system destined for luminosity of young Jupiters. Astrophys. J. 655, 541–549 (2007). planetary scattering? Mon. Not. R. Astron. Soc. 397, L16–L20 (2009). 30. Malhotra, R. in Solar System Formation and Evolution (eds Lazzaro, D., Vieira Martins, 12. Reidemeister, M. et al. A possible architecture of the planetary system HR 8799. R., Ferraz-Mello, S. & Fernandez, J.) 37–63 (ASP Conf. Ser. Vol. 149, 1998). Astron. Astrophys. 503, 247–258 (2009). Supplementary Information is linked to the online version of the paper at 13. Fabrycky, D. C. & Murray-Clay, R. A. Stability of the directly imaged multiplanet www.nature.com/nature. system HR 8799: resonance and masses. Astrophys. J. 710, 1408–1421 (2010). 14. Chambers, J. E. A hybrid symplectic integrator that permits close encounters Acknowledgements We thank the Keck staff, particularly H. Lewis, B. Goodrich and between massive bodies. Mon. Not. R. Astron. Soc. 304, 793–799 (1999). J. Lyke, for support with the follow-up observations. We thank G. Laughlin and D.C. 15. Dodson-Robinson, S. E., Veras, D., Ford, E. B. & Beichman, C. A. The formation Fabrycky for discussions. Portions of this research were performed under the auspices mechanism of gas giants on wide orbits. Astrophys. J. 707, 79–88 (2009). of the US Department of Energy by LLNL and also supported in part by the NSF Center 16. Kratter, K. M., Murray-Clay, R. A. & Youdin, A. N. The runts of the litter: why planets for Adaptive Optics. We acknowledge support by NASA grants to UCLA, LLNL and formed through gravitational instability can only be failed binary stars. Astrophys. Lowell Observatory. The data were obtained at the W.M. Keck Observatory. This J. 710, 1375–1386 (2010). publication makes use of data products from the Two Micron All Sky Survey and the 17. Stamatellos, D. & Whitworth, A. P. The properties of brown dwarfs and low-mass SIMBAD database. hydrogen-burning stars formed by disc fragmentation. Mon. Not. R. Astron. Soc. Author Contributions The authors contributed equally to this work. 392, 413–427 (2009). 18. Lafrenie`re, D. et al. The Gemini Deep Planet Survey. Astrophys. J. 670, 1367–1390 Author Information Reprints and permissions information is available at (2007). www.nature.com/reprints. The authors declare no competing financial interests. 19. Nielsen, E. & Close, L. M. A uniform analysis of 118 stars with high-contrast Readers are welcome to comment on the online version of this article at imaging: long-period extrasolar giant planets are rare around Sun-like stars. www.nature.com/nature. Correspondence and requests for materials should be Astrophys. J. 717, 878–896 (2010). addressed to C.M. ([email protected]).