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Images of a fourth planet orbiting HR 8799

High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible due to the wide orbits (>25 AU) and youth (<100 Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. A major area of contention in the extrasolar planet community is whether outer planets (>10 AU) more massive than Jupiter form via one-step gravitational instabilities or, rather, via a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory to study the formation and evolution of giant planets at wide>10 AU separations.

New near-infrared observations of HR8799, 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 Figure 1. A fourth planet, designated HR 8799e, is detected at six different epochs at an averaged projected separation of 0.368 ± 0.009" (14.5 ± 0.5 AU). Planet e is bound to the star and is orbiting counter-clockwise (see  13 . This conclusion is based on common Galactic space motions and age indicators for stars located between the previouslyknown Columba members and HR8799. The younger age suggests smaller planet masses, but to be conservative, we use both age ranges (30  Myr (Columba association) and 60[30-160] Myr 1 ) to derive the physical properties of planet e. HR 8799e is located very near planets c and d in a K s versus K s -L' color-magnitude diagram, suggesting that all three planets have similar spectral shapes and bolometric luminosities. We, therefore, adopt the same luminosity for these three planets; however, given the larger photometric error-bars and sparse wavelength coverage associated with planet e, we have conservatively assigned to it a luminosity error (Table 1) twice as large as those for planets b, c, and d 1 . This luminosity estimate is consistent with empirically calibrated bolometric corrections for brown dwarfs 14 , although such corrections may be ill-suited for young planets with distinct spectra and colors. Using the two overlapping age ranges outlined above and the evolutionary models described in the HR 8799bcd discovery article 1 , we estimate the mass of planet e to be 7 [5][6][7][8][9][10]  Stability analyses [8][9][10] have shown that the original 3-planet system may be in a mean motion period resonance with an upper limit on planetary masses of ~20 M Jup assuming an age of up to 100 Myr. With the discovery of a fourth planet, we revisit the stability of this system. We searched for stable orbital configurations with the HYBRID/Mercury package 23 using the 30 Myr (5, 7, 7 and 7 M Jup for b, c, d and e) and 60 Myr (7, 10, 10 and 10 M Jup ) masses. In our preliminary search we held the parameters for b, c, and d fixed to those matching either the single resonance (1:2 resonance between planets c and d only) or double resonance (1:2:4 resonance between planets b, c, and d) stable solutions found to date 10 , but allowed the parameters for e to vary within the regime allowed by our observations. In 100,000 trials, seven solutions for e were found using the 30 Myr masses, based on the single-resonance configuration, that are stable for at least 160 Myr (the maximum estimated age of the system), and an additional five solutions were found that are stable for over 100 Myr. All maximally stable solutions have a semimajor axis of ~14.5 AU, with planets c, d and e in a 1:2:4 resonance (planet b not in resonance). A set of 100,000 trials were also performed using the 60 Myr masses, but only two solutions were found that are stable for over 100 Myr, each of which requires a semimajor axis of ~12.5 AU, 4-sigma away from our astrometry. This is suggestive that a younger system age and lower planet masses are preferred, although a much more thorough search of parameter space is required (see the Supplementary Information for tables of stable solutions).
The mechanism for forming this system is unclear. It is challenging for gravitationalinstability fragmentation to occur at a < 20-40 AU 24,25 -ruling that out for in situ formation of planet e. In addition, disk instability mechanisms preferentially form objects more massive than these planets 25,26 . If the HR 8799 system represented low-mass examples of such a population, brown-dwarf companions to young massive stars would be plentiful. Nearby young star surveys [27][28][29] and our nearly-complete survey of 80 stars with similar masses and ages to HR 8799, have discovered no such population of brown dwarf companions. HR 8799e and possibly d are close enough to the primary star to have formed by bottom-up accretion in situ 24 , but planets b and c are located where the collisional time-scale is conventionally thought to be too low for core accretion to form giant planets before the system's gas is depleted. A hybrid process with different planets forming through different mechanisms cannot be ruled out but seems unlikely with the similar masses and dynamical properties of the four planets. It is possible that one mechanism dominated the other and the planets later migrated to their current positions.
The HR 8799 debris disk is especially massive for a star of its age or any older age main sequence star 17 , which could indicate an extremely dense protoplanetary disk. Such a disk could have induced significant migration, moving planets formed by disk-instability inward or the disk could have damped the residual eccentricity from multi-planet gravitational interactions that moved core-accretion planets outward. The massive debris disk and the lack of higher-mass analogs to this system do suggest that HR 8799 represents the high-mass end of planet formation. The HR 8799 system does show interesting similarities with the Solar system with all giant planets located past the system's estimated snow line (~2.7 AU for the Solar system and ~6 AU for HR 8799) and their debris belts located at similar equilibrium temperatures (see Figure 4). With its more massive planets, massive debris belts and large scale, the HR 8799 planetary system is also an amazing example of Nature's ability to produce extreme systems. This publication makes use of data products from the Two Micron All Sky Survey and the SIMBAD database.

Author Contributions
The authors contributed equally to this work.     luminosity-versus-age tracks for planet evolution models 11 . Objects above 13 Jupitermass 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 limit 15 17,18 ; such an age would also require planetary masses far too high for long-term stability 10  underestimated if the planets formed by core-accretion, though "cold start" core-accretion models 19 do not reproduce the observed luminosity for any combination of mass and age.

Author Information
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 discovering and following in orbit planet-mass companions at ages when formation processes are important.