HR 8799 Christian Marois,1,2,3* Bruce Macintosh,2 Travis Barman,4B
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Direct Imaging of Multiple Planets Orbiting the Star HR 8799 Christian Marois,1,2,3* Bruce Macintosh,2 Travis Barman,4B. Zuckerman,5 Inseok Song,6 Jennifer Patience,7David Lafrenière,8 René Doyon9 1National Research Council Canada Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada. 2Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA. 3Astronomy Department, University of California, Berkeley, CA 94720, USA. 4Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA. 5Physics & Astronomy Department and Center for Astrobiology, University of California, Los Angeles, CA 90095, USA. 6University of Georgia, Physics and Astronomy, 240 Physics, Athens, GA 30602, USA. 7University of Exeter, School of Physics, Stocker Road, Exeter, EX4 4QL, UK. 8Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON, M5S 3H4, Canada. 9Département de Physique and Observatoire du Mont Mégantic, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC, H3C 3J7, Canada. *To whom correspondence should be addressed. E-mail: [email protected] Direct imaging of exoplanetary systems is a powerful There is indirect evidence for planets in orbits beyond technique that can reveal Jupiter-like planets in wide 5 AU from their star. Some images of dusty debris disks orbits, can enable detailed characterization of planetary orbiting main-sequence stars (the Vega phenomenon) show atmospheres, and is a key step toward imaging Earth-like spatial structure on a scale of tens to hundreds of planets. Imaging detections are challenging due to the astronomical units (2). The most likely explanation of such combined effect of small angular separation and large structure is gravitational perturbations by planets with semi- luminosity contrast between a planet and its host star. major axes comparable to the radius of the dusty disks and High-contrast observations with the Keck and Gemini rings [see references in (3)]. telescopes have revealed three planets orbiting the star The only technique currently available to detect planets HR 8799, with projected separations of 24, 38, and 68 with semi-major axes greater than about 5 AU in a reasonable astronomical units. Multi-epoch data show counter- amount of time is infrared imaging of young, nearby stars. clockwise orbital motion for all three imaged planets. The The detected near-infrared radiation is escaped internal heat low luminosity of the companions and the estimated age of energy from the recently formed planets. During the past the system imply planetary masses between 5 and 13 times decade, hundreds of young stars with ages ! 100 Ma have that of Jupiter. This system resembles a scaled-up version been identified within ~100 pc of Earth (4, 5), and many of of the outer portion of our solar system. these have been imaged in the near-IR with ground-based adaptive optics (AO) systems and with the Hubble Space During the past decade various planet detection techniques— Telescope. Direct imaging searches for companions of these precision radial velocities, transits, and microlensing—have stars have detected some objects that are generally considered been used to detect a diverse population of exoplanets. to be near or above the mass threshold 13.6 M dividing However, these methods have two limitations. First, the Jup planets from brown dwarfs [see (6) for an example and (7) for existence of a planet is inferred through its influence on the a list of known substellar objects orbiting stars], and one star about which it orbits; the planet is not directly discerned planetary mass companion that is orbiting a brown dwarf, not [photometric signals from some of the closest-in giant planets a star (8). Recently, Lafrenière et al. (9) have detected a have been detected by careful analysis of the variations in the candidate planet near a young (5 Ma) star of the Upper integrated brightness of the system as the planet orbits its star Scorpius association, but a proper motion analysis is required (1)]. Second, these techniques are limited to small (transits) to to confirm that it is bound to the host star and not an moderate (precision radial velocity and microlensing) planet- unrelated low-mass member of the young association. In this star separation. The effective sensitivities of the latter two issue, Kalas et al. report the detection, in visible light, of a techniques diminish rapidly at semi-major axes beyond about candidate planetary mass companion near the inner edge of 5 AU. Direct observations allow discovery of planets in wider the Fomalhaut debris disk (10). Non-detections of the orbits and allow the spectroscopic and photometric candidate companion at near-IR wavelengths suggest that the characterization of their complex atmospheres to derive their detected visible flux may be primarily host-star light physical characteristics. scattering off circumplanetary dust rather than photons from / www.sciencexpress.org / 13 November 2008 / Page 1 / 10.1126/science.1166585 the underlying object. A statistical Bayesian analysis of a When planets form, gravitational potential energy is dedicated AO survey of nearby young F-, G- and K-type stars released and turned into heat in their interior. As planets do shows that exoplanets are relatively rare at separations not possess any internal nuclear energy source to maintain >20 AU around stars with masses similar to the Sun (11). their temperature, they cool down and become less luminous Bright A-type stars have been mostly neglected in imaging with time. For massive planets, this self-luminosity can surveys since the higher stellar luminosity offers a less dominate over their stellar insolation for hundreds of millions favorable planet-to-star contrast. However, main sequence A- or billions of years. With some assumptions on the initial type stars do have some advantages. The higher-mass A stars conditions at the time of formation, a planet’s mass can be can retain heavier and more extended disks and thus might derived simply by estimating the planet’s luminosity and the form massive planets at wide separations, making their system age. Our age estimate for HR8799 is based on four planets easier to detect. Millimeter interferometric continuum lines of evidence: the star’s galactic space motion, the star’s observations of the nearest Herbig Ae stars, the precursors to position in a color-magnitude diagram, the typical age of A-type stars, indicate that these are encircled by disks with $ Boo and # Dor class stars and the large mass of the masses up to several times the Minimum Mass Solar Nebula HR 8799 debris disk. (12), the minimum amount of solar abundance material Most young stars in the solar neighborhood have Galactic (0.01MSun) required to form all planets in the solar system space motions (UVW) that fall in limited ranges. HR 8799’s (13). Associated millimeter line observations resolve these space motion with respect to the Sun, as calculated from gas disks and indicate that their outer radii are 85-450 AU published distance, radial velocity and proper motion, is (12). The most exceptional example of a young A-star disk is UVW = (–11.9, –21.0, –6.8 km s%1) (16, 22). This UVW is the one orbiting IRAS 18059-3211, which is estimated to similar to that of other stars with an age between that of the have a mass of 90 times the Minimum Mass Solar Nebula and TW Hydra association [8 Ma (4, 5)] and the Pleiades [125 an outer radius extending to ~3,000 AU (14). Radial velocity ±8 Ma (23)]. The UVW of HR 8799 is similar to that of surveys of evolved A stars do seem to confirm these members of the 30 Ma old, southern hemisphere, Columba hypotheses by showing a trend of a higher frequency of and Carina Associations (5). Calculations of the UVW of the planets at wider separations (15). In this article, we describe young stars HD 984 and HD 221318, which lie near HR8799, the detection of three faint objects at 0.63"", 0.95"", and 1.73"" show that their space motions are similar to that of HR8799. (24, 38, and 68 AU projected separation) (see Fig. 1) from the We estimate the ages of HD 984 and HD 221318 to be 30 and dusty and young A-type main sequence star HR 8799, show 100 Ma, respectively, while the FEPS team estimates the age that all objects are co-moving with HR 8799, and describe of HD 984 to be 40 Ma (24). Overall, the UVW of HR 8799 their orbital motion and physical characteristics. is clearly consistent with those of young clusters and HR 8799 stellar properties. HR 8799 [also V342 Peg & associations in the solar neighborhood. Of course, in this HIP114189, located 39.4 pc (16) from Earth] is the only star UVW range of young stars, there are also older stars with known that has simultaneously been classified as # Doradus random motions; so other, independent, methods must also be (variable), $ Bootis (metal-poor Population I A-type star) and employed to place limits/constraints on the age of HR 8799. Vega-like (far-IR excess emission from circumstellar dust) HR 8799 is also found below the main sequence of the (17, 18). A fit to the Infrared Astronomical Satellite (IRAS) Pleiades, & Per (70 Ma) and IC2391 (50 Ma) on a and Infrared Space Observatory (ISO) photometry indicates Hertzsprung-Russell diagram. This is consistent with a that it has a dominant dust disk with temperature of 50 K (3, younger age compared to that of the Pleiades (25). Even with 19). Such blackbody grains, in an optically thin disk, would the more recent Tycho measurement and correcting for the reside ~75 AU ( ~2"") from HR 8799.