HR 8799 is a roughly 30 million-year-oldmain-sequencestar located 133.3light-years (40.9parsecs) away fromEarth in theconstellation ofPegasus. It has roughly 1.5 times theSun's mass and 4.9 times its luminosity. It is part of a system that also contains adebris disk and at least fourmassive planets. These planets were the firstexoplanets whose orbital motion was confirmed bydirect imaging. The star is aGamma Doradus variable: itsluminosity changes because of non-radial pulsations of its surface. The star is also classified as aLambda Boötis star, which means its surface layers are depleted iniron peakelements. It is the only known star which is simultaneously a Gamma Doradus variable, aLambda Boötis type, and aVega-like star (a star withexcess infrared emission caused by acircumstellar disk).
HR 8799 is a star that is visible to the naked eye. It has a magnitude 5.96 and it is located inside the western edge of thegreat square of Pegasus almost exactly halfway betweenBeta andAlpha Pegasi. The star's name ofHR 8799 is its line number in theBright Star Catalogue.
The star HR 8799 is a member of theLambda Boötis (λ Boo) class, a group ofpeculiar stars with an unusual lack of "metals" (elements heavier than hydrogen and helium) in their upper atmosphere. Because of this special status, stars like HR 8799 have a very complex spectral type. The luminosity profile of theBalmer lines in the star's spectrum, as well as the star'seffective temperature, best match the typical properties of anF0 V star. However, the strength of thecalcium II Kabsorption line and the other metallic lines are more like those of anA5 V star. The star's spectral type is therefore written askA5 hF0 mA5 V;λ Boo.[3][4]
Age determination of this star shows some variation based on the method used. Statistically, for stars hosting a debris disk, the luminosity of this star suggests an age of about 20–150 million years. Comparison with stars having similar motion through space gives an age in the range 30–160 million years. Given the star's position on theHertzsprung–Russell diagram of luminosity versus temperature, it has an estimated age in the range of 30–1,128 million years.λ Boötis stars like this are generally young, with a mean age of a billion years. More accurately,asteroseismology also suggests an age of approximately a billion years.[10] However, this is disputed because it would make the planets become brown dwarfs to fit into the cooling models. Brown dwarfs would not be stable in such a configuration. The best accepted value for an age of HR 8799 is 30 million years, consistent with being a member of theColumba association co-movinggroup of stars.[11]
Earlier analysis of the star's spectrum reveals that it has a slight overabundance ofcarbon andoxygen compared to the Sun (by approximately 30% and 10% respectively). While some Lambda Boötis stars havesulfur abundances similar to that of the Sun, this is not the case for HR 8799; the sulfur abundance is only around 35% of the solar level. The star is also poor in elements heavier thansodium: for example, the iron abundance is only 28% of the solar iron abundance.[12]Asteroseismic observations of other pulsating Lambda Boötis stars suggest that the peculiar abundance patterns of these stars are confined to the surface only: the bulk composition is likely more normal. This may indicate that the observed element abundances are the result of the accretion of metal-poor gas from the environment around the star.[13]
In 2020, spectral analysis utilizing multiple data sources have detected an inconsistency in prior data and concluded the star carbon and oxygen abundances are the same or slightly higher than solar. The iron abundance was updated to 30+6
−5% of solar value.[8]
Astroseismic analysis using spectroscopic data indicates that the rotational inclination of the star is constrained to be greater than or approximately equal to 40°. This contrasts with the planets' orbital inclinations, which are in roughly the same plane at an angle of about20° ± 10°. Hence, there may be an unexplained misalignment between the rotation of the star and the orbits of its planets.[14] Observation of this star with theChandra X-ray Observatory indicates that it has a weak level ofmagnetic activity, but the X-ray activity is much higher than that of an A‑type star likeAltair. This suggests that the internal structure of the star more closely resembles that of an F0 star. The temperature of thestellar corona is about 3.0 million K.[15]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (years) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| Dust disk | 15±5[20]AU | — | — | |||
| e | 7.4±0.6 MJ | 16.25±0.04 | ~45 | 0.1445±0.0013 | 25 ± 8° | 1.17+0.13 −0.11 RJ |
| d | 9.1±0.2 MJ | 26.67±0.08 | ~100 | 0.1134±0.0011 | 28° | 1.2+0.1 −0 RJ |
| c | 7.8±0.5 MJ | 41.39±0.11 | ~190 | 0.0519±0.0022 | 28° | 1.2+0.1 −0 RJ |
| b | 5.7±0.4 MJ | 71.6±0.2 | ~460 | 0.016±0.001 | 28° | 1.2+0.1 −0.1 RJ |
| Dust disk | 135–360[21]AU | — | — | |||
On 13 November 2008, Christian Marois of the National Research Council of Canada'sHerzberg Institute of Astrophysics and his team announced they had directly observed threeplanets orbiting the star with theKeck andGemini telescopes inHawaii,[22][23][24][25] in both cases employingadaptive optics to make observations in theinfrared.[b] Aprecovery observation of the outer 3 planets was later found in infrared images obtained in 1998 by theHubble Space Telescope'sNICMOS instrument, after a newly developed image-processing technique was applied.[26] Further observations in 2009–2010 revealed the fourth giant planet orbiting inside the first three planets at aprojected separation just less than 15 AU,[16][27] which has been confirmed by multiple studies.[28]
The outer planet orbits are inside a dusty disk like the SolarKuiper belt. It is one of the most massive disks known around any star within 300 light years of Earth, and there is room in the inner system forterrestrial planets.[24] There is an additional debris disk just inside the orbit of the innermost planet.[16]
The orbital radii of planets e,d,c, andb are 2–3 times those ofJupiter,Saturn,Uranus, andNeptune's orbits, respectively. Because of theinverse square law relatingradiationintensity to distance from the source, comparable radiation intensities are present at distances√4.9 ≈ 2.2 times farther from HR 8799 than from the Sun, the upshot being that corresponding planets in the solar and HR 8799 systems receive similar amounts of stellar radiation.[16]
These objects are near the upper mass limit for classification as planets; if they exceeded 13 Jupiter masses, they would be capable ofdeuteriumfusion in their interiors and thus qualify asbrown dwarfs under the definition of these terms used by theIAU's Working Group on Extrasolar Planets.[29] If the mass estimates are correct, the HR 8799 system is the first multiple-planet extrasolar system to be directly imaged.[23] The orbital motion of the planets is in an anticlockwise direction and was confirmed via multiple observations dating back to 1998.[22] The system is more likely to be stable if the planets e, d, and c are in a 4:2:1 resonance, which would imply that the orbit of the planet d has an eccentricity exceeding 0.04 in order to match the observational constraints. Planetary systems with the best-fit masses from evolutionary models would be stable if the outer three planets are in a 1:2:4 orbital resonance (similar to theLaplace resonance between Jupiter's inner threeGalilean satellites:Io,Europa, andGanymede as well as three of the planets in theGliese 876 system).[16] However, it is disputed if planet b is in resonance with the other 3 planets. According to dynamical simulations, the HR 8799 planetary system may be even an extrasolar system with multiple resonance 1:2:4:8.[19] The 4 young planets are still glowing red hot from the heat of their formation, and are larger than Jupiter and over time they will cool and shrink to the sizes of 0.8–1.0 Jupiter radii.
The broadband photometry of planets b, c and d has shown that there may be significant clouds in their atmospheres,[27] while the infrared spectroscopy of planets b and c points to non-equilibriumCO /CH4 chemistry.[16] Near-infrared observations with theProject 1640 integral field spectrograph on the Palomar Observatory have shown that compositions between the four planets vary significantly. This is a surprise since the planets presumably formed in the same way from the same disk and have similar luminosities.[30]
An additional planet candidate was found in cycle 1 withNIRCam, 5arcseconds south of HR 8799. Follow-up observations with NIRCam are planned to confirm or reject this candidate.[31]
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A number of studies have used the spectra of HR 8799's planets to determine their chemical compositions and constrain their formation scenarios. The first spectroscopic study of planet b (performed at near-infrared wavelengths) detected strong water absorption and hints of methane absorption.[32] Subsequently, weak methane and carbon monoxide absorption in this planet's atmosphere was also detected, indicating efficient vertical mixing of the atmosphere and a disequilibriumCO /CH4 ratio at the photosphere. Compared to models of planetary atmospheres, this first spectrum of planet b is best matched by a model of enhancedmetallicity (about 10 times the metallicity of the Sun), which may support the notion that this planet formed through core-accretion.[33]
The first simultaneous spectra of all four known planets in the HR 8799 system were obtained in 2012 using the Project 1640 instrument at Palomar Observatory. The near-infrared spectra from this instrument confirmed the red colors of all four planets and are best matched by models of planetary atmospheres that include clouds. Though these spectra do not directly correspond to any known astrophysical objects, some of the planet spectra demonstrate similarities with L- and T-typebrown dwarfs and the night-side spectrum of Saturn. The implications of the simultaneous spectra of all four planets obtained with Project 1640 are summarized as follows: Planet b contains ammonia and/or acetylene as well as carbon dioxide, but has little methane; planet c contains ammonia, perhaps some acetylene but neither carbon dioxide nor substantial methane; planet d contains acetylene, methane, and carbon dioxide but ammonia is not definitively detected; planet e contains methane and acetylene but no ammonia or carbon dioxide. The spectrum of planet e is similar to a reddened spectrum of Saturn.[30]
Moderate-resolution near-infrared spectroscopy, obtained with the Keck telescope, definitively detected carbon monoxide and water absorption lines in the atmosphere of planet c. The carbon-to-oxygen ratio, which is thought to be a good indicator of the formation history for giant planets, for planet c was measured to be slightly greater than that of the host star HR 8799. The enhanced carbon-to-oxygen ratio and depleted levels of carbon and oxygen in planet c favor a history in which the planet formed through core accretion.[34] However, it is important to note that conclusions about the formation history of a planet based solely on its composition may be inaccurate if the planet has undergone significant migration, chemical evolution, or core dredging.[clarification needed] Later, in November 2018, researchers confirmed the existence of water and the absence ofmethane in the atmosphere ofHR 8799 c using high-resolution spectroscopy and near-infrared adaptive optics (NIRSPAO) at the Keck Observatory.[35][36]
The red colors of the planets may be explained by the presence of iron and silicate atmospheric clouds, while their low surface gravities might explain the strong disequilibrium concentrations of carbon monoxide and the lack of strong methane absorption.[34]
In January 2009 theSpitzer Space Telescope obtained images of the debris disk around HR 8799. Three components of the debris disk were distinguished:
The halo is unusual and implies a high level of dynamic activity which is likely due to gravitational stirring by the massive planets.[37] The Spitzer team says that collisions are likely occurring among bodies similar to those in the Kuiper Belt and that the three large planets may not yet have settled into their final, stable orbits.[38]
In the photo, the bright, yellow-white portions of the dust cloud come from the outer cold disk. The huge extended dust halo, seen in orange-red, has a diameter of ≈ 2,000 AU. The diameter of Pluto's orbit (≈ 80 AU) is shown for reference as a dot in the centre.[39]
This disk is so thick that it threatens the young system's stability.[40]
The disk was first resolved with ALMA in 2016[41] and was later imaged again in 2018. These later observations were more detailed and were studied by a team of astronomers. The disk has according to this team a smooth inner edge and a smooth outer edge. These also observed a possible inner dust belt.[21] This inner belt was confirmed withMIRI observations, which measured a radius of 15 au of the inner disk.[20]
Up until the year 2010,telescopes could onlydirectly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger thanJupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation. However, in 2010 a team fromNASAsJet Propulsion Laboratory demonstrated that avortex coronagraph could enable small telescopes to directly image planets.[42] They did this by imaging the previously imaged HR 8799 planets using just a 1.5 m portion of theHale Telescope.
In 2009, an oldNICMOS image was processed to show a predicted exoplanet around HR 8799.[43] In 2011, three furtherexoplanets were rendered viewable in a NICMOS image taken in 1998, using advanced data processing.[43] The image allows the planets' orbits to be better characterised, since they take many decades to orbit their host star.[43]
Starting in 2010, astronomers searched for radio emissions from theexoplanets orbiting HR 8799 using the radio telescope atArecibo Observatory. Despite the large masses, warm temperatures, andbrown dwarf-like luminosities, they failed to detect any emissions at 5 GHz down to a flux density detection threshold of 1.0 mJy.[44]
Media related toHR 8799 at Wikimedia Commons
