HR 8799 is a roughly 30 million-year-old main-sequence star located
129 light years (39 parsecs) away from
Earth in the constellation of
Pegasus. It has roughly 1.5 times the Sun's mass and 4.9 times its
luminosity. It is part of a system that also contains a debris disk
and at least four massive planets. Those planets, along with
Fomalhaut b, were the first extrasolar planets whose orbital motion
was confirmed via direct imaging. The designation
HR 8799 is the
star's identifier in the Bright
Star Catalogue. The star is a Gamma
Doradus variable: its luminosity changes because of non-radial
pulsations of its surface. The star is also classified as a Lambda
Boötis star, which means its surface layers are depleted in iron peak
elements. This may be due to the accretion of metal-poor
circumstellar gas. It is the only known star which is
simultaneously a Gamma Doradus variable, a Lambda Boötis type, and a
Vega-like star (a star with excess infrared emission caused by a
1 Stellar properties
2 Planetary system
2.1 Planet spectra
2.2 Debris disk
2.3 Vortex Coronagraph: Testbed for high-contrast imaging technology
3 See also
6 External links
HR 8799 is a member of the Lambda Boötis (λ Boo) class, a
group of peculiar 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 the
Balmer lines in the
star's spectrum, as well as the star's effective temperature, best
match the typical properties of an F0 V star. However, the
strength of the calcium II K absorption line and the other metallic
lines are more like those of an A5 V star. The star's spectral
type is therefore written as kA5 hF0 mA5 V; λ Boo.
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
Hertzsprung–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. 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 the Columba Association co-moving
group of stars.
Detailed analysis of the star's spectrum reveals that it has a slight
overabundance of carbon and oxygen compared to the
approximately 30% and 10% respectively). While some Lambda Boötis
stars have sulfur 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 than sodium:
for example, the iron abundance is only 28% of the solar iron
abundance. 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.
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 about 20° ± 10°. Hence, there may be an unexplained misalignment
between the rotation of the star and the orbits of its planets.
Observation of this star with the
Chandra X-ray Observatory
Chandra X-ray Observatory indicates
that it has a weak level of magnetic activity, but the X-ray activity
is much higher than that of an A-type star like Altair. This suggests
that the internal structure of the star more closely resembles that of
an F0 star. The temperature of the corona is about 3.0 million K.
HR 8799 planetary system
(in order from star)
On 13 November 2008, Christian Marois of the National Research Council
Herzberg Institute of Astrophysics
Herzberg Institute of Astrophysics and his team announced
they had directly observed three planets orbiting the star with the
Keck and Gemini telescopes in Hawaii, in both cases
employing adaptive optics to make observations in the infrared.[note
3] A precovery observation of the outer 3 planets was later found in
infrared images obtained in 1998 by the Hubble Space Telescope's
NICMOS instrument, after a newly developed image-processing technique
was applied. Further observations in 2009–2010 revealed the
fourth giant planet orbiting inside the first three planets at a
projected separation just less than 15 AU  which has now also
been confirmed in multiple studies.
The outer planet orbits inside a dusty disk like the Solar Kuiper
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 for
terrestrial planets. There is an additional debris disk just
inside the orbit of the innermost planet.
The orbital radii of planets e, d, c and b are 2 to 3 times those of
Jupiter, Saturn, Uranus, and Neptune, respectively. Because of the
inverse square law relating radiation intensity to distance from the
source, comparable radiation intensities are present at distances
displaystyle scriptstyle sqrt 4.9
= 2.2 times farther from
HR 8799 than from the Sun, meaning that
corresponding planets in the solar and
HR 8799 systems receive similar
amounts of stellar radiation.
These objects are near the upper mass limit for classification as
planets; if they exceeded 13
Jupiter masses, they would be capable of
deuterium fusion in their interiors and thus qualify as brown dwarfs
under the definition of these terms used by the IAU's Working Group on
Extrasolar Planets. If the mass estimates are correct, the HR 8799
system is the first multiple-planet extrasolar system to be directly
imaged. The orbital motion of the planets is in an anticlockwise
direction and was confirmed via multiple observations dating back to
1998. 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
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 the Laplace resonance
between Jupiter's inner three Galilean satellites: Io, Europa and
Ganymede as well as three of the planets in the
Gliese 876 system).
However, it is now believed that planet b is not in resonance with the
other 3 planets. If confirmed, the
HR 8799 planetary system would be
the second extrasolar system to be observed with multiple resonances.
The 4 planets are still glowing red hot due to their young age and are
Jupiter and over time they will cool and shrink to the
size of 0.8 to 1.0
The broadband photometry of planets b, c and d has shown that there
may be significant clouds in their atmospheres, while the infrared
spectroscopy of planets b and c pointed to non-equilibrium CO/CH4
Near-infrared observations with the Project 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.
The spectrum is that of a giant exoplanet, orbiting around the bright
and very young star HR 8799, about 130 light-years away. This spectrum
of the star and the planet was obtained with the NACO adaptive optics
instrument on ESO’s Very Large Telescope.
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, which
indicates a hydrogen-rich atmosphere. Weak methane and carbon monoxide
absorption in this planet's atmosphere was also detected, indicating
efficient vertical mixing of the atmosphere and a disequilibrium
CO/CH4 ratio at the photosphere. Compared to models of planetary
atmospheres, this first spectrum of planet b is best matched by a
model of enhanced metallicity (about 10 times the metallicity of the
Sun), which may support the notion that this planet formed through
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-type
brown 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.
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 C and O in planet c favor a history in which the
planet formed through core accretion. 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.
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.
Spitzer infrared image of HR 8799's debris disk, January 2009. The
small dot in the centre is the size of Pluto's orbit.
In January 2009 the
Spitzer Space Telescope
Spitzer Space Telescope obtained images of the
debris disk around HR 8799. Three components of the debris disk were
Warm dust (T ~ 150 K) orbiting within the innermost planet (e). The
inner and outer edges of this belt are close to 4:1 and 2:1 resonances
with the planet.
A broad zone of cold dust (T ~ 45 K) with a sharp inner edge orbiting
just outside the outermost planet (b). The inner edge of this belt is
approximately in 3:2 resonance with said planet, similar to Neptune
and the Kuiper belt.
A dramatic halo of small grains originating in the cold dust
The halo is unusual and implies a high level of dynamic activity which
is likely due to gravitational stirring by the massive planets.
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.
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.
This disk is so thick that it threatens the young system's
Vortex Coronagraph: Testbed for high-contrast imaging technology
Direct image of exoplanets around the star
HR 8799 using a vortex
coronograph on a 1.5 m portion of the Hale telescope
Up until the year 2010, telescopes could only directly image
exoplanets under exceptional circumstances. Specifically, it is easier
to obtain images when the planet is especially large (considerably
larger than Jupiter), widely separated from its parent star, and hot
so that it emits intense infrared radiation. However, in 2010 a team
Jet Propulsion Laboratory
Jet Propulsion Laboratory demonstrated that a vortex
coronagraph could enable small scopes to directly image planets.
They did this by imaging the previously imaged
HR 8799 planets using
just a 1.5 m portion of the Hale Telescope.
In 2009, an old
NICMOS image was processed to show a predicted
exoplanet around HR 8799. In 2011, three further exoplanets were
rendered viewable in a
NICMOS image taken in 1998, using advanced data
processing. The image allows the planets' orbits to be better
characterised, since they take many decades to orbit their host
List of extrasolar planets
Direct imaging of extrasolar planets
^ The star is a member of the Lambda Boötis class of peculiar stars,
thus the observed abundance may not reflect the abundances of the star
as a whole.
^ The eccentricity is given for the case that the planet is in a 2:1
HR 8799 c, as suggested by stability constraints.
^ The planets are young and therefore they are still hot and bright in
the near-infrared part of the spectrum .
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Media related to
HR 8799 at Wikimedia Commons
HR 8799 system
HR 8799 e
HR 8799 d
HR 8799 c
HR 8799 b
Stars of Pegasus
BD +17 4708
BD +28 4211
Coordinates: 23h 07m 28.7150s, +21