HR 8799
Encyclopedia
HR 8799 is a young (~30 million-year-old) main sequence
star
located 129 light years (39 parsec
s) away from Earth
in the constellation
of Pegasus
, with 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
. It, along with Fomalhaut b
, were the first extrasolar planet
s 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
element
s. 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 circumstellar disk
).
(λ Boo) class, a group of peculiar star
s with an unusual lack of metals in the upper atmosphere. Because of this special status, stars like HR 8799 have a very complex spectral type. The shape of the hydrogen line
and the star's effective temperature
best match the typical spectrum of an F0 V star. However, the strength of the calcium
II K line and the metallic lines are more like those of an A5 V star. The star's spectral type is therefore written as .
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 the Hertzsprung–Russell diagram
of temperature versus luminosity, 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, astroseismology 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 configuation. The best accepted value for an age of HR8799 is 30 million years, consistent with being a member of the Columba Association.
Detailed analysis of the star's spectrum reveals that it has a slight overabundance of carbon
and oxygen
compared to the Sun (by 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 planet's 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 it's planets. Observation of this star with the 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.
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
.The planets are young and therefore they are still hot and bright in the near-infrared part of the spectrum . 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.
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 planet
s. 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
= 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 mass
es, they would be capable of deuterium
fusion
in their interiors and thus qualify as brown dwarf
s 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 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 chemistry.
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. The Spitzer team says that collisions are likely occurring among bodies similar to those in our 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.
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
s 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 2011, around that same star, three exoplanets were rendered viewable in a NICMOS image taken in 1998, using advanced data processing. The exoplanets were originally discovered with the Keck telescopes and the Gemini North telescope between 2007 and 2010. The image allows the planets' orbits to be analyzed better, since they take many decades, even hundreds of Earth years to orbit their host star. In other words, the NICMOS image is especially useful because it is older.
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
located 129 light years (39 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s) away from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Pegasus
Pegasus (constellation)
Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
, with roughly 1.5 times the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...
and at least four massive planets
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
. It, along with Fomalhaut b
Fomalhaut b
Fomalhaut b is an extrasolar planet approximately 25 light-years away in the constellation of Piscis Austrinus. The planet was discovered orbiting the A-type main sequence star Fomalhaut in 2008 in photos taken by the Hubble Space Telescope...
, were the first extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s whose orbital motion was confirmed via direct imaging. The designation HR 8799 is the star's identifier in the Bright Star Catalogue
Bright Star Catalogue
The Bright Star Catalogue, also known as the Yale Catalogue of Bright Stars or Yale Bright Star Catalogue, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. It is currently available online in its 5th...
. The star is a Gamma Doradus variable
Gamma Doradus variable
Gamma Doradus variables are variable stars which display variations in luminosity due to non-radial pulsations of their surface. The stars are typically young, early F or late A type main sequence stars, and typical brightness fluctuations are 0.1 magnitudes with periods on the order of one day...
: its luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
changes because of non-radial pulsations of its surface. The star is also classified as a Lambda Boötis star
Lambda Boötis star
A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc. The prototype is Lambda Boötis....
, which means its surface layers are depleted in iron peak
Iron peak
The iron peak is a local maximum in the vicinity of Fe on the graph of the abundances of chemical elements, as seen below....
element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s. This may be due to the accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
of metal
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
-poor circumstellar gas. It is the only known star which is simultaneously a Gamma Doradus variable, a Lambda Boötis
Lambda Boötis star
A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc. The prototype is Lambda Boötis....
type, and a Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
-like star (a star with excess infrared emission caused by a circumstellar disk
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
).
Stellar properties
The star HR 8799 is a member of the Lambda BoötisLambda Boötis star
A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc. The prototype is Lambda Boötis....
(λ Boo) class, a group of peculiar star
Peculiar star
In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.Chemically peculiar stars are common among hot main sequence stars...
s with an unusual lack of metals in the upper atmosphere. Because of this special status, stars like HR 8799 have a very complex spectral type. The shape of the hydrogen line
Hydrogen line
The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...
and the star's effective temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...
best match the typical spectrum of an F0 V star. However, the strength of the calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
II K line and the metallic lines are more like those of an A5 V star. The star's spectral type is therefore written as .
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 the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
of temperature versus luminosity, 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, astroseismology 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 configuation. The best accepted value for an age of HR8799 is 30 million years, consistent with being a member of the Columba Association.
Detailed analysis of the star's spectrum reveals that it has a slight overabundance of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
compared to the Sun (by approximately 30% and 10% respectively). While some Lambda Boötis stars have sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
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
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
: for example, the iron abundance is only 28% of the solar iron abundance. Asteroseismic
Asteroseismology
Asteroseismology also known as stellar seismology is the science that studies the internal structure of pulsating stars by the interpretation of their frequency spectra. Different oscillation modes penetrate to different depths inside the star...
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 planet's 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 it's planets. Observation of this star with the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
indicates that it has a weak level of magnetic activity
Stellar magnetic field
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively...
, 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
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
is about 3.0 million K.
Planetary system
On November 13, 2008, Christian Marois of the National Research Council of Canada's Herzberg Institute of AstrophysicsHerzberg Institute of Astrophysics
The NRC Herzberg Institute of Astrophysics is the leading Canadian centre for astronomy and astrophysics.Named for the Nobel laureate Gerhard Herzberg, it was formed in 1975 as part of the National Research Council of Canada in Ottawa, Ontario...
and his team announced they had directly observed three planets
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
orbiting the star with the Keck and Gemini
Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...
telescopes in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
, in both cases employing adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
to make observations in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
.The planets are young and therefore they are still hot and bright in the near-infrared part of the spectrum . A precovery
Precovery
Precovery is a term used in astronomy that describes the process of finding the image of an object in old archived images or photographic plates, for the purpose of calculating a more accurate orbit...
observation of the outer 3 planets was later found in infrared images obtained in 1998 by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
's NICMOS
Near Infrared Camera and Multi-Object Spectrometer
The Near Infrared Camera and Multi-Object Spectrometer is a scientific instrument for infrared astronomy, installed on the Hubble Space Telescope , operating from 1997 to 1999, and from 2002 to 2008...
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.
The outer planet orbits inside a dusty disk like the Solar Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
. 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 planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s. 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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, respectively. Because of the inverse square law relating radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
intensity
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
to distance from the source, comparable radiation intensities are present at distances
= 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 mass
Jupiter mass
Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....
es, they would be capable of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
in their interiors and thus qualify as brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s under the definition of these terms used by the IAU
IAU
IAU may refer to:*International Astronomical Union*International American University*International American University College of Medicine*International Association of Universities*International Association of Ultrarunners...
'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
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
(similar to the Laplace resonance between Jupiter's inner three Galilean satellites: Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
and Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
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 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 chemistry.
Debris disk
In January 2009 the Spitzer Space TelescopeSpitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
obtained images of the debris disk around HR 8799. Three components of the debris disk were distinguished:
- Warm dust (T ~ 150 K) orbiting within the innermost planet (e). The inner and outer edges of the 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 (a). The inner edge of this belt is approximately in 3:2 resonance with said planet.
- A dramatic halo of small grains originating in the cold dust component.
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 our 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.
Vortex Coronagraph: Testbed for high-contrast imaging technology
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 JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
s Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
demonstrated that a vortex coronagraph
Vortex coronagraph
A Vortex coronagraph is a type of optical instrument, that allows seeing very faint objects near very bright objects, that would normally obscured by glare. For example, extrasolar planets near their host star, as seen from Earth or space telescopes in Earth's solar system...
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
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
.
NICMOS images
In 2009, an old NICMOS image was processed to show a predicted exoplanet around the star HR 8799, thought to be about 130 light-years from Earth.In 2011, around that same star, three exoplanets were rendered viewable in a NICMOS image taken in 1998, using advanced data processing. The exoplanets were originally discovered with the Keck telescopes and the Gemini North telescope between 2007 and 2010. The image allows the planets' orbits to be analyzed better, since they take many decades, even hundreds of Earth years to orbit their host star. In other words, the NICMOS image is especially useful because it is older.
See also
- List of extrasolar planets
- Direct imaging of extrasolar planets