HD 217107
Encyclopedia
HD 217107 is a yellow subgiant star
approximately 65 light-year
s away from Earth in the constellation
of Pisces
(the Fish
). Its mass is very similar to the Sun
's, although it is considerably older. Two planets have been discovered orbiting the star: one is extremely close and completes an orbit every seven days, while the other is much more distant, taking eight years to complete an orbit.
astrometric
satellite measured its parallax
as 50.71 Milliarcsecond
s, which corresponds to a distance of 65 light years. Its apparent magnitude
is 6.17, making it just barely visible to the naked eye
under favourable conditions.
Spectroscopic
observations show that its spectral type
is G7 or G8, which means its temperature is about 5,000 K
. Its mass is thought to be roughly the same as the Sun's, although its estimated age of 7.7 billion years is rather older than the Sun's 4.6 billion years, and it is thought to be beginning to evolve away from the main sequence
, having consumed almost all the hydrogen
in its core in nuclear fusion
reactions.
of HD 217107 carried out in 1998 revealed that its motion along the line of sight varied over a 7.1-day cycle. The period and amplitude of this variation indicated that it was caused by a planet
ary companion in orbit around the star, with a minimum mass slightly greater than that of Jupiter
. The companion planet was designated HD 217107 b
.
While most planets with orbital periods of less than 10 days have almost circular orbits, HD 217107 b has a somewhat eccentric orbit, and its discoverers hypothesized that this could be due to the gravitational influence of a second planet in the system at a distance of several astronomical unit
s (AU). Confirmation of the existence of a second planet followed in 2005, when long term observations of the star's radial velocity variations revealed a variation on a period of about eight years, caused by a planet with a mass at least twice that of Jupiter
in a very eccentric orbit with a semimajor axis of about 4.3 AU. The second planet was designated HD 217107 c
.
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...
approximately 65 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away from Earth 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 Pisces
Pisces (constellation)
Pisces is a constellation of the zodiac. Its name is the Latin plural for fish, and its symbol is . It lies between Aquarius to the west and Aries to the east...
(the Fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
). Its mass is very similar to 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, although it is considerably older. Two planets have been discovered orbiting the star: one is extremely close and completes an orbit every seven days, while the other is much more distant, taking eight years to complete an orbit.
Distance, age, and mass
HD 217107 is fairly close to the Sun: the HipparcosHipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
astrometric
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
satellite measured its parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
as 50.71 Milliarcsecond
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
s, which corresponds to a distance of 65 light years. Its apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
is 6.17, making it just barely visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
under favourable conditions.
Spectroscopic
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
observations show that its spectral type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
is G7 or G8, which means its temperature is about 5,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. Its mass is thought to be roughly the same as the Sun's, although its estimated age of 7.7 billion years is rather older than the Sun's 4.6 billion years, and it is thought to be beginning to evolve away from the main sequence
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...
, having consumed almost all the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in its core in nuclear 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...
reactions.
Planetary system
A study of the radial velocityRadial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
of HD 217107 carried out in 1998 revealed that its motion along the line of sight varied over a 7.1-day cycle. The period and amplitude of this variation indicated that it was caused by a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
ary companion in orbit around the star, with a minimum mass slightly greater than that 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,...
. The companion planet was designated HD 217107 b
HD 217107 b
HD 217107 b is an extrasolar planet approximately 64 light-years away from Earth in the constellation of Pisces . The planet was discovered orbiting the star HD 217107 approximately every seven days, classifying the planet as a hot Jupiter...
.
While most planets with orbital periods of less than 10 days have almost circular orbits, HD 217107 b has a somewhat eccentric orbit, and its discoverers hypothesized that this could be due to the gravitational influence of a second planet in the system at a distance of several astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (AU). Confirmation of the existence of a second planet followed in 2005, when long term observations of the star's radial velocity variations revealed a variation on a period of about eight years, caused by a planet with a mass at least twice that 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,...
in a very eccentric orbit with a semimajor axis of about 4.3 AU. The second planet was designated HD 217107 c
HD 217107 c
HD 217107 c is an extrasolar planet approximately 64 light-years away from Earth in the constellation of Pisces . The planet was the second planet to be discovered orbiting the star HD 217107...
.
External links
- Extrasolar Planet Interactions by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona