Kepler-9
Encyclopedia
Kepler-9 is a sunlike star
in the constellation
Lyra
. It is situated in the Kepler Mission
's field of view. Kepler-9 has three confirmed extrasolar planet
s, all discovered by Kepler using the transit method. The discovery of the first two planets was announced on August 26, 2010, and of the third one in January, 2011. It was the first multiplanet system to be discovered using such a method. Two of Kepler-9's planets are engaged in orbital resonance
.
that was designed to search for Earth-like
planets. Unlike stars such as Aldebaran
or Sirius
, Kepler-9 does not have a colloquial name.
In June 2010, some 43 days after Kepler came online, its operating scientists submitted a list of over 700 exoplanet candidates for review. Of those, five were originally suspected to have more than one planet. Kepler-9 was one of the multiplanetary systems; it was identified as such when scientists noticed significant variations in the time intervals at which Kepler-9 was transited. Kepler-9 holds the first multiplanetary system discovered using the transit method. The discovery of the planets was announced on August 26, 2010.
, all of which orbit a sunlike G-type
star. The outer two planets, Kepler-9b
(the inner one) and Kepler-9c
(the outer one), are low density gas giants that are respectively 25% and 17% the mass of Jupiter and around 80% the radius of Jupiter. Both planets have a density less than that of water, similar to Saturn
. The innermost planet, Kepler-9d
, is a super-Earth
with a radius that is 1.64 times that of Earth
, orbiting the star every 1.6 days. It is estimated that there is a 0.59% chance that the discoveries are false.
From Kepler-9d (closest to star) to Kepler-9b (second from star), the ratio of their orbits is 1:12. However, the ratio of the orbits of the two outer planets is 1:2, a relationship known as a mean motion resonance
. Kepler-9b and Kepler-9c are the first transiting planets detected in such an orbital configuration. The resonance causes the orbital speeds of each planet to change, and thus causes the transit times of the two planets to oscillate. The period of Kepler-9b is presently increasing by 4 minutes per orbit, while that of Kepler-9c is decreasing by 39 minutes per orbit. These orbital changes allowed the masses of the planets (a parameter not normally obtainable via the transit method) to be estimated using a dynamical model. The mass estimates were further refined using radial velocity measurements
obtained with the HIRES instrument of the Keck 1 telescope.
Kepler-9b and 9c are thought to have formed beyond the "frost line
". They are then thought to have migrated inward due to interactions with the remains of the protoplanetary disk
. They would have been captured into orbital resonance during this migration.
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...
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....
Lyra
Lyra
Lyra is a small constellation. It is one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations recognized by the International Astronomical Union. Its principal star, Vega — a corner of the Summer Triangle — is one of the brightest...
. It is situated in the Kepler Mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
's field of view. Kepler-9 has three confirmed 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, all discovered by Kepler using the transit method. The discovery of the first two planets was announced on August 26, 2010, and of the third one in January, 2011. It was the first multiplanet system to be discovered using such a method. Two of Kepler-9's planets are engaged in 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...
.
Nomenclature and history
Kepler-9 was named for the Kepler Mission, a project headed by NASANASA
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...
that was designed to search for Earth-like
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...
planets. Unlike stars such as Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
or Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, Kepler-9 does not have a colloquial name.
In June 2010, some 43 days after Kepler came online, its operating scientists submitted a list of over 700 exoplanet candidates for review. Of those, five were originally suspected to have more than one planet. Kepler-9 was one of the multiplanetary systems; it was identified as such when scientists noticed significant variations in the time intervals at which Kepler-9 was transited. Kepler-9 holds the first multiplanetary system discovered using the transit method. The discovery of the planets was announced on August 26, 2010.
Star and planetary system
There are three confirmed planetsExtrasolar 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...
, all of which orbit a sunlike G-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...
star. The outer two planets, Kepler-9b
Kepler-9b
Kepler-9b is one of the first planets discovered outside the solar system by NASA's Kepler Mission. It revolves around the star Kepler-9 within the constellation Lyra. Kepler-9b is the largest of three planets detected in the Kepler system by transit method; its mass is slightly smaller than the...
(the inner one) and Kepler-9c
Kepler-9c
Kepler-9c is one of the first seven extrasolar planets, exoplanets, discovered by NASA's Kepler Mission, and one of at least two planets orbiting the star Kepler-9. Kepler-9c and Kepler-9b were the first exoplanets confirmed to be transiting their star. The planet's discovery was announced by the...
(the outer one), are low density gas giants that are respectively 25% and 17% the mass of Jupiter and around 80% the radius of Jupiter. Both planets have a density less than that of water, similar to 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,...
. The innermost planet, Kepler-9d
Kepler-9d
Kepler-9d, formerly known as KOI-377.03, is a planet in orbit around the sunlike star Kepler-9. Initially discovered by Kepler spacecraft, a terrestrial planet-searching satellite built and operated by NASA, Kepler-9d is most likely a Super-Earth, with an estimated radius approximately 60% larger...
, is a super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...
with a radius that is 1.64 times that of Earth
Earth radius
Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...
, orbiting the star every 1.6 days. It is estimated that there is a 0.59% chance that the discoveries are false.
From Kepler-9d (closest to star) to Kepler-9b (second from star), the ratio of their orbits is 1:12. However, the ratio of the orbits of the two outer planets is 1:2, a relationship known as a mean motion 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...
. Kepler-9b and Kepler-9c are the first transiting planets detected in such an orbital configuration. The resonance causes the orbital speeds of each planet to change, and thus causes the transit times of the two planets to oscillate. The period of Kepler-9b is presently increasing by 4 minutes per orbit, while that of Kepler-9c is decreasing by 39 minutes per orbit. These orbital changes allowed the masses of the planets (a parameter not normally obtainable via the transit method) to be estimated using a dynamical model. The mass estimates were further refined using radial velocity measurements
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
obtained with the HIRES instrument of the Keck 1 telescope.
Kepler-9b and 9c are thought to have formed beyond the "frost line
Frost line (astrophysics)
In astronomy or planetary science, the frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. Depending...
". They are then thought to have migrated inward due to interactions with the remains of the protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
. They would have been captured into orbital resonance during this migration.