__NOTOC__ Kepler-9b is one of the first planets discovered outside the Solar System (

exoplanets An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first detec ...

) by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

's Kepler Mission. It revolves around the star Kepler-9 within the constellation

Lyra , from ; pronounced: ) is a small constellation. It is one of the 48 listed by the 2nd century astronomer Ptolemy, and is one of the modern 88 constellations recognized by the International Astronomical Union. Lyra was often represented on star ...

. Kepler-9b is the largest of three planets detected in the Kepler system by

transit method Methods of detecting exoplanets usually rely on indirect strategies – that is, they do not directly image the planet but deduce its existence from another signal. Any planet is an extremely faint light source compared to its parent star. For e ...

; its mass is roughly half that of the planet

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, and it is the largest planet in its system. Kepler-9b and Kepler-9c display a phenomenon called

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...

, in which gravitational pull from each planet alters and stabilizes the orbit of the other. The planet's discovery was announced on August 26, 2010.

Nomenclature and history

Kepler-9b's name denotes that it is the first exoplanet discovered in orbit around the star Kepler-9. The star, in turn, was named for the Kepler Mission, a

project designed to search for

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

-like planets. Kepler-9's planets were among 700 planetary candidates collected during Kepler's first 43 days online. The system in particular was flagged as one of five systems that appeared to have held over one transiting exoplanet. Kepler-9b's discovery was announced on August 26, 2010. It was the part of the first confirmed star system in which multiple planets transited the same star. The planet was confirmed by the Kepler satellite by the

, in which the planet passes across the face of its star in relation to Earth, dimming that star's light by a small amount; this light difference is then used to determine the planet and several of its characteristics, including size and distance from its home star. Initial estimates for Kepler-9b's mass were refined by the W. M. Keck Observatory at

Mauna Kea Mauna Kea (, ; abbreviation for ''Mauna a Wākea''); is a dormant Shield volcano, shield volcano on the Hawaii (island), island of Hawaii. Its peak is above sea level, making it the List of U.S. states by elevation, highest point in Hawaii a ...

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

. In doing so, scientists found that Kepler-9b is the larger of the two gas planets discovered in the Kepler-9 system, although in mass it is smaller than planet

Characteristics

Kepler-9b is a gas planet that has a mass approximately 43 times that of the Earth; thus, it is about one-half the mass of the planet

. It has a radius of approximately , or about 80% the radius of Jupiter. The planet orbits Kepler-9 every 19.243 days, and it lies some 0.14 AU from the star. To compare, planet Mercury's average distance from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

is .387 AU and takes 87.969 days to complete an orbit. Kepler-9b is the second-closest planet to its star in the Kepler-9 system. The first known case of

in transiting exoplanets has been noted between Kepler-9b and Kepler-9c. The two planets, whose orbits correspond in a roughly 1:2 ratio, maintain the orbit of the other by gravitational tug. Kepler-9b's orbit grows, on average, four minutes longer every

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

. Eventually, this trend will reverse and increase. Over time, it can be seen that the planets' orbits oscillate slightly above and below the 1:2 ratio. Alycia Weinberger of the Carnegie Institution has stated that the Kepler-9 gas giants probably formed further away from the star than they are, and the appearance of the orbital resonance phenomenon may help explain the history of their inward migration.

Other planets in the Kepler-9 system

* Kepler-9c * Kepler-9d

References

External links

{{Kepler-9 Exoplanets with Kepler designations Exoplanets discovered in 2010 Hot Jupiters Giant planets Transiting exoplanets Lyra 9b