Kepler-40b, formerly known as KOI-428b, is a

hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere tem ...

discovered in orbit around the star Kepler-40, which is about to become a red giant. The planet was first noted as a transit event by

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

's ''

Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...

'' spacecraft. The Kepler team made data collected by its satellite publicly available, including data on Kepler-40; French and Swiss astronomers used the equivalent to one night of measurements on the

SOPHIE échelle spectrograph The SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes, literally meaning "spectrograph for the observation of the phenomena of the stellar interiors and of the exoplanets") ''échelle spectrog ...

to collect all the data needed to show that a planet was producing the periodic dimming of Kepler-40. The planet, Kepler-40b, is twice the mass of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...

and slightly larger than it in size, making it as dense as Neptune. The planet is also nearly thirteen times hotter than Jupiter and orbits five times closer to its star than Mercury is from the Sun.

Observational history

Kepler-40 was first observed by the Kepler spacecraft, a

satellite that searches for planets in transit (crossing in front of and dimming) their host stars, from May 13, 2009, to June 15, 2009 in its first days of operation. The resulting

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

was made available to the public by the Kepler science team, revealing four transit events over 33.5 days. A team composed of astronomers from France and Switzerland used the

at the

Haute-Provence Observatory The Haute-Provence Observatory (OHP, french: Observatoire de Haute-Provence) is an astronomical observatory in the southeast of France, about 90 km east of Avignon and 100 km north of Marseille. It was established in 1937 as a national ...

in southern France to examine Kepler-40. SOPHIE uses radial velocity measurements to examine stars for exoplanets. After background light was removed and alternative causes for radial velocity variations were disproved (for example, that Kepler-40 was actually a close binary star), the team used SOPHIE to analyze the properties of the actual star. The astronomers observing the star found that it is nearing the main sequence turn-off (the star fuses the last of its hydrogen and becomes a red giant). The establishment of stellar parameters helped the astronomers extrapolate the exoplanet's parameters and prove the existence of Kepler-40b. The discovered planet was the sixth transiting planet to have been discovered in orbit around stars with a radius of more than 1.8 times that of the Sun, after planets including

Kepler-5 Kepler-5 is a star located in the constellation Cygnus in the field of view of the Kepler Mission, a NASA project aimed at detecting planets in transit of, or passing in front of, their host stars as seen from Earth. One closely orbiting ...

and

Kepler-7 Kepler-7 is a star located in the constellation Lyra in the field of view of the Kepler Mission, a NASA operation in search of Earth-like planets. It is home to the fourth of the first five planets that Kepler discovered; this planet, a ...

. The discovery of Kepler-40b demonstrated that smaller telescopes, such as SOPHIE, are effective when used as follow-ups to space missions like Kepler. The team of astronomers spent what amounted roughly to one night on a 1.93d-meter telescope and gathered all the data needed to establish Kepler-40b's existence and parameters. Kepler-40b was published in the journal '' Astronomy and Astrophysics'' on January 4, 2011, after it was sent to the journal on September 15, 2010.

Host star

Kepler-40 is an F-type evolved subgiant star located in the Cygnus constellation. The star is 1.48 times the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

of the Sun and 2.13 times its

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

. With an effective temperature of 6510 K, Kepler-40 is larger, more massive, more diffuse, and hotter than the Sun is. The metallicity of Kepler-40, e/H= 0.10, means that it has 25.9 % more iron than is measured in the Sun. Kepler-40 is nearing the main sequence turn-off; in other words, it is about to fuse the last of its hydrogen and become a red giant. Kepler-40 hosts the sixth planetary system to be discovered in the orbit of a star with a mass of over 1.8 solar masses. It lies approximately 2500

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

s (8100 light years) away from Earth.

Characteristics

Kepler-40b is a

Hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere tem ...

that is estimated to be 2.2 times the mass of Jupiter (over 700 times the mass of Earth), but 1.17 times Jupiter's radius (13.12 times the radius of Earth). Thus, the planet has a density of 1.68 grams per cubic centimeter, similar to that of Neptune (1.638 g/cm³). The planet's

equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were a black body being heated only by its parent star. In this model, the presence or absence of an atmosphere (and therefore any greenhouse effect) is ...

was initially estimated to be 1620 K, thirteen times hotter than Jupiter's equilibrium temperature. In 2015, the planetary nightside temperature was modeled to be even hotter at 2327 K.A Comprehensive Study of Kepler Phase Curves and Secondary Eclipses:Temperatures and Albedos of Confirmed Kepler Giant Planets
/ref> Kepler-40b orbits its star every 6.87 days at an average distance of 0.081 AU. It also has an

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

of 89.7°, meaning that it can be seen nearly edge-on with respect to Earth. In comparison, Mercury orbits the Sun every 87.97 days at an average distance of 0.387 AU; therefore, Kepler-40b's orbit is approximately thirteen times faster than that of Mercury's and five times closer to its host star than Mercury is to the Sun.

References

{{reflist, refs= {{cite web , url=http://kepler.nasa.gov/Mission/discoveries/ , archive-url=https://web.archive.org/web/20100527104316/http://kepler.nasa.gov/Mission/discoveries/ , url-status=dead , archive-date=27 May 2010 , title=Summary Table of Kepler Discoveries , date=2 May 2011 , publisher=NASA, access-date=3 May 2011 {{cite web , url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html , title=Mercury Fact Sheet , author=David Williams , date=17 November 2010 , work= Goddard Space Flight Center , publisher=

, access-date=3 May 2011 {{cite web , url=http://solarsystem.nasa.gov/planets/charchart.cfm , title=The Planets: Orbital and Physical Characteristics , author=Arthur Cox , date=2000 , publisher=

, access-date=3 May 2011 , archive-url=https://web.archive.org/web/20121111213013/http://solarsystem.nasa.gov/planets/charchart.cfm , archive-date=11 November 2012 , url-status=dead {{cite journal , last1=Santerne , first1=A. , last2=Díaz , first2=R.F. , date=2011 , title=SOPHIE velocimetry of ''Kepler'' transit candidates , journal= Astronomy and Astrophysics , volume=528 , pages=A63 , doi= 10.1051/0004-6361/201015764, arxiv=1101.0196 , bibcode = 2011A&A...528A..63S , s2cid=119275985 (PDF file may take a while to load on some computers) Hot Jupiters Cygnus (constellation) Transiting exoplanets Exoplanets discovered in 2010 Giant planets 40b