Kepler-70, also known as KIC 5807616 and KOI-55, is a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

about away in the constellation Cygnus, with an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

of 14.87. This is too faint to be seen with the naked eye; viewing it requires a telescope with an

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

of or more. A

subdwarf B star A B-type subdwarf (sdB) is a kind of subdwarf star with B-type main-sequence star, spectral type B. They differ from the typical subdwarf by being much hotter and brighter. They are situated at the "extreme horizontal branch" of the Hertzsprung� ...

, Kepler-70 passed through the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

stage some 18.4 million years ago. In its present-day state, it is fusing helium in its core. Once it runs out of helium it will contract to form a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

. It has a relatively small radius of about 0.2 times the Sun's radius; white dwarfs are generally much smaller. The star may be host to a planetary system with two planets, although later research indicates that this is not in fact the case.

Properties

Kepler-70 is an sdB (B-type

subdwarf A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same ...

) star with a temperature of about , equivalent to that of a B0-type star, and nearly 6 times as hot as the surface temperature of the Sun, which has a surface temperature of . It has a luminosity of , a radius of , and a mass of . The star left the

red-giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around ...

stage of its lifetime about ago. Kepler-70 is still fusing helium in its core. When it runs out of

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

, it will contract into a

Planetary system

On December 21, 2011, evidence for two extremely short-period planets, Kepler-70b and Kepler-70c (also known as KOI-55 b and KOI-55 c), was announced by Charpinet et al. based on observations from the

Kepler space telescope The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

. They were detected by the reflection of starlight caused by the planets themselves, rather than through a variation in apparent stellar magnitude caused by them transiting the star. The measurements also suggested a smaller body between the two candidate planets; this remains unconfirmed. According to the main author of the paper in ''

Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

'' that announced the discovery of the two planets, Stephane Charpinet, the two planets "probably plunged deep into the star's envelope during the red giant phase, but survived." However, this is not the first sighting of planets orbiting a post-red-giant star – a handful of

pulsar planets Pulsar planets are planets that are orbiting pulsars. The first such planets to be discovered were around a millisecond pulsar in 1992 and were the first extrasolar planets to be confirmed as discovered. Pulsars are extremely precise clocks and e ...

have been observed, including

PSR J1719−1438 b PSR J1719−1438 b is an extrasolar planet that was discovered on August 25, 2011, in orbit around PSR J1719−1438, a millisecond pulsar. The pulsar planet is most likely composed largely of crystalline carbon but with a density far greater tha ...

which orbits closer to its host star, and consequently in a shorter time than, any other planet. The two planets may have started out as a pair of

gas giants A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

which spiraled inward toward their host star, which subsequently became a red giant. This engulfed the planets, evaporating all but their solid cores, which now orbit the sdB star. Alternatively, there may only have been one gas giant engulfed in this way, with the rocky/metallic core having survived evaporation but fragmented inside the star. If this theory is correct, the two planets would be two large sections of the gas giant's core. If these planets exist, then the orbits of Kepler-70b and Kepler-70c have 7:10 orbital resonance and have the closest approach between planets of any known planetary system. However, later research suggested that what had been detected was not in fact the reflection of light from exoplanets, but

stellar pulsation Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to ded ...

"visible beyond the cut-off frequency of the star." Further research indicated that star pulsation modes were indeed the more likely explanation for the signals found in 2011, and that the two exoplanets probably did not exist. If Kepler-70b exists, then it would have a temperature of about 7288 K, the same as that of an F0 star. The hottest confirmed exoplanet and the hottest with a measured temperature is KELT-9b, with a temperature of about 4,600 K.

Notes

References

External links

Kepler mission discoveries
{{2011 in space Cygnus (constellation) 55 Hypothetical planetary systems B-type subdwarfs J19452546+4105339