Kepler-70b (formerly known as its Kepler Object of Interest designation ''KOI-55.01''; sometimes listed as KOI-55 b) is one of two postulated

exoplanets An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

orbiting the subdwarf B star (sdB)

Kepler-70 Kepler-70, also known as KIC 5807616 and formerly as KOI-55, is a star in the constellation Cygnus with an apparent visual magnitude of 14.87, and is 4200 light-years away. This is too faint to be seen with the naked eye; viewing it req ...

. The other planet is

Kepler-70c Kepler-70c (formerly called KOI-55.02; sometimes listed as KOI-55 c) is one of two postulated exoplanets orbiting the sdB star Kepler-70. Their discovery was announced in 2011. However, later research suggests that the two exoplanets probably do ...

, and both planets (if they exist) orbit very close to their host star. However, later research suggests that the two exoplanets probably do not exist, and that "pulsation modes visible beyond the cut-off frequency of the star" were a more likely explanation for the signals believed to indicate exoplanets. This has not been proven with certainty one way or the other. If it exists, Kepler-70b completes one orbit around its star in just 5.76 hours, so is an ultra-short period planet. It is also the hottest known exoplanet as of mid-2017, with a surface temperature of several thousand

Kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...

. Its density is 5500 kg/m³ which is not much different from Earth.

Characteristics

Mass, radius and temperature

Kepler-70b is likely a rocky exoplanet with a mass of 0.44 and a radius of 0.76 . It has a surface temperature of over seven thousand Kelvin, the hottest known surface temperature of any exoplanet. While the precise temperature is not known, it is expected to be hotter than the surface of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

.S. Charpinet et al.: ''A compact system of small planets around a former red-giant star'', Nature 480, 496–499, supplementary material
online

Orbit

The exoplanet has an extremely short orbit, with an orbital period of 5.76 hours (345 minutes). This comes in second to the exoplanet PSR 1719-14 b, which, coincidentally, orbits a stellar remnant, much like the planets of Kepler-70.

Host star

The host star, Kepler-70 (also formally known as ''KOI-55'', ''

2MASS The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, and ...

J19452546+4105339'' or '' KIC 5807616''), is a

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 spectral ...

B-type star that 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 o ...

stage of its lifetime – according to the Extrasolar Planets Encyclopedia – about ago. It has a surface temperature of 27730 ± 270 K, nearly 6 times as hot as the surface temperature of the Sun, which has a surface temperature of 5778 K. The star has a mass of 0.496 and a radius of 0.203 These statistics were very likely higher than what they were today when it was a red giant, the estimated mass of Kepler-70 before it became a subdwarf, would probably have been around 0.89–0.95 . It is expected to become a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

in the future, after fusing the remaining

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

in its core, and shrink in size to around the size of the Earth. The star's

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

, or how bright it appears from Earth's perspective, is 14.87. Therefore, Kepler-70 is too dim to be seen with the naked eye.

Encounters

passes away from Kepler-70b during their closest approach, possibly causing tidal forces against each other. This is currently the closest recorded approach between planets.

Cultural impact

According to the main author of the paper in ''

Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...

'' 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 – numerous pulsar planets have been observed, but no planet has been found with such a short period around any star, whether or not on the main sequence.

Origins

The two planets were most likely

gas giants A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...

that spiraled inward toward their host star, which subsequently became a red giant, vaporizing much of the planets except for parts of their solid cores, which are now orbiting the Subdwarf B star. Another theory is that only one gas giant spiralled inward into the star, and that its core fragmented inside the red giant after engulfment, with the current planets being the large core fragments.

Notes

References

External links

Exoplanet and Candidate Statistics
{{Sky, 19, 45, 25, +, 41, 5, 34 B Exoplanets discovered in 2011 Exoplanets discovered by the Kepler space telescope