Kepler-70b

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 orbiting the subdwarf B star (sdB) Kepler-70. The other planet is Kepler-70c, and both planets (if they exist) orbit very close to their host star.[1]

Kepler-70b
Discovery
Discovered byCharpinet et al.[1]
Discovery siteKepler telescope
Discovery date12/22/2011 (announced) [1]
Reflection/emission modulations
Orbital characteristics
0.006 AU (900,000 km)
0.2401 ± 0.000004 d
Inclination~65
StarKepler-70
Physical characteristics
Mean radius
0.759 [note 1][2] R⊕
Volume4.736×1015 km3
Mass2.6×1024 kg
Mean density
5,500 kg/m3 (9,300 lb/cu yd)
7.5 m/s2
Temperature7,662 K (7,389 °C; 13,332 °F)[3][note 2]
  1. Calculated assuming an albedo of 0.1.
  2. Equilibrium temperature calculated assuming an albedo of 0.1.

However, later research[4][5] 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 is not 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.[1] Its density is 5500 kg/m3 which is not much different from Earth.[6]

Characteristics

Mass, radius and temperature

Kepler-70b is likely a rocky exoplanet with a mass of 0.44 M⊕ and a radius of 0.76 R⊕. It has a surface temperature of several thousand Kelvin, one of the hottest known surface temperatures on any exoplanet. While the precise temperature is not known, it is expected to be hotter than the surface of the Sun.[7]

Orbit

The exoplanet has an orbit period shorter than any exoplanet known to date, 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 J19452546+4105339 or KIC 5807616), is a subdwarf B-type star that has left its red giant stage of its lifetime – according to the Extrasolar Planets Encyclopedia – about 18.4 million years ago.[2] 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.[8] The star has a mass of 0.496 M☉ and a radius of 0.203 R☉[note 1] It is expected to become a white dwarf in the future, after fusing the remaining helium in its core, and shrink in size to around the size of the Earth.

The star's apparent magnitude, 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

Kepler-70c passes 240,000 kilometres (150,000 mi) 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 which 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."[9] 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 which 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[9]. 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.[10]

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See also

Notes
  1. 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 M☉.

References
  1. Charpinet, S.; et al. (December 21, 2011), "A compact system of small planets around a former red-giant star", Nature, 480 (7378): 496–499, Bibcode:2011Natur.480..496C, doi:10.1038/nature10631, PMID 22193103
  2. "Notes for Planet KOI-55 b". Extrasolar Planet Database. Archived from the original on 19 January 2012. Retrieved 1 January 2012.
  3. http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators
  4. Krzesinski, J. (August 25, 2015), "Planetary candidates around the pulsating sdB star KIC 5807616 considered doubtful", Astronomy & Astrophysics, 581: A7, Bibcode:2015A&A...581A...7K, doi:10.1051/0004-6361/201526346
  5. Blokesz, A.; Krzesinski, J.; Kedziora-Chudczer, L. (4 July 2019), "Analysis of putative exoplanetary signatures found in light curves of two sdBV stars observed by Kepler", Astronomy & Astrophysics, 627: A86, arXiv:1906.03321, Bibcode:2019A&A...627A..86B, doi:10.1051/0004-6361/201835003
  6. "Kepler mission discoveries". Retrieved 23 May 2013.
  7. S. Charpinet et al.: A compact system of small planets around a former red-giant star, Nature 480, 496–499, supplementary material (online)
  8. Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
  9. "Two More Earth-Sized Planets Discovered by Kepler, Orbiting Former Red Giant Star". Universe Today. Retrieved 1 January 2012.
  10. Bear, E.; Soker, N. (26 March 2012), "A tidally destructed massive planet as the progenitor of the two light planets around the SDB star KIC 05807616", The Astrophysical Journal Letters, 749 (1): L14, arXiv:1202.1168, Bibcode:2012ApJ...749L..14B, doi:10.1088/2041-8205/749/1/L14

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