Kepler-47c
{{Short description|Temperate gas giant in Kepler-47 system}}
{{Infobox planet
| name = Kepler-47c
| image = Orbiting in the Habitable Zone of Two Suns.jpg
| caption = Artist's impression of the Kepler-47 system (sizes to scale) compared to the planets of the inner Solar System with their respective habitable zones.
| discoverer = Kepler spacecraft
| discovered = August 3, 2012
| discovery_method = Transit (Kepler Mission)
| apsis = astron
| semimajor = {{val|0.9638|0.0041|0.0044|ul=AU}}
| eccentricity = {{val|0.044|0.029|0.019}}
| period = {{val|303.227|0.062|0.027|ul=d}}
| inclination = {{val|90.1925|0.0055|0.0042|u=deg}}
| arg_peri = {{val|306|40|67|u=deg}}
| star = Kepler-47 (KOI-3154)
| mean_radius = {{val|4.65|0.09|0.07|ul=Earth radius}}
| mass = {{val|3.17|2.18|1.25|ul=Earth mass}}
| density = {{val|0.17|0.09|0.07|ul=g/cm3}}
| surface_grav = {{val|1.4|ul=m/s2}} (0.15 g)
| single_temperature = {{convert|245|K|C F}}
}}
Kepler-47c (also known as Kepler-47(AB)-c and by its Kepler Object of Interest designation KOI-3154.02) is an exoplanet orbiting the binary star system Kepler-47, the outermost of three such planets discovered by NASA's Kepler spacecraft. The system, also involving two other exoplanets, is located about 3,400 light-years (1,060 parsecs) away.
Characteristics
=Mass, radius and temperature=
Kepler-47c is a gas giant, an exoplanet that is near the same mass and radius as the planets Uranus and Neptune.{{cite web |url=http://www.space.com/17416-tatooine-alien-planet-habitable-zone-life.html |title=Newfound 'Tatooine' Alien Planet Bodes Well for E.T. Search |work=Space.com |date=September 4, 2012 |first=Charles Q. |last=Choi |access-date=January 5, 2013}} It has a temperature of {{convert|245|K|C F}}.{{cite web|url=http://www.hpcf.upr.edu/~abel/phl/hec_plots/hec_orbit/hec_orbit_Kepler-47(AB)_c.png|access-date=6 April 2023|website=hpcf.upr.edu|title=Orbit Kepler 47 (AB)}} The planet has a radius of 4.62 {{Earth radius}} and has no solid surface.{{cite web|url=http://www.spacedaily.com/reports/Astronomers_Find_First_Multi_Planet_System_Around_a_Binary_Star_999.html| title=Astronomers Find First Multi-Planet System Around a Binary Star |work=SpaceDaily |date=September 3, 2012 |access-date=January 5, 2013}} It has a mass of around 3.2 {{Earth mass}}. It orbits within the habitable zone of its system, and could have a dense atmosphere of water vapor.{{citation needed|date=May 2020}}
=Host stars=
{{main|Kepler-47}}
The planet orbits in a circumbinary orbit around a (G-type) and (M-type) binary star system. The stars orbit each other about every 7.45 days. The stars have masses of 1.04 {{Solar mass}} and 0.35 {{Solar mass}} and radii of 0.96 {{Solar radius}} and 0.35 {{Solar radius}}, respectively. They have temperatures of 5636 K and 3357 K. Based on the stellar characteristics, an estimated age of 4–5 billion years for the system is possible. In comparison, the Sun is about 4.6 billion years old{{cite web |url=http://www.universetoday.com/18237/how-old-is-the-sun/ |title=How Old is the Sun? |first=Fraser |last=Cain |date=16 September 2008 |work=Universe Today |access-date=19 February 2011}} and has a temperature of 5778 K.{{cite web |url=http://www.universetoday.com/18092/temperature-of-the-sun/ |title=Temperature of the Sun |first=Fraser |last=Cain |date=September 15, 2008 |work=Universe Today |access-date=19 February 2011}} The primary star is somewhat metal-poor, with a metallicity ([Fe/H]) of −0.25, or 56% of the solar amount. The stars' luminosities ({{solar luminosity|link=y}}) are 84% and 1% that of the Sun.
The apparent magnitude of the system, or how bright it appears from Earth's perspective, is about 15.8. Therefore, it is too dim to be seen with the naked eye.
=Orbit=
Kepler-47c orbits around its parent stars every 303 days at a distance of 0.99 AU from its stars (nearly the same distance that Earth orbits from the Sun, which is about 1 AU).{{cite web|url=https://www.bbc.co.uk/news/science-environment-19401891 |title=Tatooine-like double-star systems can host planets |work=BBC |date=August 29, 2012 |access-date=January 5, 2013}} The planet receives about 87.3% of the amount of sunlight that Earth does. Unlike majority of circumbinary planets, Kepler-47c does not seem to have undergone a significant migration since its formation.{{citation|arxiv=1402.0509|title=Forming Circumbinary Planets: N -Body Simulations of Kepler-34|year=2014|doi=10.1088/2041-8205/782/1/L11|last1=Lines|first1=S.|last2=Leinhardt|first2=Z. M.|last3=Paardekooper|first3=S.|last4=Baruteau|first4=C.|last5=Thebault|first5=P.|journal=The Astrophysical Journal|volume=782|issue=1|pages=L11|bibcode=2014ApJ...782L..11L|s2cid=119214559}}
Habitability
File:Artist's concept illustrates Kepler-47, the first transiting circumbinary system.jpg
{{see also|Habitability of binary star systems}}
Kepler-47c resides in the circumbinary habitable zone of the parent stars. The exoplanet, with a radius of 4.63 {{earth radius}}, is too large to likely be rocky, and because of this the planet itself may not be habitable. Hypothetically, large enough moons, with a sufficient atmosphere and pressure, may be able to support liquid water and potentially life.
For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its star Pp must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.{{cite journal| last=Kipping| first=David| title=Transit timing effects due to an exomoon| journal=Monthly Notices of the Royal Astronomical Society| year=2009| volume=392| issue=1| pages=181–189| doi=10.1111/j.1365-2966.2008.13999.x| doi-access=free| bibcode = 2009MNRAS.392..181K |arxiv = 0810.2243| s2cid=14754293}}{{cite journal| last1=Heller| first1=R.| title=Exomoon habitability constrained by energy flux and orbital stability| journal=Astronomy & Astrophysics| volume=545| year=2012| pages=L8| issn=0004-6361| doi=10.1051/0004-6361/201220003| arxiv = 1209.0050 |bibcode = 2012A&A...545L...8H | s2cid=118458061}} Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star.{{cite web| url=https://skyandtelescope.org/astronomy-news/habitable-moons/ | work=Sky & Telescope | title=Habitable Moons |first=Andrew J. |last=LePage |date=August 1, 2006}} In the case of Kepler-47c, this would be practically the same to have a stable orbit.
Tidal effects could also allow the moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature{{cite web| last=Glatzmaier| first=Gary A.| title=How Volcanoes Work – Volcano Climate Effects| url=http://www.geology.sdsu.edu/how_volcanoes_work/climate_effects.html| access-date=29 February 2012| archive-date=23 April 2011| archive-url=https://web.archive.org/web/20110423214804/http://www.geology.sdsu.edu/how_volcanoes_work/climate_effects.html| url-status=dead}}{{cite web| title=Solar System Exploration: Io| url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Io| archive-url=https://web.archive.org/web/20031216112404/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Io| url-status=dead| archive-date=16 December 2003| work=Solar System Exploration| publisher=NASA| access-date=29 February 2012}} and create a geodynamo effect which would give the satellite a strong magnetic field.{{cite web| last=Nave| first=R.| title=Magnetic Field of the Earth| url=http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html| access-date=29 February 2012}}
To support an Earth-like atmosphere for about 4.6 billion years (the age of the Earth), the moon would have to have a Mars-like density and at least a mass of 0.07 {{Earth mass}}.{{cite web| url=http://www.xs4all.nl/~carlkop/habit.html|title=In Search Of Habitable Moons| publisher=Pennsylvania State University| access-date=2011-07-11}} One way to decrease loss from sputtering is for the moon to have a strong magnetic field that can deflect stellar wind and radiation belts. NASA's Galileo's measurements hints large moons can have magnetic fields; it found that Jupiter's moon Ganymede has its own magnetosphere, even though its mass is only 0.025 {{Earth mass}}.
The minimum stable star to circumbinary planet separation is about 2-4 times the binary star separation, or orbital period about 3-8 times the binary period. The innermost planets in all the Kepler circumbinary systems (e.g. Kepler-16b, Kepler-451b and such) have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.{{cite journal|last1=Welsh|first1=William F.|last2=Orosz|first2=Jerome A.|last3=Carter|first3=Joshua A.|last4=Fabrycky|first4=Daniel C.|title=Recent Kepler Results On Circumbinary Planets|journal=Proceedings of the International Astronomical Union|volume=8|issue=S293|year=2014|pages=125–132|issn=1743-9213|doi=10.1017/S1743921313012684|arxiv=1308.6328|bibcode=2014IAUS..293..125W|doi-access=free}} Kepler-47c lies well outside this critical limit, so its orbit is extremely likely to remain stable for billions of years.
Discovery
Kepler-47c, as well as Kepler-47b, was first discovered by scientists, from both NASA and the Tel-Aviv University in Israel, using the Kepler space telescope.{{cite web|url=http://www.timesofisrael.com/new-worlds-discovered-courtesy-of-us-israel-team/ |title=New worlds discovered, courtesy of US-Israel team |work=The Times of Israel |date=August 30, 2012 |first=David |last=Shamah |access-date=January 5, 2013}} Additionally, the planetary characteristics of both objects were identified by a team of astronomers at the University of Texas at Austin's McDonald observatory. Both planets were discovered after transiting their parent stars, and they both seem to be orbiting along the same plane.
Significance
Before the discovery of Kepler-47c, it was thought that binary stars with multiple planets could not exist. Gravitational issues caused by the parent stars would, it was believed, cause any circumbinary planets to either collide with each other, collide with one of the parent stars, or be flung out of orbit. However, this discovery shows that multiple planets can form around binary stars, even in their habitable zones; and while Kepler-47c is most likely unable to harbor life, it may have habitable moons, and other planets that could support life may orbit binary systems such as Kepler-47.
See also
References
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{{Sky|19|41|11.5|+|46|55|12}}
{{Kepler-47}}
{{2012 in space|state=collapsed}}
Category:Exoplanets discovered in 2012
Category:Giant planets in the habitable zone