User:Laufleming/Circumbinary Planets
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 An artist's picture of Kepler-16b. |
A circumbinary planet is a planet that moves around, or orbits, a binary star. A binary star is a system of two stars that orbit their center of mass. The brighter star is called the primary star, and the less bright star is called the companion star. A circumbinary planet orbits both stars in the system, not just one.
Observing Circumbinary Planets
changeEclipses
changeOne property of a binary star that scientists can measure is the amount of its light that reaches Earth. The amount of light is not constant. In some binary stars, one star moves between the Earth and the other star during their orbits. This is called an eclipse. When one star eclipses the other, it blocks some of the light from the other star. The total amount of light that reaches Earth from the binary star decreases. The amount of light reaching Earth returns to normal when the star is no longer in front of the other star. [1] [2]
Binary stars are not necessarily in an arrangement where the stars eclipse each other. However, if they are in that arrangement, scientists know how much light each star blocks when it eclipses the other star. If a planet moves in front of either star, it also blocks some light of the binary star from reaching Earth. A planet is much smaller than a star, so it will block less light. If scientists observe eclipses that only decrease the amount of light by a small amount, they can conclude that there is a planet eclipsing the binary star. [1] [2] [3] [4] This is the only direct way that scientists have observed circumbinary planets so far. The first circumbinary planet that was found this way was Kepler-16b in 2011. [4]
Timing of Periods
changeThe amount of time it takes for a star to move around its orbit and return to its starting place is called the period. The period for each star in a binary star should be almost constant. However, if there is a planet orbiting the two stars, the period of the stars could become longer or shorter.[5] This is because there is gravity between the stars and planet. The gravity between the planet and a star will push or pull the star, which will make the star complete its orbit faster or slower.
Scientists use computer programs to figure out how gravity makes binary stars move. They can guess where each star will be at a certain time by comparing the information about the orbits with the results from the computer program. If there is a difference between their guess and the actual position of the stars, it is possible that there is a planet in the system that is also adding a force to the stars. [6] [7]
A planet would have to be very large for it to change the period of a star enough for a scientist to observe the change. If scientists see a change in the period of binary stars, they may guess that the system has a planet. [2] This is an indirect way to find circumbinary planets but it is not always accurate. [8]
Forming Circumbinary Planets
changePlanets are made from much smaller pieces of rock and dust near stars. If two pieces of rock crash into each other, they will break apart if they are moving too fast. If they move more slowly, they may stick together to make a larger rock. Dust can also stick to rocks and make them larger. If many pieces of rock get stuck together, they can form a planet [9].
Rocks and dust move faster if they are near two stars. When two rocks hit each other near binary stars, they usually break apart instead of joining together [10] [11] [12] Planets could form further away from the stars, where pieces of rock move more slowly. Scientists have observed circumbinary planets that are close to the binary star, in the area where rocks break apart when they hit each other. Scientists think that the planets formed further away and moved closer to the star [10] [11] [2].
Stability of Orbits
changeLarge rocks are only called planets if they continue to orbit the stars for a very long time. If a rock moves faster than the escape velocity of the binary star, it leaves its orbit and escapes the gravity of the binary star. It could also fall into one of the stars, but this is less likely. Those orbits are unstable. If the orbit of the binary star is almost a circle, a planet orbiting the stars will probably have a stable orbit. If the binary star has an orbit that looks like an ellipse, a planet orbiting it is more likely to have an unstable orbit [13].
Habitable Zones
changeThe habitable zone of a star is the nearby area where water on a planet could be liquid. Stars radiate energy. If a planet is too far away from the star, there is not enough energy for water to be liquid. Instead, it would be ice, which is a solid. If the planet is too close to the star, there is enough energy for the water to become water vapor, which is a gas.
For single stars, the habitable zone forms a sphere. In binary stars, the habitable zone for planets orbiting only one star is almost a sphere. The habitable zone for planets orbiting both stars is more complicated because different amounts of energy from the stars reaches the planet at different points in its orbit [14]. Scientists can only find the habitable zone of a binary star by calculating the energy from the binary star everywhere near the star. If a place has enough energy for water to be liquid, the place is a part of the habitable zone.
Kepler-34b, Kepler-35b and Kepler-47c are always too close to the stars for liquid water to exist on them. Most of the orbit of Kepler-16b is in the habitable zone of Kepler-16. Kepler-47b is in the habitable zone of Kepler-47. Kepler-16b and Kepler-47c could possibly support life [12] [15] [2].
References
change- ↑ 1.0 1.1 William F. Welsh et al., "Transiting Circumbinary Planets Kepler-34 b and Kepler-35 b," Nature 481 (7382), 475-U85 (2012).
- ↑ 2.0 2.1 2.2 2.3 2.4 Jerome A. Orosz et al., "Kepler-47: A Transiting Circumbinary Multiplanet System," Science 337 (6101), 1511-1514 (2012).
- ↑ Jerome A. Orosz et al., "The Neptune-Sized Circumbinary Planet Kepler-38b," Astrophys.J. 758 (2), 87 (2012).
- ↑ 4.0 4.1 Laurance R. Doyle et al., "Kepler-16: A Transiting Circumbinary Planet," Science 333 (6049), 1602-1606 (2011).
- ↑ P. Sybilski, M. Konacki and S. Kozlowski, "Detecting circumbinary planets using eclipse timing of binary stars numerical simulations," Monthly Notices of the Royal Astronomical Society 405 (1), 657-665 (2010).
- ↑ S-B Qian et al., "Circumbinary Planets Orbiting the Rapidly Pulsating Subdwarf B-Type Binary NY Vir," Astrophys.J.Lett. 745 (2), L23 (2012).
- ↑ S-B Qian et al., "A Circumbinary Planet in Orbit Around the Short-Period White Dwarf Eclipsing Binary RR Cae," Mon.Not.Roy.Astron.Soc. 422 (1), L24-L27 (2012).
- ↑ Horner et al., "A Dynamical Analysis of the Proposed Circumbinary HW Virginis Planetary System," Mon.Not.Roy.Astron.Soc. 427 (4), 2812-2823 (2012).
- ↑ Elisa V. Quintana and Jack J. Lissauer, "Terrestrial planet formation surrounding close binary stars," Icarus 185 (1), 1-20 (2006).
- ↑ 10.0 10.1 Stefano Meschiari, "Planet Formation in Circumbinary Configurations: Turbulence Inhibits Planetesimal Accretion," Astrophys.J.Lett. 761 (1), L7 (2012).
- ↑ 11.0 11.1 Sijme-Jan Paardekooper et al., "How Not to Build Tatooine: The Difficulty of In Situ Formation of Circumbinary Planets Kepler 16b, Kepler 34b, AND Kepler 35b," Astrophys.J.Lett. 754 (1), L16 (2012).
- ↑ 12.0 12.1 B. Quarles, Z. E. Musielak and M. Cuntz, "Habitability of Earth-Mass Planets and Moons in the Kepler-16 System," Astrophys.J. 750 (1), 14 (2012).
- ↑ Samuel Doolin and Katherine M. Blundell, "The dynamics and stability of circumbinary orbits," Monthly Notices of the Royal Astronomical Society 418 (4), 2656-2668 (2011).
- ↑ Stephen R. Kane and Natalie R. Hinkel, "On the Habitable Zones of Circumbinary Planetary Systems," Astrophys.J. 762 (1), 7 (2013).
- ↑ Siegfried Eggl et al., "An Analytic Method to Determine Habitable Zones for S-Type Planetary Orbits in Binary Star Systems," Astrophys.J. 752 (1), 74 (2012).