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Tatooine discovered

October 15, 2011

 

In the legendary Star Wars saga, the two main characters Luke Skywalker and Anakin Skywalker (Darth Vader) hail from the fictional planet Tatooine, which orbits around two suns. The video above shows two beautiful scenes of binary sunsets on Tatooine from episodes IV and III respectively.

NASA scientists have now discovered such a circumbinary planet orbiting a binary star system using their Kepler space telescope. Kepler, with its 1-meter big mirror, monitors the brightness of more than 150,000 stars in the constellations Cygnus and Lyra. A transiting planet is detected as a tiny blip in the brightness of a star (as small as a part in one thousand), as the passing planet causes a miniature eclipse. This happens in the lucky situation when the orbital plane of the planet is oriented edge-on as viewed from earth.

The star system Kepler-16 was identified as a binary by the detection of mutual eclipses with a 41-day period. The total brightness of Kepler-16 was found to fall by about 15% every 41 days, as star B partially eclipses the bigger and brighter star A. About 20 days after every such occurence, the total brightness of Kepler-16 was found to reduce by about 1.5%, as the smaller star B was occulted by the bigger star A. The stars A and B are thus revolving around their common center of mass.

Apart from the primary and secondary stellar eclipses, two further partial eclipses were found to occur at a period of about 229 days. The total brightness dropped by about 2% and 0.1% respectively, as the planet Kepler-16b transited star A and then star B every 229 days. These eclipses varied in period about the mean by about 10 days, because stars A and B were in different positions on their mutual orbit each time the planet moved in front of them.

Based on the colors of light emitted by Kepler-16, star A was determined to be about 70% of the size of our sun, while star B was determined to be only 20% as big as our sun. Conclusive evidence for Kepler-16b being a planet came from measuring the deviations in the period of the primary and secondary stellar eclipses. They were found to depart from strict periodicity by about one minute, caused by the gravitational pull of the planet on the two stars. The planet was deduced to be similar in size to Saturn, but with a higher density. It presumably contains 50% gas (hydrogen and helium), and 50% heavy elements (ice and rock).

Although the mean distance of the planet Kepler-16b from its two parent stars is only 70% of the distance of the earth from the sun, its surface temperature is estimated to be between -100 and -70 degrees Celcius. The planetary orbit was found to be exactly coplanar with the stellar orbit, suggesting that the stars and the planet shared a common origin.

 

Doyle, L., Carter, J., Fabrycky, D., Slawson, R., Howell, S., Winn, J., Orosz, J., Pr sa, A., Welsh, W., Quinn, S., Latham, D., Torres, G., Buchhave, L., Marcy, G., Fortney, J., Shporer, A., Ford, E., Lissauer, J., Ragozzine, D., Rucker, M., Batalha, N., Jenkins, J., Borucki, W., Koch, D., Middour, C., Hall, J., McCauliff, S., Fanelli, M., Quintana, E., Holman, M., Caldwell, D., Still, M., Stefanik, R., Brown, W., Esquerdo, G., Tang, S., Furesz, G., Geary, J., Berlind, P., Calkins, M., Short, D., Steffen, J., Sasselov, D., Dunham, E., Cochran, W., Boss, A., Haas, M., Buzasi, D., & Fischer, D. (2011). Kepler-16: A Transiting Circumbinary Planet Science, 333 (6049), 1602-1606 DOI: 10.1126/science.1210923

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