Published in the Hindu on January 12, 2012
More than 700 planets orbiting a star (extrasolar planets) have been discovered till date. But a planet orbiting two stars was more in the realms of science fiction till recently.
Astronomers surprised everybody last September when they reported the first ever discovery of such a planet orbiting two stars (Kepler-16 system). Such a body is called a circumbinary planet.
In the case of exoplanets orbiting a star or Earth orbiting the Sun, only the planets are in orbit around the star. But in the case of circumbinary planets, all the three bodies (a planet and binary stars) are in motion. While the planet orbits the binary stars, the gravitationally bound pair of stars (binaries) orbit around each other.
If this discovery last year was considered as a rare find, scientists report today (January 12) the discovery of two more circumbinary planets — Kelper-34 b and Kepler-35 b. The results are published in Nature. Both the planets orbiting their respective binary stars are “low-density gas-giants.”
Millions of such planets
The latest finding proves that a circumbinary planet orbiting two stars is not extremely rare in our galaxy or a freak incident. Three such objects have so far been located from a small sample size of 750 binary star systems. With nearly 2.6 per cent of all Sun-like stars in our galaxy seen in a binary state, the authors estimate that there must be millions of planets orbiting binary stars.
The two circumbinary planets were discovered using the Kepler space telescopeace. The 0.95-m telescope monitors nearly 150,000 stars in the constellations Cygnus and Lyra.
Since the planets orbiting the stars are several thousands of Astronomical Units (average distance between Earth and Sun) away, the planetary transit method is used for locating them with certainty.
The planetary transit method is based on the premise that a planet passing in front of its star would slightly dim the star’s light or produce a miniature eclipse. When such transits occur periodically, the body is confirmed as a planet. But the planetary transit method would be able to detect a planet only if it is either massive or close to its star, or both.
The Kelper-34 b planet has 22 per cent of the mass of Jupiter and 76 per cent of the radius of Jupiter. The planet takes 289 days to complete one orbit around the binary stars.
The two stars (A and B) orbiting each other have an orbital period of 28 days. Star A is brighter and more massive than star B, and hence called a primary star.
Three transits were detected in all by the scientists. Of the three, two were made by the primary star A moving across star B, and one by the secondary star B moving across star A.
Kepler 35 b
In the case of Kepler 35 b, the transiting planet has 13 per cent of the mass of Jupiter and 73 per cent of radius of Jupiter. The planet takes just 131 days to complete one orbit around the binary stars.
The stars (A and B) have an orbital period of 21 days. Four transits by the two stars were detected. Of the four, three were made by the primary star moving across the secondary star, and one transit made by the secondary star moving across the primary star.
Too hot for life
While Kepler-16 b, which was discovered last year is slightly too cold to support life, both Kepler-34 b and Kepler-35 b are too hot, notes a news item published in the same issue of Nature.
The planets orbiting the binary stars experience extreme seasons due to the orbital motion of the two stars.
This is because the “the light received from their parent stars changes not only during the stars’ orbital periods (tens of days) and the planetary orbital period (hundreds of days), but also on much longer timescales through precession [slow changes in the rotational or orbital parameters] of the orbits due to three-body effects.”
The average amount of stellar energy received by the Kepler-34 b planet is 2.4 times the Earth’s insolation, with a variation of 250 per cent.
Similarly, in the case of Kepler-35 b, the insolation is 3.6 times the Earth’s, with a variation of 160 per cent.
It must also be noted that unlike planets in the Solar System, these bodies do not follow the same path on successive orbits. This is due to the gravitational effects between the three bodies.