Astronomers love mysteries, and Kepler-56 has been one of the great puzzles of our galaxy for years. This red giant, located about 3,000 light years from Earth, rotates too fast and with its internal structure literally twisted. Now, we have an explanation.
A star that spins wrong. To understand why Kepler-56 is so strange, just compare it with any similar star in its old age. When a star like the Sun runs out of fuel, it expands and becomes a red giant. Typically, as it increases in size, its rotation slows down, like a skater extending his arms.
However, Kepler-56 does the opposite: its outer shell rotates at an absurd speed, 10 times faster than normal for a red giant of its type. And the strangest thing is that the core of the star and its outer envelope rotate with axes inclined in different directions.
Something didn’t add up. Kepler-56 has two confirmed planets orbiting it (Kepler-56 by Kepler-56 c), gas giants that orbit very close to their host star. Until now, the theory trying to make sense of Kepler-56’s strange movements was that these two planets pulled on the star using tidal forces, accelerating its spin.
But Takato Tokuno, a researcher at the University of Tokyo, He realized that this explanation did not hold up.. For those planets to be responsible, the tidal efficiency would have to be orders of magnitude higher than what physics dictates. Another actor was needed in this crime scene.
It was bad digestion. He study led by Tokuno seems to have reconstructed what happened: the Kepler-56 system had a third planet, but the star ate it. This third planet, a hot Jupiter, orbited dangerously close to the star, closer than the two planets we see today. So much so that, when the star began to age and expand, it was absorbed.
It was not a smooth process. Tokuno explains the effect with a clear analogy: like a giant meteorite hitting the Earth glancingly. The planet would absorb the energy of the impact and accelerate its rotation. As it was engulfed, the planet transferred its angular momentum to the star’s atmosphere, causing it to spin at full speed. When hit at an odd angle, the star’s outer shell ended up spinning on a different axis than its core.
The planetary life cycle. According to mathematics, the engulfed planet must have had a mass between 0.5 and 2 times that of Jupiter, and a frenetic orbital period of between one and six days. Its end was not unusual for a planet.
We know that stars devour planets. In fact, our own Sun is expected to engulf Mercury, Venus and probably Earth within about 5 billion years. But catching a star still digesting is extremely difficult. And that’s what we’re apparently seeing.
Image | David A. Aguilar (CfA)
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