The red giant star Kepler-56 has a really weird spin, and it may be because it consumed one of its planets.
Kepler-56 already has two known exoplanets, but they may have had a long-lost sibling, Takato Tokuno, a doctoral student in the Department of Astronomy at the University of Tokyo, concluded after analyzing the star’s peculiar properties.
One strange quality of Kepler-56 is that its outer envelope is spinning incredibly rapidly — about 10 times faster than is typical for red giant stars. On top of that, the star’s core is misaligned with its outer envelope, with their spins pointing in different directions. It would be as if Earth’s crust were spinning much faster and in a different direction than its mantle.
What could possibly cause that?
The simplest explanation is that the known exoplanets are to blame. Big planets can tug on their parent stars, just as Jupiter causes the sun to wobble. Planets can also raise tides on their stars. They’re not very big tides, but it doesn’t take much — just a little tug here and there, repeated countless times over millions of years, can spin up a star’s outer atmosphere. And if the planets aren’t lined up with the star’s spin, it can cause misalignments.
But Tokuno realized that this scenario isn’t realistic, because it would require a very efficient ability for planets to raise rides and transfer rotational energy to the star — orders of magnitude more efficient than seen in other systems.
So maybe the star just got hungry.
When a star engulfs a planet, it affects the rotation of the star. Imagine if a giant meteor were to hit Earth with a glancing blow. We would absorb the energy of that impact and speed up in the process. And if the planet struck the star at an odd angle, it could also cause the misalignment between Kepler-56’s core and atmosphere.
In a new paper uploaded to the preprint server arXiv Oct. 29, Tokuno calculated that for this scenario to work, the doomed planet had to weigh somewhere between half the mass of Jupiter and twice its mass, and just before the impact, it would have to have an orbital period of between one and six days. That’s exactly in line with typical “hot Jupiter” exoplanets that are presumably on their way to sharing a similar fate.
There is another possibility: that this star was simply born with a high spin rate, Tokuno wrote in the paper, which has not been peer-reviewed. But that still doesn’t explain the misalignment between the core and the atmosphere. And it raises another question: Why was the star spinning so fast when it was born? One plausible explanation is that the star ate a planet, but while at a young age instead of near the end of its life.
Astronomers are just beginning to scratch the surface of the complex, and sometimes violent, relationships between planets and their parent stars. And every time we uncover a new scenario, as gruesome as it may be, we learn more about planetary life cycles.
