You’d be surprised what a planet can live through

The inside of a subdwarf B star like Kepler-70. Credit: Uwe W.

The inside of a subdwarf B star like Kepler-70. Credit: Uwe W.

Just one week left for Camp NaNoWriMo. I’m still on schedule with 40,641 words.

In about 5 billion years, our Sun will run out of hydrogen in its core turn into a red giant, expanding by about 200 times until it eats Mercury and Venus, though probably not Earth. Mercury and Venus will eventually be vaporized by the intense heat. After all, what kind of planet could survive being inside a star? Well…it turns out you just need a planet with a little more heft.

Kepler-70 is a peculiar type of star called a subdwarf B star. This is actually a little later in a star’s life than the red giant stage. Normally, after a while, a red giant shrinks down from 200 times the size of the Sun to just 10 when it stars burning helium. This is called a horizontal branch star.

But sometimes, the star loses most of its hydrogen atmosphere so that all that left is a much smaller (only a fifth the diameter of the Sun), much hotter star made mostly of helium. This is a subdwarf B star. How does a star lose it’s atmosphere like that? There are different reasons, but in the case of Kepler-70, it might have had something to do with a couple of planets crashing into it!

Long ago, we believe that Kepler-70 had two gas giant planets: hot jupiters or even warm jupiters in orbits like Mercury and Venus. As the star expanded, it engulfed these two planets. But the outer layers of a red giant’s atmosphere are very thin, and a planet can still orbit inside the star without too much trouble, if it can stand the heat.

But there is a little bit of drag from the star’s atmosphere. This drag force blew away the planets’ atmospheres, leaving only the heavy metal cores, and it caused them to spiral deeper and deeper into the star. At the same time–maybe–the gravity of the planets might have been disrupting the star’s atmosphere, causing it to blow away into space.

Then, about 18 million years ago, Kepler-70 shrank into the subdwarf B star we see today, leaving the planets safely outside it. There’s not much left. Both planets have been burned down to balls of iron about half the mass of Earth, and both of their orbits would fit inside our Sun. These burned down planetary cores are called chthonian planets, and they may still be boiling away. The inner planet, Kepler-70b, orbits the star in just under 6 hours, and it is the hottest planet known at over 7000 K. That’s hotter than the surface of our Sun and more than enough to boil iron.

Eventually, Kepler-70 will turn into another type of red giant called an asymptotic giant branch star, and its planets might boil away long before then, but the fact that they’re still there now is impressive. Planets, at least big ones, are tougher than they look.


About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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