Sometimes stars eat worlds… Sometimes they spit them out

Pamela L. Gay, Ph.D.
5 min readJul 9

A few years ago, an audience member pointed out that I always seem a little too happy when discussing death and destruction from the sky. I’m not actually excited about our universe trying to kill off humanity, I’m excited about science, and it just turns out that a lot of science involves… the universe trying to murder us in myriad different ways.

8 UMi: The star that didn’t eat its planet

One of the weirder puzzles of astronomy is the presence of large planets snuggled up next to their stars. In young solar systems, we can at least say the star likely migrated there and isn’t destined to live very long.

But the system 8 UMi is an elderly star with a large planet on an orbit a lot like Mercury’s, and this particular combination just isn’t expected.

Artists Illustration. Credit: Pamela L. Gay

As stars evolve, they can change in radius as the cores burn hotter or cooler. Someday, our own Sun will bloat up and likely consume Mercury and Venus, and possibly even Earth. Much later, it will collapse down to an object just the size of the Moon and we’d expect to see an empty swath of space surrounding it.

8 UMi appears to be burning Helium in its core, which means it should have gone through a red giant phase in the past and emptied out its surroundings. The existence of a nearby planet says something weird has to be going on.

In a new paper in the journal Nature, researchers led by Marc Hon offer up 3 different possibilities. On one hand, it may just be that this planet managed to survive being engulfed by the star. While this seems insane, it’s not impossible. The outer layers of a star just aren’t that dense and it’s possible that the planet got moved around, but not destroyed when the star engulfed it.

It’s also possible that instead of seeing a star that used to be a red giant, we’re seeing a star that used to be two stars. A merger could have prevented the resulting object from ever ballooning up into a red giant.

It’s also possible that while the star is old, the planet is young, and somehow formed in the star system’s old age as part of a second generation of star formation.

However this planet got there, we now know solar systems come in even more diverse designs…

Pamela L. Gay, Ph.D.

Astronomer, technologist, & creative focused on using new media to engage people in learning and doing science. Opinions & typos my own.