How Do White Dwarf Stars Shred Planets?

White dwarfs have almost all the mass of a normal star squished into a ball the size of our Earth -- and that comes with a lot of gravitational power. Learn how they can tear planets to shreds in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/white-dwarfs-shred-planets.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio, Hey Brainstuff, Lauren Bolebaum. Here.
When our Sun runs out of hydrogen fuel in roughly
five billion years, it will swell into a huge, red
giant star, violently shedding hot layers of plasma and cooking
the inner planets to a crisp as it goes. All

(00:24):
that will be left behind is an expanding bubble of
cooling gas, creating a beautiful planetary nebula with a white
dwarf in the middle, shining bright like a stellar diamond.
Though we know this is the fate of our star,
what of the planets in our Solar system? But what
exactly will happen to Earth long after we're gone. Astronomers

(00:48):
from the University of Warwick in the UK took a
stab at answering this question back in twenty nineteen and
came up with a rudimentary warning guide for planets that
find themselves in this grim scenario. While our planet's fate
isn't necessarily clear, the study, which is published in the
journal Monthly Notices of the Royal Astronomical Society, revealed that

(01:08):
when it comes to contending with a white dwarf star,
only the tiniest worlds will survive. Why is that, Well,
we know that many white dwarf star systems have quantities
of dust surrounding them, and through spectroscopic measurements, dust has
been found polluting these star's atmospheres. The implication is clear.

(01:30):
These star systems used to have rocky planets, plus asteroids
and comets and orbit, but through extreme tidal interactions with
their white dwarf, were torn to shreds and ground to dust.
But why do planetary bodies get blended when they're in
the orbit of a white dwarf. These exotic stellar objects

(01:52):
contain nearly the entire mass of the dead star that
they came from, in a blob of degenerate matter only
the size of Earth. With this extreme density comes an
incredibly powerful gravitational field and tidal forces. Anything that strays
too too close to a white dwarf will be pulled
in by that powerful gravity. But there's a much wider

(02:14):
zone of destruction around such a star within which planets
or other orbiting bodies will be destroyed. Within this zone,
a planet, for example, will experience a much more powerful
tidal force on the star facing side than on the
side facing away, depending on what that planet is made
of and how well it holds together due to its

(02:35):
own gravity and a number of other factors. At a
certain distance, the tidal shear through the planet will be
too much, and it will be literally pulled like taffy
until it's pulled right apart. This is known as the
destruction radiusmarked by an ominous dusty ring around a white dwarf.

(02:55):
To understand where a variety of planets of different sizes
might be safe, the researchers carried out dynamic simulations of
different planets in orbit around a star like our Sun,
as it dies and passes through the red giant phase
to become a white dwarf. This violent phase of a
star's life will disturb the orbit of the planets around it,
possibly dragging them to their dusty deaths or flinging them

(03:18):
to wider orbits. Interestingly, the researchers found that it isn't
just the mass and composition of planets that affect how
sensitive they are to the tidal shear. It's also their
viscosity or the resistance they have to being deformed. Think
if you had a glass of water and a glass
of nacho cheese, If you poked the surface of the water,

(03:40):
it would easily deform around your finger. You'd feel basically
no resistance at all. This is low viscosity. Now, if
you poked the nacho cheese, I mean, you'd still be
able to deform its surface, but it would give you
a little bit more resistance because it has a higher viscosity. Now,
think about if you poke the glass itself, It's not

(04:02):
going to deform at all from a mere poking. Of
the three, it has the highest viscosity under these particular circumstances. Anyway,
the physics is complicated. But back to white dwarfs. The
researchers found that if all other variables were controlled for
low viscosity, exoplanets of a similar consistency to say, Saturn's

(04:23):
moon Enceladus, which they called a relatively homogeneous dirty snowball
because of its thick iso layers surrounding a small core,
would be dragged to its doom if it resides within
anywhere up to five times of the white dwarf's destruction radius.
At the other extreme, a high viscosity world might live
comfortably if it orbited the white dwarf at just twice

(04:44):
its destruction radius. Recently, astronomers discovered a dense, heavy metal
object around a white dwarf that's embedded inside a dusky disc.
It's believed that this object, which isn't much bigger than
a large asteroid, was the metal core of a larger
planet that was destroyed by tidal shear, leaving its high
viscosity metallic core behind. As the search for exoplanets, that

(05:10):
is planet's orbiting other stars becomes more sophisticated, we're going
to observe more worlds in white dwarf star systems, So
the researchers hope that these simulations will act as a
guide that will help us understand what those exoplanets are
made of. Although this simulation has provided some key insights
to what it takes to avoid being dragged to a

(05:31):
dusty death, it only simulated relatively homogeneous objects. When it
comes to our planet, the problem becomes more complex because
of all the layers of atmosphere, water, rock, and inner
metallic core that our planet contains. But in summary, it
pays to be tiny and mighty and composed of heavy

(05:51):
metals if you want to have a snug orbit around
a white dwarf without being dragged to your death. As
for Earth's fate, we'll have to wait and see, but
in all honesty, you probably won't want to be there
when our red giant sun switches to broil. A note
that long before the Sun runs out of hydrogen and
puffs up into a red giant let alone before it

(06:13):
becomes a white dwarf, it will become a lot hotter
than it is now, irradiating the inner planets. This, combined
with powerful solar winds, will likely blast away our atmosphere,
undoubtedly destroying any and all life that remains. So today's

(06:33):
episode is based on the article white dwarfs can shred
planets to pieces on HowStuffWorks dot com, written by Ian O'Neil.
Brainstuff is production of iHeartRadio in partnership with hostuffworks dot
Com and is produced by Tyler klang A. Four more
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