62 ASTRONOMY • APRIL 2021
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
IS IT SAFE TO SAY EARTH WILL
BE LONG GONE BY THE TIME
OUR STAR REACHES ITS FINAL STAGE:
A BLACK DWARF?
Andrew Nemec
Palm Coast, FloridaAI
It’s hard to say. After the Sun exhausts the
hydrogen in its core, it will balloon into a
red giant, consuming Venus and Mercury. Earth will
become a scorched, lifeless rock — stripped of its atmo-
sphere, its oceans boiled off. Astronomers aren’t sure
exactly how close the Sun’s outer atmosphere will come
to Earth. If it comes too close, friction between Earth
and the outer layers of the Sun will slow our planet’s
orbit, causing it to slowly spiral into our star, dissolving
like a sugar cube in a hot cup of coffee.
If Earth manages to survive the Sun’s giant phase, it
will find itself orbiting a hot white dwarf barely larger
than our planet. For eons, Earth will continue to orbit
the Sun. But, eventually, as the Sun cools and dims to a
black dwarf, Earth’s orbit will decay due to the emission
of gravitational waves. Over a trillion trillion years, our
once-blue planet will spiral into the dead Sun — a grand
f ina le as the solar system goes dark forever.Earth’s
ultimate fate
The Sun doesn’t have the final
say in what happens to Earth,
however. While the Sun won’t
become a red giant for another
5 billion years, a lot can happen
in that time. Gravity keeps the
planets in orbit around the Sun,
but it also attracts the planets to
each other. Although much
smaller than the Sun’s pull, these
forces between planets tweak
their orbits over millions of
years, making them f lex and
d r i f t. It ’s pos sible t hat t he se f lex-
ing orbits could cause the solar
system to destabilize and eject
planets, Earth included.
Alternatively, while other stars
typically stay light-years away
from the solar system, it’s possi-
ble one could drift nearby in the
next few billion years. The solar
system may be a well-choreographed waltz of planets,
but the galaxy is more like a rock concert where the stars
mosh in a circle. The gravity of even a small star (or black
hole) could ruin orbits and kick out planets if it gets too
close. Thankfully, the odds of that happening are pretty
low because there is so much space in between stars.
The ultimate fate of Earth is something only time
can tell. The one thing astronomers are certain of is that
any far-future inhabitants of our solar system won’t
want to be around to find out!
Matt Caplan
Assistant Professor of Physics, Illinois State University,
Normal, IllinoisSirius is a binary
star system, home
to Sirius A (left), a
main sequence star,
and Sirius B (right),
a white dwarf.
Eventually, Sirius B
will cool enough
that it no longer
gives off visible
light, becoming a
black dwarf. The
same fate awaits
the Sun trillions of
years in the future.
NASA, ESA AND G. BACON (STSCI)
Our closest neighbor, Proxima Centauri, shines brightly in this
Hubble image. Just one-eighth the mass of the Sun, Proxima
Centauri is a red dwarf. ESA/HUBBLE & NASA