Page 99 - How It Works - Book Of Amazing Answers To Curious Questions, Volume 05-15

Space

What is it like on Beneath the clouds

lonely planets? planets, even without a star, if
Life could still exist on rogue
the atmosphere and
radioactive decay keep the
world warm enough.

Rogue planets are worlds without Thick atmosphere
If a rogue planet has a thick
stars to orbit, but instead are lost atmosphere, it could help to retain
warmth and preventing the world
in interstellar space from freezing over, even in the dark
depths of space.

here are around 200 billion stars in our Hypervelocity
The gravity of the black
galaxy, but astronomers have
hole is able to throw
Testimated that there are 100,000 times the planet away at a
more rogue planets than there are stars. What tremendous speed,
amounting to a
are these lonely planets like?
few percent of the
You might expect them to be cold and speed of light.
lifeless, but this is not necessarily
the case. Some planets do not
need a sun to stay warm. If
their atmosphere is thick Lonely among many Heat from the core
enough, coupled with It’s thought that there are trillions The core of a rogue planet can remain
heating from the decay of of rogue planets lost in the galaxy, molten and radioactive elements in the
radioactive elements in and many more have been ejected rock can decay and potentially produce
enough heat to maintain liquid oceans.
from the galaxy altogether.
the ground or from
volcanoes, they could
stay warm enough for
liquid water and
microbial life to exist. Runaway worlds
There is a good chance Some rogue planets are in a real hurry,
they might still be racing away from the Milky Way at 30
hanging onto the dense million miles per hour
atmosphere of hydrogen
and helium they were born
with, because they do not Black hole
have the stellar wind of a When a star with a planet
nearby star to blow the gets too close to the
supermassive black hole in
atmosphere away. the Milky Way, it fl ings the
So what causes these worlds to become planet away from its star.
runaways? There are at least three ways. The
first is that they could be born alone – some of

the biggest gas giants are very similar to Brown dwarfs: overgrown An artist’s impression of WISE
brown dwarfs, which are failed stars that form planets or failed stars? 0855-0714, which is 7.2 light
like stars, direct from a collapsing gas cloud. years away and is the closest
sub-brown dwarf to Earth
Another way is that a star that wanders too Brown dwarfs are gas giants that are too small to become
a star – they cannot reach the temperatures and
close to the black hole at the centre of the
pressures within their core to begin fusing hydrogen into
Milky Way could have its planet pulled away helium. The smallest brown dwarfs are about 13 times the
from it and sent hurtling through space and mass of Jupiter, and anything smaller should be a planet,
out of the galaxy at 48 million kilometres (30 but the dividing line is blurred. Some rogue planets have
million miles) per hour. Finally, planets can be formed like a star or a brown dwarf, condensing out of a
cloud of gas. Astronomers call these sub-brown dwarfs,
kicked out of solar systems by larger planets
and the closest one found so far is called WISE 0855- © Penn State University/NASA/JPL-Caltech; NASA/JPL-Caltech
migrating in their orbits. The migrating 0714 and is about 7.5 light years away. Its mass is
planet’s gravity forces other, smaller worlds between three and ten times the mass of Jupiter.
out of the planetary system. Some astronomers Astronomers have found many planets with the same
mass orbiting stars. WISE 0855-0714 is a cold object,
even think our Solar System once had fi ve
with temperatures between minus-13 to minus-48
giant planets, but that one was ejected as the degrees Celsius (8.6 and minus-54.4 degrees Fahrenheit).
others migrated.
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