The truth is that nothing on Earth lasts forever, and that includes all the objects in our sky.
It is on borrowed time that the Sun, the source of light and heat for every planet in our
Solar System, shines. By converting small amounts of mass into pure
energy, the Sun gets its energy by fusing hydrogen into helium in its core.
As the fuel in the core is finite, this can't last forever. The Sun has already lost the
equivalent mass of Saturn through this process and will run out of its core fuel in 5 to
7 billion years. Nearing the end of its life, after it becomes
a red giant, it will eventually blow off its outer layers, creating a planetary nebula,
with its core contracting down to a white dwarf. However, this will lead to catastrophes
throughout our Solar System. The first thing you need to know about a red
giant is that it is enormous. Compared to the size of a red giant, the size of our Sun
is nothing. Red giants measure 1.4 million kilometers across and weigh 300,000 times
as much as our Sun. Using the same mass, our Sun will grow to
over 100 times its present size, taking up Mercury as well as Venus. In the event of
the Sun expanding and losing mass, the Earth will likely be pushed out, and while it may
be engulfed, scientists disagree about its survival.
However, if it does, both Earth and Mars will become charred, barren worlds. The Oceans
and atmospheres of these planets will boil away, and we will become airless, roasting
worlds just like Mercury is right now. Red giants aren't just large, they are thousands
of degrees while shining with thousands of times the brightness of our Sun.
About one-third to half of the mass of the Sun is ejected at high temperatures into the
outer reaches of our Solar System. Gas giants are massive enough to continue
to hold onto their gas envelopes, possibly even growing as the Sun enters this phase.
The planets we find today around red giant stars are all gas giants and are much larger
than even Jupiter. Seeing these kinds of worlds may be a selection
effect - meaning we tend to notice them because they're easiest to recognize, but it may also
be a natural consequence. When huge amounts of mass leave the Sun, they
will encounter these giant worlds with large gravitational fields.
In many cases, a matter that encounters these atmospheres will make a cosmic "splat," causing
the size and mass of these worlds to grow. At the end of the day, Jupiter, Saturn, Uranus,
and Neptune may all be larger and more massive than they are today.
The Sun will be so hot and so bright that much of the outer Solar System will be destroyed.
All four gas giants have rings, although Saturn's is the most famous. Rings are mostly made
of various ices, including water ice, methane ice, and carbon dioxide.
Because of the extreme energies released by the Sun, not only will these ices melt away,
but the individual molecules will also be so energetic that they will be ejected from
the Solar System. The same applies to moons that are rich in
the water around these worlds. The frozen surface of Europe with water-ice beneath it
will completely melt. It will be the same for Enceladus, which should see its entire
surface vaporized except for its rock-and-metal core.
The moons of Jupiter, Saturn, Uranus, and Neptune will all experience significant size
reductions as their atmospheres boil away, their outer layers melt, and only the rock-and-metal
cores of these satellite worlds remain. In some cases, a moon made entirely of volatiles
may end up completely extinguished. Even the largest, most well-known objects
in the faraway Kuiper belt aren't immune to these problems. At their surface, worlds like
Triton and Eris will receive more than four times the energy Earth receives today, despite
their immense distances. The atmospheres and surfaces of the worlds,
which are currently laden with ices and oceans beneath the surface, will also boil away entirely.
Pluto is a bit different. Pluto is currently surrounded by frozen elements that make up
the solar system, including solid water, solid carbon monoxide, solid nitrogen, and solid
methane. However, Pluto may warm up to a habitable
300 Kelvin by the time the Sun reaches peak luminosity. As it approaches that peak, it
might spend millions of years between the freezing and boiling points of water (assuming
a thick atmosphere). When a frozen object is heated, it will lose
a great deal of sublimated material to the vacuum of space. Water, carbon monoxide, and
so on will just disappear. It is nonetheless true that even a low gravitational
surface acceleration like Pluto's (about 1/12th Earth's) will cause some atmospheric buildup.
It is also effective at encouraging more atmosphere by making it harder for molecules to make
it to space from the surface. Pluto could, in effect, develop a thicker
envelope, and possibly more clement conditions. However, all of this newfound status would
be fleeting. Pluto would have no more than a million or
two years to bask in the glory of being the last habitable world in the solar system.
Then it too would be returned to the cold eternity of the cosmos.
During hundreds of millions of years, there might be hope for more temperate conditions
out in the outer Kuiper belt: about 80 to 100 Earth-Sun distances away. During this
brief cosmic period, objects at that distance will receive roughly the same amount of sunlight
as Earth does at its surface today. Sunlight isn't enough to make a habitable
world; you need enough mass, the right size, and the right ingredients. Even though they
receive nearly identical amounts of energy per square meter, the Moon and Earth fare
very differently when it comes to habitability. Although even a hypothetical Planet Nine would
be too far away for life to survive, everything at the right distance is too small to support
life. After the Solar System melts, only the stripped
cores of planets, moons, and other objects will remain. While the gas giants may grow
and swell, losing their rings and many satellites, everything else will be nothing more than
metal-rich junk. I'm afraid you're going to be disappointed
if you were hoping these frozen, outer worlds of our Solar System would get the chance to
shine. When the Sun reaches the end of its life, those words will lose their meaning.
Let's just hope humans have migrated elsewhere by that time.