In around five billion years, the Sun will turn into a red giant, and it will be doomsday for Earth. But something interesting will happen in the outskirts of the Solar System: the temperature will become warm enough for icy planets and satellites like Pluto, Europa and Titan to become temperate. They will spend some time in the “habitable zone”, with temperatures suitable for liquid oceans. These periods of time will be short compared to the time scale of life on Earth, but will still be counted in millions of years.
Water ice is the main component of the satellites of Jupiter and Saturn, and of dwarf planets like Pluto, so there is no shortage of water to form oceans when the temperature becomes warm enough. Could life evolve again from scratch on one of those oceans in the far future? Even if it doesn’t, it is awesome to realise that after Earth’s demise, ocean planets will grace the solar system for the fist time (in the nomenclature of exoplanets, Earth is not an ocean planet, being neither made primarily of water nor surrounded by a water atmosphere, it is a rocky “aqua-planet”, a dry rock covered with a thin layer of water).
I could only find two papers on the topic, one about Pluto and the Kuiper-belt objects melting during the initial “giant branch” stage of the life of the Sun as a red giant (S. A. Stern, 2003, Astrobiology 3, 317, Delayed Gratification Habitable Zones), the other about Titan under a red giant Sun in the “horizontal branch” stage (Lorenz & Lunine 1997, Geophys Res Lett. 24, 2905-8, Titan under a red giant sun: a new kind of “habitable” moon).
The far future of Pluto
Using the Dartmouth models for the future evolution of the Sun, I calculated the migration of the habitable zone (first plot below) and the evolution of the temperature of Pluto during the red giant stage (second plot).
Having exhausted its hydrogen fuel, the Sun is expected to grow in size and luminosity until it reaches the point where the helium in its core will ignite. This is the “red giant branch” stage. The Sun will reach more than a thousand time its present luminosity for a short time (short on a cosmic scale of course).
As the Sun turns into a red giant, the “habitable zone” will move past the orbit of Saturn and Jupiter, and reach the orbit of Pluto.
For a few million years during this stage, the temperature of Pluto will exceed the melting point of water. The ice rocks on the surface will melt, underground methane and ammonia will fizzle through the surface in giant geysers. A global ocean might form.
Because Pluto has no magnetic field to protect it from the Solar wind, and a weak gravity, a lot of the lighter gases might escape into space, producing an enormous cometary tail. In the red light of the dying Sun, Pluto will look like a gigantic comet.
After the helium flash, the Sun will contract towards the longer-lived “horizontal branch” red giant stage, and the temperature on Pluto will drop again far below the freezing point of water. The oceans will freeze over. The surface will be much smoother, purer ice than before, cleared of all craters and mountains, and cleaned of methane and ammonia. Pluto might then look like Ganymede or Europa do today, with very smooth, white surfaces of ice criss-crossed by fault lines.
What about Titan and the satellites of Jupiter under a red giant Sun? To be continued …
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