Second chance planets: Iceball worlds that thaw out when their stars go red giant

A while back I wrote a series of posts called How planets die.

It was about all the ways planets can be sterilized or destroyed.  I even made a “planetary death scale”.  Gruesome stuff.

From How planets die (Eek!)

Let’s liven things up with a new mini-series on Second-Chance Planets.

These are planets that get a second chance at life.  At first something is off — they are too hot or too cold, or are missing key ingredients for life.  But the things change and the planets become habitable.

These are cosmic underdog stories.  Planets that get a second chance to win against all odds.

Second chance planets would be movie characters like Daniel-San in the Karate Kid. Like Rocky or Rudy. Or William Wallace in Braveheart.

These are the scrappy, lovable planets you’ll want to root for!

Our first second-chance planets start off too cold for life.

They are covered in ice. Think Hoth.

“There isn’t enough life on this ice cube to fill a space cruiser!” — Han Solo, about Hoth, from the Empire Strikes Back. Credit: Lucasfilm.

We’re talking icy planets or moons too far from their stars to be warm enough for liquid water.

Way out past the outer edge of the habitable zone.

The habitable zone is the Goldilocks-esque belt of orbits around a star in which a planet could have liquid water on its surface.  Not too hot, not too cold.  It looks something like this (in very simple terms):


The Sun’s habitable zone extends from inside Earth’s orbit to past Mars’s orbit.  (Note: Mars is indeed within the habitable zone — its lack of surface water is probably due to the loss of most of its atmosphere).

The habitable zone is closer around small stars because they are faint, so a planet must be closer-in to have the right temperature (for example, the Trappist-1 system is super compact but its central star is so puny that it has 3 habitable zone planets).  And — you guessed it — the habitable zone is farther around more massive, brighter stars.

Of course, liquid water (and maybe life) can exist on planets or moons that are not in the habitable zone.  For example, subsurface oceans are thought to exist under the ice crusts of a number of Solar System moons (and possibly on some free-floating planets).

The habitable zone is all about water on planets’ surfaces, and it’s useful in the search for life around other stars.  We can’t see down into subsurface oceans on Jupiter’s moons, let alone on exoplanets.  It’s life on planets’ surfaces that we can hope to detect.

But the habitable zone is just a snapshot. 

Stars evolve. The Sun today is 30% brighter than 4.5 billion years ago.  Its habitable zone has been slowly creeping outward, and Earth is perilously close to the inner edge.

Here is how stars like the Sun evolve:

Credit: unuplusunu via Wikimedia Commons.

These days the Sun is in its pleasantly-boring main sequence phase.  It’s busy burning Hydrogen into Helium in its nuclear furnace of a core.  As a whole, the Sun is getting brighter but only very slowly.

In about 7 billion years things will get nuts.  The Sun will run out of Hydrogen fuel and puff up into a red giant star the size of Earth’s orbit.  Mercury and Venus will fall into the Sun.

Earth is on the cusp: it may be pushed away or it may fall into the Sun.  It doesn’t really matter because Earth will be long dead, as the oceans will have boiled away billions of years earlier.

The red giant phase lasts for a few hundred million years. After that, the Sun will shed its outer layers and all that will be left is its core, a white dwarf that won’t do anything except cool off over eons…

Some asteroids and comets will probably crash down and contaminate the outer layers of the white dwarf.  Their spectral signatures may be the last signs of the Solar System’s planets.

Jeez, that’s depressing.  But there’s a bright side.

As the Sun evolves, the habitable zone evolves along with it.

Since the Sun gets brighter the habitable zone moves outward:

The habitable zone moves outward as stars get brighter with age.  Credit: Mandy Fischer.

Researchers have used models of how stars evolve to determine how the habitable zone shifts in time.

The Sun will be so much brighter as a red giant, its habitable zone will be drastically different than the present-day one.  Instead of being centered on the Earth-Mars region, it will be centered on the Jupiter-Saturn region.

It will look something like this:

Top: the habitable zone around the Sun today.  Bottom: the habitable zone around the Sun once it becomes a red giant in about 7 billion years. Credit: Wendy Kenigsberg

The Sun doesn’t just jump from the main sequence to the red giant phase.  A more complete (and more complicated) graph is included at the bottom of this post.

All told, Jupiter will have about 370 million year span in the habitable zone as the Sun evolves.  Saturn will get about 200 million years.  This means that….

… Jupiter and Saturn’s large icy moons are second-chance planets!

And they’ve got some nice big ones:

Large moons of the Solar System, with Earth for scale. The moons are ordered by which planet they orbit. From Wikipedia.

Jupiter’s four Galilean moons are each close to our Moon’s size (or larger), and Ganymede is more massive than the planet Mercury! Saturn’s moon Titan is similar in size.

These moons all contain a mix of iron, rock and ice.  As I’ve discussed before, Io is the most volcanic object in the Solar System and doesn’t have a ton of water.  But all of those other large moons are thought to be very water-rich, and several even have global oceans under a layer of ice (as does Titan):

Models of the interiors of Jupiter’s Galilean moons.  Credit: John Brodholt.

What will these moons look like when they enter the habitable zone?

Let’s see — they all have a *lot* of water. So as they heat up, their oceans should melt and they’ll become mini-ocean worlds.  They could look something like this:

The view from a hypothetical habitable moon.  Source: Wikimedia Commons.

The planets in these moons’ skies would be big.  Io is about the same distance from Jupiter as the Moon is from Earth.  Except that Jupiter is 40 times bigger than the Moon.  So in Io’s sky, Jupiter looks 40 times larger than the full Moon!  That’s about 20 degrees across!

These big moons would have abundant liquid water and potentially habitable.  Of course, their atmospheres would slowly leak away into space because their gravity isn’t too high, but that process would probably take about as long as the few hundred million years they’ve got in the habitable zone anyway…

Boom!  Second-chance planets!

That was a pretty good story, right?  We could just stop there.


I want to leverage this idea to build a planetary system in which the star’s evolution is a good thing.

An Ultimate Second-Chance Solar System.

It’s pretty simple.  This system has planets in the “normal” habitable zone — during the star’s boring main sequence phase (the phase the Sun is in right now), these are the planets that could have liquid water.

But this system will also have planets on more distant, outer orbits.  These will be frozen during the star’s main sequence phase.  But they will be in just the right place when the star goes red giant.

Let’s go retro.  For the main sequence habitable zone I’ll use good ol’ Ultimate Solar System 1, and for the red giant habitable zone I’ll use Ultimate Solar System 2.  Remember those?  They had 60 habitable zone planets between them.

Our system looks like this:

A planetary system in which two different habitable zones are populated with planets.  The inner (green) habitable zone is where water can be liquid during the star’s main sequence phase.  The yellow zone is where the habitable zone will be when the star becomes a red giant.  For details on how the planets’ orbits were chosen, see the Ultimate Solar System series here.

It’s tempting to want to build a system with rings of planets like in the Ultimate Engineered Solar System.  But I’m not convinced that those will stay stable as the star evolves.

(Side note: I wonder if there is a system without a gap in the middle, in which the habitable zone simply moves outward as the star evolves.  There could be a particular type of star for which that happens but if it exists I haven’t found it.)

Moving on.

After the red giant phase, stars like the Sun puff off their outer layers.  All that remains are the cores: white dwarfs.

White dwarfs are tiny.  They do have a habitable zone (see here) but it’s so close-in that it would take something quite special for a planet to end up there.  But crazy things happen, so it’s definitely worth thinking about.

Now, not all stars evolve like the Sun.

This story is different for low-mass stars, some of which evolve so slowly that they won’t become red giants for trillions of years!

Fast-evolving high-mass stars go through so many different phases that, even though the habitable zone jumps around, there would be little time for planets or moons to adjust.

This is why one of the posts in the How planets die series was planets being roasted, toasted and swallowed by their evolving stars.


Questions?  Comments?  Words of wisdom? 



How the habitable zone will shift during the Sun’s dramatic evolution in about 7 billion years (the Sun’s current age is about 4.5 billion years).  The red giant phase is the ~200 million year-long period centered on 12.6 billion years.  Credit: Ramses Ramirez.

9 thoughts on “Second chance planets: Iceball worlds that thaw out when their stars go red giant

  1. In the March 1994 Analog there is a story _Waterworld_ by Lee Goodloe & Jerry Oltion, in which some humans in an STL starship are investigating such a world, and trying to figure out how to do some In Situ Resource extraction from it to save themselves.

  2. I wonder how much Jupiter and Saturn will puff up with the extra heat. Would IO be swallowed up?

    1. You say “get” (present tense) but perhaps more interestingly, no such stars have ever existed… yet. Red dwarfs with total internal convection live for hundreds of trillions of years, our cosmos is only around 13 billion years old.

  3. I love this one. Question – I know the theory is Mars was possibly habitable millions of years ago. When the Sun starts its path down the Red Giant phase, could Mars become habitable again?

    1. Although Mars is currently in the habitable zone today, it is not currently habitable for humans. Also, unlike Europa (for instance), there are also no large frozen oceans on Mars that could melt with a more luminous Sun (although Mars does have small ice caps and some amount of sub-surface water).
      The habitable zone will sweep outward as the Sun gets larger, extending to approximately Earth’s orbit or so at its peak size. Mars will burn to a crisp as this happens.

  4. The trouble here is that the red giant phase of a sunlike star lasts only many millions, not several billions, of years. Perhaps life could evolve (or, if there was already life there, perhaps *photosynthesis* could evolve) there, but

    Also I remember reading a paper a while ago that suggests that icy worlds with high albedos, such as a Snowball Earth or a Europa, will NEVER be habitable. The insolation increases, but the planets are reflective enough that they stay frozen until the insolation is much more than on modern Earth, and then they very rapidly melt and the low albedo of the ocean and the greenhouse volatiles released will cook the world very rapidly.

    This isn’t as much of a problem for a desert planet with very little surface ice, and if the world is something like Callisto, where it’s icy but has a low albedo, it may melt much earlier.

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