Life and Death in The Goldilocks Zone: The Shifting Scope of Planetary Habitability

Have you ever wondered how long the Earth will be able to support life? I don’t mean to suggest that human activity will render the planet uninhabitable (although it might), I’m talking about the natural cycle of the planet and how long it will be able to keep things nice and cozy for air-breathing organisms like us.

According to a recent study in the journal Astrobiology the answer might only be another 1.75 billion years. On first reading you might think I am being pretty liberal with my use of the word “only,” but consider the fact that life has existed on Earth for nearly 4 billion years already. That means that with 1.75 billion years left on the clock, Earth is about 70% of the way through with its hosting duties.

The problem doesn’t have as much to do with the Earth as it does with the Sun. The thing about stars is that they tend to snowball in terms of the light and heat they produce. Young stars start out dim and cool compared to the steamy blinders they will eventually become. Subsequently, as they age their “habitable zones” get further and further away.

We first heard about habitable zones in our discussion of exoplanets but the Coles Notes explanation is that they are the area around a star where things are warm enough for water to be liquid (rather than ice) and cool enough for it not to boil away. More kid-friendly scientists tend to call this space the “Goldilocks Zone” because things are just right.

The Sun’s habitable zone is moving outwards at an estimated rate of 1 meter per year and recent models suggest that Earth is closer to the inside edge than previously thought. The upshot is that in a little less than 2 billion years, the oceans will have boiled away and anything left of Earth will have a pretty rough go of things.

It’s a pretty depressing thought that one day this lush blue-green ball that we all call home will be a blistered chunk of rock orbiting an aging hot-shot star, but it is a fact that humans will eventually have to face if we want to continue existing. Fortunately not all scientists agree about when check-out time might actually be, so don’t start packing up the little bottles of shampoo just yet.

Caleb Scharf, an astrobiologist at Columbia University thinks we might be getting ahead of ourselves. “It’s the age-old problem of over-interpreting a single data point” says Scharf. The fact is, we may not know as much about habitability as we think we do.  There are many factors outside of location relative to a habitable zone that dictate whether or not a planet can support life. Atmospheric conditions, plate tectonics, and the history of life on the planet all have their role to play. This has lead University of Victoria planetary climatologist Colin Goldblatt to comment, “If you want me to build a habitable planet where Venus is, I can do that; if you want me to build a dead planet where Earth is, I can do that.”

That may say as much about Colin Goldblatt’s confidence in his terraforming abilities as it does about the current state of things on Earth, but it gives us some hope. Even as the Sun heats up and the Goldilocks Zone drifts out towards Mars, organisms on Earth may have a bit of extra time to pack our bags and jump ship.