Wider ‘Goldilocks zone’ boosts odds of finding water on other planets


The bad news: Earth’s oceans will evaporate and dry up. The good news: It won’t happen for another billion years or so.

Those are the conclusions of a new study into the so-called Goldilocks zone — the distance from a star at which water on a rocky planet can exist as a liquid rather than as permanent ice or vapor. As in the fairy tale, a planet’s temperature must neither be too hot nor too cold, but just right for sustaining the stuff needed for life as we know it.

Jeremy Leconte of the Pierre Simon Laplace Institute in Paris was investigating a well-known phenomenon in astrophysics: As a star ages, it increases in brightness. What he found, however, was that the zone might cover a greater area than thought.

In his simulation of Earth, the sun’s rising luminosity will eventually cause a runaway greenhouse effect, he found.

Water vapor is a greenhouse gas. This means that beyond a certain point, increasing vapor from the warming oceans will stoke Earth’s surface temperature — which in turn causes more seawater to evaporate, and thus adds to the warming, and so on.

In around a billion years, liquid water on the surface of the planet will have completely disappeared, leaving an utterly desiccated surface, according to the simulation.

The time estimate for ocean loss is “several hundred million years later” than previously thought, France’s National Center for Scientific Research (CNRS) said in a press statement.

The high-tech modeling includes a 3-D simulation that factors in solar heat per square meter, the seasons and the vapor cycle.

Previous models have tended to treat the Earth as having a simple and uniform climate system. They usually place the start of the evaporation as soon as 150 million years from now, which is relatively brief in geological terms.

But the study, published in the journal Nature on Wednesday, concludes that the Goldilocks zone may be somewhat bigger than thought.

The precious zone starts at 0.95 astronomical units for a star that is the size and present age of the sun, it says. One AU is 149.6 million km, the mean distance between the Earth and the sun throughout its orbit.

By comparison, Venus, Earth’s sister planet in size, lies just a bit too close to the sun at 0.75 AU.

In its infancy, Venus may have had oceans, when solar luminosity was less than today, some astrobiologists believe. Today, scorched, bone-dry and barren, it is shrouded with thick, roiling clouds of carbon dioxide.

The findings could be useful for understanding exoplanets, or planets that orbit stars outside our solar system, say the authors. The hope is to locate a rocky planet in a Goldilocks zone; if the zone is wider than thought, the statistical chances improve.

So far, astronomers have only discerned uninhabitable planets made of gas, or rocky planets that are so close to their stars that any atmosphere they had most likely would have already been stripped away.