'Goldilocks' clue to habitable planets

December 11, 2013
The Earth and Venus, although not very different in size, have had a very different climate history. In the future, the Earth may look like Venus. Credit: Jeremy Leconte

The bad news: Earth's oceans will evaporate away. 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 vapour.

As in the fairy tale, a planet's temperature has to be not too hot and not too cold, but just right for sustaining the stuff for life as we know it.

Jeremy Leconte of the Pierre Simon Laplace Institute in Paris investigated a well-known phenomenon in astrophysics: as a star ages, it increases in brightness.

In their simulation of Earth, the Sun's rising luminosity will eventually cause a runaway greenhouse effect, they found.

Water vapour is a greenhouse gas. This means that beyond a certain point, increasing vapour from the warming oceans will stoke Earth's surface temperature—which in turn causes more sea water 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 their model.

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

Simulation of the increase with time of the surface temperature (here at the spring equinox) caused by the increase in solar flux. Around 1 billion years, the runaway greenhouse climate instability is predicted to vaporize all the oceans until a very high temperature is reached (The mean temperature in panel 3 is an extrapolation of the temperatures after oceans are fully vaporized). Credit: Jeremy Leconte

The high-tech modelling includes a 3-D simulation that factors in solar heat per square metre, the seasons and the vapour cycle.

Previous models have tended to simulate 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.

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 (AU) for a star that is the size and present age of the Sun, it says.

One AU is 149.6 million kilometres (92.95 million miles), being the average distance between the Earth and the Sun—the orbit of our planet is slightly elliptical.

By comparison, Venus, Earth's sister planet in size, lies at 0.75 AU: it is just a bit too close to the Sun.

In its infancy, it 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 in the Goldilocks zone, so if the zone is wider than thought, this boosts the statistical chances.

So far, astronomers have only discerned uninhabitable planets made of gas, or rocky planets that are so close to the Sun that any atmosphere they had will have probably been stripped away.

Explore further: Greenhouse effect could extend habitable zone

More information: Paper: dx.doi.org/10.1038/nature12827

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4.3 / 5 (4) Dec 11, 2013
Heh. The maps makes the unchanged plate tectonics, the reference plates of "now", very clear.

But nice to know that the clouds that makes exoplanet atmospheric observation so fuzzy also fuzzifies the HZ. These modern global circulatory 3-D models have already solved the Archean "faint young sun" problem, by all accounts.
3.7 / 5 (7) Dec 11, 2013
Note to the Editor: At no point in the atmosphere of Venus do carbon dioxide clouds form. The clouds are made of sulfuric acid droplets, with dissolved sulfur trioxide, with occasional high wispy water ice clouds.

Minor note: Venus is not at 0.75 AU from the Sun, but resides near 0.72 AU.
1.8 / 5 (10) Dec 11, 2013
Stick a bunch of unverified and or unverifiable information into a computer, hit enter and viola, you have a computer model - and real science suffers on.
2 / 5 (4) Dec 12, 2013
Quite right, the atmosphere contains 96+% CO2 but not the clouds! And why the comparison? Venus is though to be much younger planet than Earth and has yet unexplained characteristics. I think they are getting mixed up with Earth's crust being younger due to recycled elements.
2.3 / 5 (6) Dec 12, 2013
Dug, so true. True science doesn't begin on a computer.
not rated yet Jan 07, 2014
And, they published this study? Where are plate techtonics and why have they not been factored into the equations? Venus's atmosphere is over 90 times denser than that of earth. Venus with its small percentage of Nitrogen in its atmosphere has more Nitrogen in its atmosphere than does earth because of said atmospheric density. CO2 in the atmosphere of Venus is over 965,000 ppm at over 90 times atmospheric density of earth.

They did get one thing right, though. Earth almost certainly will become desiccated as the Sun ramps up its fusion.

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