In the zone

Who hasn’t enjoyed the story of the silver-haired old woman who breaks into a cottage in the woods owned by three bachelor bears and proceeds to eat their porridge and sleep in their beds?

That is, after all, the fairy tale the poet laureate of the United Kingdom, Robert Southey, recorded in 1837. He called his version of the old oral tale “The Story of the Three Bears.”

I know, what about the little blonde girl who is so particular about her meal and sleeping arrangements? Goldilocks comes into the picture in a collection of English fairy tales retold by Flora Annie Steel in 1922, although the golden-haired girl was already being called Goldilocks in an earlier nursery rhyme. And the three gentleman bears eventually became the family of Papa, Momma and Baby Bear.

There’s quite an interesting backstory to this fairy tale. But the versions all tell how the young or old intruder eats the bears’ porridge. The Papa’s porridge in the big bowl is too hot. Momma’s in a middle-sized bowl is too cold.

Ah, but Baby Bear’s porridge in the smallest bowl, is just right – not too hot nor too cold.

Turn our attention to astronomy and astrobiology, and that concept of not too hot, nor too cold, is used to describe the circumstellar habitable zone. In the CHZ, planetary surfaces can support liquid water given sufficient atmospheric pressure.

Lucky for us, the Earth is in that habitable zone or as it’s often casually called the “Goldilocks zone,” a term that began to be used in the 1970s.

Mars is also in the Goldilocks zone, and Venus is on its inner edge. But just because a planet is in the zone doesn’t mean it’s as life-filled as Earth. It may have been different for Mars and Venus in the past, but at this point, not so much.

So as the hunt for exoplanets grows more intense, the scientists are especially interested in those rocky worlds in the Goldilocks zones of their stars. Liquid water on their surfaces may be possible, and if so, possibly life exists there. There are many other factors to check out, but the radius of a planet’s orbit is a big key.

Can complex or intelligent life exist outside the relative comforts of the Goldilocks zone? Perhaps. But the evolution of multi-cellular life likely depends on surface water. We’re lucky Earth has that in abundance.

As Goldilocks might say, “It’s just right!”

Image credits

Illustration in "The Story of the Three Bears" second edition, 1839, published by W. N. Wright of 60 Pall Mall, London

Petigura/UC Berkely, Howard/UH-Manoa, Marcy