Robert Frost’s classic 1923 poem ”Nothing Golden Can Stay” is certainly true for planets that are in the habitable zone — or shall we say the golden zone — around their parent stars.

A planet’s lease on life runs out when the evolving star grows too hot for the world hold onto water for sustaining life as we know it. With increasing stellar luminosity, the habitable zone sweeps outward beyond the planet’s orbital radius, leaving paradise worlds to bake to death as surviving extremophliles burrow underground.

What’s sobering is that Earth has already spent 70 percent of its habitable years inside the sun’s golden zone.  And it took that long for intelligent life to appear on the surface.

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We’ve got about 1.7 billion years left, according to a paper published by Andrew Rushby and co-authors in Astrobiology Magazine. When the sun reaches 118 percent the brightness of what it is today our oceans will evaporate away and Earth will be desiccated, resembling the terrain on Saturn’s moon Titan.

The scientists say the best place buy real estate for long-term habitability is around a red dwarf star. A planet can remain cozy for advanced life for a stretch of time that is five times greater than for Earth. All other thing being equal, this suggests that SETI searches should target red dwarfs to see if they are home to advanced civilizations that do not have to worry about the clock running out.

There have been numerous news announcements, mostly from NASA’s Kepler planet hunting observatory, about the discovery of so-called “Earth-like” worlds nestled inside a star’s habitable zone.

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Kepler reports on a planet’s orbit size, radius, and apparent size. We know next to nothing about the physical characteristics of these worlds including tectonic and magnetic activity, atmosphere composition, axial tilt, and other conditions affecting climate.

But two reliable metrics are pinpointing the location of the planet in the star’s habitable zone and the star’s age. Unfortunately, stellar ages are not that well know for candidate habitable planets.

Based on their assessment, the researchers say that the planet Gliese 581g is the “most habitable exoplanet found to date.” It’s located smack dab in the middle of a red dwarf’s habitable zone. the world’s temperatures will be moderate for another 5 billion years longer than the span of Earth’s habitability window.

The planet may be as small as Earth and therefore have similar gravity, and surface topography. At only 20 light-years away it would be a logical destination for the first interstellar reconnaissance probes. So far, the planet has been the target of (unsuccessful) SETI searches.

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But do long-lived planets inevitably host intelligent life? This assumes intelligence is a pinnacle to which biological evolution aspires. But it is terribly anthropomorphic in concept say the authors. Finding strong chemical biosignatures on exoplanets would indicate that Darwinian evolution has kicked in, but does it deterministically advance toward self-awareness?

When all the variables for determining the habitability of candidate planets are woven together, astronomers might eventually come up with a classification scheme, as was done  a century ago for star types. “The habitable zone metric need to be incorporated into a broader Habitability Index for exoplanets,” say the authors.

Nearly 50 years ago The Star Trek TV series was a step ahead of this idea. Earth-like planets were called “Class M.” Maybe someday real starship captains will refer to a similar classification system while plying interstellar space.

Image credit: Credit: Chester Harman, Pennsylvania State University, NASA