But nobody really knows how rare or common are planets like Earth and its brand of life. “I would be more comfortable with that argument if it were not so Earth-o-centric,” Steven Benner, a chemist at the Foundation for Molecular Evolution, said in an e-mail.

For instance, he said, low-temperature water mixed with ammonia can substitute for water alone as the liquid necessary for life. So could liquid methane, which forms lakes on Titan, Saturn’s slushy frigid moon, and Dr. Benner and others have advocated looking for life there. “We are limited by our imaginations,” said Natalie Batalha, a leader of the Kepler team.

Some scientists deplore the emphasis on animals like us, saying it is hopelessly parochial and unimaginative — the scientific equivalent of the drunk searching for his car keys under a street light because that’s where the light is.

“Animals are overgrown microbes,” said Paul Falkowski, a biophysicist and biologist from Rutgers. “We are here to ferry microbes across the planet. Plants and animals are an afterthought of microbes.” So, we should hardly be disappointed if we find our neighbors are microbes. After all, on Earth, microbes were the whole story for almost four billion years, paleontologists say, and now inhabit our intestines as well as every doorknob.

Dimitar Sasselov, an astronomer turned astrobiologist at the Harvard-Smithsonian Center for Astrophysics, said he was all for the existence of a microbial planet. “Don’t assume microbes are simple,” he said, noting that 99 percent of the genes in our bodies belong to microbes inhabiting us and without which we could not live.

Looking for Goldilocks

A blue-ribbon committee of chemists convened by the National Academy of Sciences concluded that there was only one ironclad requirement for life, besides energy: a place warm enough for chemical reactions to go on. So, determining how warm a planet’s atmosphere keeps it — through assumptions, calculations or just plain guesses — has been crucial in reaching a verdict about its potential habitability.

This is how it has gone with the potential Goldilocks planets orbiting Gliese 581, a small cool red star about 20 light-years from here in the constellation Libra that has been at the center of exoplanet fantasies and speculation for the last few years. Depending on whom you talk to, it has five or six planets, three of which have at one time or another been claimed to be habitable.