CAMBRIDGE, Mass. » What does Goldilocks want?

At least four times in the last few years, astronomers have announced they have found planets orbiting other stars in the sweet spot known as the habitable zone — not too hot, not too cold — where water and thus perhaps life are possible. In short, a planet fit to be inhabited by the biochemical likes of us, a so-called Goldilocks planet.

None of these planets is known to be habitable — let alone inhabited — yet, but astronomers who are making the discoveries with NASA’s Kepler spacecraft are meeting next week in California to review the first two years of their quest, which seems tantalizingly close to hitting pay dirt. It is only a matter of time, they say, before the inventory of potentially usable real estate in the galaxy begins to dramatically increase.

"Sooner or later, Kepler will find a lukewarm planet with a size making it probably Earthlike," said Geoffrey Marcy of the University of California, Berkeley, who spends his time tracking down candidates identified by Kepler. "We’re no more than a year away" from such a discovery, he said.

Sara Seager, a planetary astronomer at the Massachusetts Institute of Technology, put it this way: "We are on the verge of being those people who will be remembered."

All this has brought to the fore a question long debated by geologists, chemists, paleontologists and cosmologists turned astrobiologists, namely: What does life really need to get going, flourish and evolve on some alien rock?

The answer depends of course on whom we expect to be living there. We might dream of green men with big eyes, ants with hive minds, or even cuddly octopuses as an antidote to cosmic loneliness; but what we are most likely to find, a growing number of scientists say, is alien pond slime.

Microbes can spring up anywhere that is wet and warm, astronomers say, although biologists are not so sanguine. But the emergence of large creatures, let alone intelligent ones, as evidenced by the history of the Earth, depends on a chain of events and accidents — from asteroid strikes to plate tectonics — that are unlikely to be repeated anytime soon.

"If you reran Earth’s history, how many times would you get animals?" asked Donald Brownlee, an astronomer at the University of Washington. He and a colleague, the paleontologist Peter Ward, made a case that we live on a lucky planet in their 1999 book, "Rare Earth."

Single-cell life might be common, given the right simple conditions, explained Marcy in an email.

"But the steady, long-term evolution toward critters that play improvsaxophone, write alliteration poems and build heavy-lifting rocket boosters may depend on a prohibitive list of planetary prerequisites," he added.

Even warm and wet is a rare condition, however, occurring now on only one of the eight official planets in our solar system and three of the several dozen moons. Mars was once wet, but it is now a desert. And after billions of dollars spent exploring Mars and the remains of space probes littering the planet, we still do not know if a single microbe ever lived there.

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 email.

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 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 decry 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 4 billion years, paleontologists say, and now inhabit our intestines as well as every doorknob. Had alien astronomers spied the Earth at almost any time during its history, they would have found no indication that it was to blossom into such a fabulous place.

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. (Wilhelm Gliese, for whom the star was named, was a Danish astronomer who cataloged nearby stars, most of them dim red dwarfs like this one.)

The first in what would become a chain of potential Goldilocks planets, identified in 2007, was a presumably rocky ball about five times as massive as the Earth and orbiting only about 7 million miles from Gliese 581, close enough within the small star’s shrunken habitable zone to have a warm surface.

"On the treasure map of the universe, one would be tempted to mark this planet with an X," Xavier Delfosse, one of the astronomers who discovered it, said at the time.

But before budding interstellar explorers could even begin conceiving of booking passage to Gliese 581c, as the planet is poetically called, other astronomers took a closer look and concluded that if the planet’s geology and atmosphere resembled those of Earth, it would be a stifling greenhouse, no place to set solar sail for.

Attention then shifted to a farther planet in the system, Gliese 581d, which had been dismissed as too cold. Could the same greenhouse effect that would torch the inner planet thaw the outer one and make it livable? The answer was yes, but only if it had "loads of carbon dioxide" and an atmosphere seven times thicker than Earth’s, said Lisa Kaltenegger, a climate modeler at the Max Planck Institute for Astronomy in Heidelberg. Otherwise it would be freezing cold.

"So it would be a bit different from Earth," Kaltenegger said.

Meanwhile yet another planet was claimed for that system, smack between the other two, by a team led by R. Paul Butler of the Carnegie Institution and Steven S. Vogt of the University of California, Santa Cruz.

"This is really the first Goldilocks planet," Butler said at a news conference last year organized by the National Science Foundation in Washington.

But the Geneva team that had discovered the earlier Gliese 581 planets could not find any evidence of the new planet’s existence in their own data. For now, anyway, most astronomers have dismissed that planet. Pending the publication of new results by the Geneva team — one of the most prolific in the planet-hunting business — Butler said, "We are in a holding pattern."

In September, what some astronomers called the best and smallest Goldilocks candidate yet was announced by the Geneva team. About 3.6 times as massive as the Earth, it circles a faint orange star in Vela known as HD 85512 at a distance of some 24 million miles, about a quarter of the Earth’s distance from the Sun. The star was also cataloged by the Danish astronomer as Gliese 370. Kaltenegger and her colleagues calculated that this planet would be habitable if it had an Earth-type geology and at least 50 percent cloud cover.

"So, so far we only have two great targets to search for atmospheric signatures of life," Kaltenegger wrote.

IF, IF, IF

So goes the history of astrobiology, as well as its future.

The problem, as many astronomers point out, is getting any more information about these planets.

"Astronomers are going to have to learn to live with ignorance," Seager said.

Some exoplanets, like the Gliese worlds, were discovered by the "wobble method" — looking at the motions they induce in their parent stars — which allows their masses and orbits to be measured. Other planets, like the ones identified by Kepler, are found by watching for the blinks when they pass in front of their stars; that also allows their sizes to be determined.

To date, none of the Goldilocks candidates have been observed to transit their stars, and thus none have been assigned both masses and sizes, which would allow astronomers to calculate their densities and compositions and find out if they are water worlds, rocks or gassy fluff balls.

Kepler fixes its gaze on a patch of stars in Cygnus that are hundreds if not thousands of light-years away — too far for any wobble detections that would assess the abundance of Earthlike planets in the galaxy or any other close scrutiny. We are liable to never know any more about those planets than we know now, astronomers say.

The brute reality, astronomers admit, is that even if there are thousands or millions of habitable planets in the galaxy, only a few hundred of them are within range of any telescope that will be built in the conceivable future.

Luckily there is some renewed hope for life on nearby planets. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics runs a project called MEarth that looks for planets around nearby stars. He pointed out that of the 300 stars within 25 light-years of here, 260 are red dwarfs like Gliese 581. Until recently it was thought that habitable-zone planets around such stars would have to hug the star so closely that they would be tidally locked, like the Moon, keeping one face locked to the star and roasting, the other freezing. But new studies have concluded that a proper atmosphere could spread the heat around.

Which is good. "These stars," Charbonneau said, "are our only hope for studying life in the universe in the coming decades."

In the original scheme of things, Kepler was to be succeeded by a space observatory called the Terrestrial Planet Finder, which would be big enough to find and study planets up to 100 light-years distant. But plans for that telescope have collapsed, because of NASA’s continuing fiscal woes and disagreements among astronomers, as well as the technological challenges involved.

Some astronomers hope that some of these functions can be performed by the James Webb Space Telescope — NASA’s Hubble successor, overdue and over budget, now scheduled for a launch in 2018. Equipped with a "starshade" that would blot out the glare of a planet’s sun, the Webb could detect and study the pinpoint of light from an exoplanet itself.

But the starshade would be hostage to the same political and fiscal pressures that are threatening to decimate NASA’s scientific programs. At best, scientists say, the search for life elsewhere has been postponed for decades.

"I’m beginning to despair that I will see it in my lifetime," said James Kasting of Pennsylvania State University.

GEOLOGY IS DESTINY

Earth got lucky early.

Fossil evidence suggests that microbial life was already inhabiting the Earth as early as 3.8 billion years ago — only 700 million years after the planet collapsed into existence, and a geological instant after the end of a killer rain of comets and asteroids that brought just the right amount of precious water in the form of ice from the outer solar system to what would otherwise be a dry planet, astronomers say.

"The question of whether the Earth is unique because of its water abundance is perhaps the most interesting one in the arsenal of Rare Earth arguments," said Kasting, who explained that calculations showed that the planet could have easily been swamped with too much water or parched with too little.

The planet has remained comfortable ever since thanks to a geological feedback process, by which weather, oceans and volcanoes act as a thermostat. Known as the carbonate silicate cycle, it regulates the amount of carbon dioxide in the atmosphere, where it acts like a greenhouse — trapping heat and keeping the planet temperate and mostly stable. Rain washes the gas out of the air and under the ocean; volcanoes disgorge it again from the underworld.

Without greenhouse gases and this cycle — which Brownlee called "this magic thing" — the Earth would have frozen into a snowball back in its early days when the Sun was only 70 percent as bright as it is now.

Still, with all this magic, it took 4 billion years for animal life to appear on the Earth.

The seeds for animal life were sown sometime in the dim past when some bacterium learned to use sunlight to split water molecules and produce oxygen and sugar — photosynthesis, in short. The results began to kick in 2.4 billion years ago when the amount of oxygen in the atmosphere began to rise dramatically. The Great Oxidation Event, as it is called in geology, "was clearly the biggest event in the history of the biosphere," said Ward from Washington.

It culminated in what is known as the Cambrian explosion, about 550 million years ago, when multicellular creatures, that is to say, animals, appeared in sudden splendiferous profusion in the fossil record. We were off to the Darwinian races.

Whatever happened to cause this flowering of species helped elevate Earth from a run-of-the-mill Planet of the Microbes into someplace special, say the Rare Earthers. Paleontologists argue about whether it could have been a spell of bad climate known as Snowball Earth, the breakup of a previous supercontinent, or something else.

In other words, alien planets that have been lucky enough to be habitable in the first place might have to be lucky again.

"The big hurdle" for other planets, said Brownlee, is to have some event or series of events to trigger their own "Cambrian-like" explosions and then nurture the results.

Eventually though, Earth’s luck will run out. As the sun ages it will get brighter, astronomers say, boosting the weathering and washing away of carbon dioxide. At the same time, as the interior of the Earth cools, volcanic activity will gradually subside, cutting off the replenishing of the greenhouse gas.

A billion years from now, Brownlee said, there will not be enough carbon dioxide left to support photosynthesis, that is to say, the oxygen we breathe.

And so much for us.

"Even Earth, wonderful and special as it is, will only have animal life for 1 billion years," Brownlee said.

© 2011 The New York Times Company