The ongoing search for Earth-like worlds has taken another promising step. On December 5, NASA announced the discovery of the planet most likely so far to sustain life outside of the Solar System. The exoplanet, given the undramatic name of Kepler 22b, was found by NASA's Kepler spacecraft as part of its mission to seek out Earth-type planets in our galaxy. Though Kepler 22b is not the first such planet to be detected in recent years, it is the first one orbiting a star similar to our Sun and at a distance where it is capable of possessing liquid water, which most scientists regard as essential for life to exist. Though this is a significant milestone, the question remains, how good a candidate for a second Earth is Kepler 22b? Could there be life there or is it a planetary blind alley?

In the zone

Kepler 22b is not exactly around the corner. It's located about 600 light years from Earth, orbiting the star Kepler 22, which is located between the constellations of Cygnus and Lyra. (For astronomy enthusiasts, the exact coordinates are Right Ascension 19h 16m 52.2sec and Declination +47deg 53min 4.2sec.)

With a magnitude of only a little less than 12, its star isn't even visible to the naked eye. However, this distant, dim little star is important because it's orbited by Kepler 22b - the first discovered exoplanet that matches the three major factors for sustaining life as we know it. First, it is roughly Earth-sized, as opposed to the super-Jupiter planets that dwarf even the largest planet in our Solar System and make up most of the discovered exoplanets, as those found outside our system are called. Second, it circles a star that is in the same major category as our Sun, which means that the star isn't too hot or too cold, hasn't too short an existence for life to evolve, and remains relatively stable without the sort of huge changes in energy output found in many variable stars. And third, the planet's orbit is similar to Earth's - there's only one star, so the orbit is stable, and it lies in what is called the habitable zone, a range of distances where the temperatures aren't too hot or too cold.

A giant camera

The discovery of Kepler 22b was part of NASA's US$600 million Kepler mission. Named after the German Astronomer Johannes Kepler (1571-1630), who discovered the laws of planetary motion, the Kepler spacecraft was built by primary contractor Ball Aerospace & Technologies and launched from Cape Canaveral on March 7, 2009 aboard a Delta II rocket. Weighing 2,290 lbs (1,039 kg), Kepler was placed in a heliocentric orbit that trails behind the Earth with an orbital period of 372 days. The reason for this was to keep the spacecraft away from the Earth's light and gravity. That way Kepler's sensors could be made much more sensitive and the position of the craft much more stable, so it can keep pointing at one fixed spot in the sky.

Kepler is basically a giant camera. It has a 37.4 inch (0.97 m) aperture servicing a 55 inch (1.4 m) mirror and it has the largest resolution of any space camera with 95 mega-pixels and routinely downloads 100 gigabytes of data a month. But its purpose is not to take photographs. It doesn't send back beautiful images of the sort we associate with the Hubble space telescope. What it does is act as a photometer. In other words, it measures the brightness of stars and that is how it finds planets.

The idea is very simple. The stars that Kepler studies don't change in brightness very much, if at all. Think of them as being like a bare light bulb sitting on a stand in a darkened room. Now imagine that someone passes a small ball in front of the bulb. The ball isn't large enough to eclipse the bulb as the Moon does the Sun, but it does block part of the light. Even if you can't see the ball, you can detect the bulb looking slightly dimmer. If you have a photometer, a device for measuring brightness, you can measure exactly how much the bulb has dimmed and from that, you can calculate how large the ball is.

That's what Kepler does. Its photometer looks at stars, measures their brightness, and if a planet happens to pass in front of the star, it can measure how much the star's brightness dims and how fast it dims and returns to normal. With this information, scientists can then calculate how large the planet is and how far it orbits from its star.

How to find a living planet

Kepler's search for planets isn't random. In fact, it's very precise and well thought out. It continually points at one spot in the sky, scanning the constellations of Cygnus, Lyra and Draco. These were chosen for two reasons. First, they lie along the line of the Solar System's orbit around the center of the galaxy and second, this points Kepler at the "Goldilocks Zone" of our galaxy.

The Goldilocks Zone is a galactic version of the criteria used to determine if a planet can sustain life - only in this case, it applies to stars and is a little more complicated. It's based on the Rare Earth hypothesis, which states that the conditions for producing life on Earth fall inside of such a narrow range that any chance of finding other life-bearing planets can only hope to succeed if astronomers look for planets that fall inside of that range. Whether the Rare Earth hypothesis is true or not is, of course, open to debate, but NASA felt that using the hypothesis as a starting point would improve its chances.

In this case, the hypothesis says that there is only a narrow band of space inside the galaxy where planets like Earth can exist and sustain life. That 21 to 27 thousand light-year wide band is the one in which the Solar System orbits. Too close to the galactic center, and there is too much radiation, stars are too close together and there are too many heavy elements. Too far away, and there aren't enough heavy elements to form a planet like Earth. For a star to have an Earth-type planet, it must travel in a circular orbit within that band.

Using this hypothesis, the Kepler spacecraft therefore points at a nearby part of the galaxy where such stars are most likely to be found and the results so far have been good. Kepler has detected over 2,300 possible planets and 200 possible Earth-type planets with over 20 planets confirmed. However, only Kepler 22b has come the closest to being a candidate for having life.

Unfortunately, a candidate is all Kepler 22b is at the moment and there are still questions on how likely it is to be a second Earth. Some critics are outspoken in this regard with Don Pollacco, from Queen's University, Northern Ireland telling the BBC that he believes the announcement is merely a publicity stunt made as part of an effort to drum up additional funding as Kepler reaches the end of its three and a half year mission.

Is there life on Kepler 22b?

But is there life on Kepler 22b? The best answer is, perhaps, but save your money if you you're thinking about booking passage on the first colony ship. The reason is that sometimes one little number can change things completely. In this case, that number is 2.4.

True, Kepler 22b does orbit a star similar to our Sun and yes, it is at a comparable distance and it is indeed within the category of what astronomers class as "Earth type". However, this Earth-type planet has a radius 2.4 times that of the original. The Earth has a diameter of about 8,000 miles (13,000 km), that makes Kepler 22b a whopping 19,000 miles (31,000 km) in diameter. This makes it closer in size to the planet Neptune than Earth. That means that Kepler 22b is either a rocky planet with a gravity two and a half times greater than Earth (not impossible, but that raises all sorts of problems if true) or, as is more likely, the planet is probably a gas giant like Neptune with a methane atmosphere and an ocean of water, ammonia and methane ice covering a small, rocky core.

Still, that hasn't completely quashed mission optimism about the planet as Kepler team member Natalie Batalha speculates that, "it's not beyond the realm of possibility that life could exist in such an ocean." However, life on a gas giant remains as much in the realm of speculation as life hiding in a crater of the Moon.

Another problem with life on Kepler 22b is that, even if it was exactly the same size as Earth, the criteria used to evaluate it still doesn't allow astronomers to regard the planet as a candidate for life except in the broadest of terms. For example, its star, Kepler 22 is only broadly similar to the Sun, which is a G2 star where Kepler 22 is a G5. It's a quarter smaller than the Sun and a quarter colder. Kepler 22b is a quarter closer to its star than the Earth is to the Sun, so this compensates somewhat, but the difference in the makeup of the two stars may be significant.

Then there is the question of the orbit of the planet. How eccentric is it? Is it circular like the Earth's or is it more elliptical, which may cause radical climactic variations with winters where it snows carbon dioxide and summers where water boils. Does the planet have a moon? How many? How big? How strong are its tides, if any? What about water? How much? How saline is it? Does the planet rotate? How fast? Does it have a magnetic field to guard against radiation? What about an atmosphere? How active is the planet? Is it a dead lump like the Moon or is it a volcanic nightmare like Jupiter's satellite Io?

Even being in the habitable zone only goes so far. True, the Earth sits comfortably inside the Solar System's zone, but so do Venus and Mars. The former is a hell of temperatures hot enough to melt lead, sulfuric acid rains and an atmosphere like the bottom of the ocean. Meanwhile, Mars is a dead, frozen world with hardly any atmosphere, almost completely dry, seared with ultraviolet radiation on a daily basis and host to sand storms of unbelievable savagery.

This isn't even to mention what else might be in the Kepler 22 system that might be wreaking havoc. The planet could be bombarded by comets or it may be in its orbit only temporarily thanks to the gravitational pull of another giant planet that threw it there a few hundred thousand years ago and may toss it out again a few millennia in the future.

None of this takes away from the importance of what the Kepler mission has uncovered. The data from the Kepler spacecraft has vastly increased our understanding of planet formation and the nature of planetary systems - not to mention finally giving astronomers hard numbers that they can use in estimating the chances of life elsewhere after centuries of having nothing but guesses. Even if Kepler 22b proves to be another Neptune, it has brought us a step closer to finding another Earth.