To date, Planet hunters have spotted more than 200 planets beyond our solar system, but the vast majority are hot, Jupiter-sized planets that would dwarf the Earth and are almost certainly lifeless.

Astronomers may be on the brink of discovering a second Earth-like planet, a find that would add fresh impetus to the search for extraterrestrial life, according to the US journal Science. Astronomers from six major centers, including NASA, Harvard and the University of Colorado, outline how advances in technology suggest scientists are on the verge of being able to detect the presence of small, rocky planets, much like our own, around distant stars for the first time. The planets are considered the most likely habitats for extraterrestrial life.

One technique relies on observing the shift in light coming from a star as a planet swings around it. Until recently, this "radial velocity" method has only been sensitive enough to pick up planets far more massive than Earth, but improvements now make the discovery of a second Earth highly likely, said Dave Latham, a co-author on the paper at the Harvard-Smithsonian Center for Astrophysics.

"It could happen almost any time now. We have the technological capability to identify Earth-like planets around the smallest stars even now," he said.

Earlier this year, the world's largest and most prolific team of planet hunters, the Anglo-Australian, California and Carnegie Planet Searches ( AAPS), reported their findings of 37 exoplanets that have been discovered over the past couple of years, 7 of which were previously unreported brown dwarfs.

Depending on whose number you go by, the total number of exoplanets currently discovered is 212 or 240, the majority of which have been discovered by the AAPS and their colleagues in the California and Carnegie searches.

The method of discovery primarily implemented is studying the Doppler wobble of stars. As a planet orbits its parent star, its gravitational pull causes the star to wobble. Using the Doppler Effect, the scientists are able to determine the velocity of the planet. When the planet moves away from Earth, its star moves toward the Earth, causing it to emit shorter wavelengths, which appear bluer. The opposite is true as well; as a planet moves closer to Earth, its star moves further away, emitting longer (redder) wavelengths of light. The AAPS uses highly advanced, sensitive spectographs to record these very small wavelengths.

But there are things that Doppler searches cannot tell researchers. With Doppler readings, they are able to calculate the velocities of the planets being studied as they move towards and away from the Earth. What Doppler readings are unable tell researchers are the angles of inclination of the orbital planet to the line of sight. This is important information because by being able to calculate the angles of inclination of the orbiting planet, scientists are able to determine the actual physical size of the planet.

The AAPS has developed a technique to find the angle of inclination: transit searches. Transit searches are a relatively new technique which has only just begun giving them results within the past few years. As a planet transits in front of its parent star, passing our line of sight from Earth, scientists are able to calculate its angle of inclination, thereby determining its eccentricity (how elliptical or round its orbital path is). In the years to come, the method of transit searches should advance, resulting in more information about already discovered planets.

Although the next generation of techniques such as interferometric astrometry and direct imaging will be the most promising new methods of detection in the future study and discovery of extrasolar planets, as Chris Tinney of AAPS explains, the most successful and powerful form of study currently in use is complementing Doppler searches with transit searches. By doing so, “You can essentially know everything you can know about a planet. You know exactly its mass and its radius, which means you can work out its density,” and therefore, “you can make estimates as to whether it’s a gas giant or an ice giant planet, or whether it’s rocky.”

As these techniques develop, the smaller and smaller the extrasolar planets being discovered will become.

So when does Tinney expect an Earth-sized planet discovery, now that they’ve gotten down to Venus-sized planets when once they only found those with a mass that of Jupiter’s?

Tinney thinks that “finding a planet of Earth mass is probably a only couple of years away. But…”—and he emphasizes the “but,” pausing for a moment—“there’s always a ‘but.’” As he explains, all of the things they are finding of very low mass are moving in very short orbital periods, which means that they are orbiting close to their parent stars. So although there they are like Earth in terms of their mass and size, these planets are very unlike the Earth in terms of their orbit.

“To find an Earth-mass planet in an Earth-like orbit is just not going to happen with the Doppler technique,” Tinney states. It is simply beyond the technology currently developed. Essentially, it would mean that they would need to be performing measurements 100 times better than any technology is capable of doing.

So does this rule out the possibility of finding a habitable planet?

Not quite. There is a “trick” to planet hunting. Scientists can look for Earth-mass planets in short period orbits around lower mass stars. These types of stars are called M dwarfs and have a mass one tenth the size of the Sun, which means that the velocity signal is ten times larger, and therefore the radius at which the planet must be from the star in order to have water or liquid on its surface is much smaller. For now, it’s Tinney’s opinion that some of the recent reports about habitable planets being discovered “is more hype than reality,” but that the discovery of such planets “will come in due course.”

In fact, that’s precisely what Tinney is currently working on, aside from his AAPS commitment. He has convinced the Gemini Observatory—a collaboration of the US, Canada, UK, Australia, Brazil, and Argentina—to build a spectrograph on one of its largest class of telescopes. In order to perform the types of studies needed to find other Earth-mass planets, scientists would need to being studying the near infrared, rather than the green wavelengths of visible light. This new Gemini spectrograph, called the Precision Radial Velocity Spectrometer, will specifically be designed to do very high precision Doppler work in the near infrared, rather than the optical. Once that type of technology is developed, Tinney believes that rather than finding the occasional one or two Earth-sized planets around M dwarf stars, finding more and more “will be much more straightforward,” thereby dispelling some of the current hype and allowing scientists to gather actual statistics about these types of systems.

NASA's mantra of "follow the water" has defined the search for extraterrestrial life on and other planets. If water is crucial for life, then the most likely sanctuaries will be planets which lie in a "habitable zone" just the right distance from a star, so that it is neither so hot that water evaporates, or so cold that it remains permanently frozen.

Dr Latham of the Harvard-Smithsonian Center said missions such as Nasa's Kepler space observatory, which launched in early March, would have a high chance of finding Earth-like planets if they are out there.

"These are the biggest questions. Are there habitable abodes? Are we alone?" he said. "Put it like this. If we don't find anything, I'll have to rethink my agnosticism."

Posted by Casey Kazan. Image Credit: Corbis.