Scientists continue to seek signs of life outside our solar system, but how will they know when they find it?

Astronomers announced Wednesday they had found seven Earth-sized planets orbiting a star 39 light-years away. Three of the planets lie within the "habitable zone," a theoretical range in which liquid water could exist. 

The scientists plan to study the atmospheres using telescopes including the Hubble Space Telescope in an effort to determine if there is actually life flourishing on these far-off worlds.

To understand what's involved in searching for signs of life, you need to first know two definitions astronomers use: potentially habitable and habitable.

Potentially habitable means that a planet lies within an theoretical zone around a star where liquid water can exist on its surface. Habitable, exoplanet researcher Sara Seager told CBC News, means there's water vapour.

Habitable zone planets

This diagram shows how the habitable zones of larger stars are wider and farther away the hotter the star is. (NASA/Kepler Mission/Dana Berry.)

Each time astronomers discover Earth-like planets, the immediate question is: Is there life on it? But that question may be harder to answer than it first appears.

"We don't know how life emerges," Amaury Triaud, co-author of the study on the new discovery, said at a news conference Wednesday.

If life emerges from an ocean of water, then it will be easier for us to find, he said. But if life forms through other means, or can thrive in an atmosphere laden with gases in which we can't live, then we could miss it.

'There are plenty of science fiction books that say if you have oxygen you have life. But that's not true.' - Michaël Gillon

We only know of carbon-based life, life that requires water to survive. As a result, we look for gases emitted into the atmosphere that are detectable. And that's how astronomers are looking for signs of life.

"We're very Earth-centric," said Seager, a Toronto-born professor at the Massachusetts Institute of Technology.

But even then, it's not that simple.

For example, we may find an oxygen-rich atmosphere on an Earth-like planet orbiting a distant star right in that habitable zone. However, it doesn't mean that the planet is teeming with biological life. A water-rich planet that receives a lot of infrared radiation could have the water molecules split, producing oxygen molecules in the atmosphere.

Life as we don't know it

So it's not as simple as just looking for gases like oxygen, methane or carbon dioxide.

"It's really the combination of these different molecules," Michaël Gillon, lead author of the study on the new planets, said.

"Oxygen by itself is not enough. There are plenty of science fiction books that say if you have oxygen you have life. But that's not true."

Exoplanet system

This artist's conception shows what the newly discovered TRAPPIST-1 planetary system may look like. (NASA/JPL-Caltech)

Seager said that worlds with other gases could also be home to life.

"We don't know if we have another planet where there is life but there's no oxygen," she said. "We're trying to think of all the other gases. We don't have an answer … yet, but we're working on it."

Closer to home

While searching for life on planets that orbit far-off stars is exciting, we lack the ability to visit these places to see for ourselves. 

The search for life in our own solar system might yield better results, however. 

When thinking about life in our solar system, many people turn to Mars, but our neighbourhood has no shortage of possible worlds where life could be found.

Two of the most promising are not planets, but rather moons.

Enceladus, one of Saturn's 60-plus moons is one such target. The moon has hydrothermal vents erupting into space from beneath its icy surface. Recently, organic compounds were detected as well as carbon dioxide and salts. Some propose that it is the best place to search for life within the solar system.

There is also one of Jupiter's moons, Europa, which is believed to have an iron core and a global ocean.

Then there's the unusual Titan, another of Saturn's moons. This cloud-covered world is similar to Earth, with an atmosphere and an Earth-like liquid cycle. Instead of water, however, Titan's lakes and atmosphere contain methane and ethane, found in natural gas. Some suggest that life could flourish there as well.

Titan

Beneath the thick haze, Saturn's moon Titan has rivers and lakes of liquid methane and ethane. (NASA )

But if any life is to be discovered in our solar system, it would most likely be microbial.

While some astronomers are focusing on the search closer to home, others have their sights on something farther away that may one day lead to the discovery of intelligent life in our galaxy.

Wednesday's announcement likely means that astronomers will now turn to small stars, rather than larger ones like our own sun, something that the researchers believe contain a better chance in the search for life.

"To find life on a planet like the Earth orbiting a [sun-like star], we would need decades of tech development, whereas here we can do it now," said Gillon.