It's getting more and more likely that earthlings may have a neighbour next door.
Some of the Martian surface is at times covered with seasonal flows of liquid, salty water, NASA announced Monday, suggesting "it would be possible for life to be on Mars today."
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"This is a really good discovery," says Ravi Kumar Kopparapu, an assistant research scientist at NASA's Goddard Space Flight Center. "This gives me hope that there are two planets [including Earth] that can potentially host water on the surface."
'There may be other ... types of life that we don't know about that might survive in rather different conditions.' - Guillermo Torres, astronomer
The news reinforces the notion that planets in the so-called Goldilocks zone are likely to have life.
But there are factors other than location that astronomers need to look at when searching for extraterrestrial life. Just because a planet finds itself nestled in just the right spot to have liquid water — one of the building blocks of life on Earth — doesn't mean it has all the other components necessary to sustain living things, or that all living things even require water.
Planets just right for life
Mars is located in the outer edge of our solar system's Goldilocks zone.
The zone draws its name from the classic children's tale starring a precocious girl who visits the home of a bear couple and their child while they're away. She explores their home and finds three bowls of porridge (one of which is too cold, one which is too hot and one that is just right to eat) and three beds (one which is too big, one which is too small and one which is just right to sleep in).
The Goldilocks zone in a planetary system is the space containing planets that are just the right distance from a star to be neither too hot nor too cold for liquid water to be present on their surface.
Earth is the only other planet in the solar system's Goldilocks zone, which ranges from about .95 astronomical units away from the sun to 1.67. One astronomical unit is how far Earth is from the sun, placing our home planet on the inner edge of the solar system's habitable zone.
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Outside our solar system, there are 31 exoplanets known to astronomers that are within another star's Goldilocks zone, according to the habitable exoplanets catalog project by the University of Puerto Rico at Arecibo.
NASA's findings are "a good sign" for these exoplanets, says Kopparapu, who studies habitable zones. It shows there is "a good chance" they too could have surface liquid water, he says, and then potentially life.
Life could survive in 'different conditions'
But, a planet within a star's habitable zone doesn't necessarily have E.T.-like Martians or even simple organisms on it.
A planet within the Goldilocks zone must also have a habitable environment, says Guillermo Torres, a staff astronomer at the Harvard-Smithsonian Center for Astrophysics. Torres was one of several astronomers who found eight new planets within a star's Goldilocks zone in January.
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To potentially host life as humans understand it, these correctly situated exoplanets need to meet a few more requirements. They can't be more than 1.5 times the size of Earth, he says, or else it may not have a solid surface. Only 10 of the 31 exoplanets within a Goldilocks zone are about the size of Earth, according to the catalogue project.
The planet's atmosphere and the chemicals present in it can also make a difference, Torres says.
In terms of Mars, which no longer has a thick atmosphere with large quantities of oxygen like Earth, water is likely a very important clue towards proving the planet had life three or four billion years ago, he says, rather than today.
But scientists are guided by only one example of life, Torres says, that which exists on Earth.
It's possible that some type of organism that can sustain massive seasonal temperature swings caused by Mars's lack of atmosphere lives on the planet, he says. After all, even on Earth scientists have discovered so-called extremophiles, organisms that live in seemingly unlivable conditions, he says, like acid, ice or extreme heat.
"There may be other ... types of life that we don't know about that might survive in rather different conditions" on Mars or other planets, he says.
Water a logical starting point
NASA's discovery highlights these "extra complexities" of where life can potentially be found, says David Hanes, a professor at the department of physics, engineering physics and astronomy at Queen's University.
It even raises the broader question of whether all life depends on water, he says.
'There's no point in going out and looking for every imaginable life form out there.' - David Hanes, professor
"That's sort of the first order for life as we know it," Hanes says. But, it's possible some type of life could be sustained on, for example, liquid methane, he posits.
Titan, one of Saturn's moons, appears to have bodies of liquid on it made up of liquid methane, he says. While the moon sits outside of our solar system's Goldilocks zone, if liquid methane can sustain a form of life, it could be on Titan.
However, water is a logical starting point for the search for life as scientists already know it can sustain it.
"There's no point in going out and looking for every imaginable life form out there," he says.
Yet, it's important not to be narrow-minded about it.
"Simple location isn't the whole story."
This article first stated that astronomer Guillermo Torres said a star within the Goldilocks zone must also have a habitable environment. In fact, Torres said a planet. The story has been updated to reflect this.Sep 29, 2015 10:52 AM ET