By Abigail Beall For Mailonline

Published: 12:00 EDT, 16 May 2016 | Updated: 00:56 EDT, 17 May 2016

The icy worlds of Enceladus and Europa are thought to have oceans buried deep beneath their surfaces, making them two places in our solar system thought to have the most potential for alien life.

Because of how far they are from the sun, both moons are extremely cold which means sustaining any form of life would be difficult.

But this is all going to change in a few million years, according to a new study.

As the sun turns into a much bigger and hotter star, engulfing Mercury and Venus and turning Earth and Mars into sizzling uninhabitable rocks, the icy moons of Europa and Enceladus (pictured) might just turn into a place perfect for life to exist

As the sun turns into a much bigger and hotter star, engulfing Mercury and Venus, turning Earth and Mars into sizzling uninhabitable rocks, the icy moons might just turn into a place perfect for life to exist.

The ‘habitable zone’ is a term astronomers use when searching for exoplanets that could be possible hosts to alien life.

Also known as the ‘Golidlocks zone’, the term refers to a planet that sits the perfect distance from its host star for liquid water to exist on the surface. 

Earth sits in the habitable zone from our sun - not too hot like Mercury or Venus, and not too cold like Jupiter and Saturn.

Throughout the universe there are stars in varying phases and ages. 

Some of these burn brighter and hotter than others, meaning their habitable zone would be further away than our distance to the sun.

Now scientists have modelled the different locations planets would have to be around different types of stars, and how long they could stay habitable for.

The ‘habitable zone’ (shown in green) is a term astronomers use when searching for exoplanets that could be possible hosts to alien life. Also known as the ‘Golidlocks zone’, the term refers to a planet that sits the perfect distance from its host star for liquid water to exist on the surface

In a few billion years our sun will become a red giant, warming distant worlds like Jupiter, Saturn and Neptune - and their moons. This will create a newly-established red giant habitable zone, around Jupiter, Saturn (pictured) and their moons like Enceladus and Europa

Astronomers usually looked at middle-aged stars like our sun, but to find habitable worlds people need to look around stars of all ages, according to said Dr Ramses Ramirez, lead author of the study and research associate at Cornell's Carl Sagan Institute in New York.

Exoplanet hunters are especially interested in identifying planets that might be home to life, and the prime consideration for that is liquid water - the key ingredient for all life on Earth.

For each star there is a habitable zone where any orbiting planet could support oceans. 

This must be not too hot (too close) or too cold (too far away), but 'just right'. For that reason, it is sometimes called the Goldilocks Zone.

Habitable zones might also exist in orbits around massive planets, where heating takes place from gravitation forces. 

In our own solar system, the Jupiter moon Europa and the Saturn moon Enceladus both have liquid water oceans.

On Titan, another moon of Saturn, oceans of ethane and methane have been found. Some speculate that these liquids could also support life, but it would be very different from any known life on Earth.

'When a star ages and brightens, the habitable zone moves outward and you're basically giving a second wind to a planetary system,' Dr Ramirez said.

'Currently objects in these outer regions are frozen in our own solar system, and Europa and Enceladus, moons orbiting Jupiter and Saturn, are icy for now.'

Depending on the mass of the original star, planets and their moons can loiter in this red giant habitable zone for up to 9 billion years.

Earth, for example, has been in our sun's habitable zone so far for about 4.5 billion years, and it has been teeming with changing forms of life.

The oldest exoplanets found using Nasa's Kepler telescope are about 11 billion years old.

'Our model predicts that when stars eventually leave the "main-sequence" of stellar evolution, becoming red giants, the habitable zone (HZ) expands outward,' Dr Ramirez told MailOnline.

'The HZ is currently in a region that approximately spans between just beyond Venus' obit to just beyond Mars' orbit. 

'However, during the red giant stage, which for our sun will occur some 7 billion years or so from now, stars will get much bigger and brighter, so the HZ will move outward to the outer regions of the stellar system, during which time outer worlds and moons will be within it for some time. 

'Any initially frozen worlds (e.g. Europa and Enceladus analogues) would melt, allowing any pre-existing subsurface life to be exposed to the atmosphere to be potentially detected. Thus, the red giant stage may be akin to giving a "second wind" to life in the stellar system.'

Launched in March 2009, Kepler is the first Nasa mission to find potentially habitable Earth-size planets. For four years, Kepler monitored 150,000 stars in a single patch of sky, measuring the tiny, telltale dip in the brightness of a star that can be produced by a transiting planet

But he also told MailOnline that as the habitable zone moves outward, other strange things also happen that are not good for habitability.

These include intense stellar winds that will erode planetary atmospheres and planets will move outward as the red giant star loses mass.

'However, even with all of these things happening, we show that some planets can at least partially retain (if not completely retain) their atmospheres during the entirety of the red giant stage of stellar evolution.'

'It is certainly my hope that our current work here will also be used by astronomers immediately upon publication,' Dr Ramirez told MailOnline. 

'With our new work, astronomers can compile a list of known red giants and use our model predictions, assuming that the stellar ages are approximately known.'   

Kepler recently discovered 1,284 new exoplanets, doubling the number of known alien worlds. 'This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth,' said Ellen Stofan, chief scientist at Nasa. This artist's concept depicts a selection of planetary discoveries

The planetary diversity suggests around other stars, initially frozen planets could be the size of Earth and might even turn into habitable worlds once the star becomes older.

In a few billion years our sun will become a red giant, warming distant worlds like Jupiter, Saturn and Neptune - and their moons.

This will create a newly-established red giant habitable zone, around Jupiter, Saturn, and their moons like Enceladus and Europa.

'Long after our own plain yellow sun expands to become a red giant star and turns Earth into a sizzling hot wasteland, there are still regions in our solar system - and other solar systems as well - where life might thrive,' said Professor Lisa Kaltenegger, director of the Carl Sagan Institute.

'For stars that are like our sun, but older, such thawed planets could stay warm up to half a billion years in the red giant habitable zone. That's no small amount of time,' said Dr Ramirez.

'In the far future, such worlds could become habitable around small red suns for billions of years, maybe even starting life, just like Earth. That makes me very optimistic for the chances for life in the long run,' said Professor Kaltenegger.

The Kepler space telescope has discovered 1,284 new exoplanets, doubling the number of known alien worlds.

For 1,284 of the candidates, the probability of being a planet is greater than 99 per cent

Nearly 550 could be rocky planets like Earth, based on their size.

Nine of these orbit in their sun's habitable zone, which is the distance from a star where orbiting planets can have surface temperatures that allow liquid water to pool.

With the addition of these nine, 21 exoplanets now are known to be members of this exclusive group.

An additional 1,327 candidates are more likely than not to be actual planets, but they do not meet the 99 percent threshold and will require additional study. 

The remaining 707 are more likely to be some other astrophysical phenomena. This analysis also validated 984 candidates previously verified by other techniques.

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