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A new study has placed the focus on the watery planets which have been orbiting dead stars for a long time.  According to the study, these white dwarfs - which are remnants of stars that have run out of fuel - may be the best place where life is being harboured due to the presence of water.

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The study suggests that scientists should spot the shadow of a planet against a former star which has withered in size but has its water oceans for billions of years even after facing all violent final throes. 

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Explaining the characteristics of the white dwarfs, the study's lead author and the University of Wisconsin–Madison astronomy professor Juliette Becker said, "White dwarfs are so small and so featureless, that if a terrestrial planet transited in front of them, you could actually do a much better job of characterizing its atmosphere."

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"The planet's atmosphere would have a much larger, clearer signal because a larger fraction of the light you're seeing is passing through exactly what you want to study," he added.

Watery planets need to survive to host alien lives!

But for hosting any lives, such a planet needs to first survive the last days of a small- to medium-sized star, which is likely to be rough.

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When the fuel of stars like our Sun is finished, it starts a fusion reaction at its core and they start to bulge enormously.

"There are two pulses, basically, during which the star grows to 100 times its normal radius. While it does that—we can call this part Destruction Phase No. 1—it will engulf any planets that are within that radius," said Becker.

If a water-harbouring planet escapes getting swallowed by its star, it has to then survive the loss of its mass and an increase in its brightness.

"The fact that the star gets so much brighter means that all planets in the system, even ones that used to be cold in the outer solar system, will suddenly have their surface temperatures increase drastically," said Becker. "That can evaporate their oceans and cost them a lot of water," he added.

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A perfect Earth-like planet will likely be present at least roughly 5 to 6 astronomical units away from its dying star for retaining an appreciable amount of water, as per the study

After a billion or more years, the planet's once-rampaging star is likely to shrink and cool. "If you can be sufficiently far away during this dangerous time that you don't lose your surface water, that's good," said Becker. 

"But the downside is you're going to be so far away from the star that all the water is going to be ice, and that's not great for life," he added. 

"If we find a lot of white dwarfs that are good candidates to host potentially habitable exoplanets, they could be worth the time. And these theoretical techniques will help us separate the best targets, so we don't spend too much time on the uninteresting ones," explained the author.

(With inputs from agencies)