Looking for alien life? Look no further than dying stars.
Most stars, when they reach old age and start to run out of fuel, swell up to hundreds of times their usual size and engulf nearby planets. However, even planets around a dying star – or in so-called red giant neighborhoods – can still support life, according to new research.
Cornell University researchers focused on the search for the “habitable zone,” the region surrounding a star where water on the surface is liquid and telescopes can detect any signs of life from a distance.
“When a star ages and brightens, the habitable zone moves outward and you’re basically giving a second wind to a planetary system. Currently objects in these outer regions are frozen in our own solar system, like Europa and Enceladus — moons orbiting Jupiter and Saturn,” said Cornell researcher and study author Ramses Ramirez.
In about 7.5 billion years, our own sun will start to die and expand, and eventually swell to about 200 times its size now. Earth then becomes uninhabitable, and Mercury and Venus will be swallowed whole by the sun. However, current frigid environments such as those icy moons of Jupiter and Saturn, might turn out to be an ideal temperature to host life.
Long after Earth converts into a “sizzling hot wasteland” there are still regions of the solar system as well as outside where life could thrive, agreed astronomy professor and co-author Lisa Kaltenegger.
Many stars, too, become red giants and stay that way for billions of years. According to this new research, planets can remain livable for up to 9 billion years — twice Earth’s present age — in certain cases. This is deemed long enough for life forms to survive or emerge, or for humanity to perhaps continue.
While astronomers typically look at middle-aged stars like our own sun, habitable worlds have to be scoured in stars of all ages, Kaltenegger added.
What determines the habitable zone is a star’s luminosity, or how much light it emits over time. As stars swell up into red giants, their luminosity increases; the sun’s luminosity, for instance, will climb more than 4,000 times when its own time comes.
For stars like our sun, a planet could harbor new habitable zone for half a million years – probably not enough time on a far-flung planet, but could be plenty of time for life underneath ice to surface and evolve into telescope-detectable forms, explained Kaltenegger.
The researchers said they have submitted for publication a second study that enumerates 23 red giants within 100 light years of Earth as likely targets for planet seekers. Life found on a “de-frozen” planet is deemed proof of how life could actually get started subsurface.
There are considerations, though: planets within the habitable zone, for example, are not necessarily conducive to life. Venus is within this zone exactly, but its very thick atmosphere is heat-trapping and would be opposed to life as we know it.
Further, a variable in the calculation could render the rest of the proposal moot: if the expanding star strips all neighboring planets of their atmospheres.
“This is one of the things we didn’t know,” Kaltenegger told Space.com, adding their work is the first time that it was explored whether or not rocky planets slightly smaller or bigger than Earth could maintain their atmospheres as their host star morphs into a red giant.
According to their findings, certain planets, such as those very close by or with low gravity, will lose their atmospheres as their stars become red giants. The authors said, however, that those with adequate mass and are posted a safe distance away can hold their atmosphere.
The researchers hope that such worlds could be habitable in the far future and maybe even start life just like Earth – a show of optimism for life’s chances to flourish and continue in the long run.
The findings were published May 16 in the Astrophysical Journal.
