How to spot signs of life from planets spinning around dying stars

From observations of planets forming around rising stars to planets surrounding remnants of dead white dwarfs from stars like ours, astronomy provides us with the tools to analyze both our distant past and our distant future. What is clear is that in about 5 billion years, our Sun will begin its stellar machinations as a sprawling Red Giant, and will eventually leave a dying remnant of its former self.

But in a recently published article in Astrophysical journal lettersCornell University astronomers have just presented a roadmap to detect possible spectral biosignatures from exo-lands that may surround dying white dwarfs. The idea is that at least a few rocky planets can survive or form from the agony of their mother stars’ tumultuous death.

White dwarfs are only slightly larger than Earth and cool for billions of years. And as the authors point out, although newly formed white dwarfs are extremely hot with temperatures up to 100,000 Kelvin (K), they gradually cool down over time due to the lack of an internal heat source.

An average white dwarf has cooled to 6,000 K after about two billion years, but then it takes an additional eight billion years to reach 4,000 K, providing planets with nearly twice the life of Earth in one area. habitable continuous white dwarf, the authors write.

To date, astronomers have only hints that such planets surrounding white dwarfs are really out there. Neither has yet to be seen.

But if astronomers are lucky enough to find an exo-Earth in transit by a white dwarf, they can be sure that it can block up to 70 percent of the white dwarf’s surface. So the contrast ratio between the planet and the white dwarf is orders of magnitude better than around a normal hydrogen-burning star, Lisa Kaltenegger, one of the co-authors and director of the Carl Sagan Institute at Cornell, told me.

Kaltenegger is hopeful that future extremely large telescopes (ELTs) and NASA’s James Webb space telescope can characterize such planets for at least 100 light years. 130 of these white dwarfs are already known to lie within about 100 light years of the Sun, she says.

Kaltenegger says that if such extrasolar Earth-like planets surround white dwarfs, then astronomers should be able to take a spectroscopy of their planetary atmospheres using the backlight of the dim white dwarf.

The team says atmospheric detection of ozone, methane, oxygen, and nitrous oxide would be a possible biological signature of a planet orbiting a white dwarf.

How many exo-lands orbiting white dwarfs could actually be out there?

“That is a great unknown,” said Kaltenegger. She says that over 95 percent of all the stars in the universe will end their lives as white dwarfs, but astronomers have yet to find a rocky planet around a white dwarf.

But is it really likely that an exo-land that has suffered the ravages of an expanding Red Giant can still harbor life?

We don’t know if life could survive the red giant phase and, more importantly, the death of its star, says Kaltenegger.

Will any of the planets we find around the white dwarfs actually be planets that have formed after the star has already passed its Red Giant phase?

Kaltenegger says that some researchers think these planets need to be second-generation planets formed around the white dwarf after the original star has already gone through its Red Giant phase. But she says others argue that planets from the outer reaches of a solar system could survive the Red Giant phase and then be dynamically moved inward to a stable orbit.

“The big question is whether it could maintain or re-deliver water to such planets in any setting,” said Kaltenegger.

What is certain is that in a few billion years, our sun will run out of fuel, throwing off its outer layers leaving a white dwarf of its former self. As for the fate of Earth during all this? That remains a hot topic of debate.

Characterizing a rocky planet around a white dwarf will give us the first idea of ​​whether planets can survive the death of their stars, what their atmosphere is made of and whether there is water, says Kaltenegger.

“And if there are signs of life on such planets, it would tell us about the incredible toughness of life,” said Kaltenegger.