Rocky Planets Orbiting Dead Stars May Be Ideal to Search for Life

Now a new study is showing that finding those biosignatures on exoplanets that orbit white dwarf stars might give us our best chance to find them.

The new paper is titled "High-resolution Spectra and Biosignatures of Earth-like Planets Transiting White Dwarfs". The lead author is Thea Kozakis, a doctoral candidate at Cornell University's Carl Sagan Institute. The study is published in the Astrophysical Journal Letters, ScienceAlert reported.

White dwarfs are intriguing stars, especially when it comes to the hunt for Earth-like planets. Though they're considered stellar remnants - meaning they have ceased fusion - they still shine. In fact, white dwarfs can remain stable for billions of years after they've stopped fusing elements.

For billions of years they emit their stored thermal energy, warming any nearby planets. That means that any life on planets orbiting them has stability, and won't have to deal with deadly flaring or other dangerous circumstances.

White dwarfs are also small, meaning telescopes don't have to contend with a huge shining sphere when they're trying to study planets next to the dwarfs.

Now a team of scientists have come up with a sort of toolkit to help space telescopes and ground telescopes look for signs of life in planets around white dwarfs. A press release calls it "a spectral field guide for these rocky worlds".

"We show what the spectral fingerprints could be and what forthcoming space-based and large terrestrial telescopes can look out for," said Thea Kozakis in a press release.

Sun-like stars can be so bright that it's difficult to detect planets orbiting them. When a planet transits in front of its star, and when that transit is between us and the star, then spacecraft like Kepler and TESS have a chance to detect them.

There are some problems with the transit method, but it's been our most successful way of finding exoplanets. To date there are thousands of confirmed exoplanets.

A white dwarf presents its own challenges when it comes to detecting planets. While they're not as huge as a main-sequence star, and hence not as bright, their small size presents another problem.

With such a small parent star, it's even harder to see the transit of an orbiting planet. It's also less likely that a planet will pass between us and the small star.

But it's still possible to detect them.

Detecting things around white dwarfs is a rather new development in astronomy. Recently, astronomers observed debris disks around white dwarfs for the first time.

In 2015, a team of astronomers found at least one disintegrating planetesimal orbiting a white dwarf. In 2019, another study presented evidence for planets orbiting a white dwarf. Another study from 2019 presented evidence that there about 1 in 10,000 "spectroscopically detectable giant planets in close orbits around white dwarfs."

But we won't really know how many there are until we get better at detecting them. In December 2019, astronomers found a Neptune-size planet orbiting a white dwarf, though in that case, the dwarf was slowly destroying the planet.

Astronomers will likely find more and more planets orbiting white dwarf stars. Some of them will be Earth-like. And when they find those ones, they'll want to probe the atmosphere for signs of life. That's where Kozakis' new study comes in.

"Rocky planets around white dwarfs are intriguing candidates to characterize because their hosts are not much bigger than Earth-size planets," said Lisa Kaltenegger, associate professor of astronomy in the College of Arts and Sciences and director of the Carl Sagan Institute.

"We are hoping for and looking for that kind of transit," Kozakis said. "If we observe a transit of that kind of planet, scientists can find out what is in its atmosphere, refer back to this paper, match it to spectral fingerprints and look for signs of life. Publishing this kind of guide allows observers to know what to look for."

White dwarfs eventually cool down. It takes a long time, but eventually they'll become black dwarfs, and emit no heat at all. To take into account the different temperatures and colors of white dwarfs as they evolve, the pair of researchers built their spectra guide around three temperatures.

They write, "To explore WD planet evolution throughout their host's cooling, we model the photochemistry and climates of such planets using WD spectral models described in Saumon et al. (2014) for WD hosts at 6,000, 5,000, and 4,000 K."

"We wanted to know if light from a white dwarf – a long-dead star – would allow us to spot life in a planet's atmosphere if it were there," Kaltenegger said. They also created spectral models for different atmospheres.

This study focuses on spectral biosignatures created by methane, nitrous oxide, and ozone. Detecting them, however, is not that simple. There are false positives to contend with.

Still, this study "…expands scientific databases for finding spectral signs of life on exoplanets…" as it says in a press release.

But there are still some over-arching questions associated with potential life on planets orbiting white dwarfs.

Before a star becomes a white dwarf, it goes through a red giant phase. The star loses so much mass that it can't contain itself and expands. Our own Sun will go through this, and when it expands, it will consume Mercury, Venus, and maybe even Earth.

Life would likely not survive this tumultuous change. So could there really be life on an Earth-like planet orbiting a white dwarf?

Nobody knows. Planets can migrate, and it's possible that a planet could survive its star's transition from main sequence to white dwarf. Or it's even possible that life could begin again on a planet once its host star has become a white dwarf. After all, white dwarfs are very stable and long-lived.

"If we would find signs of life on planets orbiting under the light of long-dead stars," Kaltenegger said, "the next intriguing question would be whether life survived the star's death or started all over again – a second genesis, if you will."