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Some very powerful telescopes will see the first light in the near future. One of them is the long-awaited James Webb Space Telescope (JWST). One of JWST’s roles, and the role of the other upcoming areas too, is to search for biosignatures in the atmospheres of exoplanets. Now, a new study shows that finding those biosignatures on exoplanets orbiting white dwarf stars could give us our best shot at finding them.
The new article is titled “High-resolution spectra and bio-signatures of Earth-like planets transiting white dwarfs.” The lead author is Thea Kozakis, a doctoral candidate at the Carl Sagan Institute at Cornell University. The study is published in Astrophysical Journal Letters.
“We showed what the spectral fingerprints might be and what large ground and ground telescopes they can take into account.”
Thea Kozakis, lead author, Cornell University
White dwarfs are intriguing stars, especially when it comes to hunting down Earth-like planets. Although they are considered stellar remains, meaning they have stopped fusing, they still glow. In fact, white dwarfs can remain stable for billions of years after they have stopped fusing elements together. For billions of years they emit their stored thermal energy, heating nearby planets. That means that any life on the planets that orbit them has stability, and it won’t have to deal with deadly burns or other dangerous circumstances.
White dwarfs are also small, which means that telescopes don’t have to deal with a huge, shiny sphere when trying to study planets alongside dwarfs.
Now, a team of scientists has created a kind of toolkit to help space telescopes and ground-based telescopes search for signs of life on the planets around white dwarfs. A press release calls it “a spectral field guide for these rocky worlds.”
“We showed what the spectral fingerprints might be and the next large ground and ground telescopes,” Thea Kozakis said in a press release.
Sun-like stars can be so bright that it is 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, spacecraft like Kepler and TESS have a chance to detect them. There are some problems with the transit method, but it has been our most successful way of finding exoplanets. To date there are thousands of confirmed exoplanets.
“The publication of this type of guide lets observers know what to look for.”
Thea Kozakis, lead author, Cornell University
A white dwarf presents its own challenges in detecting planets. While they are not as large as a main sequence star, and therefore not as bright, their small size presents another problem. With such a small parent star, it is even more difficult to see the transit of an orbiting planet. A planet is also less likely to pass between us and the little star.
But it is still possible to detect them.
Detecting things around white dwarfs is a fairly 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 planetesimal decay in orbit around a white dwarf. In 2019, another study presented evidence of planets orbiting a white dwarf. Another 2019 study presented evidence that there are approximately 1 in 10,000 “spectroscopically detectable giant planets in close orbits around white dwarfs.”
But we really won’t know how many there are until we are better at spotting them. In December 2019, astronomers found a Neptune-sized 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 like Earth. And when they find those, they’ll want to probe the atmosphere for signs of life. That’s where Kozakis’ new studio comes in.
“The rocky planets around white dwarfs are interesting candidates to characterize because their hosts are not much larger than Earth-sized planets,” said Lisa Kaltenegger, associate professor of astronomy at the College of Arts and Sciences and director of the Carl Sagan Institute.
“We are waiting and looking for that kind of transit,” said Kozakis. “If we look at a transit of that type of planet, scientists can discover what is in its atmosphere, consult this document, compare it with spectral fingerprints and look for signs of life. The publication of this type of guide allows observers to know what to look for. “
The white dwarfs finally cool down. It takes a long time, but eventually they will become black dwarfs and will not emit heat. To account for the different temperatures and colors of white dwarfs as they evolve, the pair of researchers built their spectrum guide around three temperatures. They write: “To explore the evolution of the planet WD during the cooling of its host, we modeled the photochemistry and climates of these planets using the 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 the light from a white dwarf, a long-dead star, would allow us to detect life in a planet’s atmosphere if it were there,” Kaltenegger said. They also created spectral models for different atmospheres.
This study focuses on the spectral biosignatures created by methane, nitrous oxide, and ozone. However, detecting them is not that simple. There are false positives to deal with. Still, this study “… expands scientific databases to find spectral signs of life on exoplanets …” as stated in a press release.
But there are still some general questions associated with potential life on planets orbiting white dwarfs.
Before a star turns into a white dwarf, it goes through a giant red phase. The star loses so much mass that it cannot be contained and it expands. Our own Sun will go through this, and when it expands, it will consume Mercury, Venus, and perhaps even Earth. Life probably would 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 is possible for a planet to survive the transition of its main sequence star to the white dwarf. Or it is even possible that life can start again on a planet once its host star has turned into a white dwarf. After all, white dwarfs are very stable and long-lived.
“If we find signs of life on planets orbiting in the light of long-dead stars,” said Kaltenegger, “the next intriguing question would be whether life survived the death of the star or whether it started again, a second genesis, if you want. ” . “
