Concept art of a planet orbiting a white dwarf. Image: Jack Madden/Cornell University
The Sun is the center of our solar system and the nourisher of life on Earth, but it is not an immortal entity. Some five billion years from now, our star will expand into a red giant, shed its outer layers, and settle into an afterlife as a type of dead star called a white dwarf.But just because stars die does not necessarily mean that life in white dwarf systems will be permanently snuffed out with them. In fact, any hypothetical planets orbiting these dead stars might “have relatively stable environments for billions of years” making them “intriguing targets” to look for signs of life, according to a study published on Thursday in The Astrophysical Journal Letters.
“The key question we asked is: If life existed on such a planet, could we even spot it because it orbits around a long-dead star?” said astronomer Lisa Kaltenegger, director of Carl Sagan Institute at Cornell University and a co-author of the study, in an email. “The answer is yes, if it is there, we could spot it.”Massive stars explode into violent supernovae, but Sun-scale stars experience a somewhat milder death. When the Sun runs out of fuel, it will jettison its outer gas, forming a planetary nebula, and then become a hot and super-dense “stellar remnant” that will maintain a stellar mass, but within a sphere about the size of Earth.The remnants are considered dead stars in part because they no longer perform hydrogen fusion, which is the reaction that makes stars “live” and shine, causing white dwarfs to slowly cool down over billions of years.A team of scientists led by Thea Kozakis, a doctoral candidate in astronomy at Cornell’s Carl Sagan Institute, modeled how these cooling periods might influence planets orbiting white dwarfs.Scientists have already discovered hints of a large planet orbiting a white dwarf, and evidence that these systems may host rocky planets and asteroids, as well. These speculative worlds may have either endured their star’s death, or perhaps they are new planets formed from the ashes of dead stellar systems.“There is a lot of discussion going on as to whether planets around white dwarfs would be first generation planets, which were at the outer parts of the solar systems before the star died, or whether they are second generation planets, which formed in a disk of debris left over from the death of the star,” Kaltenegger said.
It’s not clear if any alien life that existed on such worlds could survive a star’s death, or if life could potentially emerge on a new world formed from the remains of the old system. However, it is possible to speculate about scenarios that might preserve life through the transition, or re-spark life after a white dwarf formed.“If the planet would survive the death of its star and get dynamically moved from the outer regions of the initial solar system, then there are initial ideas that life could have been sheltered in an ice-covered ocean,” Kaltenegger explained. “However, if the planet formed around the white dwarf, then signs of life would indicate a second genesis.”Next-generation observatories such as the James Webb Space Telescope (JWST) or the Extremely Large Telescope (ELT) will be better-equipped to spot these rocky Earth-sized worlds, assuming they do orbit white dwarfs. Scientists typically spot exoplanets, which are worlds that orbit other stars, by looking for a transit, or a dip in a star’s brightness that indicates a planet is crossing in front of it from our perspective.Because white dwarfs are so small, these transits would be abnormally fast and frequent, lasting only a few minutes, so they would be particularly challenging to detect. But if scientists do spot one of these blips around a star, they could study the planet’s atmosphere—backlit by the white dwarf’s light—for signs of life, known as “biosignatures.”
Kozakis and her colleagues ran simulations of exoplanet atmospheres responding to a white dwarf that cooled from 6,000 to 4,000 Kelvin over a period of several billion years.The team found that biomarkers such as ozone and methane gas could hint at the presence of life on rocky planets in these systems, and could be detectable from Earth. The study’s models suggest that the cooling of the white dwarf would likely decrease the amount of ozone in a planet’s atmosphere over time, while increasing the methane content, which has implications for habitability.“An increase in the greenhouse gas methane will help a planet stay warm when its host cools,” Kaltenegger said. “It is the opposite of the Earth's situation. The Sun becomes more luminous with time, thus the habitable zone moves outwards towards Mars. So for us, an increase in greenhouse gases makes the problem worse.”“For a planet around a white dwarf, an increase in greenhouse gases is what will help keep it warm,” she continued. “The decrease in ozone won't be a problem for surface ozone flux, because the UV radiation of the white dwarf also decreases with time, so the ozone layer that protects us from harmful UV radiation on Earth is less and less needed as the white dwarf becomes older.”Counterintuitively, the researchers also found that as white dwarfs cool, planetary surfaces would increase in temperature, because cooler dwarfs “emit a larger percentage of their light at longer wavelengths, resulting in more efficient planetary surface heating,” according to the study.The new research is intended to lay out the possible chemical fingerprints of habitability in these systems for scientists to follow in case rocky exoplanets are detected by next-generation telescopes. But whether these exotic worlds will be discovered—and whether they host life—remains a mystery for the time being.“If we find such a rocky planet in the habitable zone of a white dwarf, then scientists in the near future could use our spectra to look with JWST and ELT to spot signs of life on such worlds,” Kaltenegger said.“And of course, the intriguing question that would follow, if we were successful,” she added: “Did life survive the death of its star, or did it start all over again?”