In the next few years, our quest to discover whether life exists elsewhere in the universe will be given a boost when the next generation of Earth- and space-based telescopes become operational. In order to help interpret these future observations, astronomers from Cornell University have compiled a “spectral field guide.” This blueprint shows what signs of life (such as methane) may look like in the spectra of planets orbiting long-dead white dwarf stars.
“Rocky planets around white dwarfs are intriguing candidates to characterize because their hosts are not much bigger than Earth-size planets,” Lisa Kaltenegger, associate professor of astronomy at Cornell and co-author of the study published in The Astrophysical Letters, said in a statement.
The team assembled spectral models for different planetary atmospheres at different temperatures, targeting planets with equivalent radiation exposure to modern Earth, and who orbit their white dwarf star within its habitable zone. Potential signs of life that the team identified in their spectral template includes methane in combination with ozone or nitrous oxide.
However, before the team compiled their biosignature guide, they had to be sure that the light from a white dwarf would be enough to allow astronomers to spot life in a planet’s atmosphere, if it was there, Kaltenegger explained. As dense, dead remnants of medium and small stars that no longer undergo nuclear fusion, white dwarfs have a faint luminosity. However, by capturing an exoplanet’s brief crossing in front of one, the spectrum of its atmosphere can still be obtained.
“We are hoping for and looking for that kind of transit,” Thea Kozakis, doctoral candidate in astronomy at Cornell and co-author of the study, said in a statement. “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.”
As the authors point out in their study, white dwarfs have relatively stable environments for billions of years after initial cooling. Therefore it will be very interesting to probe the atmospheres of the terrestrial planets orbiting these stars with upcoming instruments such as the Extremely Large Telescopes and the James Webb Space Telescope.
“If we would find signs of life on planets orbiting under the light of long-dead stars, the next intriguing question would be whether life survived the star’s death or started all over again – a second genesis, if you will,” Kaltenegger said.