Next-Generation Telescopes Could Detect Signs of Life on Rocky Planets around White Dwarfs

A team of astrobiologists from the Carl Sagan Institute at Cornell University has developed a spectral field guide for Earth-like exoplanets transiting small, dense stars called white dwarfs.

An artist’s impression of a rocky exoplanet orbiting a white dwarf. Image credit: Jack Madden / Cornell University.

In just a few years, astronomers — using tools such as the James Webb Space Telescope, the Extremely Large Telescopes, as well as mission concepts like Origins, HabEx, and LUVOIR — will be able to search for life on exoplanets.

White dwarfs are similar in size to Earth and have relatively stable environments for billions of years after initial cooling, making them intriguing targets for exoplanet searches and characterization of terrestrial planet atmospheres.

Their small size and the resulting large planet transit signal allow observations with the upcoming telescopes to probe the atmosphere of such rocky planets, if they exist.

“Rocky planets around white dwarfs are intriguing candidates to characterize because their hosts are not much bigger than Earth-size planets,” said Dr. Lisa Kaltenegger, director of the Carl Sagan Institute.

The trick is to catch an exoplanet’s quick crossing in front of a white dwarf.

“We are hoping for and looking for that kind of transit,” said Thea Kozakis, a doctoral candidate in the Carl Sagan Institute.

“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.”

Kozakis, Kaltenegger and their colleague, Zifan Lin, assembled the spectral models for different atmospheres at different temperatures to create a template for possible biosignatures.

“We show what the spectral fingerprints could be and what forthcoming space-based and large terrestrial telescopes can look out for,” Kozakis said.

Chasing down these planets in the habitable zone of white dwarf systems is challenging.

“We wanted to know if light from a white dwarf would allow us to spot life in a planet’s atmosphere if it were there,” Dr. Kaltenegger said.

“Our paper indicates that astronomers should be able to see spectral biosignatures — such as methane in combination with ozone or nitrous oxide — if those signs of life are present.”

“This research expands scientific databases for finding spectral signs of life on exoplanets to forgotten star systems.”

“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.”

The team’s paper was published in the Astrophysical Journal Letters.