Thea Kozakis (left) and                                       Lisa Kaltenegger (right).

Courtesy of Thea Kozakis and Lisa Kaltenegger

Thea Kozakis (left) and Lisa Kaltenegger (right).

May 27, 2020

A Home in the Stars : White Dwarfs and the Hunt for Earth-like Exoplanets

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Ever since the Space Age of the 1960s, the question of whether there is life outside of Earth continues to fascinate and elude scientists.

“As Earth is the only planet known to support life, the best bet at discovering evidence of alien biology would be to search for Earth-like planets,” said Thea Kozakis Ph.D. ’20. “However, pinpointing where such planets could exist is quite difficult.”

In the search for habitable exoplanets and life elsewhere, a research team consisting of Kozakis, Prof. Lisa Kaltenegger, astronomy, and Zifan Lin ’20 published a paper in January identifying white dwarfs as an area of interest. They used spectral models to simulate the white dwarf and its evolution over time, assuming that a white dwarf around 0.6 times the size of the Sun has an atmosphere of pure hydrogen.

When a star runs out of its hydrogen fuel, nuclear fusion, the process responsible for the star’s internal heat, stops. The star then tends to expel all of its outer material, leaving behind its exposed core — known as a white dwarf.

The first stage of a white dwarf’s formation is extremely hot, around 180,000℉, but its surface temperature reduces over time. An average white dwarf takes around 8 billion years to cool down from 10000℉ (the temperature of the sun) to 6700℉.

In this temperature range, the white dwarf can support a long-lived and stable Habitable (Goldilocks) Zone — the orbital range around a star where a planet could have liquid water on its surface. This increases the likelihood of finding life around such stars.

White dwarfs are also only slightly larger than Earth, making it much easier to detect Earth-sized planets around white dwarfs compared to stars like the sun.

“Usually when we think of finding planets and looking into their atmospheres, we think about the transit method, where we wait for the planet to go in front of its star, and the planet is detected when it blocks a fraction of its star’s light,” Kozakis said. “Historically, it has been easier to detect planets around smaller stars, as more of the host star’s light is blocked.”

An Earth-sized planet would block over 50 percent of the light from a typical white dwarf compared to the 0.01 percent of light a star like the sun emits, indicating that the instruments used to detect such planets do not need to be very sensitive to variation in a star’s light output.

Once a planet has been detected, scientists analyze its chemical composition to see if it can support life. This can be done by studying the spectrum — or, the range of wavelengths — of the electromagnetic radiation transmitted while a planet passes the front of its star.

“[T]he chemical properties of Earth-like planets around a white dwarf are somewhat different from those around stars like the sun,” said Kaltenegger, who is the director of Cornell’s Carl Sagan Institute. “This is because the difference in the nature and amount of light transmitted by the stars changes the photochemistry and thus the chemical composition of the planets.”

The research team created a model that showed the different ratios of chemicals on a rocky Earth-like planet throughout the white dwarf’s cooling process, which could indicate if the planet was habitable.

“We looked specifically for biosignatures like oxygen and ozone with methane and nitrous oxide — which are chemical features of an atmosphere which could indicate life,” Kozakis said.

Oxygen could be present in a planet’s atmosphere for non-biological reasons, such as the decomposition of ozone caused UV radiation emitted from the white dwarf, Kozakis said. Along with oxygen, the presence of methane and nitrous oxide — created by living organisms — is also necessary for a planet to support life.

Other than analyzing chemical compositions, the team additionally looked for the presence of water — a key ingredient for life on Earth.

The model created in this paper is part of a larger database of “spectral fingerprints” for habitable worlds created by Kaltenegger’s research team. NASA plans to use this database in developing telescopes like the James Webb Space Telescope and the Extremely Large Telescope.

“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 [James Webb Space Telescope and the Extremely Large Telescope] to spot signs of life on such worlds,” Kaltnegger said.