James Webb Will Look for Signs of Life on Planets Orbiting Dead Stars


Can the galaxy’s dead stars help us in our search for life? A group of researchers from Cornell University thinks so. They say that watching exoplanets transit in front of white dwarfs can tell us a lot about those planets.

It might even reveal signs of life.

A new study presents this idea in The Astrophysical Journal Letters. The research is titled “The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope.” The lead and corresponding author is Lisa Kaltenegger, associate professor of astronomy in the College of Arts and Sciences at Cornell. Kaltenegger is also the director of the Carl Sagan Institute.

“If rocky planets exist around white dwarfs, we could spot signs of life on them in the next few years,” said Kaltenegger in a press release.

One of the goals of the James Webb Space Telescope (JWST) is to characterize exoplanet atmospheres using spectroscopy. The JWST has the power to do that with very distant planets. While other facilities can do spectroscopy, the JWST has the added benefit of doing it in the infrared. In infrared light, molecules in a planet’s atmosphere have the largest number of features in their spectra, making them easier to identify.

“What if the death of the star is not the end for life?”

Lisa Kaltenegger, Lead Author, Professor, Cornell University

But this study takes the JWST and its atmosphere-observing powers in a different direction. While exoplanet research and the search for life normally focuses on planets transiting small M-dwarfs, the authors say there might be a better way. They point out that finding white dwarfs with planets transiting in front of them is a way to advance the search for life. That’s partly because detecting potential biosignatures would be easier.

The James Webb Space Telescope, with its iconic segmented mirror, sits inside a cleanroom at NASA’s Johnson Space Center in Houston. This newest study is just one more reson we can’t wait for this telescope to get to work. Credit: NASA/JSC

Detecting biosignatures around M-dwarfs is challenging. The powerful light output from the large stars makes it harder to see what’s going on in their vicinity. And M-dwars are known for the high level of sunspot and flaring activity. All that activity could impair spectroscopic searches for biomarkers. In their paper, the authors explain that “Earth-like planets around cool small M dwarfs, such as TRAPPIST-1, are promising targets for characterization with the upcoming Extremely Large Telescopes (ELTs) and JWST. However, there remain outstanding challenges in interpreting transmission spectra of M-dwarf terrestrial planets, notably contamination from unocculted starspots.”

But white dwarfs are different. They’ve run out of nuclear fuel and have shrank down to only a remnant core. They still provide enough light for spectroscopic investigation of their exoplanet atmospheres, but they don’t overwhelm the signal with their own luminosity. And since they’re no longer actively burning nuclear fuel, solitary white dwarfs don’t flare, so they don’t impair spectroscopic results. (They can flare if they’re in a binary relationship).

The authors say that by focusing on white dwarfs, the JWST should be able to identify water and carbon dioxide—both substances correlated with living processes—in as little as a couple of hours.

“When observing Earth-like planets orbiting white dwarfs, the James Webb Space Telescope can detect water and carbon dioxide within a matter of hours,” MacDonald said. “Two days of observing time with this powerful telescope would allow the discovery of biosignature gases, such as ozone and methane.”

This figure from the study shows simulated JWST detections of biomarkers in the atmosphere of an Earth-like planet orbiting a white dwarf. Image Credit: Kaltenegger et al, 2020.
This figure from the study shows simulated JWST detections of biomarkers in the atmosphere of an Earth-like planet orbiting a white dwarf. Image Credit: Kaltenegger et al, 2020.

A few discoveries led to this potential new method of searching for signs of life.

White dwarfs go through a lot of changes as they leave the main sequence. Their progenitor star sheds its outer layers in a series of violent convulsions that should spell doom for any planets orbiting them. In its red giant phase, the star expands to envelop any planetary bodies that are too close. This will happen to our own Sun in several billion years. The Sun will envelop and destroy Mercury, Venus, maybe even Earth.

But sometimes planets might survive the process.

After several billions years, yellow suns (like ours) become Red Giants, expanding to several hundred times their normal size. Then they'll eventually become white dwarfs. Credit: Wendy Kenigsburg
After several billions years, yellow suns (like ours) become Red Giants, expanding to several hundred times their normal size. Then they’ll eventually become white dwarfs. Credit: Wendy Kenigsburg

The authors explain in their paper that “The origin and survival of close-in planets orbiting WDs have seen active theoretical study. Once a main-sequence star evolves into a WD, stable planetary systems can undergo violent dynamical instabilities, exciting planets into high-eccentricity, low-pericenter



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