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Can the dead stars of the galaxy help us in our search for life? A group of researchers at Cornell University think so. They say that observing the transit of exoplanets in front of white dwarfs can tell us a lot about those planets.
It could even reveal signs of life.
A new study presents this idea in The Astrophysical Journal Letters. The research is titled “The White Dwarf Opportunity: Solid Detections of Molecules in Earth-like Atmospheres of Exoplanets Using the James Webb Space Telescope.” The lead and corresponding author is Lisa Kaltenegger, associate professor of astronomy at Cornell College of Arts and Sciences. Kaltenegger is also the director of the Carl Sagan Institute.
“If there are rocky planets around white dwarfs, we could detect signs of life on them for years to come,” Kaltenegger said 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 infrared. In infrared light, molecules in a planet’s atmosphere have the most features in their spectra, making them easier to identify.
“What if the death of the star is not the end of life?”
Lisa Kaltenegger, lead author, professor, Cornell University
But this study takes JWST and its powers of observing the atmosphere in a different direction. While exoplanet research and the search for life typically focus on planets that transit small M dwarfs, the authors say there could be a better way. They point out that finding white dwarfs with transiting planets in front of them is one way forward in the search for life. This is partly because it would be easier to detect possible biological signatures.
Detecting biosignatures around M dwarfs is challenging. The powerful light output of large stars makes it more difficult to see what is happening in your vicinity. And M-dwars are known for high level of sunspot and flare activity. All that activity could affect spectroscopic searches for biomarkers. In their paper, the authors explain that “Earth-like planets around small cool M dwarfs, such as TRAPPIST-1, are promising targets for characterization with upcoming Extremely Large Telescopes (ELTs) and JWSTs. However, challenges remain in interpreting the transmission spectra of the M-dwarf terrestrial planets, in particular contamination from unstructured starspots. “
But white dwarfs are different. They have run out of nuclear fuel and have been reduced to just 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 are no longer actively burning nuclear fuel, the lone white dwarfs do not ignite, so they do not affect the spectroscopic results. (They can glow if they are 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 just a couple of hours.
“By observing Earth-like planets orbiting white dwarfs, the James Webb Space Telescope can detect water and carbon dioxide in a matter of hours,” MacDonald said. “Two days of observation time with this powerful telescope would allow the discovery of gases with a biological signature, such as ozone and methane.”
Some discoveries led to this potential new method of searching for signs of life.
White dwarfs go through a lot of changes when exiting the main sequence. Their progenitor star throws its outer layers in a series of violent convulsions that should spell doom for any planet that orbits them. In its red giant phase, the star expands to engulf any planetary bodies that are too close. This will happen to our own Sun in several billion years. The Sun will engulf and destroy Mercury, Venus, maybe even Earth.
But sometimes the planets can survive the process.
The authors explain in their article that “The origin and survival of nearby planets orbiting WD have been the subject of active theoretical study. Once a main sequence star evolves into a WD, stable planetary systems can suffer violent dynamic instabilities, exciting planets in high eccentricity and low pericenter orbits. These orbits can rapidly circularize due to dissipation of the tides, which in some circumstances leads to the survival of planets in close orbits. “
When astronomers discovered planets orbiting a white dwarf, things went from the theoretical to the practical; a very significant development.
At first, astronomers studying white dwarfs saw evidence of rocky debris near dead stars. The debris was orbiting in debris disks, or even closer to the star, or right in the star’s atmosphere. Scientists interpreted that as evidence of planets destroyed when the star turned into a white dwarf.
In September 2019, astronomers discovered a giant planet candidate orbiting a white dwarf. This was evidence that large planets can survive the transition from star to white dwarf. They can survive through migration. And around the same time, in November 2019, scientists discovered a planet orbiting a red giant, having survived that star’s transition to its red giant phase.
In December 2019, a team of astronomers discovered a planet the size of Neptune orbiting a white dwarf much smaller than it. They couldn’t see the planet itself, only the planet’s atmosphere when the white dwarf stripped it. The planet was probably doomed, but it showed that white dwarfs can still host exoplanets. And while this was a gas giant, and unlikely to harbor life, it shows that rocky planets can also survive around white dwarfs.
That’s where this job comes in.
“We now know that giant planets can exist around white dwarfs, and evidence dates back more than 100 years showing rocky material contaminating the light from white dwarfs. There are certainly small rocks in white dwarf systems, ”MacDonald said. “It is a logical leap to imagine a rocky planet like Earth orbiting a white dwarf.”
NASA’s TESS spacecraft is the first planet-hunting spacecraft of the day. Part of your search involves hunting for rocky planets around white dwarfs. White dwarfs are small and their planets should have short transition times, much like WD 1856 + 534, the giant planet candidate found in September 2019. That only took about two minutes to transit, and more likely planets with shorter transit times do so. to be seen.
Typically, an exoplanet is dwarfed by its star, and all that light blinds us to the planet. But with white dwarfs, that is not the case. The authors explain in their paper that “transiting planets orbiting smaller stars are generally easier to characterize, due to their larger planet-to-star size ratio.” The rapid repetition of transits facilitates the identification of biomarkers spectroscopically.
As the authors write in their article, “the rocky planets in the habitable zone WD therefore represent a promising opportunity to characterize the atmospheres of terrestrial planets and explore the possibility of a second genesis on these worlds.”
If, or when, TESS finds rocky planets orbiting a white dwarf, Kaltenegger and his colleagues will be ready. They took established Hubble Space Telescope methods for identifying gases in exoplanet atmospheres and combined them with atmospheres modeled on white dwarf planets from other research. So, once JWST is up and running, the foundations for understanding exoplanet atmospheres spectroscopically will be ready.
What if we found life on a planet orbiting a white dwarf? The implications are staggering. Given that most of the stars in the Milky Way, including our own, will end their lives as white dwarfs, the proposition is staggering. In fact, astrophysicists believe that more than 97% of the stars in our galaxy will become white dwarfs. Could life have survived on planets that survived the transition of their stars? Or, even more exciting, could life have resurfaced?
“What if the death of the star is not the end of life?” she said. “Could life go on, even after our sun has died? Signs of life on planets orbiting white dwarfs would show not only the incredible tenacity of life, but perhaps a glimpse into our future as well. “
