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.
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.
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.”
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.
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 orbits. These orbits can rapidly circularize due to tidal dissipation, leading in some circumstances to the survival of planets in close-in orbits.”
When astronomers discovered planets orbiting a white dwarf, things went from theoretical to practical; a very significant development.
At first, astronomers studying white dwarfs saw evidence of rocky debris near the 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 as the star became a white dwarf.
In September 2019, astronomers discovered a giant planet candidate orbiting a white dwarf. This was evidence that large planets can survive their star’s transition to white dwarf. They may survive via migration. And around the same time, in November 2019, scientists discovered a planet that’s orbiting a red giant, having survived that star’s transition to its red giant phase.
In December 2019, a team of astronomers discovered a Neptune-sized planet orbiting a white dwarf much smaller than itself. They couldn’t see the planet itself, just the atmosphere of the planet as the white dwarf stripped it away. The planet was likely doomed, but it proved that white dwarfs can still host exoplanets. And though this one was a gas giant, and unlikely to host any life, it shows that rocky planets may survive around white dwarfs, too.
That’s where this work comes in.
“We know now that giant planets can exist around white dwarfs, and evidence stretches back over 100 years showing rocky material polluting light from white dwarfs. There are certainly small rocks in white dwarf systems,” MacDonald said. “It’s a logical leap to imagine a rocky planet like the Earth orbiting a white dwarf.”
NASA’s TESS spacecraft is the premiere planet-hunting spacecraft of the day. Part of its search involves hunting for rocky planets around white dwarfs. White dwarfs are small, and their planets should have short transition times, just like WD 1856+534, the giant planet candidate found in September 2019. That one took only about two minutes to transit, and planets with shorter transit times are more likely to be spotted.
Usually, an exoplanet is dwarfed by its star, and all that light blinds us to the sight of the planet. But with white dwarfs, that’s not the case. The authors explain in their paper that “Transiting planets orbiting smaller stars are generally easier to characterize, due to their increased planet-to-star size ratio.” The rapid repetition of transits makes it easier to identify biomarkers spectroscopically.
As the authors write in their paper, “Rocky planets in the WD habitable zone therefore represent a promising opportunity to characterize terrestrial planet atmospheres and explore the possibility of a second genesis on these worlds.”
If, or when, TESS finds rocky planets orbiting a white dwarf, Kaltenegger and her colleagues will be ready. They took established Hubble Space Telescope methods of identifying gases in exoplanet atmospheres and have combined them with modelled atmospheres of white dwarf planets from other research. So once the JWST is operational, the groundwork for understanding exoplanet atmospheres spectroscopically is already in place.
What if we did find life on a planet orbiting a white dwarf? The implications are stunning. Since most stars in the Milky Way, including our own, will end their lives as white dwarfs, the proposition is astounding. In fact, astrophysicists think that over 97% of the stars in our galaxy will become white dwarfs. Could life have survived on planets that survived their stars’ transition? Or, even more exciting, could life have re-emerged?
“What if the death of the star is not the end for life?” she said. “Could life go on, even once our sun has died? Signs of life on planets orbiting white dwarfs would not only show the incredible tenacity of life, but perhaps also a glimpse into our future.”