Lessons to Humanity from Habitable Planets Around Old Stars
In my previous essay, posted here, I suggested that to preserve the habitability of its home planet — an advanced technological civilization might choose to move the planet away from a brightening sun-like star. The preferred planet-star separation would scale as the square root of the evolving star’s luminosity.
If this idea happens to be popular among our siblings in the family of intelligent civilizations within the Milky-Way galaxy, it might lead to an over-abundance of Earth-mass planets in the habitable zone around old Sun-like stars, as these stars evolve along the red giant branch of their Hertzsprung-Russel diagram.
But as the host sun-like star eventually turns to a faint white dwarf, the resident civilization might choose to migrate its planet closer to the dimmed furnace. This would imply an artificial over-abundance of habitable Earth-like planets around white dwarfs.
The graveyard of the Milky-Way galaxy is full of billions of corpses of dead sun-like stars in the form of old white dwarfs, each carrying about 60% of the mass of the Sun. At their typical age of a few billion years, the surface temperature of these white dwarfs is similar to that of the Sun, about 6,000 degrees Kelvin, resulting in white light which is conducive for life-as-we-know-it. The size of a white dwarf is comparable to that of Earth, but the habitable zone around it, is a hundred times bigger — amounting to 1 to 3 times that current radius of the Sun. Closer than a solar radius, the gravitational tide from the white dwarf would destroy a rocky planet.
The fact that the size of the white dwarf is comparable to that of Earth makes transits easy to detect. The probability for a transit is of order 0.6% for a habitable Earth-like planet around a white dwarf. This offers a unique opportunity for probing the composition of the atmospheres of habitable-zone Earth-mass exoplanets that transit white dwarfs.
In 2013, I co-authored a paper with Dani Maoz (published here) which showed that during a transit by an Earth-mass planet across a white dwarf, the transmission spectrum of the planet’s atmosphere would show prominent bio-markers such as molecular oxygen absorption at a wavelength of ∼ 0.76 micrometers. We calculated that a potentially life-sustaining Earth-like planet transiting a white dwarf would be detectable by the Webb telescope in about 5 hours of total exposure time, integrated over 160 two-minute transits.
A follow-up paper that I co-authored in 2014 here with my then undergraduate student Henry Lin (currently a professor at Princeton University), showed that industrial pollution is also detectable in habitable Earth-like planets around a white dwarf. In particular, tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F) are the easiest to detect chlorofluorocarbons (CFCs) resulting from technological activity. Our paper estimated that a few days of total integration time on the Webb telescope will be necessary to detect the concentration of CCl3F or CF4 for industrial pollution that is ten times higher than the current level on Earth.
The discovery of an over-abundance of habitable Earth-like planets around red giants or white dwarfs can be substantiated as a technological signature by finding industrial pollution in the related planetary atmospheres.
Finding such evidence would provide a useful guide to humanity on how to survive on Earth in the next 10 billion years, both before and after the Sun will turn into a red giant and then a white dwarf in 7.6 billion years (as calculated here).
Based on the census of white dwarfs reported here, most stars formed billions of years before the Sun. Given that we arrived late to the cosmic party, we might have missed numerous tragic incidents involving the extinction of civilizations which did not engage in a cosmic engineering project to save their planet before it lost habitability. We were not around to hear their cry for help. We better be wise to learn the lessons from the surviving civilization by studying habitable planets around red giants and white dwarfs.
As the philosopher George Santayana stated in his 1905 book titled The Life of Reason: Reason in Common Sense: “Those who cannot remember the past are condemned to repeat it.”
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
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