Are City Lights Still on Around White Dwarfs?
The Sun was born 4.6 billion years ago. In 7.6 billion years, it will become a red giant with a puffed-up atmosphere after which its core will condense to a hot metallic remnant the size of the Earth, a white dwarf. Deep images from the Hubble and Webb telescopes show that a few percent of all stars formed more than (4.6+7.6)=12.2 billion years ago. This was less than 1.6 billion years after the Big Bang, at redshifts above 4. From this early population, all Sun-like stars transformed into white dwarfs by now. The surfaces of any habitable planets around them were sterilized by excess heating during the red giant phase and could not support life any more.
A few percent of all stars in the Milky Way galaxy belong to this early population. If technological civilizations similar to ours emerged around any of these early suns, their cry for help as their red giant boiled off all liquid water on their planet fell on deaf ears. The light signal passed through Earth when it was populated with microbes and was never heard by humans. Hence, the philosophical question: “If a tree falls in the forest and no one is around to hear it, does it make a sound?”
I jog every morning at sunrise through the local woods and I see evidence for tree logs lying on the ground, which likely fell off when no one was around to hear them. The trunks, eroded by the weather, provide indisputable testimony that the debris around them was part of a living tree. In analogy, can we search the neighborhood of white dwarfs for any technological relics that may have survived?
If an Earth-sized planet orbits a white dwarf, it could block all the stellar light when it transits the line-of-sight that connects the observer to the star. This offers an opportunity to search for artificial illumination of the dark side of the planet. If past civilizations illuminated the planet’s nightside with city lights and left the lights on at the end of their life, then we might notice the artificial lights if their power supply is long lived, as in the case of a nuclear reactor. A sufficiently illuminated nightside could be detected when the white dwarf is fully occulted.
Over the past decade, a variety of surveys discovered objects blocking the view of white dwarfs. Some white dwarfs show evidence for rocky material floating in their atmospheres, others in disks of rocky debris or even orbiting very closely — interpreted as the debris of rocky planets that were scattered inwards and disrupted by gravitational tide. Recently, a Jupiter-sized planet was discovered in a tight orbit around a white dwarf.
But city lights are not the only hope for finding technological relics from civilizations that used to exist around old suns. As pointed out in a paper I co-authored 8 years ago, the Webb telescope could search for thick industrial pollution in the atmosphere of a planet occulting a white dwarf.
The last phase in the life of a technological civilization around an early sun might have resulted in a rushed attempt to launch as many crafts as possible to interstellar space, in order to escape the imminent red-giant catastrophe looming over the horizon. This vigorous exodus offers another opportunity for uncovering the fallen trees in the forest of civilizations that predated us.
Keeping that in mind, it would be prudent to search for technological equipment that was sent long ago to interstellar space and entered the Solar system by now. If the net production of spacecraft just before the red giant catastrophe is more than 30 times bigger than the total prior to that, then most technological probes in interstellar space originated from alarmed civilizations at the last phase of their existence. This is because a few percent of all Sun-like stars formed early enough to become red giants by now.
It will take less than a billion years for our five interstellar probes: Voyager 1 & 2, Pioneer 10 & 11, and New Horizons, to cross the entire Milky Way galaxy. By now, any similar probes from the oldest Milky-Way stars could have passed through the Solar system. The Galileo Project aims to study interstellar objects and find out if any of them might have been artificial in origin.
Here’s hoping that we will find relics from civilizations which had their cosmic roots in the deep Webb images, celebrated at the White House on July 11, 2022. Alternatively, we will also be delighted if the Webb telescope will detect city lights on the nightside of an Earth-like planet in orbit around a white dwarf. Even though the party may be over on that planet, we owe our thanks to those who may have left the lights on. Happy Thanksgiving!
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 chairs the advisory board for the Breakthrough Starshot project, and 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. His new book, titled “Interstellar”, is scheduled for publication in June 2023.