In about a billion years, the Sun is forecasted to brighten up and turn the Earth into a desert like Mars. Based on the star formation history of the Milky-Way galaxy, about half of the Sun-like stars formed more than a billion years before the Sun.
If some of these suns hosted a habitable Earth-like planet, and life on that planet led to the emergence of an intelligent species more than a billion years ago, then this species must have suffered from a devastating astrophysical catastrophe by now. In that case, it would be appropriate for us, as cosmic citizens of the Milky-Way galaxy, to hold a memorial service. once a year commemorating the civilizations that may have tragically died this way within our galaxy. Their number is ten billion if about a fifth of the Earth-analogs had a similar biological history to that of Earth.
This math implies roughly one dead Galactic civilization per living human currently on Earth. If we only knew the identities of these lost civilizations, then each of us could have lit one candle in memory of one of them.
Currently, our effort to protect ourselves from astrophysical threats is rather limited. The Planetary Defense Coordination Office at NASA is focused on the risk from an asteroid impact. We can mitigate that risk by deflecting killer asteroids heading towards Earth. But the inevitable brightening of the Sun is far more difficult to mitigate without a vast investment of resources in a cosmic engineering project on the megastructure scale of a protective Dyson sphere.
The level of social and geopolitical turmoil that results from an astrophysical catastrophe triggered by stellar evolution is difficult to overestimate. As the surface temperature of an Earth-analog rises, wars may erupt on desirable territories which exhibit a cooler climate. Global heating is likely to trigger mass migrations to regions like Antarctica, Greenland and Alaska on Earth. As soon as the heat becomes intolerable, multi-billionaires like the Elon Musks, Jeff Bezos or Richard Bransons of the day, would expand their business portfolio to offer trips out of Earth towards space platforms or rocky planets, like Mars, whose orbits are farther away from the brightening star.
In our own Solar system, Mars had desirable real estate a few billion years ago. The latest results from the Curiosity rover, indicate extreme levels of evaporation when Mars lost its liquid water. Old regions on Mars show signs of abundant water in the form of valleys and deltas, and minerals that only form in the presence of liquid water. Billions of years ago, the atmosphere of Mars was much denser and warm enough to form rivers, lakes, and oceans of water. As Mars cooled and lost its magnetic field, the solar wind and radiation eroded the planet’s atmosphere, turning the planet’s surface into the cold, arid desert we see today.
If intelligent animals existed before the Martian atmosphere was lost, they might have left interesting paintings on the walls of Martian caves. In that case, the delivery of humans to Mars by SpaceX might give rise to an exciting new scientific discipline of Martian archaeology.
Cosmic migration is unlikely to stay within the confines of the parent planetary system. Once a civilization develops the means for travel across astronomical scales over millions of years, it will likely embark on interstellar travel. A journey through interstellar space requires a sophisticated navigation system that takes into account the motions of the background stars. This is mandatory because the typical stellar speeds of hundreds of kilometers per second exceed by an order of magnitude the typical speed of chemical rockets. The navigation system can be anchored to the global frame of the cosmic microwave background, relative to which all velocities can be calibrated exquisitely by the Doppler effect. Nevertheless, reaching the destination of a habitable exoplanet around a distant star would be far more demanding than the navigation challenge encountered by Moses in the biblical story of his journey leading the Israelites towards the Promised Land.
In a podcast interview yesterday, I noted that Moses would have been far more impressed by a present-day cell phone than by the burning bush in the original biblical story. Not only would the cell phone give Moses an `out of this world’ experience, it would have also provided him with a practical benefit. Instead of wandering in the desert for forty years on a journey to the Promised Land, the GPS system on the cell phone would have allowed Moses to reach his destination within a matter of weeks or months, depending on the route. And whenever Israelites would have strayed from the best path, the GPS system would have announced “Recalculating”. Imagine this announcement appearing in the biblical text.
Like Moses who never reached the Promised Land, most interstellar travelers probably did not survive the journey and left behind damaged vehicles. Given that, archaeology can be extended beyond Mars to a search through interstellar objects. Some of the debris from these ambitious projects might be flying right now through the Solar system. Finding them among the numerous interstellar asteroids requires a dedicated space telescope, as I argued in a recent paper.
The discovery of interstellar technological relics would remind us of the dead civilizations that preceded us. The least we can do in their honor is to recover the pieces of the vehicles they desperately launched to space in the last century of habitability on their birth exoplanets. Commemorating their existence will remind us how vulnerable we all are. Here’s hoping that this sober realization will convince us to halt the toxicity that characterizes our short-term geopolitical interactions on Earth. From a cosmic perspective we are all in the same boat.
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.
