Permanent climate change on habitable planets

Irrespective of how well we exercise or monitor our diet, no human was documented to live longer than Jeanne Calment, who died at age 122.5 years. Similarly, irrespective on how well we mitigate climate change, deflect asteroids with DART-like NASA missions, develop mRNA vaccines for lethal viruses and avoid nuclear wars, the Earth will ultimately be sterilized by the sun in about 1 billion years.

Detailed calculations of the evolution of the sun and the response of Earth to it imply that we are entering the last quarter of terrestrial life. In 1 billion years, the water vapor content of the atmosphere will increase substantially, and the oceans will start to evaporate, triggering runaway evaporation until the oceans have boiled dry. The water vapor in the atmosphere will make its way into the stratosphere, where the solar UV radiation will dissociate the water molecules. The dissociation products will gradually escape, until most of the atmospheric water vapor has been lost. The subsequent dry greenhouse phase will raise the surface temperature, and the Earth would become a dry, lifeless planet.

Any intelligent civilization on a habitable Earth-like planet around a sun-like star, will face this existential risk. Whereas technologically savvy civilizations will abort their overheated planets, less advanced ones will show signs of distress like animals in a burning forest.

In our own history of science and technology, radio communication was developed during the same century as space travel. However, it will take a while before we could develop the infrastructure of launching large enough spaceships to carry all of humanity away from Earth under existential distress.

A more realistic scenario would resemble the biblical story of Noah’s ark, in which Noah spared his family and pairs of terrestrial animals from a world-engulfing flood. In such a limited scenario, we would preserve only a representation of what we hold precious. Noah’s spaceship, in this case, could carry an electronic archive of the DNA of all terrestrial lifeforms, as well as all human creations in the form of books, music and valuable internet content. It would be prudent to leave behind any toxic content from social media.

In this limited scenario, most of humanity will be left behind. Under these circumstances, radio and TV stations will transmit intense signals of distress as our planet gets overheated by the sun. Could we detect such signals from a distance?

By surveying stars that are currently transitioning through the evolutionary phase that the sun will reach in 1 billion years, we could search for cries for help in the form of radio or laser communication signals from other intelligent life forms that may exist.

Alternatively, we could search for accelerated technological activities on a planet that is about to get sterilized.

If intelligent life forms elsewhere in the cosmos exist and operate anything like ours, civilizations under distress would likely show more city lights and industrial pollution than expected under normal circumstances, like ants engaged in building new colonies in anticipation of harsh weather. Their scientists would be feverishly working around the clock on subsurface, sustainable bunkers with the proper food, air supply and insolation from the increasing heat emanating from their host star. The wealthy class will get access to this technology and build luxury underground compounds long before the rest of the population is protected.After the surface is sterilized, it would be difficult to detect these underground civilizations. Their lifespan would depend on how much water, food or energy they can store or extract from the rock surrounding them.

Data from the Mars Reconnaissance Orbiter implies that Mars was rippled with rivers and ponds of water about 2.5 billion years ago. NASA’s MAVEN Orbiter presented evidence that Mars lost its atmosphere around that time. So far, the Perseverance Rover has not noticed any relics of Martian technology, implying that it never experienced the above-mentioned technological aftermath of drying up. Coincidentally, at around the same time as Mars dried up, the Earth’s atmosphere was enriched with Oxygen by cyanobacteria. There was both good news and bad news about potential life in the Solar System simultaneously; too bad there were no sentient beings around to report about it in the news media.

Our main opportunity for collecting memorabilia from possible extraterrestrials is to search for artificial objects launched to interstellar space in the final existential act of technological civilizations. (Harvard’s Galileo Project, which I lead, is engaged in this search.) The success of this search depends critically on how many gadgets were sent into space by technological civilizations in the last accelerated phase of their existence, before the lights in their “Cape Canaveral” sites turned off.

Most sun-like stars formed billions of years before the sun and should have boiled off all oceans on their habitable Earth-like planets by now. The oldest stars have evolved beyond the red-giant phase, engulfed their closest planets and became white dwarfs by now. After billions of years of cooling, these metallic remnants have roughly the surface temperature of the sun and the radius of the Earth. As a result, their habitable zone is a hundred times closer in than the Earth-sun separation.

As I pointed out in a co-authored paper a decade ago, a habitable Earth-like planet would cover the entire face of the white dwarf as it transits in front of it, making the study of biomarkers in its the planet’s atmosphere feasible already with NASA’s new James Webb Space Telescope. Whereas oxygen or methane would flag primitive forms of life, I showed in a follow-up co-authored paper that industrial pollution could also be detected as the fingerprint of an advanced technological civilization, if such exists.

If we ever study technological civilizations in the habitable zone around white dwarfs, it would be particularly interesting to read the history books written by their ancestors over the past 10 billion years.

Avi Loeb is the head of the Galileo Project at Harvard University, founding director of Harvard’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, as well as 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.