But we may be missing a world of possibility, according to astronomer Lisa Kaltenegger. Several worlds, in fact.
In a paper published Monday in the Astrophysical Journal, Kaltenegger, who is director of the Carl Sagan Institute at Cornell University, and her colleague Ramses Ramirez modeled the conditions under which life could exist around stars that are close to using up their fuel — ones much older and bigger than our sun.
"We can find all these new places that may become habitable worlds," Kaltenegger said, in the dim, red glow of a slow-burning dwarf star, or on once-frozen planets thawed by a rapidly expanding red giant.
Nearly two dozen such potentially life-sustaining suns exist right in our own galactic back yard, she and Ramirez found. And they want scientists to start taking a closer look at them.
The idea that life can exist around aging stars is not a new one. In 2004, NASA scientist Alan Stern — who led the New Horizons mission to Pluto — authored a paper examining how future conditions in our solar system might change what's known as the habitable zone, where worlds are warm enough to have liquid water on their surface.
For now, of course, Earth inhabits that celestial sweet spot — close enough for life-giving warmth, but not so close that we're burnt to a crisp. But stars get bigger as they age. In 4 billion years, the sun, now a yellow dwarf star, will use up most of the hydrogen fuel at its core and become a geriatric red giant. It will balloon to 200 times its current size, practically touching Earth's orbit and bathing the once-frozen outer edges of the solar system in new levels of warmth. In this very hot, very distant future, Stern suggested, the most habitable worlds will actually be Pluto and its moon Charon, along with Neptune's moon Triton.
We won't be around to see if that happens — Earth will be a "hot, sizzling wasteland," at that point, Kaltenegger said, if it doesn't get engulfed entirely by the expanding sun. Best case scenario, humans will already be far off in space searching for somewhere else to call home. Worst case, civilization burns to a crisp. (Really worst case, we're already long dead from nuclear war/climate change/a global pandemic/the apocalyptic scenario of your choice.)
But the universe is already full of older stars with expanded habitable zones. So Kaltnegger and Ramirez developed a model to figure out what it would take for a planet to sustain life there.
They found that someplace like Europa, which is one of Jupiter's moons, or Pluto probably wouldn't be a very good home for life, even once they warmed. Both are too small to hold on to the gaseous atmospheres needed for life.
Besides, stars like our Sun age rapidly once they evolve into red dwarfs, at least relative to the timescale at which life evolves. The newly-warmed worlds would only get around half a billion years in the habitable zone before the zone shifted again — too short a time frame for life to take hold, most scientists say. Most scientists think that it took nearly 1 billion years for the first life to appear on Earth. Even the earliest, most radical estimates of life's origins place the starting point at about a half-billion years after Earth's birth.
But if life was able to evolve below the surface of those planets — warmed, perhaps, by heat from their worlds' cores — then the above-ground defrosting that happens as their star ages could expose it to human telescopes, Kaltenegger said. The thawing of the icy surface would release the gases typically associated with life into their atmospheres, where we might detect them.
"It's interesting to think that we have this opportunity to look at an unfrozen world and figure out if life could start under the frozen surface," Kaltenegger said.
Another option is to look at older red dwarfs, the low-mass, very dim stars that are capable of burning for trillions of years. The ones that exist now are pretty far from their end-of-life phases, but when they do enter that stage, these slow-burning stars move through it more slowly than our sun will, giving their outer planets a comfortable 9 billion years in the habitable zone.
Kaltenegger and Ramirez have identified 23 stars that could be home to either kind of world within 100 light-years of our own solar system. Astronomers don't know what kinds of planets orbit around them — let alone whether there are any good-size, rocky worlds within the habitable zone carrying the necessary components for life.
"We don't know if there are planets there because nobody has ever looked. Nobody ever thought of it," Kaltenegger said.
But "now that we have this list of the best targets," she continued, "we can use our telescopes to take a look and tell us if they find something there. That will be fascinating to see."