Scientists once assumed that the best place to look for extraterrestrial life would be planets within their star’s habitable, or “Goldilocks” zone, that narrow range of orbits where water remains liquid on a planet’s surface. Around our Sun, the habitable zone stretches from just outside the orbit of Venus to just outside the orbit of Mars, with Earth squarely in the middle.
But new research suggests scientists searching for habitable planets should look outside habitable zones — way outside.
In a paper published Monday in the journal Nature astronomy, researchers from the universities of Bern and Zurich conclude that certain super Earth exoplanets — rocky planets between 2 and 10 times Earth’s mass — could remain habitable for forms of life at distances 10 times that of Earth from the Sun. Some planets could even remain habitable for tens of billions of years after getting expelled from their solar system, becoming strange oases wandering interstellar space without a star to call their own.
But there’s a catch: These habitable super Earths must possess thick hydrogen-helium atmospheres that swaddle their surfaces and insulate them from the cold of space. Life on such a plan would be more like life at the bottom of a terrestrial ocean trench than a life in a city or rain forest, but it first with other recent studies suggesting that life could be possible in extreme environments in the cosmos, and that scientists could miss important science of life if they focus only on perfectly Earth-life planets.
“What we hope for with this study is to give another reminder that habitability might not necessarily have to look like it does on Earth,” said Marit Mol Lous, a doctoral student at the University of University of Zürich and lead author of the paper.
Life as we know it needs three ingredients: energy, nutrition and liquid water. Rocky planets in a habitable zone make this easy, with energy coming from their star, nutrition from their various minerals and water remaining liquid due to their perfect orbital placement. If Earth were much closer to the Sun, our water would boil away, and much further out, greenhouse gasses freeze out of the atmosphere and the planet’s water turns to ice.
For the hypothetical super Earth’s Ms Lous and her colleagues considered, their rocky surfaces provide the nutrition. What they explored in their modeling was what kind of atmosphere could provide the sufficient energy in order to keep water liquid.
Stars and their planets form from disks of dust and gas that are largely hydrogen and helium. Planets that form around a star may initially possess an atmosphere of similar composition, but planets too close to their star will see the Hydrogen and Helium evaporated by solar energy.
But planets at least several times further out than the distance of Earth to the Sun may retain their primordial atmospheres, and that comes with a benefit: At sufficient pressures, hydrogen will trap a planet’s heat — generated by the decay of radioactive elements — providing energy even without sunlight, and keeping water liquid. When modeling potential super Earths, researchers found that planets more than five times as massive as Earth with hefty hydrogen-helium atmospheres can maintain liquid water conditions for more than 50 billion years.
The longest duration of habitability the researchers calculated in their modeling was 84 billion years for a planet 10 times as massive as Earth. Scientists believe the universe is 13.8 billion-years-old so far.
“It should be noted,” the researchers write in the paper, “that it takes [10 billion years] of cooling before these conditions are reached,” the insulating hydrogen doing such a good job of heating such worlds that they remain too dense for water vapor to condense. Which is just as well, as the cosmos 100 billion years from now will be well on it’s way to becoming the cold, nearly starless expanse that is its destiny.
In all that cold and dark, life, of a sort, may yet thrive on rogue, starless planets swaddled in hydrogen envelopes. But it’s life is more likely to resemble deep sea creatures than a technological civilization.
And a technological civilization on such a planet in the far future would have a tough challenge to overcome just to see the night sky and wonder at how alone they are in the universe, according to Ms Lous. In addition to blocking out any starlight, the hydrogen-helium atmosphere that made life possible on such a world would be 10,000 times heavier than Earths.
“The surface pressures in our results are on the order of 100 to 1,000 bar, the pressure range of oceanic floors and trenches,” the researchers write in the paper. “There is no theoretical pressure limit on life, and some of the most extreme examples in Earth’s biosphere thrive at ~500 bar.”
But there’s very little scientists can say with certainty about such worlds, Ms Lous notes, and that includes how conditions might help or hinder the evolution of life.
“The planets we consider would likely not have a day-night cycle or seasons as we have them on Earth and that could have implications to how ‘complex’ the life is,” she said. “As we are not biologists however, we don’t want to jump to conclusions.”
Ms Lous is quick to point out that the new work is largely conceptual, and that future studies of actual candidate hydrogen-helium atmosphere super Earths, perhaps with the newly commissioned James Webb Space Telescope, will be crucial to understanding how well their findings reflect reality.
“We have studied a type of planet that might be very prevalent in the universe, but that is far from well understood. More complex calculations might actually show it is impossible for a planet to form with the exact conditions we need for liquid water to be present,” she said. “But who knows, we are excited to keep working on this.”