A few years ago, in a lab in Berlin, the astrobiologist Iva Vilović was thrilled when she observed garden cress sprouting under a light designed to mimic a “K-dwarf” star — an orange-hued star that’s cooler and slower burning than our yellow sun. To her surprise, the cress grew even faster and taller than it did under simulated sunlight.
Her results suggested that photosynthesis, the process that supplies the energy for Earth’s entire food web, can happen on planets orbiting these orange stars. “K-dwarf light is a very, very interesting light source, and it should be considered, if not prioritized, for any type of search for extraterrestrial life, in my opinion,” said Dr. Vilović, now at the Leibniz Institute for Astrophysics in Potsdam, Germany.
In particular, alien life may thrive in the light of these stars on planets that are slightly larger than Earth, known as super-Earths. Scientists like Dr. Vilović believe super-Earths that orbit K-dwarfs might, in fact, be the liveliest places in the galaxy — perhaps even more suitable for life than here.
Scientists call these “superhabitable worlds,” planets that would allow carbon-based life to persist longer and become even more complex and diverse than on Earth.
A superhabitable world — as scientists imagine it — is about 20 to 30 percent wider and twice as massive as Earth, and is placed at the right distance from its star for water to flow on its surface. The stronger gravity of this bigger Earth would hold on tighter to ambient molecules than Earth does, giving the planet a thicker atmosphere and a warmer and more stable climate, fueling the growth of extraterrestrial plants and animals (or their equivalents). The super-Earth’s bigger iron core would erect a stronger magnetic field around it, better blocking cosmic radiation.
As for the superhabitable planet’s surface, picture paradise. An enlarged orb would be rounder and smoother than Earth, as its stronger gravity erodes continental shelves and fills in trenches. Water would spill evenly over its surface as shallow lakes and seas.
On Earth, tropical islands and shallow seas are among the most biodiverse habitats. Now imagine a whole world of balmy archipelagos and reefs. The islands would burst with biological activity, their diverse topographies and geographic isolation fueling explosions of distinct species. Light would reach clear to the seafloor, powering abundant alien algae and plants to feed a booming marine ecosystem.
As for that light, it might be the orange glow of the planet’s K-dwarf star.
It was in the 2010s when astronomers began contemplating planets that are a little bigger, and a little nicer, than Earth, and where to find them. The whole idea was spurred in part by a question from John Armstrong a professor at Weber State University in Utah, who asked in the comments section of an astrobiology seminar on YouTube: “Shouldn’t there be planets more habitable than Earth?”
That got another astronomer, René Heller, who is now at the Max Planck Institute for Solar System Research in Germany, thinking. In the quest to discover chemical signs of life on distant worlds, he thought, astronomers have focused on finding “Earth twins,” rocky, watery, Earth-size planets orbiting sun-like stars. But when it comes to hosting life, Earth may not be the ideal archetype. After all, the fact that we’re not located at a special place in the cosmos (that we revolve around the sun, not that it revolves around us) is what the Copernican revolution was all about.
“It occurred to me that currently, humanity is looking for Earthlike planets with the same anthropocentric bias,” Dr. Heller said. “Of course, Earth is special to us. But from a universal perspective, why should Earth be a reference?”
In fact, Earth is only marginally habitable. Most astrobiologists think water must be present on a planet’s surface for life to arise there, and indeed the first bacteria appeared on Earth almost as soon as water pooled on its cooling surface roughly four billion years ago. But if Earth were just 2 percent closer to the sun, that pooling wouldn’t have happened. Instead, all of Earth’s water would have boiled away. We got lucky. But our luck will run out. The sun is expanding and brightening. Scientists calculate that in another half-billion years or so, Earth will be too hot to sustain complex life like plants and animals.
For Dr. Heller, the hunt for more habitable worlds is about “finding out if we are alone, if we could have been found, if there’s someone to talk to,” he said.
“Although I am not, at least in my everyday life, a very spiritual person,” he added, “it’s nevertheless something that at least drives me — this question, Could there be someone?”
He contacted Dr. Armstrong, and together they wrote a 2014 paper introducing the hypothesis of superhabitable worlds, which the two envisioned as archipelago-covered super-Earths. They were proposing a new target for telescopes.
“Superhabitable worlds must be considered for future follow-up observations of signs of extraterrestrial life,” they wrote.
In the paper, Dr. Heller and Dr. Armstrong reasoned that cool, petite K-dwarfs would be better host stars for superhabitable worlds than sun-like stars because they burn through their hydrogen fuel more slowly and brighten much more gradually. Our sun has only five billion years’ worth of hydrogen left before it will suddenly balloon and redden, becoming a red giant that will engulf the Earth. K-dwarfs, by contrast, shine steadily for tens of billions of years. This means life on a superhabitable planet orbiting a K-dwarf has far longer to evolve into ever more complex life forms.
What kinds of life forms? We can only imagine. All we know is earthlings have grown increasingly complex and sophisticated over the past four billion years, culminating so far in human-level intelligence.
“K-dwarfs do seem to be everyone’s sweet-spot star,” said Johanna Teske, a planetary scientist at the Carnegie Institution for Science in Washington, D.C. Planets that can sustain liquid water sit closer to K-dwarfs than they do to sun-like stars, so they are easier to observe with telescopes.
Astronomers search for life on planets tens or hundreds of light-years away by looking for gases in their atmospheres that may indicate biological activity. Current instruments, such as the James Webb Space Telescope, are mostly capable of checking the skies of rocky planets that tightly orbit tiny red stars called M-dwarfs. So far, most of these planets have been revealed as bare rocks, stripped skyless by their aggressive, nearby stars.
Probing the skies of potential superhabitable planets — namely, super-Earths orbiting at the right distances around K-dwarfs — should become feasible in future. The first step is to find nearby candidates. Dr. Heller is a leader of the European Space Agency’s Plato mission, a space telescope launching in 2027 with the purpose of detecting potentially habitable planets, including super-Earths orbiting K-dwarfs. “We will probably find a few of them — maybe a dozen,” Dr. Heller said. “But we won’t learn much about them.”
Follow-up observations from other telescopes should reveal more about the chemical makeup of these planets’ skies. The Webb telescope might be able to investigate the skies of some potentially superhabitable super-Earths in the coming years. The Extremely Large Telescope, which measures 39 meters across and is currently under construction in Chile, will be capable of seeing more of them. Even more extensive and definitive observations will become possible with the Habitable Worlds Observatory, a NASA space mission planned for the 2040s.
For Dr. Vilović, the pursuit of superhabitable planets is, in part, “an idealistic dream of a world that has beautiful conditions that are not being influenced by humans in a negative way,” she said. She thinks it’s also about a “drive to find extremes” — the most extreme version of the kind of life we know and understand.
As for whether the dream is also a reality, Dr. Vilović feels good about the odds. “I like to believe that statistically speaking, in the realm of the universe, it is likely that life exists — especially on superhabitable planets,” she said.
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