Scientists Describe 'Superhabitable' Worlds That Could Outdo Earth for Life

What makes a world 'superhabitable'

In 2020, a team led by Dirk Schulze-Makuch of Washington State University, with René Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University, proposed criteria for what they called superhabitable planets — worlds that could theoretically support more and longer-lived life than Earth, according to a summary of the study, published in the journal Astrobiology, reported by the science news site EarthSky.

EarthSky reported that an ideal superhabitable planet would be older than Earth, in the range of 5 to 8 billion years, roughly 10% larger in diameter and about 1.5 times Earth's mass, with a global mean temperature about 8 degrees Fahrenheit warmer and a wetter surface than Earth's. Such a planet would also orbit a K-type star, which can shine steadily for 20 to 70 billion years — far longer than the sun's roughly 10-billion-year lifespan — giving life more time to evolve, EarthSky said.

24 candidates, none confirmed

Searching among more than 4,000 known exoplanets located in their stars' habitable zones, the researchers singled out 24 candidates that came closest to the superhabitable criteria, including Kepler-69c and Kepler-186f, EarthSky reported. None of the 24 met every criterion, and the study's authors said the conditions described remain largely untestable with current observational tools, according to EarthSky's account of the paper. EarthSky quoted Schulze-Makuch as saying humans have a great number of complex and diverse lifeforms on Earth, but that does not mean Earth has the best of everything.

Newer research points to cooler K-dwarf systems

A more recent study by Iva Vilović of the Technical University of Berlin and René Heller of the Max Planck Institute for Solar System Research modeled planets orbiting mid-type K dwarf stars, according to the science newsletter Centauri Dreams. The study found that a planet receiving about 80% of the sunlight Earth gets from the sun — positioned between the inner edge and the center of its star's habitable zone — could hold a stable surface temperature while needing far less atmospheric carbon dioxide than a planet orbiting in the middle of a sun-like star's habitable zone, Centauri Dreams reported.

Centauri Dreams said the modeled atmosphere on such a world would contain more oxygen (25% versus Earth's 21%) along with higher methane and nitrous oxide linked to microbial activity, and that the planet would still need plate tectonics and a strong magnetic field to stay habitable, with surface temperatures about 5 degrees Celsius warmer than Earth's today.

France 24 reported that researchers simulating conditions around K-dwarf stars described hypothetical worlds with better weather, smoother terrain, abundant flowing water and abundant plant and animal life; the outlet also said some plant leaves grew faster under light simulating a K-dwarf star than under the sun's light.

Why these worlds remain out of reach

K-type stars are not rare: Centauri Dreams reported that roughly 1,000 of them exist within 100 light-years of Earth. But confirming whether any orbiting planet is genuinely superhabitable — let alone inhabited — is far beyond current capabilities. Centauri Dreams reported that detecting a paired oxygen-and-methane biosignature with the James Webb Space Telescope would require about 150 transits, or roughly 43 years of observation, for a planet around a K-dwarf star, compared with about 1,700 transits — nearly 1,700 years — for an Earth-like planet orbiting a sun-like star.

France 24 summarized the practical conclusion of the coverage: the worlds described as potentially better suited to life are located outside the solar system, placing them beyond the reach of any current or foreseeable spacecraft.