Cloud Simulation
This illustration shows simulated cloud coverage (white) on a tidally locked planet (blue) that would be orbiting a red dwarf star. Jun Yang/University of Chicago

Researchers now think there may be twice the number of habitable planets previously thought, thanks to new calculations that include the effect of clouds on alien worlds. This shift in thinking means there are 60 billion potentially habitable planets in the Milky Way alone, with countless billions of planets outside of our galaxy that may be able to support life.

Planets in the habitable zone of a parent star may have the ability to sustain liquid water on their surface. This habitable zone, sometimes known as the “Goldilocks Zone,” is neither too hot nor too cold, which increases the chance of liquid water, and life, being present on the planets residing within it. Researchers from the University of Chicago and Northwestern University used computer models of planets orbiting red dwarf stars, the most common type of star in the universe, and discovered that clouds increased the potential habitable zone of red dwarf stars.

Since red dwarf stars are smaller than stars like our Sun, which is classified as a yellow dwarf, their habitable zone is likewise smaller, but this new study points to a much-expanded zone surrounding these parent stars. According to a press release from the researchers, it was believed that for every red dwarf star there was about one Earth-sized planet orbiting within its habitable zone. This belief was based on data collected by NASA’s Kepler Mission, which looks for Earth-sized planets within a star’s habitable zone. The new research doubles that number because previous calculations did not factor in an atmosphere’s effect on creating a situation where water can exist on the planet’s surface.

The study, led by Nicolas Cowan, from Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics, Dorian Abbot and Jun Yang, from the University of Chicago, factored in atmospheric conditions in the formula for determining the habitable zone. Clouds are important to meteorological phenomena on a planet and clouds can affect the temperature on the planet’s surface. Even if a planet is outside the habitable zone, clouds could absorb heat, in the form of radiation, which could warm the surface of the planet and allow liquid water to form.

Abbot, noting the effect of clouds in warming up and cooling down Earth, said, “They reflect sunlight to cool things off, and they absorb infrared radiation from the surface to make a greenhouse effect. That's part of what keeps the planet warm enough to sustain life.” In comparison to Earth orbiting the Sun, a planet orbiting a red dwarf star would need to complete that trip every month or two, notes Cowan.

These planets will eventually become stuck with one side facing the star, much like our Moon's dark side that never faces the Earth, which would prevent the existence of liquid water due to the extreme heat. The computer simulations were used to predict the climate of a planet similar to the simulations to predict Earth’s climate, presenting a three-dimensional model of the alien planet’s climate.

The researchers discovered that water clouds helped cool down the planet. A planet originally ruled out as having the ability to sustain liquid water because it was orbiting too near a red dwarf star, may still have the ability to have liquid water on its surface. According to the simulation, water clouds are an indication that there is liquid water on the surface of the planet.

The next step for researchers is to test their simulations by measuring temperatures on an alien planet using the James Webb Telescope. The study was published in Astrophysical Journal Letters.