These alien planets may be more habitable for life than our own Earth
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These alien planets may be more habitable for life than our own Earth

Bigger, better, more suited for biology: let’s not overlook superhabitable planets with potential.

A superhabitable planet is a world that might have an even better chance at hosting life than our own home, Earth.

Because Earth is the only known inhabited planet and life on it is dependent on liquid water, efforts to find exoplanets that could support life are focused on Earth-like worlds. However, other scientists think that other types of planets may have conditions for life that are as good as — or even greater than — Earth. In fact, some scientists argue that focusing only on Earth-like worlds might be too “anthropocentric and geocentric,” blinding us to the possibilities of exobiology.

“We are so over-focused on finding a mirror image of Earth that we may overlook a planet that is even more well-suited for life,” Dirk Schulze-Makuch, an astrobiologist at Washington State University and the Technical University of Berlin, told Space.com.

HOW DO SCIENTISTS LOOK FOR SUPERHABITABLE PLANETS?

Schulze-Makuch and his colleagues examined the Kepler Object of Interest Exoplanet Archive for potentially superhabitable exoplanets, focusing on 4,500 planetary systems that have rocky planets within their stars’ habitable zones, where liquid water can persist. The researchers published their findings in the journal Astrobiology in 2020.

The scientists looked at planetary systems with yellow dwarf stars like our sun, as well as orange dwarf stars, which are cooler, dimmer, and less massive than our sun.

“Our sun is actually not the best kind of star for hosting a planet with lots of life on it,” Schulze-Makuch told Space.com.

In the Milky Way, orange dwarf stars are roughly 50% more common than yellow dwarf stars. Unlike our sun has a lifetime estimated at less than 10 billion years, orange dwarfs have lifetimes of 20 billion to 70 billion years. Since complex life took about 3.5 billion years to appear on Earth, the longer lifetimes of orange dwarf stars could give planets within their habitable zones more time to develop life and accrue biodiversity. 

The Earth is about 4.5 billion years old, so the researchers speculated the sweet spot for life would be a planet between 5 billion to 8 billion years old.

The size and mass of a planet can also affect how effectively it can support life, according to the researchers. A larger-than-Earth rocky planet would have a larger habitable surface area and possibly a thicker, more stable atmosphere. A planet 1.5 times the mass of Earth would likely retain its interior heat for a longer period of time, which would help keep its core molten and its protective magnetic field active for a longer period of time in which life may arise and evolve.

Worlds that are slightly warmer than Earth by about 8 degrees Fahrenheit (5 degrees Celsius) may be habitable, as they may have larger tropical zones, which support more species on Earth. However, warmer planets might also need more moisture, since greater heat could expand deserts.

In addition, planets with the same amount of land area as Earth but broken up into smaller continents might be more habitable. When continents become unusually large (as Earth’s ancient continent Gondwana did about 500 million years ago), their centers are often located far from oceans, resulting in vast, inhospitable deserts in their interiors. Furthermore, because Earth’s shallow waters contain more biodiversity than its deep oceans, scientists believe that planets with shallower waters could support more life.

WHAT ARE SOME SUPERHABITABLE PLANETS?

An artist’s depiction of a rocky, Earth-size exoplanet. We haven’t found any planets exactly as habitable as Earth, but some planets might be even better for life than ours: superhabitable worlds. (Image credit: NASA Ames/SETI Institute/JPL-Caltech)

In total, Schulze-Makuch and colleagues discovered 24 potentially habitable planets. None of these worlds met all of the requirements the researchers established for superhabitable planets, but one did — KOI 5715.01.

KOI (Kepler Object of Interest) 5725.01 is a 5.5 billion-year-old planet with a diameter of 1.8 to 2.4 times that of Earth, orbiting an orange dwarf 2,965 light-years away. It may have an average surface temperature approximately 4.3 degrees F (2.4 degrees C) lower than Earth, but if it has more greenhouse gases to trap heat than Earth, the researchers believe it could be superhabitable.

Schulze-own Makuch’s favorite potentially superhabitable world from these 24 was KOI 5554.01. This planet is around 6.5 billion years old, has a diameter 0.72 to 1.29 times that of Earth, and orbits a yellow dwarf 700 light-years away.

“I really liked the average surface temperature — about 27 degrees C [80 degrees F],” Schulze-Makuch said. “And it’s probably about Earth’s size, and a little bit older than Earth.”

Each of these 24 possibly habitable planets is more than 100 light-years away from Earth. This distance is too far for NASA’s Transiting Exoplanet Survey Satellite (TESS) satellite to obtain high-quality photographs of them in order to learn more about them.

However, Schulze-Makuch pointed out that future spacecraft, such as the recently launched James Webb Space Telescope, NASA’s LUVOIR space observatory mission concept, and the European Space Agency’s PLATO space telescope, could shed light on these worlds.

“We caution that while we search for superhabitable planets, that doesn’t mean that they necessarily contain life,” Schulze-Makuch said. “A planet can be habitable or superhabitable but uninhabited.”

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