Superhabitable planets - Bigger, better, more suited for biology

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Superhabitable planets
By Charles Q. Choi Contributions from Vicky Stein published 2 days ago

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


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)
Jump to:
The search
Some contenders
Additional resources

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 its life depends on 
liquid water, efforts to identify exoplanets that could host life focus 
on Earth-like worlds. But some researchers think there are other types 
of planets that could offer conditions for life as good as — or even 
better 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?
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To search for potentially superhabitable exoplanets, Schulze-Makuch and 
his team investigated the Kepler Object of Interest Exoplanet Archive, 
focusing on 4,500 planetary systems that likely possessed rocky planets 
within their stars' habitable zones, where liquid water can persist. The 
researchers published their findings in a 2020 paper in the journal 
Astrobiology.

In addition to looking at planetary systems with yellow dwarf stars like 
our sun, the scientists also looked at 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.

Orange dwarf stars are about 50% more common than yellow dwarfs in the 
Milky Way. Whereas 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.

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.

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The size and mass of a planet can also influence how well it can support 
life, the researchers wrote. A rocky planet that is larger than Earth 
would have more habitable surface area, and potentially a thicker, more 
stable atmosphere. A planet with about 1.5 times Earth's mass would 
likely retain its interior heat longer, which in turn would help keep 
its core molten and its protective magnetic field active for a greater 
timespan in which life might have the chance to arise and evolve.

Worlds that are slightly warmer than Earth by about 8 degrees Fahrenheit 
(5 degrees Celsius) might be superhabitable, since they could have 
larger tropical zones which on Earth foster more biodiversity. 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 particularly large (such as Earth's past continent 
Gondwana, about 500 million years ago), their centers are far from 
oceans, often rendering the interiors of large continents vast, 
inhospitable deserts. Moreover, Earth's shallow waters have a greater 
biodiversity than its deep oceans, so scientists speculate that planets 
with shallower waters could support more life.

WHAT ARE SOME SUPERHABITABLE PLANETS?
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All in all, Schulze-Makuch and team identified 24 potentially 
superhabitable planets. None of these worlds met all the criteria the 
researchers drew up for superhabitable planets, but one did meet at 
least two — KOI 5715.01.

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

Schulze-Makuch's own favorite potentially superhabitable world from 
these 24 was KOI 5554.01. This planet is about 6.5 billion years old, 
with a diameter 0.72 to 1.29 times that of Earth, orbiting a yellow 
dwarf about 700 light-years from Earth.

"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."

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All 24 of these potentially superhabitable planets are more than 100 
light-years from Earth. This makes them too far for NASA's Transiting 
Exoplanet Survey Satellite (TESS) spacecraft to capture high-quality 
images from to learn more about them.

Still, Schulze-Makuch noted that future spacecraft, such as the newly 
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."

ADDITIONAL RESOURCES AND READING
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If you're thrilled by the idea of exoplanets, poke around the NASA 
Exoplanet Archive yourself! If you want to read more about how different 
and bizarre other planets might be, dive in to astronomer Michael 
Summers and physicist James Trefil's 2017 book "Exoplanets: Diamond 
Worlds, Super Earths, Pulsar Planets, and the New Search for Life beyond 
Our Solar System." And for anyone looking to dream about living on 
another world, get lost in this TED playlist about becoming — and 
thriving as — an extraterrestrial.

Bibliography
Brennan, Pat. "The Habitable Zone | The Search For Life." Exoplanet 
Exploration: Planets Beyond our Solar System, April 2, 2021. 
https://exoplanets.nasa.gov/search-for-life/habitable-zone.
Schulze-Makuch, Dirk, René Heller, and Edward Guinan. "In Search for a 
Planet Better than Earth: Top Contenders for a Superhabitable World." 
Astrobiology 20, no. 12 (December 1, 2020): 1394–1404. 
https://doi.org/10.1089/ast.2019.2161.
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Charles Q. Choi
Charles Q. Choi