On Earth,
there is life virtually everywhere there is liquid water. As such, the hunt for
extraterrestrial life has focused on so-called habitable or
"Goldilocks" zones — areas around stars temperate enough for planets
to possess liquid water on their surfaces.
Since Earth
is the only inhabited world known, this planet is usually the focus of studies
on habitability. However, scientists have reasoned that worlds other than
Earth-like ones could offer conditions suitable for life to emerge and evolve.
Such worlds might even prove "super-habitable," or have better
chances at hosting life than Earth.
"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," study lead author Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman and the Technical University of Berlin, told Space.com.
To search
for potentially super-habitable exoplanets, researchers investigated the Kepler
Object of Interest Exoplanet Archive, focusing on 4,500 planetary systems that
likely possessed rocky planet within their stars' habitable zones.
In addition
to looking at planetary systems with yellow dwarf stars like our sun, the
scientists also looked at orange dwarf stars, which are somewhat cooler, dimmer
and less massive than our sun. 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 dwarfs could give planets within their habitable
zones more time to develop life and accrue biodiversity. Orange dwarfs are also
about 50% more frequent than yellow dwarfs in the Milky Way.
"Our sun is actually not the best kind of star for hosting a planet with lots of life on it," Schulze-Makuch said.
An older
planet might give life more time to evolve. Earth is about 4.5 billion years
old, so the researchers speculated the sweet spot for life is a planet that is
between 5 billion to 8 billion years old.
The size and
mass of a planet can also influence how well it can support life. A planet that
is 10% wider than Earth would have more habitable land. One that is about 1.5
times Earth's mass would be expected to retain its interior heat longer, which
in turn would help keep its core molten and its protective magnetic fields
active. A heavier planet would also have stronger gravity to help retain its
atmosphere over a longer span of time.
Worlds those
are slightly warmer than Earth by about 8 degrees Fahrenheit (5 degrees
Celsius) might be super-habitable, since they could have larger tropical zones
that could be more benign for 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 it comes to continents that are too
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 planets with shallower waters may
be super-habitable.
Super-habitable
haul
All in all,
the scientists identified 24 potentially super-habitable planets. None of them
met all the criteria the researchers drew up for super-habitable planets, but
one did meet at least two — KOI 5715.01, 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 Earth, but if it has more greenhouse
gases than Earth to trap heat, it might be super-habitable, they said.
Schulze-Makuch's
favorite potentially super-habitable world from these 24 was KOI 5554.01, a
world about 6.5 billion years old 0.72 to 1.29 times Earth's diameter 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."
All 24 of
these potentially super-habitable 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 NASA's James Webb Space
Telescope, NASA's LUVIOR space observatory and the European Space Agency's PLATO
space telescope, could shed light on these worlds.
"We caution that while we search for super-habitable planets, that doesn't mean that they necessarily contain life," Schulze-Makuch said. "A planet can be habitable or super-habitable but uninhabited."
The
scientists detailed their findings online Sept. 18 in the journal Astrobiology.