Scientists for years have focused their efforts on finding a planet in the so-called “Goldilocks” zone that the Earth finds itself in. Close enough to the Sun to possess adequate liquid water and temperatures to sustain life, far enough away not to burn and smoulder such as planets like Mercury. Since Earth is the only inhabited world known, the planet is usually the focus of studies on habitability.
This has left many other planets, not overly similar to Earth, overlooked.
Scientists, in a new study, have reasoned that these other worlds could offer conditions suitable for life to emerge and evolve – some of which might prove to be “super-habitable”, with even better potential for housing life than Earth.
Professor Dirk Schulze-Makuch, an astrobiologist, told Space.com: “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.”
To search for these potentially “super-habitable” planets, researchers sifted through the Kepler Object of Interest Exoplanet Archive, focusing on 4,500 planetary systems that likely possessed rocky planets within their stars’ habitable zones.
Astronomy: The study found at least two planets that met some of the requirements
Exoplanets: Exoplanets shown in comparison to the size of the Earth
Instead of exclusively 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 has a lifetime of slightly less than 10 billion years, whereas orange dwarfs have lifetimes of 20 billion to 70 billion years.
Researchers have reasoned that, since complex life took around 3.5 billion years to appear on Earth, the longer lifetime of orange dwarfs could give their planets more potential to sprout and develop life, and accrue biodiversity.
In the Milky Way, our galaxy among billions in our universe, orange dwarfs are about 50 percent more frequent than yellow dwarfs.
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Prof Schulze-Makuc explained: “Our Sun is actually not the best kind of star for hosting a planet with lots of life on it.”
An older planet may, researchers suggest, give life more time to evolve without the immediate worry of being consumed by its star.
Earth is about 4.5 billion years old, so the researchers speculated that the sweet spot for life is a planet that is between 5 billion to 8 billion years old.
In all, the scientists identified 24 potentially super-habitable planets.
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Biodiversity: Many of the potential planets could host a wealth of biodiversity
Earth: Our planet is located in the ‘Goldilocks’ region of the solar system
Unfortunately, none of the planets met all the criteria the researchers drew up.
However, one did meet at least two requirements – 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.
Researchers said that although it might have an average surface temperature about 2.4C degrees cooler than Earth, should the planet have more greenhouse gases than Earth to trap heat, it may prove to be “super-habitable”.
This was not Prof Schulze-Makuch’s favourite, however: that was found in 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.
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Of this find, he said: “I really liked the average surface temperature — about 27C degrees.
“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 and therefore learn more about them.
Water: Any exoplanet would need water as it is a necessity for life to survive and grow
Despite the promising findings, Prof Schulze-Makuch cautioned: “We caution that while we search for super-habitable planets, that doesn’t mean that they necessarily contain life.
“A planet can be habitable or super-habitable but uninhabited.”