The search for a habitable planet like Earth just hit a new challenge, from planets that could well be BETTER than Earth at fostering life. How could this be? We all know Earth is the template on which we base our assumptions about the chances for life elsewhere. We are searching Mars and other places in our own solar system, including Venus.
It turns out that the best habitable planet could have conditions much MORE suitable for life than Earth. A habitable planet, such as Kepler 62F, shown here, which orbits a K-type star, could have several variables that influence life. This world, and others like it, could be orbiting stars that are more slowly changing and longer life expectancies than the Sun. Astronomers have actually identified a couple of dozen contenders for the “Better than Earth” sweepstakes. They are called “super-habitable planets” and if they can be found, they’d all a little larger, warmer, and likely wetter than Earth. Those conditions make it easier for life to develop and thrive.
All of this comes from a study made by a scientist at Washington State University. Dr. Dirk Schulze-Makuch published an article in the journal Astrobiology that details the types of planets and stars that could be in the super-Goldilocks state. That is, more welcoming of life than our home planet. His contenders are more than 100 light-years away from us, so we’re going to need some superpower telescopes to observe them. That would include the upcoming James Webb Space Telescope, as well as the proposed LUVOIR space observatory and European Space Agency’s PLATO space telescope.
Searching for a Habitable Planet that’s “Super”
For now, Schulze-Makuch, a geobiologist with expertise in planetary habitability, is working with astronomers Rene Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University to answer questions about superhabitability. They needed to identify the criteria that would make a world “superhabitable” and then look through the characteristics of the more than 4500 known exoplanets to see if they fit. Those worlds may not be inhabited, but they could have the conditions to host life.
What Makes a Habitable Planet “Super”
The first thing was to look at systems with worlds that could be terrestrial, and orbit within the stars Goldilocks zone. That’s the region where liquid water could flow on the surface of a supposedly habitable planet. But, there are other factors to consider, such as the age of a planet’s star. The Sun, for example, will live about 10 billion years.
That sounds like a long time, but think about this: life started here some 3.8 billion years ago, when the Earth was pretty young. However, but it took quite a long time before the more complex life forms appeared. So, it might make sense to look for worlds where the stars live longer to search out the possibility of complex life arising on them. Shorter-lived stars wouldn’t be around long enough for complex life (and indeed, intelligent life) to form. Or, they’d start to age before then, making conditions for life worse than before.
So, longer-lived G stars would be good candidates forhosting superhabitable worlds. And, it’s possible that K-type stars could be useful here. They’re cooler, less massive, and luminous than the Sun. Their big advantage is that they live a lot longer than the Sun — somewhere between 20 to 70 billion years. That lifespan would allow orbiting planets to be older as well.
All that extra time gives life a lot longer to develop and create more complex forms. The downside here is that much older planets could have exhausted their interior heat, which would affect their ability to create a magnetic field. Given these constraints, the sweet spot for life on any world is probably between 5 to 8 billion years. At 4.5 billion years, Earth is a precocious producer of life. It might be an outlier.
More Measures of Habitability
Age is only one factor to think about when we consider whether a habitable planet could be superhabitable. Their sizes and masses are also important. A planet that is 10% larger than the Earth should have more habitable land distributed among its oceans and continents. That gives more places for life to life and thrive. If we find a planet that is about one and a half times Earth’s mass, it’s very like to retain its interior heating. That usually happens through the decay of radioactive elements in the core. Finally, such a more-massive world would have a stronger gravity. That helps it retain an atmosphere much longer than smaller, less-massive worlds.
The atmosphere is a big player in the habitability of a planet. Water is a huge factor — in fact, it’s necessary for life. A little more water in the form of moisture, clouds, and humidity is always useful when fostering life. A slightly overall warmer temperature, a mean surface temperature of about 5 degrees Celsius (or about 8 degrees Fahrenheit) greater than Earth, together with the additional moisture, would be also better for life. Of course, we can see this playing out here on Earth. Places with higher warmth and moisture have a greater diversity of life (such as in tropical rain forests) than the areas with colder, drier conditions.
Are There Superhabitable Planets “Out There”?
The scientists looked at 24 planet candidates they thought might meet all the criteria for being “better than Earth” in terms of superhabitability. Only one meets four of the criteria, but none meet all of them. So, no superhabitables out there among the 24. But, the galaxy is huge and we’ve only just started the search for exoplanets in the past 25 years. Earth is still the only place we know FOR sure has life, and it does a good job at hosting life across a huge range of conditions. Still, the ideas in the current research pave the way for future searches around stars that may even provide better conditions for life than we have here.