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Our planet is the best there is, right? Not necessarily, say researchers at Washington State University who have produced a list of 24 planets outside our Solar System that are not only Earth-like, but may even be better than Earth.
The list—which is intended to be a “to do” list for a bunch of powerful telescopes due to go live in the next few years—includes planets that are older, a little larger, slightly warmer and possibly wetter than Earth, and which orbit stars with longer lifespans than our Sun.
The researchers—whose work is published this week in the journal Astrobiology—think the worlds in the list contain some that could be called “super-habitable.” That means they could be places where life could more easily thrive than on Earth.
Cue an MVP—Most Valuable Planet—which is likely to be larger than Earth and easier to detect than Earth-like planets.
If we want to find life elsewhere in the galaxy then “superhabitable” planets may deserve higher priority than most Earth-like planets, say the researchers.
How could another planet possibly be more suitable for life than Earth? To an Earthling with only one reference point, it sounds like a crazy question.
Here’s everything you need to know about the search for “super-habitable” planets where life may not only exist, but thrive.
Where are the ‘super-habitable’ planets?
Sadly, all of the 24 planets are in star systems that are over 100 light-years from the Solar System. The researchers went through the list of the over 4,500 known exoplanets in our Milky Way. They didn’t look for life, but only for the general conditions that would be conducive to complex life—defined as multicellular macroscopic life such as animals, plants and fungi.
How could a planet be more habitable than Earth?
We live here.
We know it’s teeming with life.
There’s biomass and biodiversity everywhere we look.
How could another planet possibly be ‘better?’ Yet our planet’s environments have changed dramatically over time. The researchers argue that there could be regions of star-planet systems that could allow for planets to be even better for life than our Earth. Largely they assessed them by considering:
- Astrophysics: the kind of star they orbit and its lifespan.
- Earth’s history: what kind of planet it is Vs. the varying habitability of the natural history of our planet.
What’s wrong with our Sun?
The Sun is a “G-type” star. Our very own ball of hot plasma been happily fusing hydrogen and helium atoms for about 4.6 billion years and it’s got another 5.4 billion years before it runs out of fuel. Complex life—that’s us—appeared on Earth between five million and seven million years ago. So it took a while. What are the chances of complex life appearing during the 10 million years’ lifespan of another G-type star? That’s unknowable, but it could be unlikely. The star could die before complex life got going.
So, the researchers say, we should also look for life on planets orbiting cooler “G-type” stars—with longer lifespans than our Sun—as well as “K dwarf” stars, which are cooler, less massive and less luminous than our Sun. They have a lifespan of between 20 billion and 70 billion years. That could be where to look for complex life because it will have had a vastly longer time to develop. After all, the Milky Way has been around for 13.5 billion years.
Taking into account the fact that a planet needs to be in its prime to retain a magnetic field—which protects any life that is on its surface from lethal radiation—the researchers come out with a sweet spot of how old a planet needs to be to support life; five billion to eight billion years old.
What’s wrong with our planet?
Earth has certainly not been habitable for all of its existence. Ice ages, asteroid strikes and changes in the chemical make-up of Earth’s atmosphere (such as the “Great Oxidation Event” about 2.4 billion years ago) have all affected Earth’s habitability.
The researchers used Earth as a basis for establishing which planets could be “super-habitable,” but used some leeway. For example they narrowed down the list to planet-star systems that are thought to have rocky planets orbiting within the host star’s “habitable zone”—the region around a star a planet needs to be orbiting within for liquid water to exist on its surface. Not too hot, not too cold, so also called the “Goldilock’s zone.”
They also chose planets that are 10% to 50% larger than Earth, which would have more habitable land, would have interior heating for longer, and would also have stronger gravity to keep its atmosphere for longer.
Lastly, they chose slightly warmer planets with more moisture. After all, it’s in Earth’s warm, moist tropical rain forests that boast the most biodiversity.
So what’s the most ‘super-habitable’ planet?
Here’s the researchers’ shopping list:
- In orbit around a “K-type” dwarf star.
- About 5-8 billion years old.
- Up to1.5 more massive than Earth and about 10% larger than Earth.
- Mean surface temperature about 5°C higher than on Earth .
- Moist atmosphere with 25–30% O2 levels, the rest mostly inert gases (e.g., N2).
- Scattered land/water distributed with lots of shallow water areas and archipelagos.
- Large moon (1–10% of the planetary mass) at moderate distance (10–100 planetary radii).
- Has plate tectonics or similar geological/geochemical recycling mechanism as well as a strong protective geomagnetic field.
Though none of the 24 top planet candidates meet all the criteria for “superhabitable” planets—largely because of missing data about moons, plate tectonics etc.—the closest the researchers can get to establishing the MVP is a planet called KOI 5715.01.
About 1.8 times bigger than Earth and about 5.5 billion years distant, KOI 5715.01 orbits its star every 190 days.
However, rather sadly, KOI 5715.01 is a whopping 2,964 light-years from the Solar System.
Why do we need a list of ‘superhabitable’ planets?
Due to launch in the next few years are a three very powerful space-based telescopes that will give us the opportunity to study far-of exoplanets close-up for the first time. The most famous is NASA’s James Web Space Telescope, which is due to launch on October 31, 2021, while the European Space Agency’s PLATO space telescope (2026) and NASA’s LUVIOR space observatory (due to launch in the 2030s) are also arriving soon.
“With the next space telescopes coming up we will get more information so it is important to select some targets,” said Dirk Schulze-Makuch, a geobiologist and professor with Washington State University and the Technical University in Berlin, who led the study. “We have to focus on certain planets that have the most promising conditions for complex life.”
Can’t we just look for an ‘Earth 2.0?’
So far much of the work on exoplanets has been against the backdrop for the search for an Earth-like “twin planet” somewhere out there in the Milky Way. So we’re looking for ourselves? The very definition of that search is biased—it’s human-centric and Earth-centric.
“We have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours,” said Schulze-Makuch. “It’s sometimes difficult to convey this principle of superhabitable planets because we think we have the best planet,” he added.
“We have a great number of complex and diverse lifeforms, and many that can survive in extreme environments … it is good to have adaptable life, but that doesn’t mean that we have the best of everything.”
Wishing you clear skies and wide eyes.