Better than the earth? Are there superhabitable worlds in the Milky Way?

I've already said time and again that the Earth is the best planet in the universe. No matter where we go, we will never find a planet that is a better home for life on Earth than Earth. Of course, it's because we, and all other terrestrial life forms, have evolved in this environment. Evolution has adapted us to this planet and it is unlikely that we will ever find another planet so beneficial to us.

However, is it the best planet? Are there places in the universe that could have the conditions for more diversity of life?

The fact that we have life on Earth is pretty incredible. We are located in the habitable zone of a star of the main sequence that does not produce too many killer torches.

We have a thick atmosphere filled with oxygen and nitrogen that we can breathe. The planet is large enough to be melted in its core, with a rotating iron ball that maintains a planetary magnetic field. This, combined with a thick atmosphere, protects the planet's surface from cosmic rays, ultraviolet rays from the sun, and deadly solar storms.

We have plate tectonic systems that constantly recycle materials on the surface of the planet and bring fresh chemicals from within.

We have a relatively large moon, which probably keeps our planet more stable in its axial tilt, with tides that have helped life forms early in the transition from the oceans to the earth. But not too big moon.

We have huge oceans that help regulate the planet's climate, moving warm waters into colder regions to make them more diverse and livable.

The list is long and I am sure that some factors are not yet known.

And when it comes to the Earth, life has prospered, inserting itself into all the possible ecological niches, adapting to the evolutionary trends to cope with the intense cold, the intense heat, the intense pressures exerted on the bottom. oceans, even cities, in the immediate vicinity of human beings. beings.

But could the Earth be better? Could there be super habitable planets?

If there is one thing that the field of astronomy has taught us, it is that we are not special. We are not the center of the solar system. This is not a place or a special moment in the universe. And that probably means that the Earth is not the best place for life. It's the best place for humans, but not for life.

According to an article published in 2013, Ravi Kumar Kopparapu, an astrobiologist from Penn State, and others have calculated where should really lie the edges of the habitable zone of a star, from modern climate data. They calculated that a habitable area around a sun-like star should be between 0.99 and 1.7 times the distance from the Earth to the sun.

Which means that the Earth is actually at the inner limit of the habitable zone of the sun. Like, hardly. If it was closer to the sun, we would have a greenhouse effect that wraps around like Venus.

You probably want to be closer to the middle of the habitable zone, where orbital variations will not push your planet into extremes.

The Earth is relatively young. Given the fact that the planet has only existed for 4.5 billion years and that it has understood multicellular life only in the hundreds of millions of millions of years. # 39; years.

The sun is warming up and as we are so close, we are actually only a few hundred million years old, a billion or so before the temperature rises and the oceans evaporate. But what if life could have taken billions of years of further evolution to develop new and more diverse life forms?

You think that a platypus is unusual, imagine what you would get with 2 billion additional years of evolution. Or 20 billion.

In a 2016 article entitled Superhabitable Worlds, Rene Heller and John Armstrong describe the conditions that make the planet as habitable as possible. It's a very readable paper, with lots of cool ideas. If you are a science fiction writer looking for constructive ideas, take a look at it.

They propose that lower sun stars, classified as K stars, are probably the best candidates for diversity because they have a long life and are relatively stable. A K-star will have a lifespan of 20 to 70 billion years without these superb megaflares of red dwarfs.

You would want other planets in the star system, able to redirect asteroids and comets with their gravity to provide water and other chemicals necessary for life. Thank you for that, Jupiter.

And ideally, you want several habitable planets in the same system, able to send life. A process known as panspermia.

Make your habitable planet the moon of a gas giant to get powerful tidal forces that would prevent fresh volcanic material from surfacing.

Better yet, have a binary planet, where two worlds revolve around each other, transmitting tidal forces and exchanging life forms.

And we are just starting!

Enlarge the planet and get more surface for water to circulate temperatures (more about it in a second), but also more surface for life forms to exploit various niches.

So, we are talking about a bigger and more massive planet. Once you have about twice the mass of the Earth, plate tectonics starts to go out, so try to stay below that amount.

You also want a world big enough and warm enough inside that the movement of iron alloys in its nucleus maintains a planetary magnetosphere.

You probably worry about the surface gravity, but a planet whose mass is twice the Earth must be about 40% larger to have about the same surface gravity.

At a recent conference in Barcelona, ​​Dr. Stephanie Olson of the University of Chicago presented the work she had done to look for environments best suited for life on exoplanets.

They used a NASA tool called the ROCKE-3-D general circulation model. It's a really amazing tool and freely available to the public. You can go to the website and then see what conditions would occur on different worlds, from ancient Venus to planets orbiting Proxima Centauri.

You can simulate their air temperatures, their precipitation, their concentrations in the soil, and so on.

Let me show you some examples. This is the pre-industrial Earth, with air temperatures ranging from around 35 ° C near the equator to less than -60 ° C at the poles.

But you can replace the Earth by the ancient Venus, in the image of the planet 2.9 billion years ago, while the sun was 20% weaker than today # 39; hui. However, it was still running once every 243 days and probably had a shallow ocean reaching a depth of 310 meters across its lowlands.

And here is a planet orbiting the red dwarf star Proxima Centauri, the star closest to the sun. Because it orbits so close to its star, the planet is probably locked. This has a dramatic impact on the air temperature with one side facing the star and one side facing outward.

But if the planet has a resonant rotation, where it turns three times on its axis every 2 orbits, and if its atmosphere roughly matches that of the nitrogen and oxygen of the Earth, you will find yourself with a much more aesthetic world. comfortable to live.

Olson and his team have used this software to simulate ocean climates and habitats of different types of exoplanets. Here on Earth, the diversity of life depends on the rising of materials from the depths of the oceans, to restore them to the surface, where life can use them.

More upwelling means more biological activity, more diversity.

In other words, to find the most diverse planets in life, you must search for the worlds characterized by strong ocean circulation.

Is there anything better than the Earth? According to Olson, if a planet rotates more slowly, has a higher atmospheric density and has continents, you can increase the amount of ocean circulation.

And this gives us an idea of ​​what astronomers will look for when they look at extrasolar worlds. When NASA's LUVOIR or HabEx missions fly in the 2030s, they will be able to directly image the surface of the exoplanets. They will measure the chemicals in their atmosphere, detect water and even determine which part of the planet is covered with continents.

We really should not be surprised if we find super habitable worlds in the Milky Way, worlds clearly livable than the Earth. Once again, it turns out that we are not special. That's fine, at least we will have company.

---

---