Emma is one of my regular guest bloggers. I feel really thrilled about the possibility to post one educating post of this great blogger once a month over the next couple of months. Thank you so much, Emma, for sharing these great posts with us! If you would like to check out the previous guest posts, this amazing blogger wrote for me, head over here.
So you thought Earth was the perfect home for you? Well, you might be wrong. Earth, with its diverse climate and abundance of natural resources, is easy to take for granted. You think we live on our dream world, just the right temperature, just the right size, just the right composition. But that’s not true at all. The Earth is located on the inner rim of the “habitable zone,” a slender region where our star’s light is neither too intense nor too weak. As our star nears the end of its life and grows hotter, its habitable zone will be pushed farther out into space, and it will leave our perfect planet behind to die. Complex life will be exterminated quickly. Later on, the oceans will evaporate, leaving even simple organisms with no chance of survival. Earth will, over a billion years or so, become a dead world.
Our immediate thought is to look for planets like our own. But few have been detected. Planets orbiting other stars, previously called “exoplanets,” are now termed “extrasolar planets.” Both names make sense, “extra-” and “exo-” both meaning “outside.” Over 1,800 extrasolar planets have been detected, and statistics suggest that our galaxy harbors at least 100 billion more. However, few of these resemble Earth. However, other places in the universe, not just planets, can potentially offer us a home.
The next candidate in the running would be a very large moon. The largest moon in our solar system, Jupiter’s Ganymede, is only 2.5% as massive as Earth. But a huge planet, larger than Jupiter, may have a strong enough gravitational field to hold a moon around the size of Earth. If such a planet formed in the star’s habitable zone, it wouldn’t matter if the planet was icy, rocky, or gaseous. The moon, large enough to hold a breathable, protective atmosphere but not large enough to crush us with its atmospheric density and gravitational field, would be our safe haven. As long as it orbited within the habitable zone, it could form liquid water oceans, and existing within the gravitational fields of both its host planet and its star would provide it with multiple sources of energy. Earth, on the other hand, relies mainly on the sun. Tidal forces between the moon and its planet could drive geological activity and provide a greater chance of the formation of liquid water oceans. The geological activity, including plate tectonics, would keep the surface active and make the moon’s surface conditions very similar to those of Earth. In short, Earth could find a distant cousin in such a moon. Literally distant!
To date, no “exomoons” have been detected, habitable or otherwise. However, superhabitable planets may already exist within our catalogue of confirmed exoplanets. Superhabitable planets are worlds slightly larger than Earth, orbiting much cooler, dimmer stars. One might think that these cool stars, known as K dwarfs or M dwarfs depending on their energy output, would have even more trouble supporting life than our star does. However, the reverse is true. K and M dwarfs have less hydrogen fuel to burn, but they expend it much more efficiently than stars the sun’s size and larger. That means they would sustain the same habitable zone for far longer than our sun will, and they still give off enough light for photosynthesis to occur. Planets located within a K or M dwarf’s habitable zone would have much longer to evolve than our Earth ever would. Liquid water oceans could exist, and if life developed, natural processes could gradually make the atmosphere even more friendly to life.
Life on a superhabitable world, be it a planet or a moon, might be radically different from what we’re used to, but some basic qualities would stay the same. Plant ecosystems would thrive, and a pleasantly thick atmosphere would shield the planet from meteorite impacts. The planet would support a protective magnetic field for deflecting the star’s solar wind. The star itself would glow a dim orange or red in the sky. Plant life might be tinged darker shades of green to better absorb the star’s cool wavelengths, and it might look very alien to us, evolved from different evolutionary processes that had no connection to our particular evolutionary past. In the sky of a lunar world would be a sight to behold: the parent planet, hovering like a gigantic moon in space, perhaps with glowing white ring systems and maybe even a moon or two. Life could definitely be worth living on an extrasolar world.