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| We haven't yet discovered a truly Earth-like planet. I wish scientists would stop saying we have. |
Let's Begin with Gravity
Is gravity all that important? I don't know. I do know I don't want to live on Mars. It's gravity is less than 1/2 of Earth's gravity. If I were to spend time on Mars I'd have to exercise constantly if I hoped to come back to Earth some day. Think about it. You'll go through Mars life at one weight and then land on Earth with more than twice the weight you're used to. Can you imagine trying to walk around with twice the weight you normally carry?
I think I could tolerate an Earth-like gravity of 80% to 125% of what Earth's gravity is like. If I spent five years on a planet with 80% of Earth's gravity then returning to Earth would mean I was carrying about 25% extra of my body weight. I mean the body weight I was used to on the smaller planet.
If I spend 5 years on a planet with 125% of Earth's gravity then when I return to Earth I'll have a spring in my step.
Okay, since these planets don't exist in the solar system I won't be visiting them. But if we could travel to these planets in a short time, I would want them to have truly Earth-like gravity. The only planet in the solar system that falls into my preferred range is Venus. But there's a problem.
I Want an Earth-like Atmosphere
Venus fails this test. From what I've read, 96% of Venus' atmosphere is carbon dioxide. The Earth's atmosphere is less than 4% carbon dioxide. Think about that. All the carbon we have been flooding into the atmosphere for 250 years is warming the Earth up faster than Mother Nature was going to. You don't need to ask why Venus is so much hotter than Earth.I want an Earth-like planet to have an atmosphere that is mostly nitrogen and oxygen. I guess we could tolerate some other gases that are not very common here on Earth. Earth's air is about 78% nitrogen and 20% oxygen.
The good news about Venus, I guess, is that the laws of physics say most of that carbon dioxide could be converted to some sort of chalky substance. Most of Earth's carbon dioxide is stored in rocks like limestone, which were made from the shells of dead marine life. If we could kickstart a calcite-forming process on Venus, eventually its atmosphere would become more Earth-like. Venus and Earth have about the same amount of nitrogen.
That means if we find a planet like Venus that isn't as close to its star as our Venus is close to Sol, it might be a habitable world. Someday.
If we could pull most of the carbon out of Venus' atmosphere the surface pressure would drop from its current 90 bars to a more Earth-like 1 bar.
But there is still a problem with our Venus.
I Want Water on the Planet
Mars has water. Earth has water. As far as we can determine Venus doesn't have water, although it probably once did. What happened to all of Venus' water? It seems it was blown away by the solar wind. Maybe there is still water trapped in the planet's crust but we don't know.Mars may not have as much water as it did a few billion years ago. There used to be a large ocean on Mars. But now what water remains is frozen in a kind of permafrost. We don't know how much water is left. The problem with Mars' water is that it cannot liquify on the surface any more.
If Venus still has water then as soon as it escapes into the atmosphere it will rise up to the top and eventually be blown away by the solar wind. The solar wind may also have blown away much of Mars' atmosphere and water.
So a truly Earth-like planet must have liquid water, preferably at least one ocean. At least with an ocean you will have a hydration system. The water will flow up into the sky and rain back down.
I Want Tectonic Plates and Magnetism
Mars and Venus may have tectonic plates that have frozen into place. We don't fully understand how tectonics work. What we know is that large portions of the Earth's crust move around. The edge of one plate slides under the edge of another plate. And Each plate is being pushed by forces under the surface.Scientists are not sure if this ever happened on Venus. They believe that Mars may have once had tectonic plates. We know that Mars has volcanoes. Volcanoes may be a sign of tectonic activity.
Tectonic movement means there is a hot, swirling mantle underneath the crust. That could mean there is a hot metal core that is still spinning. Scientists believe Mars' core has stopped moving, or almost so. Earth's magnetic field is generated by the immense metal core as it churns beneath the magma. The magnetic field protects the Earth from most of the solar and cosmic radiation that bombards Mars and Venus.
Even if we could convert Mars and Venus to watery worlds with Earth-like atmospheres, they both lack magnetic fields. We would have to find a way to generate similar magnetic fields. Most likely that would have to be done with satellites. I don't think physicists have any idea of how to turn a planet's iron core into swirling, molten metal again. Even if the movies say they can do it, I don't think that is possible.
Finally, I Want a Sol-like Sun
I'm not sure but I think the "Earth-like" planets that have been discovered so far all orbit red dwarf stars. There are about 10 times as many red dwarf suns in the Milky Way galaxy as all other stars combined. But since we don't live under a red dwarf sun I don't think we would feel very much at home. The "goldilocks zone" for red dwarf stars is much smaller and closer to the stars. Their planets are usually tidally locked, meaning one side always faces the star.A truly Earth-like planet will have about a 24-hour day. I can live with somewhat shorter or longer days, but not too much shorter or longer.
And it would be nice if an Earth-like planet had a moon. We may not need a moon to be able to live on a planet like Earth, but it would make the night sky more familiar and home-like.
If I ever find myself living on another world, I won't call it Earth-like unless I can check off all these important points.
