We live in a part of the solar system called the Goldilocks zone, where it’s not too hot that all the water evaporates away but it’s also not too cold where it freezes instead. And unless you are an astronaut the big spherical rock will be your home for life. But what if that home suddenly changed? What if in a split second the earth doubled in size? Let’s assume that the density remains constant so the mass of the earth increases as well. How would that change things?
First off, the gravity would be very different; the earth would have 8 times as much mass and gravity would be twice as strong. So if you stepped on a scale you’d suddenly weigh twice as much as before. It would be much harder for you to walk and you get much more tired easily. After all, you’d feel like you were giving another version of yourself a piggyback ride all day every day.
Over the years, human bones, especially in the legs, would have to become stronger to support this increased weight. Meanwhile, trees would start to collapse and any new trees that group in their place might not grow as tall. There is a limit for how tall a tree can grow, usually around 400 feet or so. This limit is determined by gravity, since the taller the tree is the more energy is required to transport water from the roots to the top. If the amount of energy the tree gains from photosynthesis is more than the energy it takes to transport the water, it will keep growing. Otherwise, it will simply stop. So, if the earth suddenly got twice as big the trees like the California Redwood may not be as impressive.
Luckily, even though the gravitational pull of the earth will be stronger, it still won’t be enough to break the moon apart. In order for that to happen the moon would have to be in the earth’s Roche limit. This is the minimum distance the moon can approach the earth without being torn apart by tidal forces. This happens because the near side of the moon is pulled in harder than the far side. When the moon gets close enough to the earth the gravity holding it together is overcome by this cosmic tug. Inside that limit, the earth’s gravity will tear the moon apart, giving us a ring like those around Saturn.
The bigger the earth is, the more it will heat up. If there are more unstable elements in the crust and interior, more heat will reach the surface and there may be much more volcanic activity.
So, it seems it would probably not be that exciting to live on an earth that is twice as big. What do you think?