Mars

"Turns out when we found a whole other earth in the mirror plane, this big ball of red dust wasn't worth all that much anymore."

The fourth planet in the solar system, Mars has long been an object of space exploration as well as a theorized terraforming target. Despite lacking a strong magnetic field, the planet had water, a rocky surface, and enough sunlight to potentially sustain life. To that end, missions were launched throughout the 21st century to study the planet and to prepare for a terraforming mission.

Terraforming was always going to be a complicated and expensive process, and even as late as 2080 was still considered impractical to attempt in a reasonable timeframe. Theorized methods would've taken centuries to bear fruit, were uncertain to work, and would've been extremely expensive. Meanwhile, Mars remained a scientific outpost, too tempting to abandon entirely yet too uncertain to commit to.

In 2085, the first Tein Machine was constructed, and in 2098, the first Tein Gate was built to another system with an earthlike planet. Interest in Mars dropped like a stone, and over the decades since even many of the arcologies already built on Mars have been allowed to fall into disrepair. Though not quite abandoned, Mars no longer holds the allure or potential it once did. After all, why spend all that money and effort terraforming Mars when one day, eventually, another viable Tein Gate will be found, and humanity will have yet another home.