A Mars-Like Planet Might Be Hiding in the Outer Solar System

If the 21st century has taught us any astronomical lessons, it’s that counting planets is hard. In 2000, there were nine planets, and now there are eight, but that might not last. Astronomers have been on the hunt for a theorized ninth planet in the extreme outer solar system, and now a study suggests there might be another planet out there. Unlike the massive (and completely hypothetical) Planet Nine, this one is believed to be a small, rocky world like Mars.

All the planetary uncertainty lies in the outer reaches of the solar system, beyond the orbit of Neptune. This is where Clyde Tombaugh discovered Pluto, which we thought was a planet for decades but has since been demoted to a dwarf planet. It was still a notable discovery as the first known representative of the Kuiper Belt, a ring of icy rocks that includes other big planetoids like Makemake and Eris.

To make sense of the mishmash of objects out there, scientists often turn to simulations that can search for signs of undiscovered planets. And there could be a lot to find out there. “It seems unlikely that nature created four giant planet cores, but then nothing else larger than dwarf planets in the outer solar system,” the study says.

The team found that models capable of closely approximating the current state of our solar system start with at least one extra planet, something vaguely Earth or Mars-like. This world was bounced around in the outer solar system by the intense gravity fields of Neptune and Saturn until it ended up in a far-out orbit where we can’t see it. It’s also possible the planet (or planets) were ejected from the solar system.

We are only beginning to understand how solar systems like ours form, but it’s become apparent that planets don’t stay in the same orbit forever — they might migrate in or out depending on conditions and interactions with other objects. The simulations underpinning this study show that the four large gas giants may have rearranged as they gained mass. Jupiter moved inward, and the others moved outward. In about half the simulations, all the extra rocky planets were kicked out into interstellar space, but in the other half, one of them remained in the Kuiper Belt region.

The existence of this extra planet doesn’t preclude the existence of Planet Nine and vice versa. We won’t know which (if either) of them exist until someone can find them out there. The upcoming Vera Rubin Observatory might be able to see these objects when it begins surveying the sky in 2023. The ESA’s Gaia star mapping satellite might also see evidence of extra planets, but only if it distorts the light from distant stars as Gaia happens to be watching.

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