Planet 9 Might Be an Immigrant

The question is: From where?

Artist's concept of Planet 9.

In a new paper, Caltech researchers Michael Brown and Konstantin Batygin put some constraints on the possible existence of Planet 9, which they believe is a Super-Earth planet orbiting in the far reaches of the outer solar system. Planet 9, if it is confirmed to exist, would nicely explain the unconventional motions of distant objects in the Kuiper Belt and beyond, and would address the long-standing question of why the planets of our solar system orbit in a plane tilted six degrees from the Sun’s equatorial plane.

Brown and Batygin used extensive computer simulations to constrain the mass and orbit of Planet 9, and found that only a narrow range of orbits can explain their observations. They now believe the planet has a mass somewhere between 5 and 20 Earth masses, and that it orbits between 150 and 350 AU from the Sun (1 AU is the average distance between the Sun and Earth). They also conclude that the planet is likely to be inclined to the plane of Earth’s orbit by about 30 degrees. What’s more, they think it should be detectable, especially when the Sun, Earth, and Planet Nine are lined up with each other.

It is intriguing that the the weird motions of such distant objects as Eris and Sedna are better explained with Planet 9 than without it.  But there’s a problem: According to current thinking about solar system formation, the outer Solar System would not have enough mass for a Super-Earth planet to form. So, assuming Planet 9 exists (and the case is getting stronger that it does), where did it come from?

There are two main possibilities. One is that it formed in the inner reaches of our solar system and migrated outward to its present position. This would nicely explain why our solar system has no Super-Earth-type planet, when most other solar systems seem to have one or more. It would also open up the possibility that Planet 9 may be at least partially rocky, rather than being a Neptune-type gas giant. Perhaps it was even habitable before it migrated to the outer solar system, in which case evidence from that early time may be frozen in place.

The other possibility is that it was caught by our Sun from elsewhere—a rogue planet that originated in some other solar system. This seems somewhat unlikely, as it would be difficult for our Sun to catch such a “heavyweight” so far out. But it’s still a possibility.

So, if and when Planet 9 is actually discovered and confirmed, we’ll still have plenty of questions.

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