Planet 9 probably isn’t from another star, according to researchers from the UK and Switzerland.
According to a paper on the Cornell University Library archive, while previous studies show how planet-sized objects can be captured during the early evolution and dissolution of stellar nurseries, it seems several constraints reduce the probability of interstellar planet-swapping to a near-zero value, in the case of Planet 9.
Not long ago, scientists like Alexander Mustill of Lund University observed unusual orbital characteristics of several objects in the Edgeworth-Kuiper Belt — a cloud of frozen objects beyond the orbit of Neptune — and hypothesized the presence of an interstellar interloper; essentially, a (former) exoplanet snatched up by the Sun’s gravitational forces soon after it was born, roughly 4.5 billion years ago.
Roughly 20 times the mass of Earth, this ninth planet’s effects on other distant bodies would place it in an eccentric elliptical orbit, up to 60 degrees off the solar orbital plane. If confirmed, Planet 9 is estimated between 150-350 AU away. That’s at least 150 times farther from the Sun than the Earth.