Have Astronomers Found Pluto’s Replacement?

In 1930, American astronomer Clyde Tombaugh discovered a large body in the outer solar system. After analyzing astronomical photographs of the night sky, Tombaugh and colleagues determined this body to be the ninth planet in the solar system. Named Pluto, this planet ranked as the smallest and most distant planet in the solar system for over 75 years. The 2006 discovery of several objects larger than Pluto compelled astronomers to reconsider the characteristics of planets more carefully, which eventually led to Pluto’s reclassification to dwarf planet. This reclassification left our solar system with eight planets. However, exciting recent developments suggest that another object in the solar system (even more distant than Pluto) fits all the requirements for planet designation. What would such a discovery mean?

On January 20, Caltech published a news article describing evidence that pointed to a large planet occupying the distant recesses of the solar system. According to the article, the hypothetical planet is 10 times more massive than Earth, orbits about 20 times farther from the Sun than does Neptune, and takes between 10,000 and 20,000 years to orbit the Sun.

While the news of a ninth planet set social media abuzz, it’s worth mentioning that astronomers have not actually detected a planet. Instead, by analyzing the orbital parameters of the most distant Kuiper Belt objects (KBOs), astronomers recognized that six of those objects had elliptical orbits that all shared a common inclination (angle from the plane of the solar system) and also pointed to the same direction in space. The chance coincidence of these two features is remote, so it was best explained by the existence of a large, remote planet they’re now referring to as Planet Nine. A distant planet with the features like Planet Nine would shepherd the KBOs in question into the shared region of space. The simulations explained the orbits of Sedna-like objects as well as other objects with orbits perpendicular to the plane of the solar system (found by previous observations).1

With the caveat that the hypothetical Planet Nine might not exist, let’s explore some of the apologetic implications. First, the mass of Planet Nine puts it in the super-Earth category, which means that it has a mass between 1 and 10 times that of Earth. Two of the first extrasolar planets found are super-Earths and many astronomers have discovered many since. This class may be one of the more common sizes of planets, yet none of the eight known planets within our own solar system fall into this class. If Planet Nine proves real, it would give our solar system a super-Earth member.

However, a planet that large would not form in the outer solar system so it must have formed closer and been ejected to its more distant orbit. Some scenarios speculate that the solar system started with five giant planets (Jupiter, Saturn, Uranus, Neptune, and one unidentified giant planet) packed into fairly tight orbits. As four of these planets migrated to their current locations, they ended up ejecting the fifth member. Based on observations of exoplanets, this type of migration usually disrupts the orbits of potentially habitable planets, but in our solar system it enhanced, rather than destroyed, Earth’s habitability.

The Caltech news article brings exciting evidence pointing to the possibility of a ninth planet. Yet actual observations of Planet Nine must happen before astronomers will acknowledge its existence. Depending on Planet Nine’s location in its orbit, that could occur quickly or take quite a while. Given our current knowledge of the solar system’s fine-tuning, we can expect that continued research in this effort will provide further evidence of the Creator’s work.


  1. Konstantin Batygin and Michael E. Brown, “Evidence for a Distant Giant Planet in the Solar System,” Astronomical Journal 151 (January 20, 2016), doi:10.3847/0004-6256/151/2/22.

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