A Giant Planet Ejected From Our Solar System

Based on studying the lunar craters and the bodies in the Kuiper belt beyond Neptune, scientists have deduced that something big happened in our solar system when it was about 600 million years old, throwing off the orbits of the giant planets and sending smaller bodies flying every which way, including to impact the moon. Now one astronomer showed that the best model for how this could occur involves having a fifth giant planet that was ejected from our solar system.

This means that the early sky from an Earth viewpoint 4 billion years ago would’ve been rather different – having another planet in the sky, having the gas giants in different orbits, and having a much less dinged up moon, at least. If you ever time travel to the dawn of life on Earth, remember not to be too startled by this. 

You can read the full deal on this at Wired or see the press release from the Southwest Research Institute. Wired has a related article from May about the discovery of “orphan planets,” planets wandering the galaxy without a star to orbit. Maybe we can see our orphaned planet out there? 

Our neighbours in the solar system. The interplanetary distances are definitely not to scale, and not depicted are the other 334 moons in the solar system (168 around the planets). Image courtesy of jasonirwin.ca.

While we’re here, let’s see what Wikipedia can teach us about the Kuiper belt. You’ve probably heard of the asteroid belt, which is a region between Mars and Jupiter occupied by, naturally, a lot of asteroids. It’s the division between the four inner and four outer planets, or the terrestrial planets and the gas giants. About a third of the mass of the asteroid belt is in Ceres, the solar system’s smallest dwarf planet and largest asteroid, which is about 950 km in diameter. A space probe from NASA was launched in 2007 and will investigate Ceres in 2015.

Meanwhile, the Kuiper belt is past Neptune, the farthest planet in the solar system, and is about 20 times as wide as the asteroid belt. Its existence was first hypothesized in 1930 shortly after the discovery of Pluto, by an astronomer who thought Pluto looked awfully lonely. Starting in 1992, he was proven right as asteroids in the Kuiper belt were discovered. Now there are four bodies labeled as dwarf planets in the Kuiper belt – Pluto, Eris, Makemake, and Haumea, although more may be added in the future.

The more scientists learned about the Kuiper belt – particularly the discovery of Eris, which is larger than Pluto – the harder it was to maintain that Pluto was, in fact, a planet comparable to the others, leading to its downgrading in 2006 and the creation of the “dwarf planet” classification. Although this upset quite a few people, it’s not the first time this has happened – asteroids used to be classified as planets before we discovered how many of them there were, in the early 1800s. The asteroids that were once considered planets are Ceres, Pallas, Juno and Vesta, all in the asteroid belt. 

I guess the lesson in all this is that there’s a lot going on in the solar system, and we have a lot to learn about our neighbourhood, so don’t get too emotionally attached to your solar system model. If you thought downgrading Pluto from a planet to a dwarf planet was sad, imagine how people in the 1500s felt about downgrading Earth from the center of the universe to a planet. Oh the Facebook groups they would have made…


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