Learning the parts of the solar system used to be a simple thing — basically, all that really mattered was the sun and nine planets. The mnemonic My Very Educated Mother Just Served Us Nine Pizzas helped keep things straight, with each letter representing one of the planets (in order from the sun): Mercury, Venus, Earth, Mars…
But things got complicated 12 years ago, when Pluto was reclassified as a dwarf planet. Now there are two kinds of planets, and some scientists are still haggling over what the word “planet” even means.
According to the International Astronomical Union (IAU), which establishes official names and terms for things in space, a celestial object is considered a planet only if it meets three specific criteria. It must orbit the sun. It must be massive enough — and generate sufficient gravity — to pull itself into a spherical shape. And it must have “cleared the neighborhood” of its orbit, meaning that its gravity has swept away any other similar objects.
Dwarf planets pass the first two tests but flunk the third: They share their region of the solar system with other objects like them, so they lack the uniqueness of Earth or Mars.
Pluto was always an oddity. It’s much smaller than any other planet in the solar system — just 1,477 miles in diameter and a mere 1/500th the mass of Earth. It circles the sun in an oddly tilted, looping orbit. And starting in 1992, astronomers began to find additional objects orbiting the sun beyond Neptune, meaning Pluto isn’t one of a kind.
Things came to a head in 2003, when Caltech astronomer Michael Brown discovered Eris, an object almost identical in size to Pluto, in the same outer region of the solar system. “It was instantly obvious that there were not going to be nine planets anymore,” Brown says. “It was less obvious how many there were going to be.”
In 2006, an IAU committee coined the term “dwarf planet” and applied it to both Pluto and Eris. The distant, frigid zone beyond Neptune where they reside is now known as the Kuiper Belt, and the many smaller objects that surround them are called, simply, Kuiper Belt Objects.
No, they’re not. “Minor planet” is an outdated term once applied to any small object in the solar system that wasn’t a comet or moon. “Asteroid” generally refers to rocky objects that orbit the sun at or within the orbit of Jupiter, primarily in the asteroid belt between Jupiter and Mars.
Most asteroids are too small to pull themselves into a spherical shape, so they don’t qualify as dwarf planets. One notable exception is Ceres, whose 588-mile diameter makes it the largest asteroid by far. Many scientists regard Ceres as a fossil protoplanet, the last survivor of the swarm of flying rocks that came together to form the eight main planets 4.5 billion years ago.
Officially, there are five: Pluto, Eris, Ceres and two other Kuiper Belt Objects, Haumea (HOW-may-uh) and Makemake (MAH-kay MAH-kay). But there are many others awaiting classification, and there could be still more out there that haven't been discovered.
Astronomers keep finding sizable objects in the Kuiper Belt and beyond. Measuring these dim, extremely remote things to figure out if they’re big enough to be dwarf planets is difficult and time consuming.
Also, the IAU doesn’t seem to be in a hurry to certify a new batch of dwarf planets, which it hasn’t done since Pluto’s reclassification in 2006. While we wait, Brown keeps a running tally of dwarf planet candidates on his website. It includes five “nearly certain” dwarf planets, and a staggering 661 “possible” dwarf planets.
In addition to these, Brown and other astronomers suspect that something much bigger may be lurking on the fringe of the solar system. But if this hypothetical body, known as Planet Nine, exists, it isn’t a dwarf planet. “It would have something like five to seven times the mass of the Earth, so it’ll clearly be a planet,” Brown says.
During the IAU’s 2018 meeting, astronomer Eric Mamajek of NASA’s Jet Propulsion Laboratory shared an expanded definition that would apply not just to objects in our solar system but to those orbiting other stars. He calls it “a working definition.”
One big obstacle to making Pluto a planet again is figuring out a way to deal with the profusion of planet-like objects out there. Many scientists ridicule the idea of hundreds of planets in the solar system — not to mention the dubious logic of lumping Texas-sized iceballs in the same category with hulking, 88,000-mile-wide Jupiter.
But don’t weep for Pluto. Alan Stern, who led the New Horizons mission to the former ninth planet, points out that dwarf planets are planets, too. And Pluto is a fascinating little world, no matter what you call it.
And for those struggling to remember the updated arrangement of eight planets in the solar system, Indiana University astronomer Phyllis Lugger offers a tweaked mnemonic: My Very Educated Mother Just Served Us Nachos.