Pluto, the Kuiper Belt, and the Outer Solar System

Young Astronomers Blog, Volume 29, Number 6.

Remember Pluto? Pluto was discovered in 1930 and for over seventy years was considered the ninth planet. Pluto is small, and from the beginning, really didn’t fit in with the rest of the planets. It was an oddball located way out in the Solar System. But, for many years, no one knew of anything else out there. So, Pluto was considered to be a planet.

It turned out that Pluto wasn’t completely alone. It has moons! In 1978, James Christy and Robert Harrington noticed a “bulge” on the side of Pluto. They soon discovered it was a large moon orbiting Pluto.

Moons, particularly large ones, need names, so it became time to name this moon. Charlene is Christy’s wife, and her nickname is “Char”. So, he added an -on to the end, like prot-on and neutr-on, and came up with Char-on. Now you can’t name a moon after your spouse, there are rules. So, he looked Charon up and discovered it was the ferryman from Greek mythology who took the souls of the dead to the underworld. Therefore, the name fit and was accepted. The problem was that it is pronounced “Khar-on” (like Care-on). Well, that wouldn’t work, so many people today pronounce it “Shar-on” (like Cher-on). Although, as expected, James Christy pronounces it “Char-on” (like Charlene).

Years later, in 2005, 2011, and 2012, four small additional moons were discovered. They were eventually named Nix, Hydra, Kerberos, and Styx. Therefore, rather than just Pluto, there is an entire “Pluto system” consisting of Pluto and its five moons.

The discovery of Charon didn’t disqualify Pluto from being a full-fledged planet. Then things started to get complicated.

For many years, astronomers, including Kenneth Edgeworth, Gerard Kuiper, and others hypothesized the existence of a belt of objects, possibly similar to the asteroid belt, out beyond the orbit of Neptune. Soon astronomers began to refer to this group of objects as the “Kuiper Belt” or sometimes the “Edgeworth-Kuiper Belt”. However, it remained just a theory until the early 1990s.

In 1992, David Jewitt and Jane Luu, noticed a small object moving way out in the solar system. They had discovered the first Kuiper Belt object other than Pluto and its moon. They wanted to name it Smiley, but the name was taken by an asteroid, so the object remained unnamed for many years and was referred to as 1992 QB₁ (Q-B-1). In early 2018, QB₁ received the official name Albion from the works of William Blake. QB₁ is an extremely small object, so no one was worried at the time about Pluto’s status as a planet.

As the turn of the 21^st century loomed, support for Pluto as a full-fledged planet began to wane. First, there was Brian Marsden, who for many years, was the Director of the International Astronomical Union’s (IAU) Minor Planet Center (MPC). The MPC keeps track of and names all the objects in the Solar System other than the official planets. Objects are initially given a provisional name such as QB₁. Once their orbits have been confirmed, they receive an official number using a scheme that began with 1 Ceres, 2 Pallas, 3 Juno, and so on many years ago. Eventually, some will receive an official name suggested by the discoverer. For example, QB₁ is officially minor planet 15760 Albion.

In February 1999, minor planet number 10000 would soon be assigned, so Marsden thought it would be nice if it were given to Pluto. He announced the suggestion to the world. The IAU demurred and immediately issued a press released reassuring the public that there were no plans to “demote” Pluto.

The next year, New York’s Hayden Planetarium opened its Rose Center for Earth and Space. In it, solar system objects were classified according to their characteristics and, as such, Pluto was relegated to merely a Kuiper Belt object. Everything was fine for a while. Then a reporter got involved and an article appeared in the New York Times alerting us to the change in Pluto’s status with the headline “Pluto’s Not a planet? Only in New York”.

Many people give Neil deGrasse Tyson, the Director of the Hayden Planetarium, the credit (or is it the blame?) for reclassifying Pluto. But maybe Astronomer Mike Brown had more to do with it. In fact, Dr. Brown wrote a book entitled How I killed Pluto and Why It Had It Coming.

Over the next few years Dr. Brown and his colleagues discovered several new Kuiper Belt objects. Things were started to look a bit bleak for Pluto as it became more and more just one of many Kuiper Belt objects, albeit still the largest one.

In June 2002, Chad Trujillo and Mike Brown found what they thought was an object larger than Pluto out around the Kuiper belt, although recent estimates have shown it to be smaller. It was initial given the designation 2002 LM₆₀. The object was later named Quaoar after the creation force in the mythology of the Tongva people, who predated the Europeans in the Los Angeles area. Quaoar has one moon, Weywot, who in Tongva mythology, was created by Quaoar.

The most controversial discovery was object 2003 EL₆₁. It was found by Mike Brown and his team in December 2004 from observations made in 2003. Because it was discovered in December, the object was nicknamed “Santa”. Before a formal announcement could be made, a team led by J. J. Ortiz of Spain sent a July 27, 2005 email to the International Astronomical Union’s Minor Planet Center announcing the discovery of the object.

The controversy over this discovery remains. The place of discovery is listed as the Sierra Nevada Observatory (Instituto de Astrofisica) in Spain, but neither team is given credit for the discovery. In September 2008, “Santa” received the official name Haumea, a deity in Hawaiian mythology, chosen by Brown’s team. Haumea has two moons, Hi’iaka and Namaka, named after the daughters of Haumea. Their initial nicknames were “Rudolph” and “Blitzen”.

In March 2005, Brown and his team discovered object 2005 FY₉ nicknamed ‘Easter Bunny”. In July 2008, it was officially named Makemake from the mythology of the Rapa Nui (Easter Island) people in the South Pacific. In 2016, a small moon was discovered around Makemake. It has yet to receive an official name and is referred to as MK2.

Other objects such as, Varuna (Robert McMillian in 2000), Ixion (Deep Ecliptic Survey in 2001), MS₄ (Trujillo and Brown in 2002), Salacia (Row, Brown, and Barkume in 2004), and Orcus (Brown, Trujillo, and Rabinowitz in 2004) were also found.

The biggest surprise, and the one that finally did Pluto in, was the discovery of 2003 UB₃₁₃. It was announced in July 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz. Shortly after the announcement, NASA sent out a press release proclaiming the discovery of the 10^th planet. Sky & Telescope followed soon later with a similar article.

The object needed a name. So, Brown and his team decided to temporarily refer to it as “Xena” after the TV warrior princess. However, Xena wasn’t acceptable as an official name. The name Eris, the goddess of strife and discord from Greek mythology, was eventually chosen. It proved to be rather fitting given the chaos its discovery caused for Pluto and the Solar System. Eris has one moon, Dysnomia, named after Eris’ daughter, the demon spirit of lawlessness. It was initially nicknamed “Gabrielle”.

With the discovery of Eris, the outer part of the Solar System was getting crowded. In addition, Eris is almost the same size as Pluto, and a few of the other objects are large enough to be spherical. So, if Pluto is a planet, what about these new objects? This takes us to the International Astronomical Union’s August 2006 general meeting in Prague and a fateful decision. That story will continue in a future article.

After the Prague meeting, object 2007 OR₁₀ was discovered by Meg Schwamb, Mike Brown, and David Rabinowitz. It was nicknamed “Snow White”. In 2016, Snow White was found to be slightly larger than Makemake and Haumea. So, it needed an official name. The public was asked to make the final choice and it was Gonggong, a Chinese water god. In early 2020, the name was approved by the IAU. Oh, and Gonggong’s moon is named Xiangliu, after a minister who served Gonggong.

For more specifics about each of these objects, see JPL’s Small-Body Database Browser. Also see Mike Brown’s list of Kuiper Belt objects and potential dwarf planets.

We now know that the Kuiper Belt is a “donut-shaped” area, ranging from 30 AU to 50 AU out beyond the orbit of Neptune, with icy objects left over from the formation of the Solar System. It is somewhat like the more “disk shaped” Asteroid Belt but contains more material including larger objects such as Eris and Pluto.

There are more Trans-Neptunian objects (TNO) out there. Several so called “Inner Oort Cloud” objects, such as Sedna and 2012 VP₁₁₃, have been identified over the past few years. However, we’ll leave these to another article.