Meet Antipholus and Antipholus, a Very Odd Kuiper Belt Couple

2001 QW322 is a highly split Kuiper Belt pair, orbiting eachother at a distance of 125,000 km

The Kuiper Belt is an eerie, mysterious and cold region of the Solar System. In it, there are billions of small pieces of rocks with lots of fancy names. As a general designation, all objects in the Kuiper belt are called “Kuiper-belt objects” (KBO’s for short). As the Kuiper belt is located in a region just beyond Neptune, they may also be known as trans-Neptunian objects (TNO’s). Inside the Kuiper belt, we have Pluto-like objects known as “Plutoids”, classical KBO’s called “Cubewanos” (the largest being the recently discovered Makemake) and a whole host of other objects such as icy objects soon to become the next generation of periodic comets.

We are only scraping the surface, finding only a small portion of KBOs. We know of a thousand, but astronomers believe there may be as many as 70,000 measuring over 100km in diameter, plus countless other smaller objects.

Therefore, it is not very surprising that some rather strange KBOs exist, and possibly the oddest one has just been observed. From the same team that discovered KBO 2008 KV42 — a piece of rock orbiting the wrong way in a one-way Solar System — a binary Kuiper belt object has been found with a huge orbit…

2001 QW322 is a binary KBO. This means there are two objects orbiting around a common point. Binary asteroids and KBOs aren’t uncommon, but 2001 QW322 is a rarity as the orbital distance is very big. To put this distance in perspective, if two baseballs were in deep space, gravitationally bound in a binary system, they would have an orbital distance of 200 km. The 2001 QW322 pair have a separation of 125,000 km.

The 2001 QW322 binary has also been given a special name: Antipholus. The Antipholus twins are characters from one of William Shakespeare’s earliest plays, The Comedy of Errors, based on the mistaken identities of two sets of twins. Antipholus (of Ephesus) and Antipholus (of Syracuse) is one set of twins where many of the farcical mishaps are focused on. Therefore, 2001 QW322 is also known as “Antipholus and Antipholus,” as they are very hard to distinguish apart. In fact, astronomers have to be very careful to know which one is which!

The twin KBO was discovered in August 2001 with the Canada-France-Hawai‘i Telescope and it has since been tracked by the Canada France Ecliptic Plane Survey (CFEPS) using the VLT and Gemini observatories. As the orbital distance is so big, the Antipholus system has a long orbital period of 25-30 years, so it has taken seven years to understand its orbital characteristics better. This research has led the CFEPS team to reveal that 2001 QW322 has a remarkably circular orbital eccentricity of less than 0.4.

The Antipholus twins are not small chunks of rock either. According to the CFEPS team, as they know the brightness of both the objects (magnitude of 23.7) and the distance of 2001 QW322 from the Earth is 43.4AU, both objects have a radius of 54km (they are therefore two of those 100km+ KBO’s mentioned earlier). The brightness comparison between the objects differs by only 1–5%, suggesting they are of very similar size. The observations also have a tinge of blue in the brightness data, meaning they have icy surfaces.

So how did this unique KBO binary form? The formation of 2001 QW322 requires the density of the Kuiper belt to be much higher than it is now; it is therefore theorized that this binary formed between 300 million to 1 billion years ago. However, it is unclear whether the two objects have been orbiting in this circular configuration for all this time, or it is an evolving system, where it used to be a more tightly orbiting binary.

Either way, this is an exciting observation, proving that several international observatories can be used to great effect when studying the same KBO. I wonder what other strange objects are hiding inside the mysterious Kuiper belt…?

For more:
Gemini press release »
CFEPF press release »
Publication abstract (Science) »