Xena Reignites the Debate About the Number of Planets in Our Solar System

Astronomers are reviving the debate about whether our solar system contains a 10th planet. A team of US scientists have discovered a Pluto-sized world billions of miles beyond Neptune and named it Xena after a fictional television character, reigniting a debate about the definition of a planet.

Scientists Michael Brown and Chad Trujillo made the discovery in 2003 while observing the Kuiper belt using telescopes at Palomar Observatory and Gemini Observatory. They spotted a new object in the outer region of our solar system and initially classified it as a dwarf planet. Brown and his colleagues were big fans of the TV show Xena, Warrior Princess, so they decided to name the tiny world after the hero. The discovery reignited the debate about whether there are nine fully fledged planets in our solar system, or if Xena should be considered a jumped-up asteroid.

Although the astronomers have not yet determined what Xena is made of, it is clearly bigger than Pluto and has enough mass to hold a moon. The team has a new look at the Xena with the Hubble Space Telescope and was able to measure its diameter and determine that it is about 1,490 miles across, 30 percent larger than Pluto’s. Xena also appears to be gray, which suggests that it may have a surface of frozen methane gas.

Xena’s elongated orbit also suggests that it has a long period of rotation, indicating that it is icy and not solid. Its distance from the Sun means that it is very cold and one year there would take five Earth years, so it takes an extremely long time to complete a trip around the solar system.

In the meantime, Brown and his colleagues have discovered that Xena has a small companion, called Gabrielle, which is believed to be about 60 times smaller than Xena and revolves very closely around the dwarf planet. Its discovery could help astronomers to determine the size of Xena’s core, which is thought to be rocky rather than icey.

Several of the Kuiper belt objects that have been found have moons, but the team had to keep the existence of Xena’s as well as its naming a secret until they had more precise measurements from other telescopes and a decision about how to classify it. The technical information was being kept on telescope databases that were only accessible to astronomers with the right credentials, but this security was breached and a media frenzy forced the astronomers to make the announcement early.

The team’s official name for Xena is now Eris, after the Greek goddess of lawlessness. The new name will take some getting used to, but it is a good match for the TV series and it pays tribute to actress Lucy Lawless, who played Xena on the popular TV show. Astronomers have also found that Xena/Eris has a large number of satellites, but it is too early to tell if any of them are in elliptical orbits like those of the major planets.