Plutoid on the Makemake

The International Astronomical Union (IAU) has named its first plutoid. The Kuiper belt object formerly known as 2005 FY9 has been rechristened “Makemake” and classified as both a dwarf planet and plutoid. Makemake is fifty times further away from the Sun than we are and its orbital period is 310 Earth years. It has an apparent magnitude of about 16.7, which makes it bright enough to be visible using high-end amateur telescopes. The plutoid is named for a Polynesian God and is pronounced “mahki-mahki”.

The IAU announced the new category of plutoid last month at a meeting of its executive committee in Oslo. The category covers what it calls “transneptunian dwarf planets similar to Pluto”. Plutoids orbit the Sun at a distance generally greater than that of Neptune and have a minimum defined magnitude of brightness. Makemake now joins the two other two known and named plutoids Eris and Pluto itself. The IAU fully expects that more plutoids will be named as science progresses and new discoveries are made.

The plutoid Makemake was discovered in Easter 2005 by astronomers at California’s Palomar Observatory. According to Mike Brown, who led the Caltech team that found the object, they nicknamed it “Easterbunny”. The IAU preferred to give it the interim name of 2005 FY9 but after six months of lobbying they accepted a proposal from Brown's team to rename it to Makemake. Brown says the planet is two thirds the size of Pluto and is the brightest object in the Kuiper Belt after Pluto itself. He says the surface is “covered with large amounts of almost pure methane ice, which is scientifically fascinating, but really not easily relatable to terrestrial mythology.”

Brown is also the discoverer of Eris which ultimately led to Pluto’s demotion from the major planets. The existence of Eris drastically heated up the debate over how to define a planet with over a dozen other candidates vying for acceptance. In 2006, it seemed as if the IAU was going down the path of greatly expanding the list of solar system planets but instead decided there were would only be eight. Pluto and Eris (then called 2003 UB313) joined a new category called dwarf planets which also included Ceres which lies in the asteroid belt between Mars and Jupiter. However because of its location, Ceres can not be considered a plutoid.

In keeping with the original “Easterbunny” nickname for the newest plutoid, Makemake is named for the supreme god of Rapa Nui (Chile's Easter Island). Makemake was the creator of the first humans and the patron of the Tangata bird cult. He was worshipped in the form of sea birds, which were his incarnation. His material symbol, a man with a bird's head, can be found carved in petroglyphs on the island. With his divine power he makes the plants and animals grow. Some believe that the huge statues on the island are connected to his cult. Easter Island was first visited by Europeans on Easter Sunday 1722, exactly 283 years before the plutoid was discovered.

the Solar System gets busier

A long-standing fact beloved of school-children across the world is about to change. Ever since American astronomer Clyde Tombaugh discovered Pluto in 1930, it has been commonly understood that the Solar System contains 9 planets. In order of distance from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (although Pluto occasionally goes nearer to the Sun than Neptune due to its eccentric orbit). However, that list is about to grow. On August 16, the International Astronomical Union (IAU) announced that it is planning to add three new members to the exclusive club of large celestial objects orbiting our Sun.

The additions come after the organisation concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the new definition is approved by the astronomers gathered at the triennial IAU General Assembly in Prague (14-25 August 2006), the Solar System will include 12 planets, with more to come. These will be the 8 closest planets to the Sun, the largest asteroid (Ceres) and three remote ‘plutons’. This new category contains Pluto itself, its erstwhile moon and now double planet Charon and the provisionally sexy-named 2003 UB313. Plutons are distinguished from classical planets in that they reside in highly tilted orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). The draft "Planet Definition" Resolution will be discussed and refined during the General Assembly and then it will be presented for voting on 24 August.

The word "planet" comes from the Greek word for "wanderer”. There is no formal definition for the term but it is generally considered to be a relatively large mass of accreted matter in orbit around a star. Astronomers also drew a distinction between planets, and large asteroids (sometimes confusing called minor planets). With a diameter of almost a thousand kilometres, Ceres, discovered by an Italian monk in 1801, is by far the largest of the asteroids. For the next fifty years it was classified as a planet but once its true size was known, it was reclassified as an asteroid. Ceres has remained an asteroid for the last 150 years.

But the meaning of the word planet has come under increasing scrutiny due to recent discoveries. Pluto was initially added to the list of 8 planets because it was believed to be as big as Earth. Later its diameter was measured to be just 18% of Earth’s. Pluto is smaller than the Moon which is 25% of Earth’s diameter. From the 1990s onwards, astronomers became aware of a vast population of small bodies orbiting the sun beyond Neptune. But the trouble really started with the discovery of 2003 UB313. As the name would suggest it was discovered in 2003 by a team at Mt Palomar observatory in San Diego. Dubbed “Xena” by its discoverers, it is currently classified as a “Scattered Disk Object” but because it is slightly larger than Pluto and even has its own moon, many have argued that it too should be given planet status. Xena is three times further away from the Sun as Pluto.

The discovery drastically heated up the debate over how to define a planet. Some astronomers claim Pluto is just an overgrown Kuiper-belt object and there are really only eight planets. However, if the new definition is accepted, then the list will rise quickly from 12. Currently a dozen other "candidate planets" are listed on IAU's "watchlist" which keeps changing as new objects are found and the physics of the existing candidates becomes better known. The number could rapidly accelerate to the thousands as objects in the Kuiper belt are identified by prospective planet-finders. This should make an interesting dilemma for those attempting to stay within the current naming convention: all planets must be named after Roman gods.

The days of an easily memorised and numbered Solar System are itself numbered.