Is 2003 UB313 the Tenth Planet of Our Solar System?
Is the recently discovered Kuiper belt object (KBO) 2003 UB313 the tenth planet of the Solar System? If it is the first KBO known to be larger than Pluto, perhaps astronomers should proclaim it a planet.
Because of its relatively large distance from the Earth and its relatively small diameter, it's a challenge to measure the diameter of 2003 UB313. The most straightforward way to measure its diameter is to measure its brightness. The larger the brightness, the larger the hemispherical area reflecting sunlight to us, and so the larger the diameter of 2003 UB313. There is a well known mathematical relationship between the brightness and the diameter of a Solar System body.
Unfortunately, there is a complication. We need to know not only the brightness, but also the albedo of the body's surface. The albedo is a measure of how efficient the surface is at reflecting sunlight. It's a measure of the darkness of the body's surface. A surface as dark as charcoal has a very low albedo whereas a surface as bright as freshly fallen show has a very large albedo. By only measuring the brightness of 2003 UB313 and not knowing its albedo, we can't distinguish between a body with a large diameter and a dark surface and a body with a small diameter and a bright surface.
On the other hand, if we use our brightness measurement and assume the largest possible albedo for 2003 UB313 so that we get the smallest possible diameter for 2003 UB313 , and we get a diameter for 2003 UB313 that is still larger than Pluto, then we will know 2003 UB313 is larger than Pluto.
On the night of 2005 October 31, we used the Vatican Advanced Technology Telescope to measure the brightness of 2003 UB313. We measured V = 18.76 ± 0.02. Such a brightness means that for the largest possible albedo, 2003 UB313 has a diameter as large as Pluto, 2,300 km. If we assume a smaller and more likely albedo, similar to that of Pluto, we find 2003 UB313 has a diameter of 2,900 km. There's no doubt 2003 UB313 is larger than Pluto.
Relative sizes of the sun and planets M V E M Jupiter Saturn Uranus Neptune Pluto
So if Pluto is a planet, and 2003 UB313 is slightly bigger than Pluto, then 2003 UB313 must be a planet. Right? Not so fast.
Maybe we shouldn't call Pluto a planet. As the figure above shows, there is an architecture to our Solar System - four small, rocky planets on circular orbits close to the Sun (Mercury, Venus, Earth, and Mars) followed by four giant, hydrogen-rich planets on circular orbits farther from the Sun (Jupiter, Saturn, Uranus, and Neptune). Pluto and 2003 UB313 are the largest members of an ancient debris disk of icy objects orbiting the Sun at and beyond Neptune. Because of Neptune's disruptive gravitational influence, Pluto, 2003 UB313, and the numerous smaller icy objects did not accrete into the core of a giant planet. Pluto and 2003 UB313 are the largest known remaining building blocks of a giant planet that never formed at all.
In addition, if we classify Pluto and 2003 UB313 as planets, we create another problem. There are a number of KBOs with diameter smaller than Pluto. At what diameter do we say an object is large enough for classification as a planet? 1,000 km, 2000 km, or perhaps the diameter of Pluto at 2,300 km. Nature gives us a continuum of KBO sizes and picking a particular size for the transition between KBO and planet is arbitrary. It's like trying to define the size at which a pebble becomes a rock.
So Pluto is not a planet and neither is 2003 UB313. Right? Not so fast.
We've called Pluto a planet for 75 years. Perhaps only for historical reasons we should allow Pluto to keep its status as a planet. Pluto's definition as a planet is part of our culture. In addition, perhaps it's best not to promote 2003 UB313 too soon. Who knows what new discoveries may be lurking in the outer Solar System that will require us to rethink all we know so far. The final decision as to whether 2003 UB313 is a planet rests with the International Astronomical Union.