Is there a ‘Planet X’?

Forget about X-ray vision and gamma ray defence beams- Planet X isn’t a superhero home world but a hypothetical 10th planet in our solar system.

“I think this question tends to be intriguing because of X – the unknown,” says astronomer John Percy. “But right now, we just don’t know if there is another planet lurking on the edges of the solar system.”

Percy says there are thousands of small objects orbiting the Sun beyond Neptune and Pluto. Called Kuiper Belt Objects, astronomers do not consider these to be planets because of their relatively small size – the largest, Quaoar, is half the size of Pluto. They are icy and disintegrate if diverted by another cosmic body into an orbit closer to the Sun. If this occurs, he adds, they appear to us as comets.

If Kuiper Belt Objects aren’t planets, then what is? According to Percy, a planet is a large object that orbits the Sun but, unlike the Sun, does not emit any significant energy. “If the orbiting object is small, it is classed as an asteroid (or minor planet) if it is rocky, or as a Kuiper Belt Object or comet nucleus if it is icy.”

There are nine known planets in our solar system. “Most people remember them in order from the sun,” says Percy. He suggests the mnemonic device “My Very Educated Mother Just Showed Us Nine Planets” as a useful way to remember the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

According to Percy, the two most distant planets in our solar system – Neptune and Pluto – were discovered because scientists noticed differences in the motion of nearby planets. “In the case of Neptune, it was discovered that the motion of Uranus seemed to be influenced by some unseen massive object. Two astronomers independently calculated where Neptune would likely be, and it was found shortly thereafter.”

As for Pluto, astronomers thought they detected similar irregularities in Neptune’s motion, initially believed due to the influence of a ninth planet. Although this hypothesis was eventually found to be incorrect, the search for a ninth planet had begun, resulting in the discovery of Pluto in 1930. Pluto, Percy adds, is so small – even smaller than our Moon – that it would not likely be considered a planet if it were discovered today.

While his academic research revolves around the nature and evolution of stars, Percy is a firm believer in bringing the excitement of science to the public. “Stars and planets and the beginning of our universe are topics that fascinate just about everyone. People love looking up in the sky and seeing light that has been travelling toward us for thousands of years.”

Percy holds a grant from the NSERC PromoScience Program to promote public awareness of science as well as a grant from the Ontario government to promote young peoples’ interest in science. “Now that astronomy is a required part of Ontario’s elementary and secondary school science curriculum, I also do work with the Faculty of Education, the Ministry of Education and teachers and the Science Teachers’ Association of Ontario to encourage and assist teachers.”

Schools are not the only organizations to profit from Percy’s dedication and expertise. He and his students in his Science Education (SCI398Y) course presented a Science Day for the Girl Guides in the Peel Region in March and 140 girls completed their science badge requirements.