Puzzling Pluto

Ken Tapping, 2nd September, 2015

In the sky this week…

  • Saturn lies low in the southwest in the evening.
  • Venus and Mars lie low in the dawn glow.
  • If you have dark skies, you can explore the arch of the Milky Way, which passes almost overhead at this time of year. Get out the binoculars.
  • The Moon will reach Last Quarter on the 5th.

In the 19th Century, astronomers noticed something funny about the orbit of Uranus, the 7th planet out from the Sun. We live on the 3rd.  Uranus was not turning up at the predicted places in the sky at the right times. Something was perturbing its orbit. French astronomer Urbain Le Verrier calculated that these perturbations could be explained by the gravitational attraction on Uranus by an unknown planet, and worked out where in the sky to look for it. The new planet was discovered in 1846 by German astronomer Johann Galle, very close to the predicted position. It was named Neptune. Then, over following years astronomers reported that Neptune’s orbit was being perturbed by something, probably another unknown planet. They calculated where in the sky to look for it, and the search started. In 1930 American astronomer Clyde Tombaugh found it, again quite close to the predicted position. The new world was named Pluto. Then problems started to appear. First of all Pluto was far too small to cause the perturbations that led to its discovery. Secondly, the perturbations were found to be due to something else entirely. The process that led to Pluto’s discovery was utterly spurious and scientifically meaningless, yet the new world was found close to the predicted location. That was just the beginning; since then Pluto has provided us with one puzzle after another.

Over the years following Pluto’s discovery, astronomers made careful measurements of its position in order to determine its orbit. It soon became clear there were problems with that too.  All the other planets move around the Sun like runners on a racetrack. They all stay in their own lanes. None of their paths cross the orbits of other planets – except for Pluto’s. Part of its path around the Sun lies inside Neptune’s orbit, and part lies outside. A collision at some point in the future cannot be ruled out. One possible explanation for the unusual orbit is that Pluto was once a satellite of Neptune, which somehow escaped to orbit the Sun independently.

As telescopes improved, more objects that seemed Pluto-like turned up in the outer Solar System, and it was deemed that Pluto was just one of the largest and nearest members of a new class of body.  Since they are located in what became known as the Kuiper Belt, named after its discoverer, Pluto was recategorized as a “Kuiper Belt Object”. The argument as to whether Pluto is a planet or something else goes on. A recent vote among astronomers at the International Astronomy Union has not quelled the dispute.

The Kuiper Belt Objects are believed to be leftover construction material from the formation of the Solar System, which is why we want very much to examine one of them close up. This was one of the motivations for the New Horizons space mission. It was logical to add Pluto the list of destinations. The journey to Pluto took almost ten years; the space probe flew close by it and is now sending back its observations. This is a slow process because the space probe is now a very long way away, about 5,900 million kilometres.

It is now moving further out into the Kuiper Belt to observe some additional objects. Pluto is not a deep-frozen piece of ice and rock that has been unchanged for billions of years; it is geologically active. If the other Kuiper Belt Objects resemble Pluto, then we will know how to classify it and can use Pluto as a more or less typical example for study. On the other hand, if Pluto continues to be unique we will have a lot of new questions to answer. Actually, that is not a bad thing. Having a new discovery raise ten new questions is what makes science so exciting.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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