No: Pluto hasn’t gone anywhere. It’s still visible

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 August 2020

Mike Njenga asks a straightforward question “Why isn’t Pluto visible anymore?” The straightforward answer is: Pluto is still visible from Earth! But why did Njenga ask this question? I suspect two possibilities.

Firstly, Njenga might be referring to the fact that Pluto is no longer considered a planet. This change of classification was adopted in August 2006 when astronomers defined a planet as an object that (a) orbits the sun, (b) is massive enough for its own gravity has pulled it into a round shape, and (c) has cleared the neighbourhood of its orbit. Pluto does not satisfy the last requirement.

Furthermore, even the description of Pluto as “the ninth planet” was seriously misleading because, at certain times, it comes nearer to the sun than Neptune, making it the eighth. This last happened in the period from 1979 to 1999.

The second reason why Njenga might have asked his question is that, on two occasions this year, Pluto has “hidden” behind Jupiter and so became “invisible” from Earth, albeit for short durations. These “conjunctions” happen every 12 to 13 years.

The occur when Earth, Jupiter and Pluto line up. Thus, observers from Earth will not be able to see Pluto since Jupiter (which is nearer) is blocking their view. The first conjunction of 2020 happened on 5th April, the second one on 30th June and the third one is expected on 12th November. Apart from those three instances, Pluto remains visible from Earth.

The reason for this apparent “dance” is that we are viewing two objects moving at different speeds while sitting on a moving body! Indeed, the word planet is derived from the Greek word planētēs which means “wanderer”.

Ancient Greek astronomers observed that, while all the stars moved across the sky at the same speed and in the same direction, a few of them (the wanderers) didn’t: sometimes faster or slower and other times in the opposite direction!

These wanderers are the planets. So, when Pluto was discovered in 1930, it was immediately called a planet, because it was also a wanderer. But astronomers changed this after clearly defining what a planet is.