An eye for an i #278 Children

When Neptune became the farthest...

An optical illusion Image shows the top view of the orbit of Pluto, along with that of Neptune, Uranus, Saturn and Jupiter. The apparent crossing-points of Neptune and Pluto in two dimensions are optical illusions, created by the fact that the two orbits are steeply inclined when compared to each other.  

If you were to ask your parents and grandparents, or even me for that matter, which planet in the solar system is farthest according to the textbooks we used at school, we wouldn’t hesitate to say Pluto. For from the time Pluto was discovered in 1930, till it was demoted to the status of a dwarf planet in 2006, Pluto was considered the farthest planet in our solar system. Naturally then, our textbooks reflected that.

But in 2006, as you might be aware, the International Astronomical Union (IAU) changed the definition of a planet. With that, Pluto lost its status as a planet and handed over the claim of being the planet farthest from the sun in our solar system to Neptune, the rightful owner.

Quirk in Pluto’s orbit

You might be tempted to say that for those 76 years, therefore, Pluto must have been the farthest planet. But a quirk in Pluto’s orbit, however, means that that isn’t true either.

On January 21, 1979, Pluto moved inside the orbit of Neptune, thereby making the latter the farthest planet from the sun. Pluto slipped beyond Neptune once again in February 1999, becoming the farthest planet. In fact, Pluto’s orbit crosses inside of Neptune’s orbit for 20 years out of every 248 years that it takes to go around the sun. But Pluto’s demotion implies that this tussle for being the farthest planet has ended, and it now solely belongs to Neptune.

In case you are wondering what is happening here, it is Pluto’s elliptical orbit that takes it closer to the sun than Neptune for 20 years during its revolution. This means that at its perihelion, or closest distance to the sun, Pluto is closer to the sun than Neptune.

Will Neptune, Pluto collide?

The next question that creeps into our head at this point is whether Neptune and Pluto will ever collide with each other as a result of this. The answer is no and there are two reasons for it.

Firstly, while planets deviate only slightly from their orbit in the vertical and radial directions, Pluto takes large excursions. In fact, Pluto’s orbit around the sun takes it 17 degrees above and below the plane in which the planets revolve. Effectively, this means that Neptune and Pluto can’t collide even when Pluto gets closer to the sun than Neptune.

To understand this better, get hold of a Frisbee and make your way to a circular area that is fenced. If you were to throw the Frisbee high up against a strong wind, there are chances that the Frisbee might cross the fence while still in air, before descending and making its way back into the circular area. The Frisbee goes outside the circle and comes back in, to be on both sides of the fence without ever hitting it. The behaviour of Pluto and Neptune is similar to that of the Frisbee and the fence.

Secondly, Neptune’s orbital period is 164.8 years. As a result, Neptune completes three orbits around the sun for every two that Pluto completes. This puts them in a gravitational resonance leading to Neptune and Pluto speeding up or slowing down as they approach each other. This alters their path, preventing them from coming closer than 18 AU – a whopping 2,600 million kilometres.

While Pluto might no longer be considered a planet, there is nothing stopping it from coming within Neptune’s orbit and being closer to the sun than Neptune for a period of 20 years. But none of us will be around when Pluto next repeats this process, for there is a waiting time of over 200 years remaining!

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Printable version | Feb 28, 2021 5:08:53 PM | https://www.thehindu.com/children/when-neptune-became-the-farthest/article22489536.ece

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