New Horizons' mission to Pluto challenges our views about the solar system

Even a few months ago, the astronomical object Pluto was a small piece of rock of frozen gases that seems exceptionally uninteresting. It orbited the Sun in a highly elliptical orbit going way outside the plane of rotation of all the planets. It was so small that a few years ago, it was even demoted from the list of planets and called a dwarf planets. We had seen several of these and were not really expecting much. Images from Hubble telescope and some occultation studies had told us that as many as 5 objects along with Pluto went around each other.

But NASA did something almost 10 years ago that everyone else thought was not a great idea. It sent a mission to Pluto. The argument was that we really did not know anything about Pluto and currently Pluto had just passed its point of closest approach to the Sun in 1989. So if we don’t send a mission now, we would not get similar opportunity for another 200 years when Pluto is so close to the Sun.

But going to Pluto was not easy. It is about 2400 km in size and about 4.7 billion kilometres away, moving at the speed of 4.7 km/s. Also, it would have been impossible to reach Pluto in a reasonable time, if we went directly to Pluto since the speed that we can give to our interplanetary missions is just not enough. It had to be speeded up by another planet.

So New Horizons used the gravity of Jupiter to accelerate it. It went to Jupiter whose gravity helped it to get a sharp left turn at significantly higher velocity to point it towards Pluto. Even then, at 16.26 km/s, New Horizons was the fastest spacecraft we had sent out. It gained an additional speed of 4 km/s by flying past Jupiter in a slingshot manner. By any standards, this is an extraordinary achievement of people who planned this trajectory and got it right. 

As a technological marvel, it is comparable to Europe’s Rosetta Mission that landed on a comet September last year. Rosetta (after having flown past Mars, and two asteroids), trailed comet 67P and eventually not only orbited that comet but also landed a small probe on the comet ten years after the mission was launched. New Horizons that visited Pluto was launched about nine and a half years ago but went much farther from the Sun. New Horizons is not the farthest a manmade object has gone. Pioneer and Voyager missions sent out in 1970’s have left the solar system.

When New Horizons was sent to Pluto, our text books called it a planet. Since then, we have seen several objects of comparable size and in fact Pluto itself is a member of a large family of objects called Kuiper Belt objects consisting of several hundreds of thousands of objects of varying sizes. Some of the long period comets in the solar system come from this region. So while New Horizons was approaching Pluto, scientists on Earth were busy demoting it to a dwarf planet. So it seemed that the scientists were not expecting much to be found by New Horizons. Scientists had a firm belief of Pluto being a ‘hum ho’ planet and New Horizons, while being a technological marvel, was not going to provide nothing exciting. They could not have been more wrong.

So what did New Horizons do? First and foremost, it once again humbled the scientists. There were so many things that we saw that we were not expecting that we were caught by surprise. We knew for example that Pluto has 5 moons and goes around the sun in an elliptical orbit. But here are some of the things we were not expecting.

1) With a diameter of 2370 km, we now know for sure that amongst all the Kuiper belt objects, Pluto is probably the largest but it is not the heaviest. The heaviest is Eris which has a diameter of 2326 km which is almost 30% heavier than Pluto.

2) Pluto’s Moon Charon which is quite close to Pluto is much smaller than Pluto but is so close to it that the two seem to exchange gases. We have found potential traces of gases on Charon that would normally evaporate away in Charton’s low gravity but are present on Pluto. So they must be being transported from Pluto.

3) There are very few meteor craters on Pluto. If Pluto was born with the solar system some 4.5 billion years ago and is unchanged since then, we would expect quite a few meteors impact craters. So where are they? There are two explanations, either Pluto has been cleaning itself up over time, or that Pluto itself was born only recently and that the entire Pluto and its 5 moon system is of recent origin.

4) Pluto with is surface temperature that varies between -218 C (55K) and -240 C (33 K) has a thin nitrogen atmosphere which probably freezes over when Pluto is farther away from the Sun.

5) It has mountains probably as tall as 3.5 km tall on its surface.

6) It also has huge flat plains that cover a large fraction of its surface.

But by far the biggest mystery is that Pluto’s surface is not dead and that even though it has some volatile material like nitrogen and methane, the amount of material is small. This has severe consequences. 

Let us first begin with the clean surface of Pluto. Why is it so smooth and where are all the impact craters? 

This can happen if either Pluto is very young or that it can wipe its surface clean. Both suggestions have interesting consequences. If Pluto is young, it means that Pluto and all its moons were probably a larger chunk of rock which broke up by collision with another object less than a hundred million years ago (compared to the age of the solar system which is four and a half thousand million years). This would imply that in spite of such a long time gap, the outermost parts of the solar system are not fully settled and large scale collisions do seem to be occurring. The obvious question then is, what kind of forces can keep solar system so unstable. Is there something unknown lurking in the outer reaches of the solar system that is disturbing it?

If Pluto is indeed old and is cleaning its surface, then there are additional problems. Since Pluto has quite tall mountains suggesting that it is geologically active since old mountains, like the meteor craters would have been wiped out. Earth is geologically active but that is due to immense amount of heat that it stored in its belly when it was born. Does Pluto also have a hot belly? This is not easy to explain since the hot belly of the earth is a result of its proximity to the Sun and its large size. Pluto is about 3.5% the size of earth (1185 km radius compared to Earth’s radius of 6378 km), it is also 40 times as far away from the Sun as the Earth is. So any internal heat would have been lost by now. There are only two ways which we can imagine for keeping Pluto warm, one is that it has a lot of radioactive material in its core or that the gravity of its moon is continuously causing a tide like effect on its surface, moving large chunks of methane ice from one place to another producing heat that keeps it going. Either case is problematic. Our present understanding of the formation of the solar system does not suggest that large quantities of radioactive material can be found so far from the centre of the solar system. If the moons of Pluto are keeping it hot, then such a system cannot last for long – for example, due to tidal effects, the Earth is slowing down in its rotation and the Moon is drifting away. Such effects would be far more dramatic on Pluto. So the Pluto system should be more unstable (and therefore young). 

Whichever way we look at it, Pluto has challenged several of our basic views on how solar system was formed, how it exists and where is it headed. If collisions that can form Pluto can occur, then our present confidence on the serenity of the Solar System will be clearly shaken. If Pluto has really been around from the birth of the Solar System, then clearly, it has properties we never thought it could possess. Our science is limited by what we think is possible, Nature has no such limitations.