How was the Kuiper belt discovered

The Kuiper Belt

The image of the outer solar system has fundamentally changed several times over the past hundred years, most recently since the 1990s. In 2006, Pluto was classified as a dwarf planet and has not been considered the ninth planet in our solar system since then. This definition was preceded by new discoveries in the region beyond the planet Neptune.

The most important celestial bodies in our solar system are often listed according to increasing distance from the sun. The rock planets Mercury, Venus, Earth and Mars are followed by the asteroid belt, and finally the four gas planets Jupiter, Saturn, Uranus and Neptune. For a long time Pluto was considered the ninth planet, but today it is only seen as part of a whole region called the Kuiper Belt.

In order to describe the distances from planets or regions in the solar system, the so-called "astronomical units" are usually used. The mean distance between the earth and the sun - around 150 million kilometers - corresponds to exactly one astronomical unit. Jupiter orbits the sun five to five and a half astronomical units away, and the outermost planet, Neptune, is about 30 astronomical units from the sun.

Hermann Böhnhardt

Hermann Böhnhardt from the Max Planck Institute for Solar System Research in Göttingen describes the position of the Kuiper Belt in the solar system:

“The Kuiper Belt is part of the planetary system, it is the outer edge of the area in which the planets reside. It is mainly located between about 39 times the distance to the earth and 48 times the distance to the earth. There are parts that are included in the Kuiper Belt that are a little closer than these 39 astronomical units, and there is also a good part that goes further out than the 48. "

The story of the discovery of Pluto, the first known Kuiper Belt object, began in the 19th century. The discovery of Neptune, which was made in 1846, had been predicted: due to disturbances in the orbit of Uranus, discovered almost 65 years earlier, the position of Neptune had been successfully calculated. When Neptune's orbit appeared to be abnormal, astronomers looked for another large planet that could explain them.

Thus, when Pluto was discovered in 1930, it was mistaken for such another large planet. In the following decades, however, it turned out to be much smaller than initially thought. A targeted search for other bodies in Pluto's environment began a good 60 years later.

Small bodies of the outer solar system

“To date, the series of discoveries has found almost 1,500 such bodies. Incidentally, a good half of them are not precisely determined in terms of railway dynamics, so that they cannot be found with certainty after a year. But this is something that I believe will be eliminated in the next 5 to 10 years by search programs with 6-meter telescopes, because these objects will be found back. This developed slowly from 1992 - in the beginning actually with good search programs on existing 2 to 4 meter telescopes that have small fields of view and therefore had to search for a long time. There are now wide-field cameras that work more efficiently and, of course, find more with a conscious search. "

Difficult discoveries despite many objects

One difficulty with recognizing objects in the outer solar system is that they take a particularly long time to orbit the sun. For an observer on earth, they move very slowly across the sky and are sometimes indistinguishable from stars in the background.

“Pluto itself, that roughly defines the edge of the planetary system with an orbital period of a little more than 200 years, 230 or 240 years, in the order of magnitude - that is the typical period of rotation. That is, Pluto, discovered in 1930, has not yet made a full orbit since its discovery - that will take well over 100 years. And of course this orbital time increases the further the objects are outside of Pluto orbit. "

If the observation time is not sufficient to determine its orbit precisely, it can therefore be difficult to find an object later in the sky. The exact measurement of the orbits in the Kuiper belt is therefore an upcoming task for astronomy.

Eris and Dysnomia

In addition, these objects can best be found with particularly large telescopes, as they are very faint. The amount of light that an object reflects from the sun towards us depends on the fourth power of its distance from us. Some celestial bodies in Pluto's vicinity are a few hundred kilometers in size - similar to the largest asteroids in the inner solar system - but due to their distance, reflected sunlight only reaches us many thousands of times weaker than from the asteroid belt.

Because the amount of reflected sunlight also depends on the size of a body, so far only the largest objects in the Kuiper Belt have probably been observed - those with a radius of 100 kilometers or more. How many bodies there are in total in the Kuiper Belt can therefore only be estimated:

“If you calculate now: 50 kilometers radius, how many objects could there be? This can be extrapolated from the size distribution known from larger objects. If you assume that it continues like this, then you come to the order of 300,000 that exist up to 50 kilometers. Up to a radius of 20 kilometers you are at 3 million, in the order of magnitude - well, it increases very quickly, whereby the radius naturally decreases significantly. "

Pluto: planet or dwarf planet?

Around 2005 several objects were discovered in the Kuiper Belt that were comparable in size to Pluto. In addition to the objects Makemake and Haumea, which are over 1,000 kilometers in diameter, Eris in particular caught the attention of astronomers. With a diameter of over 2,300 kilometers, Eris is about the same size and even heavier than Pluto. In 2006, a vote by the International Astronomical Union finally led to a definition of planets according to which neither Pluto nor Eris belonged to this category and the number of planets in the solar system was reduced to eight again.

The dwarf planet Pluto

Planet or not, when the New Horizons spacecraft was sent on its ten-year voyage to the Kuiper Belt in 2005, Pluto was its first target, and it became the first Kuiper Belt body ever observed at close range. Researchers are currently further evaluating the data from the flyby from July 2015.

“I actually expect New Horizons to provide a first, more detailed picture of a dwarf planet in this vicinity of the Kuiper Belt. Pluto is not the only representative of a dwarf planet in the Kuiper Belt. Of course you have to take certain characteristics as individual, but I believe that will be the main result: the description of a dwarf planet in the outer planetary system. "

Some of the objects in the Kuiper Belt even have certain properties in common with the large planets - for example, that they have moons. In addition to four small companions, Pluto has the moon Charon, which is unusually large for its circumstances and with whom it circles around a common focus. About fifty other Kuiper belt objects are also known to have moons, such as Eris and his companion Dysnomia.

“And atmosphere - yes, we only have one example so far, namely Pluto itself has an atmosphere, probably of a temporary nature. That is, an atmosphere that occurs now that it is relatively close to the sun, and possibly when it is then in the point of its orbit that is far from the sun, will freeze out again, that is, it will precipitate as ice on the surface. You have to be patient, you can see that; in the course of a lifetime that could turn out to be. "

Water and ice

A property that also reveals something about the development of the objects in the Kuiper Belt is their density. Previous observations have shown that the smaller bodies tend to have a mass-to-volume ratio of around one gram per cubic centimeter - this roughly corresponds to the density of liquid water. Larger bodies, on the other hand, show densities up to twice as high.

“This is probably because the larger bodies were able to create a kind of life of their own inside that the smaller bodies could not achieve, and thus became more compact. The life of its own, of which Pluto is spoken of, for example, is that it is probably not evenly mixed inside, but, like the earth, has a core that contains heavier material and then becomes lighter and lighter towards the surface. That is a guess right now, based on certain evidence or evidence we have. "

The five known moons of Pluto

The material from which the objects in the Kuiper Belt are made is often summarized as "ice" - meaning substances that would be liquid or even volatile at higher temperatures, but are in solid form in the cold outer solar system. For example, nitrogen ice is very common on the surface of Pluto; there is also carbon dioxide and carbon monoxide ice. There are also small amounts of substances at the transition between organic and inorganic chemistry, such as methane or methanol.

Especially on Pluto as in the rest of the Kuiper Belt, ordinary water ice is very common - but no surprise for Hermann Böhnhardt:

“One should never forget: The most common molecule in space as a whole - except for H.2 - is water. So it shouldn't come as a surprise that water is mentioned so often. It's just there everywhere. "

It is possible that the Kuiper Belt can not only be examined on site in the outer solar system, but also much closer to Earth: There are indications that the gravity of the large planets could transport objects from the Kuiper Belt through the solar system.

"Scattered by Neptune, passed on to Uranus, who can then scatter again - maybe outwards or inwards - and so it goes, so to speak, in a chain through the large planets, to Jupiter, who then makes the final selection and you either as a comet recruited or thrown into the sun or catapulted very far out. "

If this theory applies, the investigation of near-Earth comets - such as the current Rosetta Philae mission to comet 67P / Churyumov-Gerasimenko - could also help to understand the Kuiper Belt.