A whole new world on the edge of the solar system

From my point of view, there is no point in pulling one or more objects from the crowd and calling them by some special name, and not part of this crowd.
- Mike Brown, aka "Pluto Killer"

In our solar system in its inner part there are four rocky planets, and in the outer - four gas giants. But beyond Neptune, thousands of icy and stony worlds - including Pluto, the former ninth planet - make up the huge and wide ring known as the Kuiper belt .



His story began with several ice worlds, but then it turned out that there was a very dense movement in this place - since 1992, hundreds of worlds have been discovered there.


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And although some of these objects are as big and rounded spheres as Pluto, most of them simply do not have enough gravity to bring themselves into such a shape.

Of course, there are funny exceptions. For example, the dwarf planet Haumea , which has enough gravity to give itself a spherical shape, but it rotates so fast that it stretches into an ellipsoid!



One of the ways to explore another world, even occupying no more than one pixel in an image obtained from a camera, is to observe the change in its light curve.

Imagine our moon.



No matter how far you are from the moon, as long as you can see it somehow, you can measure the amount of light it reflects. And if the moon turns so that you see it from different angles from different angles, you may find that the observed amount of light changes over time. It changes with periodicity, depending on what part of the moon you are looking at.

What about Haumea, the uneven ellipsoid? Let's explore its light curve.



The curve shows that it has not only one side brighter than the other (left brighter than the right), but also that we see different angles of the planet (therefore there are bright peaks and deep dips on the curve).

It can also be concluded that the darker side is more red because more red light is emitted from it than white. The same team created a video based on this data, which demonstrates how this dwarf planet should look like.



Haumea is not the only uneven object. Not so far from our planet, you can find well-known asteroids, characterized by a large scatter in mass.

In 1989, we studied on the high-resolution radar one of the asteroids, 4769 Castalia , and our discoveries surprised us very much.



We found a new class of asteroids, known today as a tight binary system . Such asteroids consist of two small objects, the masses of which are not enough to give them a spherical shape, touching each other! Since then, many other similar objects have been found, including the largest of the Trojan asteroids of Jupiter (those located in the vicinity of its Lagrange points, 60 degrees ahead of and behind Jupiter): 624 Hektor .



Of course, you have the right to question the realisticness of the picture depicted by the artist. But we not only discovered many such close binary systems, but also visited one of them! Meet the famous asteroid 25143 Itokawa .



It is believed that Itokava was a close double system of two heaps of cobblestones, and now, under the influence of the combined gravity, its halves began to fuse together and gave it such an irregular shape, resembling a potato.

This behavior of asteroids is not frequent, but it occurs. But until recently, we have not yet discovered similar objects in the Kuiper belt. We had no data, no suitable resolution.



But in 2011, at the European Congress of the planetology community, Pedro Lacerda [Pedro Lacerda] published the light curve of a distant object from the Kuiper belt, 2001QG298.

What shape should an object have to produce such a strange curve? According to Lacerde:

Imagine that you glued two eggs to the tops - something like 2001QG298 will have this shape. It is a bit like an hourglass.
The object is removed so that its form we do not consider. But its brightness fluctuations, the light curve, show its strange shape during its rotation. Sometimes an object becomes dimmer because one part of it is hidden behind the other, so sunlight is reflected from a smaller surface. When the hidden part reappears, the hourglass shape can be seen completely. The reflective area increases and the subject becomes brighter.

In other words, it is a tight binary system, rotating in such a way that its halves are periodically hidden from our field of view.



You, of course, noticed that the light curve from 2004 differs significantly from the curve of 2011. Why would it?

Recall that, like all objects in the solar system, KBO 2001QG298 revolves around the sun! And in the process of rotation we see its different parts, which means a different amount of light in general!



And since, as a percentage, we know a very small fraction of Kuiper belt objects, it is possible that such a shape is not uncommon for them. From the press release :

The most important consequence of the discovery was the possibility that this form of double Kuiper belt objects can occur very often. When Sheppard and Juitt discovered 2001QG298 in a set of 34 Kuiper Belt objects in 2004, they realized that they were lucky to consider its double essence. If he had not been turned by his side to us during their observations, they would not have recorded strong fluctuations on the light curve. They estimated that approximately 10% of Kuiper belt objects are close binary systems if their slopes are randomly arranged.

But Lacerda believes that their slopes may not be random, and that there may be more such objects.

“It was a surprise for us that 2001QG298 is tilted by 90 degrees, but this is not the first time to see such a tilt in a tight double system,” he argues. “There is another well-known double object, the large Trojan asteroid 624 Hektor, also tilted almost 90 degrees.”

If close binary systems are strongly inclined, then the chances of detecting their variable light curves are reduced — this can be done only twice in their entire orbit. The discovery of such an object in a small set hints that there may be more close binary systems than Sheppard and Juitt had first thought. Lacerda suggests that about 25% of the Kuiper belt objects are tight binary systems.

From two sets of observations, Lacerda recreated the appearance of this object from our point of view and even made a video of its rotation around the center and the circulation around the Sun!



So, although this is the first close binary system that we found in the Kuiper belt, it definitely will not be the last!