Read the excerpt “Journey to the Outer Solar System”: Between Two Worlds

Another possibility is the presence of an object the size of an unknown planet in regions farther from the solar system, which determines the orbits of this object by the influence of its gravity. Speculations about an unknown planet have often been expressed over the past few decades. In the past few years alone, discussion has resumed about Planet Nine which is said to lie in regions beyond 200 AU from the Sun. There is no evidence of such a body, but the evidence appears to be mounting.

Perhaps, according to the third attempt at interpretation, it is not about an object still present, but rather a large planet (possibly a gas giant) that was formed with other planets, but was then thrown out of the Solar System through interactions with them. This may be enough to post the DO to its current location.

Although these things are hard to spot, we’ve already identified quite a few. The most famous of these is Sedna, the next destination on our journey. This body defines a new “subclass” of separate objects. All those DOs whose perihelion exceeds 75 astronomical units, as in Sedna, are referred to as “Sednoids”. Currently only a few Sednoids are known (see Table 9.1).

9.2 Sedna

Sedna is a strange world we encounter about 500 astronomical units away¹ (Fig. 9.1, not included in this excerpt). It takes sunlight approximately three days to reach this distant object. We have already seen in the previous section that this “discrete body” has a highly eccentric orbit (Fig. 9.2, not included in this sample). At its closest to the Sun, it reaches our central star up to 76 astronomical units, but then continues its journey into the outer solar system to an almost unimaginable 922 astronomical units.