In April 2016 I published a post Our Solar System, an update. At the end of this post I wrote about the New Horizons mission, that it was on its way to the Kuiper Belt, after a successful flyby of {luto.

Here is the Kuiper Belt, a ring of (mainly) small icy bodies orbiting the sun beyond the orbit of Neptune. The distance scales are in Astronomical Units (AU), where 1 AU is 150 million km. the average distance between Earth and the Sun. The locations of Jupiter, Saturn, Uranus, and Neptune are given. Earth and the other planets are inside the yellow blob in the center. The grey cloud between this blob and Jupiter represents the asteroid belt.

More than 3000 Kuiper Belt Objects (KBOs) have been found and that number is increasing yearly. Many of them are (relatively) small, like, for instance, the 2014 MU69, mentioned in my 2016 post as the next destination for New Horizons. Another flyby on 1 January 2019 was very successful. Here is an image of 2014 MU69, taken by New Horizons. It is a contact binary, dimensions ~40x20x10 km, now renamed Arrokoth. See the appendix about naming (and renaming) objects in the Solar System.

The official name for any object orbiting the Sun beyond Neptune is Trans-Neptunian Object (TNO). Some of them can be quite large. In 2005 Eris was discovered, with a diameter of ~2300 km, about 1/5th of Earth’s diameter, similar in size to Pluto. A heated discussion among astronomers led in 2006 to the demotion of Pluto as a planet and the introduction of a new concept: dwarf planet. Pluto was always an odd one out with its elliptical orbit. It is now a dwarf planet, like Eris. Here is the Outer Solar System.

More dwarf planets have been discovered in the region beyond Neptune. A fascinating one is Sedna, discovered in 2003. Its orbit is extremely elliptical, its distance to the Sun varying between 76 and 937 AU, far outside the Kuiper Belt. One orbit takes 11.400 years, Various estimates for its diameter, Wikipedia gives >1000 km. Here is the orbit of Sedna in orange. The Outer Solar System is now so tiny, the Kuiper belt is marked in blue.

Also shown is the orbit of 2012VP113, in red. Discovered in 2012, diameter ~600 km. Again very elliptical, distance to the Sun between 80 and 460 AU. One orbit takes ~4500 years. You may wonder how astronomers discover such a remote object and even determine some of its properties. In the picture you can see how. Three images, taken by a powerful telescope, with a 30-minute interval, have been superimposed. Look at the small dot in the center. That is 2012VP113, moving against the background of stars

One more extreme TNO, 2015TG387, was discovered in 2015. Its aphelion (the farthest distance from the SUn) is a staggering 2114 AU. The orbital period is about 40.000 years.

More of these extremely elliptical TNOs have been found. What can have been the cause? . Not the giant planets or the Kuiper Belt, they never come close enough to feel their gravitation.

In 2016, two astronomers, Batygin and Brown, came up with an interesting hypothesis. A planet with a mass of about ten times that of Earth, orbiting the Sun in an elliptical orbit between 280 and 1120 AU, orbital period of 5000 years, could explain the orbits. In the diagram, the orbit of this hypothetical Planet Nine is shown.

P[anet Nine has not yet been found and it will not be easy. Not all astronomers are convinced that it exists, but it generated a lot of interest in extreme TNOs.

Recently, a new one has been found 2017 OF201. First observed in 2017. Distance to Sun between  45 and 1630 AU. Orbital period 24.000 years. Here are again three superimposed pictures, this time taken with an interval of 1 hour. Estimated diameter ≈ 550 to 850 km.

In an appendix I will tell more about the interesting way this extreme TNO was discovered and how an estimate could be made about its size. The evidence presented was so convincing that on 21 May TNO 2017 OF201 was accepted by the authoritative International Astronomical Union (IAU) as a new dwarf planet. I flurry of articles in magazines and newspapers followed. Some are accurate, like the EarthSky one, others contain errors, like the Yahoo!News one.

Here the orbit of 2017PF201 is added in red to the other TNOs. The supposed orbit of PLanet 9, here called Planer X, is shown in black.

As you see, the orientation of this new TNO is completely different from the others! And that is a serious challenge for the Planet 9 theory. Model calculations show that Planet 9 would strongly disturb the orbit of 2017 OF201 and, in the future, would kick it out of the solar system. So, does Planet 9 really exist?

That was the ending I had in mind for this post.

But, very recently another interesting article was published, claiming that PLanet 9 may have been found! Click here for the original publication (quite technical). The idea is “simple”. Planet 9 will be cold, but still it emits (thermal) infrared radiation. The authors use data from two infrared missions, IRAS (1983) and Akari (2006), comparing them, filtering out all known infrared sources and looking for an area, that doesn’t move within a few months, the operating time of both missions, but is found in a different location after 23 years. They find one suitable candidate, which fits with the theoretical orbit of Planet 9. Amazing.

I am sure that this is not the end of the story 🙂 .

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Appendix 1 Naming/renaming of astronomical objects in the solar system

All astronomical solar system objects (except comets), smaller than planets, are called minor planets (planetoids). Asteroids, KBOs, TNOs, and dwarf planets. The Minor Planet Center keeps track of them. When a new minor planet has been discovered, it gets a name. For the new dwarf planet, this (provisional) name is 2017 OF₂₀₁. Here is the explanation. 2017 was the year it was first observed. Followed by two letters.

The first letter, O, tells in which half-month of that year it was discovered, in the second half pf JUly 2017. The second letter gives the order of discovery for that half-month. The F would naan that it was the sixth minor planet discovered in that half-month. But wait. When this coding was designed ( in 1925), it could handle 25 discoveries in a half-month, but nowadays, with modern technology, there are many more. That’s why the subscript is added. 201 x 25 = 5025 +8 = 5033. This dwarf planet was the 5033th discovery in the second half of July 2017!

When the orbit is determined accurately enough, this provisional designation is replaced by a (sequential) number. The team that discovered the minor planet can then suggest a name. The minor planet 2014 MU69, visited by New Horizons, is now named 486958 Arrokoth. Using the coding given above, you should be able to check that Arrokoth was the 745th discovery in the second half of June 2014.

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Appendix 2 T The discovery of dwarf planet 2017 OF₂₀₁.

The original article can be found here. The authors use data from the Dark Energy Survey project, which itself is not related to the solar system. To find objects in the solar system, you must look for objects that move. Using the survey data, already ~800 TNOs have been found. The next step is to find the distance of the object. For that we use the apparent motion of the object against the background of the stars. It is called parallax. Due to Earth’s orbit around the Sun, the position of the object changes. From how much it changes, the distance can be calculated. For 2017 OF₂₀₁ this distance is at the moment about 90 AU.

The combined effect of parallax and real motion for 2017 OF₂₀₁ is shown in this diagram

The oval (due to parallax) is moving throughout the years. Observation dates are indicated.

From the amount of light, combined with the distance, a rough estimate can be made of the size. For 2017 OF₂₀₁ this results in a diameter of 550 to 850 km . Big enough to call the object a dwarf planet.