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Comment by nkoren | Hacker News Reader

If it holds up, this is fantastic. Given how interesting the Pluto system has turned out to be, I can only fantasise about a potential super-earth system relatively nearby.

However, to be completely pedantic: would this actually be a planet? Or still a dwarf planet, despite its massive size? Keep in mind that the definition of planethood is not only that it's large enough to be rounded by its own gravity, but that it has also "cleared its orbit". I get the impression that this would cut through broad swathes of the still-cluttered Kuiper belt, and thus would only qualify as a "dwarf" despite its massive size.

I checked the original papers for references to whether it had cleared its orbit, and couldn't find any. Correct me if I'm wrong?

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The criteria for clearing an orbit are apparently somewhat involved[1]. It can't just be a matter of no other bodies having the same distance from the sun or Pluto would disqualify Neptune and Jupiter would be disqualified by its trojans. Because the planets in these cases mass so much more than other other nearby bodies those other bodies dance to the planet's tune and aren't counted against the planet in terms of clearing it's neighborhood. But if you look at the large bodies in the asteroid belt or the TNOs then their influence is very much mutual.

It's not entirely obvious if the new body would be a planet under the listed criteria since I don't think we have a good handle on the total trans-Neptunian population. In fact by Soter's µ criteria we might just need a single Pluto-sized object to cross this planet's path to disqualify it.

[1]https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

I was misremembering Pluto's mass as being 1/10 of Earth's when it's really 1/500th. It would take a large number of TNOs to push this hypothetical object from being a planet to being a dwarf planet.

> However, to be completely pedantic: would this actually be a planet?

This is one of the paper's author's answer to that question.

http://www.findplanetnine.com/2016/01/is-planet-nine-planet....

For those that won't click through, a very short explanation given by the author: "Is Planet Nine gravitationally dominant? I think it is safe to say that any planet whose existence is inferred by its gravitational effects on a huge area of the solar system is gravitationally dominant."

He then goes on to give a more technical argument.

> would this actually be a planet? Or still a dwarf planet

The NY Times article covers this question in some detail, and says the experts are certain this would be a ninth planet:

http://www.nytimes.com/2016/01/21/science/space/ninth-planet...

From what I understand, it has cleared the space but there are still harmonic crossings that can't be cleared. Interesting challenge to the new definition - must the orbit be clear, or only clear sailing through the orbit.

Even the Earth still has Lagrange points on its orbit that it can't clear. 2010 TK7 is a known asteroid in Earth/Sun Lagrange point [1].

[1] https://en.wikipedia.org/wiki/2010_TK7

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