Not The Ninth Planet
Posted by on Thursday, February 25, 2016 Under: Astronomy
I feel that I must being this article with a disclaimer. Although I usually try to keep my science news articles neutral, some of the comments in this particular article should be considered more of an opinion piece on the silliness of certain definitions in astronomy. Those who only wish more serious, formal articles might want to skip this one...
By now even the most casual of astronomy fans will have heard the reports that astronomers believe they have discovered the ninth planet in the solar system, located well beyond the orbit of Pluto. This supposed planet has not been directly observed, but its presence is implied by the effect that its gravitational field has on other objects around it.
What we do know about it is that it is about three times the size of the Earth, that its orbit brings it from roughly 200 times further from the sun than the Earth to a furthest point of as much as 1200 times further out. And it has a year that lasts for 20,000 Earth years. Other than that we know nothing about it. We don't even know where it is in its orbit right now.
And yet astronomers and media outlets are calling it the ninth planet. And they are wrong.
Ten years ago the International Astronomical Union (IAU) held a controversial vote in which a few hundred non-experts decided on the formal definition of a planet. They had two criteria that were generally accepted - that the planet must be held in a roughly spherical shape by its own gravity and that it has to orbit the Sun. (The formal definitions are much more precise than these rough summaries, but in content they are the same).
However the great controversy came in the third criteria. It was decided that a planet must have cleared all other objects out of its local neighbourhood. And so even Jupiter, the largest planet in our solar system, would lose its planetary status if it entered an asteroid field. (And in fact some astronomers have argued that the asteroids that cross Jupiter's orbit should disqualify it as a planet, although these comments are usually made by curmudgeons like me who wish to demonstrate the silliness of this criterium). The most notable effect of this rule is that the planet Pluto lost its planet status since it has not absorbed the Kuiper belt objects (aka big rocks) that cross its orbit. In defining planets, location matters most.
And that is why I do not agree with the claim that this new discovery is the ninth planet. If the three criteria of the IAU are applied, then the new object is still going to pass through the Kuiper belt and therefore has not cleared its neighbourhood of other objects, in which case it by definition is not a planet.
And if we use only the two generally accepted criteria, then Pluto remains the ninth planet. And so do the tenth, eleventh, and twelfth planet-like objects that were previously discovered beyond the orbit of Pluto. In this case the new object is perhaps the fifteenth to twentieth planet of our solar system.
Of course it is possible that when the new object is directly observed it will be found to be further out than the furthest edge of the Kuiper Belt, in which case it might get to be a planet under either definition. But at present that really does seem unlikely given the data that has been released so far.
So in spite of the media hype and attention, this is definitely not the ninth planet!
By now even the most casual of astronomy fans will have heard the reports that astronomers believe they have discovered the ninth planet in the solar system, located well beyond the orbit of Pluto. This supposed planet has not been directly observed, but its presence is implied by the effect that its gravitational field has on other objects around it.
What we do know about it is that it is about three times the size of the Earth, that its orbit brings it from roughly 200 times further from the sun than the Earth to a furthest point of as much as 1200 times further out. And it has a year that lasts for 20,000 Earth years. Other than that we know nothing about it. We don't even know where it is in its orbit right now.
And yet astronomers and media outlets are calling it the ninth planet. And they are wrong.
Ten years ago the International Astronomical Union (IAU) held a controversial vote in which a few hundred non-experts decided on the formal definition of a planet. They had two criteria that were generally accepted - that the planet must be held in a roughly spherical shape by its own gravity and that it has to orbit the Sun. (The formal definitions are much more precise than these rough summaries, but in content they are the same).
However the great controversy came in the third criteria. It was decided that a planet must have cleared all other objects out of its local neighbourhood. And so even Jupiter, the largest planet in our solar system, would lose its planetary status if it entered an asteroid field. (And in fact some astronomers have argued that the asteroids that cross Jupiter's orbit should disqualify it as a planet, although these comments are usually made by curmudgeons like me who wish to demonstrate the silliness of this criterium). The most notable effect of this rule is that the planet Pluto lost its planet status since it has not absorbed the Kuiper belt objects (aka big rocks) that cross its orbit. In defining planets, location matters most.
And that is why I do not agree with the claim that this new discovery is the ninth planet. If the three criteria of the IAU are applied, then the new object is still going to pass through the Kuiper belt and therefore has not cleared its neighbourhood of other objects, in which case it by definition is not a planet.
And if we use only the two generally accepted criteria, then Pluto remains the ninth planet. And so do the tenth, eleventh, and twelfth planet-like objects that were previously discovered beyond the orbit of Pluto. In this case the new object is perhaps the fifteenth to twentieth planet of our solar system.
Of course it is possible that when the new object is directly observed it will be found to be further out than the furthest edge of the Kuiper Belt, in which case it might get to be a planet under either definition. But at present that really does seem unlikely given the data that has been released so far.
So in spite of the media hype and attention, this is definitely not the ninth planet!
In : Astronomy
I am a theoretical physicist & mathematician, with training in electronics, programming, robotics, and a number of other related fields.
