Friday, January 22, 2016

Parsing "Planet Nine" - It's Not Like Earth

Artist's conception of 'Planet Nine' - predicted by Caltech astronomers

"Planet Nine" is the name given to the newest potential member of our solar system by astronomers at Caltech. Don't call it "Planet X", because to these guys - like Michael Brown (former 'slayer' of Pluto) it reeks of aliens and sci-fi.   Planet Nine then simply designates the recognition of the ninth actual planet belonging to the Sun's family - after Pluto was summarily ejected some 8 years ago for being a "dwarf".

Let's also get it into our heads Planet Nine has not been "discovered", as in actually training the Palomar telescope on a portion of the sky and seeing it. No, its existence has been inferred from the marvelous science of celestial mechanics, based on the gravitational effects detected on smaller objects in the Kuiper Belt - and from mathematical modeling techniques and computer simulations.

According to the Caltech site, the two scientists - Konstantin Batygin and Brown  describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt. Those who wish to access the actual paper can get the .pdf here:

To say the paper is a work of stupendous scientific inference and deduction is an understatement. In fact, the current work didn't launch until after two earlier researchers (one a postdoc of Brown) noted that 13 of the most distant objects in the Kuiper Belt are similar with respect to an obscure orbital feature. To explain that similarity, they suggested the possible presence of a small planet. Brown's interest was piqued and he enlisted the help of Batygin - whereupon the two commenced a year and a half long collaboration.

The pair quickly found that the six most distant objects from the original collection of 13  all followed elliptical orbits that pointed in the same direction in physical space. That was particularly surprising because the outermost points of their orbits moved around the solar system, and they traveled at different rates.  According to Brown:

"It's almost like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place,"

 The odds of that happening were something like 1 in 100, but on top of that, the orbits of the six objects were also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. "Basically", Brown said, "it shouldn't be happening randomly. So we decided to look for something else shaping the orbits."

After a number of trials they noticed almost by accident that if they ran their simulations with a massive planet and an anti-aligned orbit (i.e. the orbit's perihelion is 180 degrees opposite the perihelions of all the other objects and planets) then the 6 distant Kuiper Belt objects assumed the alignment actually observed.

In other words, a massive planet (10 times Earth's mass) was needed in order to have alignments observed jibe with the data. It was somewhat like finding the piece to a puzzle which finally fits with the rest of the pieces.

Brown and Batygin estimate the planet takes from 10,000 to 20,000 years to make one orbit around the Sun. Using Kepler's harmonic law, e.g.

 One can then work out the range for the semi-major axis. With a 10,000 year period it turns out to be 434 AU (astronomical units) and at 20,000 years it's 736 AU. Since Neptune is at about 40 AU, that makes Planet Nine up to almost 20 times more distant. From the nearer distance the Sun would appear in the sky as barely a 2nd magnitude star in brightness (about like Polaris). Needless to say the temperatures would be unthinkably frigid - think close to absolute zero Kelvin, or -273 C. With an order of magnitude greater mass than Earth a 200 lb. Earth astronaut would weigh in at nearly 290 lbs. there.

Now, it remains for someone to actually observe the planet (estimated at about +6.5 absolute magnitude) and maybe even lay claim to naming it. Once that actual observation occurs we will be able to finally say the planet has been "discovered". I'd try myself, but my eyesight simply isn't that great and my remaining telescope is only a 2.4" Tasco refractor - mainly used  to observe the occasional unusual astronomical event - like five of the planets (Mercury, Venus, Mars, Jupiter and Saturn) lined up in the morning sky right now.

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