Saturday, February 6, 2016

Imaging Planet X: Is It Possible?

The news is that there may be a ninth planet. It's being called "Planet X" for historical reasons, with the X designating "unknown" rather than the number 10. It gets a planet designation because even though this inferred object probably resembles a trans-Neptunian object it's thought to be be bigger than Earth.  (Please don't confuse it with the utterly fictional "Planet X" known to some as Nibiru.)

The ex-planet Pluto is easy to image as are many of the other dwarf planets; what about this new denizen?

Factors that dictate a planet's total brightness (as opposed to its surface brightness) are its distance from the Sun and from us, its average albedo, its size, and its phase. In the case of these distant objects phase is essentially always full and can be ignored; likewise the 2 AU annual change in the distance from Earth is neglected. To keep this simple I'll make a comparison between X and Pluto to estimate the former's relative brightness and magnitude.

Lets's start with some simplifying assumptions and say that X and Pluto have the same albedo and that X lacks a moon that contributes significantly to its brightness. (Charon represents about 1/5 of the Plutonian system brightness--if Pluto were to become invisible Charon would be easily imaged!)

So what are the speculated physical characteristics of X?

Size: Larger than Earth, smaller than Neptune. If it's simply a scaled up version of Pluto with 4500 times the mass, it should be about 16 times the size of Pluto. this puts it about 2.8 times the size of Earth and about 73% the size of Neptune. Close enough. This gives X a reflecting cross section about 256 times that of Pluto; we'll knock that down to 200 to account for its assumed lack of a bright moon.

Orbit: There's a lot of uncertainty here; the suggestion is that as X's orbit caries it between 200 to 1200 AU from the Sun. Unfortunately no one knows it's present distance, so we'll make estimates for both the extremes. At present Pluto's distance to the Sun is about 33 AU and it's magnitude 14.2. The distance factors are (33/200) to the fourth power and (33/1200) to the fourth power for perihelion and aphelion, respectively, using the assumption that X will resemble a point source such as a star.

Perihelion: 0.148 the brightness of Pluto, giving it a magnitude of about 16.3. This is easily within the range of amateur imaging.

Aphelion:  0.000114 the brightness of Pluto, with a magnitude of about 24.1. This is probably beyond amateurs. The deepest stars I've ever imaged are near magnitude 20; X would require a total exposure time about 40 times greater than that for a 20th magnitude star to image at its most distant point.

That perihelion magnitude is encouraging, but there are complications. Not much is known about X's orbit, so even if X is near perihelion we don't know were to look for it. Given that its orbit is probably fairly eccentric it will usually be found closer to aphelion, and therefore is usually very, very dim. And don't even think about waiting for the next perihelion, the orbital period is many times a human lifetime.

My recommendation is to leave X to the professional deep sky surveys. On the other hand, if you're feeling really lucky and have a whole lot of time, you could be the next Clyde Tombaugh.


It's been a miserably cloudy winter here. I've had only one night out to image, and that was to try to catch Barnard's Loop. Overhead power lines and a neighbor's lights really degraded the image.

Barnard's Loop (in hydrogen alpha)
 As you can see, above and left of center there are diagonal artifacts thanks to the power lines and at lower right, air traffic. Too much signal was lost trying to reject the obstructions. If the weather improves I might get another chance this winter.

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