The Kuiper Belt Controversy continues, with the lastest round showing up in the Times today: Planet Discovered Last Year, Thought to Be Larger Than Pluto, Proves Roughly the Same Size:
The object — still unnamed more than a year after its discovery but tagged with the temporary designation 2003 UB313 and nicknamed Xena by the discoverer — covered an area only 1.5 pixels wide in the digital image, taken by the space telescope in December. But that was enough to extract the diameter: 1,490 miles, give or take 60 miles.
A previous estimate by a team of German researchers, based on measurements taken from the ground, had put the diameter at 1,860 miles, or 30 percent more than Pluto’s 1,422 (as measured by the telescope).
This, of course, fans the controversy over whether Pluto ought to be officially called a “planet” back into a roaring smolder. If it’s just one of many similarly-sized ice-balls out in the distant regions of the Solar System, why should Pluto get a special name? And should anybody actually care?
The far more interesting aspect of this, to my mind, is in the first paragraph: The thing is only a pixel and a half across, and they get the diameter to a few percent. That’s some nifty curve-fitting, and some serious faith in your image-processing capabilities.
Let’s keep it simple. A planet must meet at least three criteria:
1) A planet must have an elliptical orbit with the sun at one focus.
2) A planet must have a minor radius larger than that of Earth’s moon.
3) A planet’s orbit must not intersect any orbit of any planet on the average closer to the sun.
Pluto is a visible lump, but Pluto is only Kuiper belt debris. El Ultimo Presidente Boosh should propose a vast brave national effort to thermonuke Pluto out of creation to restore heavenly perfection. Expansive expensive studies must begin immediately.
(Maintaining 5.626% decay/year tritium booster potency during voyage duration is left as an exercise for the alert reader.)
I was just about to comment on the estimated accuracy of their diameter calculation. It reminds me of TV police shows where they take a grainy security camera video and process it so you can read a license plate on a car a block away. On the other hand, I can conceive of some diameter estimation techniques if the image is moving across the focal plane, so maybe it’s not quite so imaginative.
Actually it was 1.5637 pixels across; they just rounded to the nearest half-pixel.
It would be terribly disappointing if the answer to “Is there a 10th planet?” would be that there isn’t any 9th.
I prefer “A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.”
That would put the fun in the game back. Why stop now? I think it’s legitimate to ask if not planetary hunting promotes science more than cataloging objects.
I heard Michael Brown (leader of the team that discovered 2003 UB 313) give a talk that mentioned this a few weeks ago. As is usually the case with press-release science, the truth is more complicated than what the press release would have you believe.
What they’re trying to say with the “1.5 pixels wide” is that the _deconvolved_ source would be 1.5 pixels wide, where a true point source would be exactly 1 pixel wide. The actual source on the image is significantly larger due to the diffraction effects.
What they did in this particular case is image 2003 UB 313, then image a star on exactly the same place on the detector (since optics aren’t perfect, the point-spread function — the appearance of a point source — will be subtly different from point to point). This let them get a good determination of the PSF at that point on the detector at that point in time; they then used that to deconvolve the UB313 image. So yeah, I believe their numbers, and it’s not really comparable to the Hollywood infinite-magnification image processors.
Torbjorn, your definition would turn Ceres and probably a few other asteroids into planets. Which is fine, if that’s what you want, but it strikes me as difficult to argue that _this_ rocky body between Mars and Jupiter is a planet, while _that_ one with a 10% smaller diameter is not.
I’m in the “call Pluto a planet because it’s traditionally been called a planet, even if we adopt planet-criteria for new discoveries which would exclude Pluto” camp. Kinda like how we call “Lake” Huron and “Lake” Michigan two different lakes, when hydrologically they constitute a single lake.
Andrea,
Yes, but won’t you always have a demarcation problem? Roundness may be easy to define, but perhaps self gravity roundness will be density and historical temperature dependent, and so there will be bodies that have different masses.
I nicked the definition from Wikipedia, but I liked it since it will keep established planets and the “game” going. Other benefits may be that it is easier to establish planethood, and that it won’t flipflop if other comparative bodies are discovered. But I’m not an astronomer, so I should probably bow out from a discussion that is way over my head and let you heavenly forces continue instead.