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The hypothetical Oort cloud is an explaination for long period comets. It presumably is made up of icy bodies orbiting up to 50,000 AU from the Sun. If so, shouldn't the Spitzer IR telescope have detected the radiation generated by solar energy from such a spherical cloud?

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""have detected the radiation generated by solar energy from such a spherical cloud"" What is Your idea about that "radiation generated by sun energy"? I do not understand that. – Georg Apr 8 '11 at 22:18
Georg: I assume Oort bodies/particles will absorb or reflect solar energy and emit something in the IR range of sufficient quantity to be detectable. – Michael Luciuk Apr 8 '11 at 22:53
Oort cloud objects can be expected to be cold. Very cold. How low does Spitzer's detection band go? – dmckee Apr 9 '11 at 1:15
dmckee: According to the Spitzer site, its bandwidth coverage is 3 - 180 microns. – Michael Luciuk Apr 9 '11 at 1:35
Yeah, I went and answered my own question. See below. – dmckee Apr 9 '11 at 1:36
up vote 2 down vote accepted

Wikipedia puts the Spitzer Cold mission imaging capability in the band 3 -- 180 micrometers.

I calculate the blackbody temperature that would have a peak emission at the extreme of that band like this:

The frequency is

$$ \nu = \frac{3.0 \times 10^8 \text{ m/s}}{180 \times 10^{-6} \text{ m}} = 1.7 \times 10^{12} \text{ Hz} $$

And from that a corresponding temperature of

$$ T = \frac{1.7 \times 10^{12} \text{ Hz}}{59 \times 10^9 \text{ Hz}} = 28 \text{ K} $$

OK. What to compare that to?

Wolfram Alpha puts the surface temperature of Pluto at around $44\text{ K}$, so Spitzer Cold should have been able to image Pluto.

Unfortunately, Wolfram also puts the radius of Pluto's orbit at around 40 AU. The Oort cloud objects would somehow need to be 2/3 or Pluto's temperature to have been (just barely!) visible to Spitzer Cold. This is not my specialty, but I kinda doubt it.

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Thanks dmckee. At the same WIKI Wien site, lambda max = 2.89E-3 / T which yields T=16K rather than your 28K. Regardless, I'm not sure how cold Oort bodies 50,000 AU away would be from solar radiation. – Michael Luciuk Apr 9 '11 at 2:00
@Michael: I would guess they are much closer to the temperature of the CMB than to that of Pluto. But like I said, This is very far from my specialty. – dmckee Apr 9 '11 at 2:03
dmckee: I agree. T would likely be in the 3K range, too low for Spitzer detection. – Michael Luciuk Apr 9 '11 at 2:21

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