Joseph Lazio wrote:
"c" == chornedsnorkack writes:

c Joseph Lazio wrote:

c What would happen if Epsilon Indi B orbited Sun, not Epsilon Indi
c A? [...]
c Would anyone discover it as a modest source of infrared?

[What about Jupiter?]
c Jupiter, at 5 a. u. is about 120 K temperature and radiates twice
c the heat received from Sun. So, if Jupiter were at 1500 a. u. it
c would still have temperature of about 100 K.

Above, I concluded that a 500 K object at a distance of 1500 AU
would have a J-band magnitude of 1.5. A 100 K object would be
about 7 magnitudes dimmer, or magnitude 8.5.

c Does this logic hold? Yes, the total bolometric magnitude of a body
c 5 times colder would be 7 magnitudes lower. But one would expect
c that a very much biger portion of the radiation would be in far IR,
c nor in near IR like J-band.

Steve Willner corrected my post on this point. Yes, J band is not the
best band, but one would not have to go into the far IR. At 12
microns, one of the bands surveyed by IRAS, our hypothetical object
would still be one of the brightest objects in the sky.

[...]
c Would it be seen in IR in absence of visible reflection showing
c where to look for IR radiation?

Yes. One doesn't need to see something via visible reflection in
order to look for IR radiation from it. One can observe the sky at
IR wavelengths without regard to how the sky looks at visible
wavelengths. That was part of the point behind IRAS, 2MASS, and
other surveys in the IR. I encourage you to visit the 2MASS site
at URL: http //www.ipac.caltech.edu/2mass/ .

c But are the detection thresholds for weak objects identical for
c general all-sky surveys and searching for a specific location?

I don't understand your reference to "weak objects." A hypothetical
solar companion, similar to eps Ind Ba or eps Ind Bb, would be one of
the brightest objects in the IR sky. A Jupiter-mass companion would
be somewhat fainter, but I suspect far above the IRAS detection
threshold.

Then where is the threshold for such surveys, in respect to gas giants?

There are the small, roughly 2000 km in diametre (and cold) objects
Xena and Sedna, at about 90 a. u. They were discovered in visual
surveys - not IR surveys - but once found, they could, with difficulty,
be seen in IR.

Sedna has aphelion at 900 a. u., so there can be large bodies so far
out...

At 1500 a. u.... Jupiter radiates a lot in IR. So does Saturn, with
luminosity something like a fourth of Jupiter. Neptune radiates, too -
though I think its luminosity is something like 50 times smaller than
Saturn.

But Uranus, very close to Neptune in mass and density, does NOT
radiate, except of course because of solar heating.

Would an Uranus-like giant at 1500 a. u. be visible in IR? What about
Neptune?