Detecting Nearby Faint Objects

[Dr Wu]

I could be mistaken here, but I did glean from somewhere (probably the net) that gazing up at the night sky doesn't reveal the full picture about the many faint celestial objects that may be lurking unseen in nearby space - that is to say, the space between the Centauri system and the outer margins of our Oort Cloud. These objects might range from Neptunian gas giants, through to low-mass brown dwarfs, and maybe even one or two extremely dim red dwarfs?

I mention the above in passing because I would like to know just how far (preferably in terms of AU rather than light-years and parsecs) our most powerful telescopes are able to detect such "nearby" objects. I guess the answer would depend in part on luminosity, albedo and angular size. Nevertheless, I would appreciate some kind of yardstick when when it comes to understanding the current resolving power we can bring to bear on any sizeable object beyond (say) the Kuiper belt.

Many thanks.

 

[newjerseyrunner]

Our telescopes are capable of observing large objects very far out there. Halley's comet is actually used to perfect methods for finding very faint objects because of it's known orbit. It can be observed everywhere in it's orbit, but it only has a 75 year orbit, and it's really big. There have been a few studies to attempt to rule out any more large players in the solar system. There are no brown dwarfs within 10ly of us, and no Saturn size objects out to at least 10000AU source

Objects the size of neptune could still be lurking out there, and should be visible to us, we just got to spot them.

 

[Dr Wu]

Thank you. I had no idea Halley's comet was observable throughout its orbit - likewise that brown dwarfs were rarer in our cosmic neighbourhood than I'd supposed. Nearby space has suddenly become a great deal emptier!

 

[newjerseyrunner]

If it's any consolation, there could be a number of rogue planets in our neighborhood that aren't detectable yet. :) There are also most likely some more really big objects far out there in the Oort cloud, maybe the size of Neptune, but almost certainly a few more Plutos out there.

 

[|Glitch|]

I agree with newjerseyrunner, that if there were objects larger than the planet Saturn within a few light years of the Sol system, we would have detected them by now. However, I am compelled to point out that there is indeed a brown dwarf binary pair within 10 light years of Earth - Luhman 16AB (WISE 1049−5319), 6.59 light years away.

As newjerseyrunner also pointed out, there are lots of objects that are smaller than the planet Saturn. I have no doubt that there are rogue asteroids/comets and small planets flying around in interstellar space. Our current understanding of solar system formation suggests that the early solar system was extremely chaotic, with Jupiter migrating in its orbit, and even causing Neptune and Uranus to switch orbits. Asteroids and comets were not only flung in towards the inner planets by the gas giants, they were also flung out of the solar system. So think of the Late Heavy Bombardment period, only in reverse with the same number of asteroids/comets being flung outward.

There are bound to be more objects like 90377 Sedna, which is 524.4 ± 1.0 AU away, has an orbit of ~11,400 years, and has almost the same size as Pluto.

 

[Dr Wu]

Just out of interest, how would astronomers detect a Jupiter-sized gas giant at the distances being discussed here? I would imagine that the reflective sunlight coming off them must be so faint as it be practically undetectable. Or would infrared telescopes be able to home in on any heat they might radiate? Then there are gravitational perturbations, I guess - although again I wonder how these 'nudges' might be observable over distances stretching across several thousands of AU.

 

[|Glitch|]

Just out of interest, how would astronomers detect a Jupiter-sized gas giant at the distances being discussed here? I would imagine that the reflective sunlight coming off them must be so faint as it be practically undetectable. Or would infrared telescopes be able to home in on any heat they might radiate? Then there are gravitational perturbations, I guess - although again I wonder how these 'nudges' might be observable over distances stretching across several thousands of AU.

When you are talking about the space between solar systems, there isn't much chance of them being detected by reflected starlight. Infrared would be the only way to detect such objects. Which is precisely what the Wide-field Infrared Survey Explorer (WISE) was designed to do. It was because of WISE that we were able to detect Luhman 16AB.