PDA

View Full Version : How big bodies can exist in outer Solar System?



chornedsnorkack
2009-Feb-15, 01:35 PM
Well, how big things can exist unseen?

We did see Pluto - but very importantly managed to not see Eris till very recently. What else may we not see?

Eris at 95 a. u. was found recently. The total luminosity of a reflective object decreases with fourth power of distance, so that we could miss an Earth-sized object at about 200 a. u., Neptune sized object at 450 a. u. and Jupiter sized object at 800 a. u.

There is stuff out there at 900 a. u., because that is where Sedna´s aphelion is. Can we observationally rule out a brown dwarf star at Sedna´s orbit? Nothing is bigger than Jupiter except red dwarf and bigger stars, something which we do have observational evidence against (a star the size of Proxima Centauri and as far from Sun as Proxima is from Toliman would be a naked eye object, though an inconspicuous one).

We know that Neptune has some inner heat. But then, we know where to look. Would a body as big and hot as Neptune (50 000 km and 50 K) located far enough to be missed in reflected visible light (say 500 a. u.) be seen in infrared?

parallaxicality
2009-Feb-15, 02:10 PM
We can't rule out anything. Ideas of what's out there are based on computer models and hypotheses about how the Solar System formed, which could very well be wrong. That said, comprehensive searches of the sky have yet to turn up anything. Even Mike Brown's most recent "Sedna search" hasn't found any new bodies on Seda-like orbits, which is odd in itself.

tony873004
2009-Feb-16, 04:45 AM
...a star the size of Proxima Centauri and as far from Sun as Proxima is from Toliman [Alpha Centauri B] would be a naked eye object, though an inconspicuous one...
Inconspicous is the key. It matters more how quickly it moves against the background stars than how bright it is. For example, Eris is the 4th brightest KBO, yet was about the 500th one discovered. Why were nearly 500 dimmer ones discovered before it? Because it's appearant motion is slow. An object's motion against the background stars is largly due to Earth's motion. So the farther the object, the slower it appears to move. Astronomers have computers scan their images for moving objects. To weed out false positives, they place thresholds on the appearant velocities. Sedna barely squeeked past the threshold and hence it was discovered. This inspired Mike Brown to lower the threshold, and this led to the discovery of Eris. Eris has since been identified in pre-discovery photos, and propably appears as a bright object in the photographic plates that Lowell used to discover Pluto nearly a century ago.


...Even Mike Brown's most recent "Sedna search" hasn't found any new bodies on Sedna-like orbits, which is odd in itself.
It's not too odd since objects in Sedna-like orbits will spend most of their time near aphelion, and rush through perihelion. Only at perihelion are they close enough to discern from background stars due to their appearent motion. Sedna is currently a few decades away from perihelion, in an orbit that has a period of about 10 thousand years. That's why it was discovered. 99% of the time, Sedna is too far to be discoverable with our current techology and techniques.

JustAFriend
2009-Feb-16, 02:34 PM
Doesn't matter how big the objects are out there ---- it's DARK out there that far from the sun.

If the object doesn't reflect a lot of light and there isn't a whole lot of light out there, you're going to have a tough time detecting it....

chornedsnorkack
2009-Feb-16, 04:39 PM
It's not too odd since objects in Sedna-like orbits will spend most of their time near aphelion, and rush through perihelion. Only at perihelion are they close enough to discern from background stars due to their appearent motion. Sedna is currently a few decades away from perihelion, in an orbit that has a period of about 10 thousand years. That's why it was discovered. 99% of the time, Sedna is too far to be discoverable with our current techology and techniques.
Even at aphelion, Sedna has annual parallax of 220 arc seconds. While the closest fixed stars have parallaxes of 3/4 arc seconds, and already in 1830-s, a parallax of 1/8 arc seconds was seen.

How many visible objects do not have observational upper bound to proper motion and annual parallax?