[meteorite-list] Astronomers Find Another Small Icy Body Out Past Pluto (2015 KH162)

[Ron Baalke]

http://www.slate.com/blogs/bad_astronomy/2016/02/25/_2015_kh162_a_distant_icy_world_past_pluto.html

Astronomers Find Another Small Icy Body Out Past Pluto
By Phil Plait
February 25, 2016

Astronomers have found an interesting new world orbiting the Sun - no, it's 
not Planet Nine, but it's still cool. Called 2015 KH162, it's a small(ish) 
object orbiting the Sun far, far past Pluto.

It's not clear how big it is. You can figure out an object's size if you 
know how far away it is (which we do) and how reflective it is, a number 
called its albedo. If it's shiny it can be small and still look bright; 
if it's dark it has to be much bigger to appear as bright.

Albedos can be difficult to determine, but given the albedos of other 
objects in that part of the solar system KH162 could be as small as 500 
kilometers across, or as large as 1,000. Either way, it's much smaller 
even than Pluto (which, at 2,300 kilometers, is smaller than our own Moon). 
It's probably even smaller than Pluto's moon Charon (which is 1,270 kilometers 
in diameter). Still, given that it's almost certainly mostly ice and rock, 
it's big enough that it's probably close to spherical.

Judging just from its brightness, it's probably in the Top 20 objects 
by size we know of so far out past Neptune. Not huge, but not just a bit 
of fluff, either.

[Diagram]
The orbit of KH162 keeps it well away from the Sun. The Earth's orbit 
is too small to even show on this scale.
NASA/JPL-Caltech

The orbit is interesting. KH162 is on a fairly elliptical orbit that's 
tilted quite a bit to the plane of the solar system (the major planets 
all orbit the Sun in essentially the same plane; if you looked at the 
solar system from the side it would look flat, like a DVD seen on edge). 
It gets as far from the Sun as 12.5 billion km, but at its closest it's 
a mere 6.2 billion km out. That's very interesting; that means sometimes 
it's closer to the Sun than Pluto gets!

Not that they'll ever collide. KH162's orbit is tipped enough that their 
paths don't physically cross.

[Diagram]
Seen from the side, the solar system planets' orbits form a flat disk. 
You can see how tilted KH162's orbit is. Pluto's (in white/gray), too.
NASA/JPL-Caltech

It was first observed using a telescope on Mauna Kea in Hawaii back in 
May 2015. It was seen again many times over the next few months, enough 
times to establish an orbit - an object has to be observed many times 
to nail down the orbital shape, and KH162 moves so slowly that this took 
a while. It takes 489.6 years to orbit the Sun once.

In general objects out this far are called trans-Neptunian objects, or 
TNOs. There are different populations of them; for example, the Kuiper 
Belt is a torus-shaped region past Neptune where objects like Pluto and 
Eris dwell. Past that is the scattered disk, which has objects with more 
elliptical, tilted orbits. Over billions of years, some of these objects 
interacted with Neptune, and the giant planet's gravity flung them into 
such orbits. I talked about this for Crash Course Astronomy:

I noticed something right away about KH162: Its orbital period is almost 
exactly three times the period of Neptune's orbit (489.6 versus 164.8 
years). That sort of simple ratio of orbital periods is called a resonance. 
This is certainly not a coincidence; resonances are common and usually 
the result of gravitational interactions. I talked with astronomer David 
Nesvorny, who studies how small objects out past Neptune interact with 
it, and he directed me to a paper he just published.

The details are complex, but the bottom line is that billions of years 
ago, there may have been lots of Pluto-size objects past Neptune as well 
as countless smaller ones. If so, as Neptune scattered the Pluto-size 
objects one by one, the big planet got a bit of a kick, too. Every time 
that happened it would have moved a tiny amount in its orbit (what we 
call migration). Eventually it kicked all those objects away, leaving 
just a couple (which is what we see, specifically Pluto and Eris). Due 
to the weird nature of orbital mechanics, many of the smaller objects 
in certain orbits would've been spared. This includes the 3:1 resonance; 
the orbit KH162 is in!

So it may be a survivor of Neptune's wrath, in a lucky orbit that kept 
it away from the much bigger and more persuasive planet. How about that?

On the "how amazed should I be by this discovery?" scale I'd rate it somewhere 
around "hey, that's pretty cool!" It's pretty interesting. In fact, I'd 
say its discovery is important for two reasons. One is that we don't know 
of many objects this size that far out - they're faint, and really hard 
to detect. Every one we find is an important addition to the inventory, 
and tells us more about how the early solar system behaved.

But another reason this excites me is that it shows that there still are 
relatively massive objects out there left to be found. The scattered disk 
extends far, far past KH162, so there could be many bigger objects out 
in that region that are simply too faint to be found.

Yet. It's a big sky, and we've only been looking for these worlds for 
a few years. KH162 certainly has a lot of siblings (including bigger siblings), 
and I have no doubt we'll find many more.