Bonus Factismal – Somewhere Out There

If you’ve been paying attention to the science news lately, then you’ve probably heard about the discovery of yet another small chunk of rock and ice out in the distant reaches of the Solar System. Known as “2012 VP113” (because it was the 2,825th object sighted in the first half of November, 2012) or “Biden” for short (because he was “VP” when it was first seen), this chunk of stuff is probably about 250 miles across which makes it just barely large enough for a planetologist to call it a planet. (Astronomers don’t recognize these objects as “real” planets because there are more than ten of them.) Biden has generated a lot of interest for two reasons:
1) It is pink.
2) It is really, really far away even when it is close.

The first point is just a by-product of where and how Biden formed. It came together far out in the Solar System and so didn’t heat up much as the various bits smashed together into a ball-shaped mass. As a result, Biden is rich in primitive compounds like those found in comets, with lots of methane, ethane, amino acids, and other organic goodies. When those things are exposed to ultraviolet radiation (like that from the Sun), they break down into a reddish compound; you might think of it as the planetary equivalent of “sunburn”. And the color came as no surprise; Sedna, which is also in a far out orbit, is even redder than Biden.

The images that discovered 2012 VP113 (Image courtesy NASA)

The images that discovered 2012 VP113
(Image courtesy NASA)

The second point is where the surprise lurks. If you go back ten years, most astronomers and planetologists thought that the Solar System formed with a bit of a gap between the “inner” bits (all of the junk inside of Pluto’s orbit plus the small chunks of rock and ice in the Kuiper belt) and the “outer” part (the Oort cloud where long-period comets come from). (NB: I put “inner” in quotes for a reason; astronomical convention is to call the four terrestrial planets the “inner Solar System” and the planets from Jupiter to Pluto the “outer Solar System”. As we’ve learned more, what used to seem far, far away turns out to be fairly close.) The reason for the gap was that it was thought most of the stuff inside that region was either scattered farther out or captured by stuff closer in. But the discovery of Sedna and Biden calls that simple view into question.

The orbits of Sedna and Biden (Image courtesy NASA)

The orbits of Sedna and Biden
(Image courtesy NASA)

If just one planet had been found in that region, then it could have been a fluke. But finding two means that we’ll probably find dozens if not hundreds of planets in that region. And that means that our view of how the Solar System formed may need to be updated. Perhaps our Solar System came close to another one while it was forming, pulling some of the Oort cloud planets into the empty zone. Or maybe there’s a large planet hidden out in the Oort cloud; though astronomers are relatively certain that there’s nothing as big as Saturn out there, something just five times the size of Earth could have created the odd orbits and wouldn’t have been detected with our current telescopes. Or perhaps a rogue planet barreled through the Solar System, disturbing orbits. Until we get more data, we’ll never know for sure.

What we do know is that Biden has the farthest known perihelion (closest approach to the Sun); it never gets any closer to the Sun than 42,910,527,500 miles. But just how close is that? Most of us (astronomers included) have a hard time grasping something as small as a hundred miles; how are we supposed to understand 42 billion of them? The answer, of course, is that we don’t; we just us a scale to put everything into perspective. So here’s a scale for you to use when trying to understand just how far away Biden is.

Let’s start by drawing a circle two inches in diameter on a big piece of paper. In the center of the circle, put a dot that is one one-hundredth of an inch across (about the size of a pencil point). That dot is the Sun and the circle is the Earth’s orbit, both drawn to the same scale. At that scale, the entire Earth-Moon system fits neatly within width of the line that you used to draw the circle. The distance from the center of that circle to its edge has a special name in astronomy; it is called an astronomical unit and is equal to the distance from the Earth to the Sun (roughly 93,000,000 miles). Now start putting dots on the paper to represent some parts of the Solar System:

Object Distance (km) Distance (AU) Distance (scaled)
 Sun (radius)  1,392,684 0.01 0.01″
 Earth’s orbit 149,800,000 1.0  1″
 Jupiter’s orbit 778,547,200 5.2 5.2″
 Pluto’s orbit  5,874,000,000 39 3′ 3.2″
 Sedna’s current distance 13,422,080,000 90 7′  5.7″
 Sedna’s closest distance (perihelion) 11,438,728,000  76 6′ 4.4″
 Sedna’s farthest distance (aphelion) 140,362,600,000  938 78′ 2.3″
 2012 VP113 (“Biden”) current distance 12,433,400,000 83 6′ 11.1″
 Biden’s closest distance (perihelion) 12,028,940,000  80 6′ 8.4″
 Biden’s farthest distance (aphelion) 69,057,800,000 462 38′ 5.6″
 Pioneer 10’s current distance 11,984,000,000 80 6′ 8.2″
 Voyager 2’s current distance 15,676,265,000  105 8″ 8.8″
 Voyager 1‘s current position 19,051,866,500 127 10′ 7.3″

As I said, use a big piece of paper! Once you’ve got everything drawn up, you’ll have a better idea of just how vast the reaches of space are and just how empty most of it is. (One problem: this will forever ruing the asteroid belt scene in Star Wars for you. Sorry about that!) But it will also give you a better idea of just how neat it is that we can actually see these things and learn from them – and that’s what makes science fun!

One thought on “Bonus Factismal – Somewhere Out There

  1. Pingback: January 20 – Scientists Discover New Planet! (Not) | Little facts about science

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