Posts Tagged ‘Stars’

What happens when two stars fall into each other? [Science]

19 Mar
The universe is mostly empty, but once in a great while stars get close enough to crash into each other. Two stars recently combined to form Sco V1309. Find out what happened when they did. More »

How Many Dwarfs Are There?

15 Dec
(Image courtesy Wikipedia)

Most people know by now that Pluto has been downgraded. Astronomers have decided that, conceptually, we should reserve the word "planet" for the small number of dominant bodies in the solar system. Pluto doesn't come close to making the cut. But it didn't just get shoved into the corner as "insignificant object," it got to be part of a brand new class of objects never before defined, the "dwarf planets."

Now, before you complain that, clearly, by virtue of the power of the English language, a "dwarf planet" must certainly be a planet first, a dwarf second, I would just like to mention two things. First all adjective noun combination in the English language are not noun first, adjective second. A matchbox car is, in fact, not a real car. It's OK if a dwarf planet is not a real planet. Second, though, I will acknowledge that the language is unfortunate and misleading. I preferred the term "planetoid" myself, rather than the (intentionally?) misleading "dwarf planet."

Still, forgetting the vagaries of language, we are left with dwarf planets which are not planets. How many are out there besides Pluto? And what is a dwarf planet? The International Astronomical Union (the group responsible for all astronomical nomenclature) has officially declared there to be five dwarf planet (in order of mass: Eris, Pluto, Makemake, Haumea, Ceres), and we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.

Now is a good time, then, to remind ourselves what a dwarf planet really is.

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet. The official definition is that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction.

Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object which is geologically dead and one which might have interesting processes worthy of study.

[Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.]

So how many dwarf planets are there? Five, of course. The IAU says so.

But let's ask the more scientifically interesting question: how many (non-planet) objects in the solar system are large enough to be round due to their own gravitational pull?

Still five, right?

Well, no. Here is where the IAU and reality part ways.

There are many more objects that precisely fit the definition of dwarf planet but that the IAU chosen not to recognize. But if the category of dwarf planet is important, then it is the reality that is important, not the official list. So let's examine reality.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. Vesta, the next largest, is close, but has a large crater blasted out of its side that makes it distinctly oblong. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round.

The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. We might be a bit off in one direction or another, but 400 km seems like a good estimate.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of now, about 50 objects this size or larger are known in the Kuiper belt (including, of course, Eris, Pluto, Makemake, and Haumea). Our best estimate is that a complete survey of the Kuiper belt would double this number, so there are roughly 100 dwarf planets in the Kuiper belt, of which 50 are currently known.

The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

Does it matter how many dwarf planets we say there are?

I think the answer is "yes." If you believe that there are only 4 dwarf planets in the Kuiper belt then you place an oversized importance on those 4 objects and you get an exceedingly warped picture of what the outer solar is like. The important thing about the Kuiper belt is that beyond Neptune there are many many many objects with hundreds being large enough to be round. The four "IAU Dwarf Planets" in the outer solar system are all fascinating objects -- hey! I discovered 3 of them, I must think there are at least a little interesting -- but it would be a gross exaggeration to think of them as the only objects, or even the only important objects, in the fascinating region of space beyond Neptune.

I love dwarf planets. All hundred of them or so.


Look Up!

11 Dec

Did anybody catch Mercury for the first time last night? I had just enough hazy cloud on my western horizon last night that Mercury was lost in the much. If you missed it, keep trying. And if you still can't find it, don't fret: your assignment for tonight is much, much easier.

The planets all travel around the sun in flat disk. Since we sit inside this disk too, when we go outside and look for planets they will all lie along one giant circle around us. Planets move slowly, so waiting for one of them to trace out the giant circle can take a while, but the Moon takes only a month to circle around us, so we can use it to trace the paths of the planets in the sky.

If you've been watching the moon the last few days, you have seen it climbing in the evening sky still growing towards its first quarter (which comes up on Monday - so quickly! Wasn't it a tiny sliver just days ago?).

The earthshine is fading away, as the view of the Earth from the Moon is also moving from full to third quarter.

As the moon has moved eastward, it might have been hard for you not to notice the incredibly bright star that the moon has been getting closer and closer to. It will be at its very closest on Monday night. That star is a great marker for helping you really visualize how fast the moon is moving across the sky. On Monday night, if you look right and sunset and then again a few hours later, you will even be able to notice the different positions in a single night.

That super bright star, though, is more than just a convenient sign post. And it's not a star. It's Jupiter. Jupiter! I think so many of us have gotten used to the fact that NASA and others provide us so many beautiful pictures of planets from spacecraft and telescope that we have forgotten that these things are really there, up in the sky, night after night after night.

Now that you know where Jupiter is (and, again, don't worry if you don't see it tonight; it is going to be the brightest thing gracing our evening skies for the rest of the year) you have a chance to see one of the most spectacular sights in the sky. Go back in and grab some binoculars. If you don't have binocular go back in and call your favorite present-giver and remind him or her that binoculars really would make the perfect present for you. Go back outside with your binocular and find a place where you can hold them good and steady. I like to lean against a wall, but you can try lying on the ground or setting them on a fence or anything that works for you. Now find Jupiter.

If you can get your binoculars steady enough, the disk of Jupiter will come into view. And it will clearly be a disk. Strung out in a line beside the disk will be four little orbs. Stars? Nope. Moons. These are the four moons of Jupiter that were first discovered by Galileo.


On the left, close together are the oddly magnetic Ganymede and the icy ocean filled Europa, close on the right is Io, the most volcanically active place in the solar system, and furthest of all on the right is Callisto, which is, well, just Callisto.

Come back tomorrow and everything is different. There are only three moons. Io and Europa have swapped places, Callisto hasn't moved much, and Ganymede is now so close to Jupiter that you probably won't be able to see it. The next night? All different again.

If you have been paying extra close attention you might even notice that the line that the four moons make basically points in the same direction as the line that our moon is tracing across our sky. Those moons of Jupiter are in the same disk as the planets of the solar system.

This amazing sight - Jupiter and its moons dancing across the sky - is, to my mind, one of the most wonderful things you can see in the solar system, on par with the Grand Canyon or Iguazu Falls or eruptions on Kilauea. Chances are you've never seen it, but it's just outside your door. It's free. Go outside. Look up!


Directly Observing Exoplanets Just Got Easier [Astronomy]

17 Oct
So that Gliese Goldilocks Zone planet may not exist. Sad. Cheer up though, because Arizona University astronomers have discovered a new technique that could make spotting exoplanets a bit easier. Which is great, because right now it's really frickin' hard. More »