Tag: Pluto

Attack of the Pluto!

By Phil Plait | July 25, 2012 1:21 pm

I was interviewed once again on the G4 TV program "Attack of the Show!" – I guess they didn’t learn their lesson the first time – where my pal Matt Mira (from Nerdist!) and I talked about planets around other stars, and whether Pluto is a planet or not.

Here’s the segment (you may need to refresh this page to load it):

I pretty much restated my case that I’ve been making about Pluto for quite some time: you can’t really define what a planet is, so the argument over whether Pluto is one or not is the wrong thing to be talking about. The fact that a fifth moon was recently found is irrelevant; "planet" is more of a concept than a defined object. Trying to draw firm borders around a fuzzy thing like this only guarantees more arguing and less light shed on the topic.

Anyway, it was a fun interview, and it’s always a good day when I can get one good zinger in, especially at Matt’s expense.

Related Posts:

Mars Attacks of the Show!
In which we define what a planet is
A fifth moon for Pluto!
Shining shoes for NASA
My Nerdist episode is online!

A fifth moon for Pluto!

By Phil Plait | July 11, 2012 10:12 am

Astronomers have just announced that tiny Pluto has a fifth moon! It was discovered using the Hubble Space Telescope:

You can see it in that image (click to enhadesenate) in the green circle. Pluto was targeted by HST for several observations in late June and early July, and P5 – also called S/2012 (134340), the moon’s designation until it gets a proper name – was seen moving around the tiny world. This image is from July 7.

As moons go, it isn’t much: it’s probably only about 10 – 25 kilometers (6 – 15 miles) across, making it one of the smallest moons detected in the entire solar system. That’s actually pretty amazing, given Pluto was 4.7 billion km away (2.8 billion miles) when these images were taken!

Pluto was observed in part to look for more moons. In 2015, the New Horizons probe will zip past Pluto, and scientists want to know as much about the system as they can before it gets there. The odds are low of them hitting any of those moons – space is big, and the moons and spacecraft are small – but a) better safe than sorry, and 2) if there are more targets to observe we want to know now so they can be added to the itinerary!

Observations like this are good for discovering moons and getting their locations, but size is a different matter. Literally. We know how far away the moon is, and how bright, but it’s far too small to directly get the size. Its diameter has to be estimated by assuming how reflective the surface is. If it’s dark like coal, it has to be bigger to be so bright, and if it’s shiny like ice, it’s smaller. That’s why we don’t know P5’s size to even within a factor of 2! But once New Horizons zips past, it may be able to nail down the size far better.

The first moon of Pluto, Charon, was discovered in 1978. Nix and Hydra were found using Hubble in 2006, and the fourth moon just last year, in 2011.

As for the argument about Pluto being a planet or not, this will no doubt provide grist for the mill. However, number of moons does not a planet make; Mercury and Venus have none and they’re planets. Mars has twice as many as Earth does, but it’s not twice the planet! And many very small asteroids have moons, too.

My feelings about this are on record: the word "planet" is not and can not be defined; it’s a concept, not a definition. It’s like the word "continent": it’s more of an idea than something you can rigidly define. There is no sharp border that you can use to divide objects into planet and not planet.

So I actually don’t care if you call Pluto a planet or not. It is what it is: a very cool object, perhaps the biggest in the Kuiper Belt of frozen icy comet-like bodies past Neptune. It’s an oddity, since it’s so bright, and yes, has so many moons.

And it’s absolutely worthy of study, no matter what you call it.

Image credit: NASA, ESA, and M. Showalter (SETI Institute)

Related Posts:

Pluto has another moon!
The unbearable roundness of being
BAFact math: How bright is the Sun from Pluto? (and the followup, How big does the Sun look from Pluto?)
Shining shoes for NASA

BAFact math: how big does the Sun look from Pluto?

By Phil Plait | March 16, 2012 10:16 am

[On January 4, 2012, I started a new features: BAFacts, where I write an astronomy/space fact that is short enough to be tweeted. A lot of them reference older posts, but some of the facts need a little mathematical explanation. When that happens I’ll write a post like this one that does the math so you can see the numbers for yourself. Why? Because MATH!] 


Today’s BAFact:

From Pluto, the Sun is so far away it would appear to be a point in the sky like a star, though an incredibly bright one.

Yesterday, I showed how the Sun would still be painfully bright even from Pluto, far brighter than the full Moon looks from here on Earth. But how big would it look in the sky?

It turns out, that math is even easier than it was to find the brightness! The size of an object on the sky depends on how big it really is, physically, and how far away it is. If you double the distance to an object, it will appear half the size. Easy peasy*.

So, as I established yesterday, on average Pluto is about 39 times farther from the Sun than the Earth, so if you were standing on Pluto (hopefully, in a well-heated and insulated spacesuit!) the Sun would appear 1/39th as big, or 0.026 times as big as it does from Earth.

What would that look like?

Well, the size of the Sun in the sky from Earth is about a half a degree — remember, there are 360° in a circle. So from the horizon to the zenith is 90°, and your outstretched fist is very roughly 10°. The Sun is about 0.5°, so you can block it with a single finger held at arm’s length.

From Pluto, though, it’s far smaller: less than 1 arcminute in size (a degree is divided into 60 arcminutes, so from Earth the Sun is about 30 arcmin across). That brings up an interesting point: the smallest size the human eye can easily resolve is something about an arcminute across. Anything smaller than that looks like a dot.

So from Pluto, the Sun would look like a star — that is, a point of light — albeit an intensely bright one. Looking at it would certainly be painful, and probably make your eyes tear up.

But wait! I also mentioned yesterday that Pluto’s orbit is an ellipse, and it goes from 4.4 billion to 7.3 billion km from the Sun. That’s a factor of 29 to 49 times the Earths distance from the Sun. So that shrinks the size of the Sun accordingly. When Pluto is farthest from the Sun (called aphelion) the Sun is far less than an arcminute in size, and looks like a dot. When Pluto is closest to the Sun (perihelion) it will actually be just about one arcminute in diameter. Someone with sharp eyes might be able to perceive it as a disk rather than a point of light… though that would still be really tough to do, because the Sun’s still so bright. If you had a filter in your spacesuit visor you’d be able to see the disk of the Sun.

If you’re curious, blogger Burton MacKenzie made a simple diagram showing how big the Sun is from each of the planets (thumbnail shown here; click to ensolarnate). He didn’t put Pluto on it, but from there the Sun would look even smaller on average than it does from Neptune.

Never forget: the solar system is big! The New Horizons probe was launched in early 2006, is screaming across the solar system at 15 km/sec (fast enough to cross the entire US in about 5 minutes!) but still won’t pass Pluto until mid-2015.

Space is deep, vast, and empty. From far enough away, even the Sun itself would be dimmed to invisibility. If there’s a life lesson in there somewhere, feel free to find it.

Image credit: ESO, annotated by me

* Well, almost easy peasy. This only works well if the object is far enough away that it appears small to you. There’s actually a trigonometric formula to do this exactly, but it hardly matters; for something the size of the Sun, even at Mercury’s distance, saying its apparent size changes linearly with distance is OK.

CATEGORIZED UNDER: Astronomy, BAFacts, Cool stuff, Debunking
MORE ABOUT: angular size, Pluto, Sun

BAFact math: How bright is the Sun from Pluto?

By Phil Plait | March 15, 2012 11:07 am

[On January 4, 2012, I started a new features: BAFacts, where I write an astronomy/space fact that is short enough to be tweeted. A lot of them reference older posts, but some of the facts need a little mathematical explanation. When that happens I’ll write a post like this one that does the math so you can see the numbers for yourself. Why? Because MATH!]

Today’s BAFact:

From Pluto, the Sun is fainter than it is from Earth, but still can be 450x brighter than the full Moon.

I remember reading a science fiction story many years ago which took place on Pluto. The author described the Sun as being so faint that it looked like just another bright star (too bad I don’t remember the name of the story anymore). I was thinking about that again recently, and wondered just how bright the Sun does look from Pluto. This turns out to be pretty easy to calculate!

First, you need to understand how an object like the Sun — really, any source of light — dims with distance. The Sun emits light in all directions, so as you get farther away from the Sun, that light gets spread out. Imagine a sphere perfectly encasing the Sun right at its surface. Each square centimeter has a certain amount of light passing through it. If I double the size of the sphere, there’s a lot more surface area to that sphere, but the total amount of light passing through it hasn’t changed. Therefore the amount of light passing through each square centimeter has dropped. Since I doubled the sphere’s diameter, I can figure out how much its dropped, too!

The formula for the surface area of a sphere is

Surface area = 4 × π × radius 2

If I double the size of the sphere, everything on the right side of the equation stays the same except for the radius, which is now twice as big. Therefore the area increases by 22 = 4. So the light passing through each square centimeter of the bigger sphere drops by a factor of four. Someone standing on that sphere would see the Sun being 1/4 as bright as if they were on the surface.

If I make the sphere ten times bigger, the area goes up by 10 × 10 = 100 times, and the brightness drops by 100. You get the picture.

So now we’re ready to figure out how bright the Sun is from Pluto!

The Earth orbits the Sun, on average, at a distance of about 150 million km. Pluto has a very elliptical orbit, but has an average distance of about 5.9 billion kilometers, or roughly 39 times the Earth’s distance from the Sun. Using the method above, the Sun must be 392 = about 1500 times fainter, or more grammatically correctly, 0.00065 times as bright. That’s pretty faint!

Or is it? Well, let’s compare that to how bright the full Moon looks from Earth. To us here at home, the Sun is about 400,000 times brighter than the full Moon, so even from distant, frigid Pluto, on average the Sun would look more than 250 times brighter than the full Moon does from Earth!

Pluto’s orbit is also highly elliptical, stretching from 4.4 billion km to just over 7.3 billion km from the Sun. Doing the math again, that means the Sun goes from being 0.0012 to 0.0004 as bright as it is from Earth: a range of roughly 150 to 450 times as bright as the Moon from Earth. That’s a change in brightness by a factor of three!

Still, given that you can read by the light of the full Moon, obviously the Sun from Pluto is still pretty dang intense. It would hardly look like just any other star: it would greatly outshine everything else in the sky. Painful to look at, most likely. So the short story I read was wrong, but at least we learned something. That’s a decent trade.

And let me leave you with a question: From Pluto, how big would the Sun look? Ah, that’s a BAFact for another day. Tomorrow, actually!

Image credit: Dan Durda, showing Pluto, its moon Charon, and the Sun.

CATEGORIZED UNDER: Astronomy, BAFacts, Cool stuff, Top Post

Your chance to lick Pluto

By Phil Plait | March 6, 2012 10:50 am

I got a note from astronomer Alan Stern, who is the Principal Investigator on the Pluto New Horizons probe currently on its way for a 2015 flyby of the diminutive world. There’s a drive to get a US postal stamp made to honor Pluto, and Alan was letting me know the petition is doing well, but has a long way to go: they almost at 10,000 signatures, but they want 100,000!

You can read more about this in my post from early February. I think this is a pretty nifty idea, and if you do too go and sign the petition. If we can get this rolling now, a stamp will be issued to surplant the one made in 1990 that said Pluto has "Never been Explored"… just in time for that to be no longer true.

But hurry — your last chance to sign the petition is March 13, in just a week. Go!

CATEGORIZED UNDER: Astronomy, Cool stuff

Give Pluto your stamp of approval

By Phil Plait | February 1, 2012 11:00 am

In 2015, NASA’s New Horizons spacecraft will zip past Pluto, giving us our first close-up view of this tiny world.

The team behind the space probe have a nice idea to help raise awareness of it: make a new US Post Office stamp commemorating it. My friend Dan Durda, both an accomplished astronomer and artist, created this lovely design of the stamp:

[Click to enhadesenate. Note: the word "Forever" means the stamp is always good for first class postage, and is crossed out here to prevent forgery.]

It shows the spacecraft going by Pluto and its (relatively) freakishly large moon Charon. I like how he didn’t go for photorealism, but instead used an oil paint-like feel for it. The stamp is meant as a followup — I might even say send-up — of a US stamp issued in 1990 about Pluto that has the label "Not Yet Explored".

I like this stamp! I’d love to see it made official, too. Alan Stern, the head guy for the mission, created a petition to help that along. It takes more than just a nice stamp design to get the PO’s notice; it has to have public support as well. I signed the petition, and if you want to, please do.

I’ll note that I expect this to raise the specter of whether Pluto is a planet or not. I have some thoughts on that, and I’ll be posting again soon on that topic.

Related posts:

Pluto has another moon!
Find cold, distant worlds with Ice Hunters
Pluto still may be the biggest dwarf planet
Percy, Percy, me

CATEGORIZED UNDER: Astronomy, Cool stuff

Pluto has another moon!

By Phil Plait | July 20, 2011 8:57 am

This is pretty nifty: astronomers have discovered a fourth moon orbiting Pluto!

The tiny chunk of ice was found in Hubble images taken just a few weeks ago, and was clearly seen among the three previously known satellites:

It doesn’t have a name yet — it’s designated S/2011 P1 (or just P4 informally) — and it’s only about 13 – 34 km (8 to 21 miles) across. The size is estimated by measuring its brightness and assuming it’s icy like Pluto itself — a more reflective (white or icy) object would appear brighter than a darker object if they are the same size. Since its actual reflectivity isn’t known, the size has a wide range. But it’s still pretty dinky. For comparison, Pluto itself is 2300 km across, and its biggest moon Charon is well over 1000 km in size. I’ll note our own Moon is 3470 km across, so even Pluto is pretty small.

The thing is, in that single image above you can’t be sure if the object is a moon or a coincidentally placed background star. The solution: take a second image! That was done, clinching the moon’s identity:

See how it’s moved? Mind you, in the week or so between these two images Pluto moved substantially compared to background stars, and the moon moved along with it around the Sun at the same time it’s going around Pluto. You can see the motion of the other moons as well.

In the image, the diagonal lines are an optical effect inside the telescope itself. Pluto is very bright, so the astronomers used some processing techniques to make it appear much fainter, taking multiple images and subtracting one from the other to remove the glare of Pluto (it doesn’t work perfectly, which is why there is a black strip across the image; that blocks unwanted noisy light). I did this myself on many images when I worked on Hubble. It’s amazing how well it works, as you can see in the image above.

Mind you, Pluto was 5 billion km (3 billion miles) from Earth when this image was taken! But we’ll soon get much better pictures: the New Horizons probe will fly past the tiny world in 2015, snapping away as it does. We’ll probably learn more about Pluto in a few hours than we have since its discovery in 1930.

I wonder what they’ll name this iceball? The two moons Nix and Hydra (discovered in 2005) were named after Roman mythological characters associated with night time and Pluto. Cerberus seems like an obvious choice, but there’s already an asteroid with that name. Maybe they can change the spelling a bit to Kerberus to get around that. There’s already an asteroid named Persephone, too, if you’re curious. We’re running out of good names!

Well, whatever it’ll be called, it’s there, and we’ll see it up close in personal in just a few more years.

Credit: NASA, ESA, and M. Showalter (SETI institute)

Related posts:

Ten Things You Don’t Know About Pluto
The Unbearable Roundness of Being (about the definition of "planet")
Pluto may still be the biggest dwarf planet
Pluto wanders into a Messier situation

CATEGORIZED UNDER: Astronomy, Cool stuff, Science, Top Post

Find cold, distant worlds with Ice Hunters

By Phil Plait | June 21, 2011 12:42 pm

In July 2015, the New Horizons probe will fly past Pluto, snapping pictures and taking data of this icy world. Whether you think Pluto is a planet or not* it’s still a fascinating object and I can’t wait to see what New Horizons sends back.

But what happens after that? The space probe will still be traveling into deep space at high speed… but space out there isn’t empty. Beyond Pluto lie the solar system’s coldest denizens, the Kuiper Belt Objects. These are lumps of ice, frozen chunks that may take millennia to circle the Sun once. We’ve identified over a thousand of these KBOs, and there are tens of thousands more waiting to be discovered. Scientists with the New Horizons mission are hoping to find one near enough to the probe’s path to plan a flyby, so we can finally see one of these things up close.

And that’s where you come in!

A new website, Ice Hunters, has been put together to help you find potential KBOs for New Horizons to visit. It’s part of the Zooniverse; a citizen science project that gets people involved in real astronomy. In this case, you can examine images from the giant Magellan and Subaru telescopes to look for targets. It’s actually not terribly hard; here’s one image I looked at:

Basically, KBOs move over time, so two images are taken some time apart. One is digitally subtracted from the other, so stars tend to go away (though they don’t erase perfectly, leaving those ugly residuals). Any whitish blobs left are things that have changed between the two images: variable stars, asteroids, cosmic rays, and, hopefully, KBOs. When you find something you simply tag it by clicking it. A circle is placed around it, and the location is logged. You can see the object I found in the image above.

Humans are pretty good at this, while computers are easily confused by the messy residuals. But just to make sure every click is saved and compared to the work of other people. The more an object is clicked, the more likely it is to be something real and worth following up. The website explains how all this works.

That’s all there is to it! You have to register to do this (unless you’re already a Zooniverse member); it’s free and easy. And who knows? You may literally be the person who finds an icy world that will get a visit from New Horizons!

New Horizons image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

*My opinion: defining the word "planet" is a bad bet if not impossible. If you want the longer story, check out this article I wrote for Discover Magazine.

Related posts:

Hubble spots a chunk of ice 6.7 billion km away
YOU can find extrasolar planets

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Space

Pluto still may be the biggest dwarf planet

By Phil Plait | November 16, 2010 7:00 am

Is Pluto a planet?Mike Brown is an astronomer, and in my opinion is mainly responsible for kick-starting the demotion of Pluto as a planet — he and his team found Eris, an object in the outer solar system that was apparently bigger than Pluto. It was this discovery which set in motion the events that led to the foofooraw about Pluto, and the vote that turned it (and Eris and many others) into "dwarf planets".

Mike continues to observe Eris and other dwarf planets (as well as search for new ones). These objects are small and far away — did you know our own Moon is substantially larger than Pluto? — and therefore hard to analyze. Even with huge telescopes, these objects are hardly more than dots.

However, a fortuitous event landed in the laps of astronomers recently: Eris passed directly in front of a faint star. To us on the ground, it appeared as if the star winked out as the dwarf planet passed in front of it. By carefully timing the duration of this mini-eclipse, the size of Eris could be estimated.

And, to everyone’s shock, Eris looks to be roughly the same size as Pluto. Mike describes all this on his blog.

What does this mean for Pluto? Read More

CATEGORIZED UNDER: Astronomy, Piece of mind, Science

Big Fat Whale has the scoop on Pluto

By Phil Plait | October 5, 2010 2:41 pm

bigfatwhale_plutoSpeaking of Pluto, one of my favorite web comics, Big Fat Whale, has a pretty funny one up about tabloid science.

Poor Pluto. First Mike Brown demotes it, then it’s caught in a custody battle.

I’m a big fan of satire, and BFW is a great mirror on society. I suppose some of it is NSFW, but then, that’s one of the reasons I work from home. Everything is SFW for me!

MORE ABOUT: Big Fat Whale, Pluto

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