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back to article Stargazers spy retrograde planetary bloater

Astronomers have spotted what they claim is the first exoplanet in a retrograde orbit around a star - a bloated body which is also the "least-dense planet currently known". Dubbed WASP-17b, the body measures 1.5-2 Jupiter radii but weighs in at just half its mass, meaning it's about a dense as polystyrene and "if you could place …

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WTF?

but weighs in at just half its mass

Jupiter is 90% hydrogen and 10% helium. I can't see how a larger gas giant can be so much lighter.There are no lighter elements.

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Coat

@ AJ - It's not lighter

It's less dense...

Mind you, I can't help thinking, you'd need a bloody big bathtub...

The vacuum rated one, please!

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Go

@lighter

It's more spread out, duh. Jupiter is close to being as large as a planet can be, and if we found a way to add significant mass to Jupiter, it would get *smaller*, not larger, as the gases became more compressed.

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@ac

If it's less dense it'll be lighter. Easy.

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@Anonymous John

As the article says, it is a lot less dense than jupiter. twice the size, but half the mass, suggests that it's got a lot more empty space in it, it could even be made of much heavier elements than jupiter, just less of it and very spread out.

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*Anonymous John

I imagine it's because the super-close orbit (1/8th that of Mercury) causes both tidal effects and heating to literally inflate the planet.

What I can't picture is how it can be at 150 degrees off the orbital plane. Surely it only reaches 90 degrees off, and then it's just 90 degrees off in the other direction?

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Headmaster

space

Density, Anon John. If the gas is more disbursed than the gas of Jupiter, then it can weight less.

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Flame

talking hot air

Jupiter is just that - a gas giant with a small , dense liquid core. The density of an ideal gas like hydrogen is proportional to its temperature, all other parameters being constant. This planet is so close to its sun, it must be much hotter. It probably has its core heated by gravitational and magnetic effects as well. The only surprise is that it hasn't boiled off into space. Perhaps it has only entered this orbit recently, and will be boiled away in a few (tens of ) million years.

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Re: but weighs in at just half its mass.

Same stuff, less densely packed.

See the bit about tidal forces causing it to remain large rather than collapsing under gravity to a more sensible size for its mass.

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Boffin

How dense is it?

I suppose the tidal forces causing the bloat have done something to the density of the planet. Larger (i.e. higher volume) than Jupiter, but less mass. So it must be a lot less dense than Jupiter.

I'm not a physicist, but guess that the closeness to the sun means that gravitational effects on the planet pull some of its mass away from its centre - causing those tidal forces the boffins were talking about, and reducing the density of the gas giant.

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re: Anonymous John

It's not the mass, it's the density.

The atmosphere must just be thinner.

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Maybe...

Maybe it's made of Philadelphia soft cheese, apparently that's *really* light. Or Maltesers.

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@ Anonymous John

Most of Jupiter's *atmosphere* is hydrogen and helium, there's presumed to be a much denser core comprising anything between 5 and 15% if Jupiter's total mass (that's 15 to 45 Earth masses) made of elements such as silicon and iron.

If this new planet either doesn't have a rocky core, or has a very small core (like Uranus) it would have a lower density. Saturn, for instance, has a bulk density lower than water; again because it has a relatively small core.

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Coat

So close to it's sun?

Maybe it's the planetary equivalent of popcorn?

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Bah!

The only real light element is Tungsten.

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@ Anon John

By being more diffuse?

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Boffin

Lighter - hotter - bigger

Have less matter but heat it up so it expands and occupies more volume.

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@Anon John

Perhaps it's only got half as much hydrogen and helium? Jupiter is quite big and heavy, so it's entirely possible for much lighter gas giants to exist (Saturn, Neptune?).

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Density

"...but weighs in at just half its mass, meaning it's about a dense as polystyrene".

Planet? Or another example of HP's packaging run amok! It's probably just a replacement mouse-ball misdelivered to the wrong solar system...

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Joke

Oh my god...

Bloated body spinning in the opposite direction? Hmm, this isn't a planet, it's the Death Star built by the Dark Lord himself - Steve Ballmer.

The lack of mass must be down to it housing the Microsoft PR and marketing departments.

(Sorry I couldn't resist that)

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Boffin

@Anonymous John: it's a matter of compression

"There are no lighter elements."

It's a matter of compression: if you squeeze hydrogen at hundreds of thousands of atmospheres pressure at some temperature, it will be more dense than hydrogen at the same pressure but a much higher temperature.

This planet is MUCH hotter than Jupiter - thus the hydrogen in this planet will be MUCH less dense than the hydrogen in Jupiter.

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@ Anonymous John

Which weighs more, a balloon full of steam, or an ice-cube? Yet they're both made of water.

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mass, density, temperature.... or somefink

i dunno. something like if you have a hyperthetical ballon, and you are able to gently heat the gas inside the ballon without it bursting, then when the ballon is twice the size, it will be half the denisty for the same given mass.

so this thing is having half the mass of jupiter, is twice the size of jupiter, so maybe its structure is carrying about 4 times the heat energy of jupiter, if it has the same material composition.

given that it is so close to the sun, from what my skim reading told me, it's orbital period is under four earth days, an increase in planetary heating i'd think is a given.

but then if it had the same mass as jupiter, its gravitational effect would be twice as strong, and so it would be half its current size? ie, equal to jupiter? i dunno. maybe at room temperature, or in the bath big enough to float this monster - rather than the hostile irradiated vaccum of space maybe?.

what i think is amazing is that the planet's magnetosphere is still strong enough to maintain a gasous body this close to the star without it being blown away by the solar wind/radiation.

but then it is probably also driven by the same tidal forces from within the star that have given it such a funky orbital disposition in the first place.

but then, i have the mental capacity to be entertained by an elastic band untill home time

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@ Anonymous John

It could be made of Hydrolium. It's a new gas that lets you compress large gas giants down into the size of a bathtub.

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@Richard_C

"Planet? Or another example of HP's packaging run amok! It's probably just a replacement mouse-ball misdelivered to the wrong solar system..."

You son-of-a-b......

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Joke

@SynnerCal

"The lack of mass must be down to it housing the Microsoft PR and marketing departments."

No, housing those groups would surely increase it dense-ness....

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Composition

Maybe it's made of things lighter than hydrogen: Ballmer's insight, Gates' integrity, Jobs' humility, Blogojovich's honesty, Gordon Brown's utility, Dell's innovation, NASCAR's excitement, actress' intelligence, ...

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Anonymous Coward

"orbiting its star every 3.7 days"

Wouldn't that be 1 year, rather?

Or does its year have 3.7 days?

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Yes, yes

I do know about density.

Which is much less than any of our four gas giants.

Its proximity toíts sun could account for that, I suppose.

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Coat

Bathtub

"You get this bath. Right. A large round bath. And it's made of ebony."

"Where from?" said Arthur, "Harrods was destroyed by the Vogons."

MInes the coat with the copy of THHGTTG in the pocket.

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Alien

You're all wrong

Ask a creationist, or scientologist. It is obviously made of elements our "handed down" science haven't found. That our creationist overlords have built in another of their "Lab worlds"

That explains it's size and composition far easier that things like tidal forces, thermic expansion or gravitational fields.

How can our puny minds comprehend this fact.

Anon in case the chuch of nut- scientology comes looking.

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Welcome

@Maybe & Anon John

How long is a piece of string (cheese)?

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