# Actual spin doctors eye up ALIEN WORLD Beta Pictoris b: Young, hot ... and really FAST

Alien world Beta Pictoris b is spinning so fast that its day lasts just eight hours as its equator whips around at almost 100,000km an hour. Artist's impression of Beta Pictoris b Boffins – specifically doctors of astronomy – have managed to figure out the rotation rate of the exoplanet for the first time using the ESO's …

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By "10 times bigger than Jupiter" do you mean ten times the radius of Jupiter, ten times the volume of Jupiter or ten times the mass of Jupiter?

#### Mass

Long answer: It is mass, since Jupiter is the about the biggest volume an object, that is not a star, can reach, regardless of mass. Thus, it is pointless to speak of volume for gas planets, and then of course radius.

The reason for this is that if you add more mass you will increase it's gravity, increasing it's gravity will compress it further. It is like if you throw lose snow in a pile, the more you add, the denser it becomes, but the height doesn't really increase for a while.

Beta Pictoris B is about 10 times the mass of Jupiter (current estimates range from 4 to 11x Jupiter's mass). It has about 1.65x Jupiter's radius, so about 4.5x Jupiter's volume.

As a rule of thumb, few objects with masses between Jupiter and small red dwarfs (100x Jupiter's mass) get much larger in diameter than Jupiter unless very high temperatures make them fluffier. With increasing mass comes increasing density, which creates enough gravity to compress the planets / brown dwarfs / red dwarfs in that mass range. This produces oddities like red dwarf stars that have "surface" gravities many times higher than Sol's gravity and brown dwarfs that are ten times more massive than Jupiter but scarcely larger in diameter.

unless very high temperatures make them fluffier

Hot bunny planets!

If it's that big, and spinning that fast, can I safely assume that aliens get flicked off if they go near the equator?

'unless very high temperatures make them fluffier'

Hot bunny planets!

I wonder if there is a pink one out there.

If it's that big, and spinning that fast, can I safely assume that aliens get flicked off if they go near the equator?

Proper answer: if it was spinning fast enough for aliens near the equator to get flung off, then the equator itself would also get flung off. In fact, a high enough spin rate to fling stuff off would have prevented the planet from forming so large in the first place.

#### @cray74 - So over four times the volume isn't "much larger"?

Think I disagree on that point, David verses Goliath springs to mind.

You got an up vote anyway.

"Proper answer: if it was spinning fast enough for aliens near the equator to get flung off, then the equator itself would also get flung off."

Not if the planet's made of something interesting, like silicone, or chewing gum.

"If it's that big, and spinning that fast, can I safely assume that aliens get flicked off if they go near the equator?"

Alas, no. Even in Hal Clement's "Mission of Gravity," its super-fast giant world couldn't tear itself apart or flick aliens off. Gravity only lowered to about 3Gs at the equator and was pretty epic near the poles. [Edit: actually, it's pretty close.]

The problem for centripetal alien-flinging planets is that on the scale of planets centripetal forces get fairly weak. The equation is Acceleration = Velocity x Velocity / Radius. You want that acceleration to match or exceed gravity for aliens to start flinging off the planet. For a planet with 1.65x Jupiter's radius (about 120,000,000 meters, rounding up), you need very high velocities to equal gravity.

A quick approximation of relative gravities is: relative mass / (relative radius x relative radius. Beta Pictoris b (BPb hereafter, or maybe "Beepee") is 10x the mass of Jupiter and 1.65x the radius, so it should have about: 10 / (1.65 x 1.65) = 3.67x Jupiter's gravity. That's nice to know, but not handy for comparing to centripetal acceleration, which the prior equation presents in meters per second (or traditional El Reg units, and I'm less familiar with those than a Lockheed engineer is with metric units). Jupiter's surface gravity is 2.53x that of Earth, so Beepee has 3.673 x 2.53 = 9.3G's at its gaseous "surface," give or take some rounding errors. A final conversion to m/s/s gives 91m/s/s.

Alrighty then, back to spinning in circles. Beepee is spinning at 100,000km/hr, which, now that I think about it, translates to 27,777 meters per second or more than twice Earth's escape velocity. On a smaller planet, aliens would be flinging off the equator and make amusing strings of craters on any moons that got in the way. Very Kerbal.

But Beepee is not a lesser planet. It has a surface gravity of 9.3Gs. The equatorial centripetal acceleration is (27,777m/s x 27,777 m/s) / 120,000,000m = 6.43G. 9.3G - 6.43G still leaves almost 3Gs at the equator.

Wow, that does get close, close where someone should doublecheck my number crunching. Earth's spin effects only lowers perceived gravity by a small percent at the equator so I didn't anticipate such a drop on Beepee. But, no, aliens cannot flee Beepee's gravitational might simply by vacationing at the equator. There's still an unhealthy margin of remaining gravity there.

If it's that big, and spinning that fast, can I safely assume that aliens get flicked off if they go near the equator?

Obligatory xkcd: http://what-if.xkcd.com/92/

#### Bespin

Bespin has a 12-hour day. Just sayin'.

#### Suggestion

Maybe large gaseous planets form mostly from coalescence of rotating gas clouds, whereas rocky planets form mostly from collisions - a large rotating gas cloud would conserve its angular momentum as it contracts, speeding up the rotation. You can see this effect yourself if you sit in an office chair with your legs sticking out and spin yourself round. If you then pull your legs in, your rotation speeds up.

In a field of rocky objects, even if is rotating, as it coalesces, the components will collide and rebound in different directions. Some will be sped up in the direction of rotation, and possibly escape, whilst others will be slowed down, and fall towards the centre. Perhaps someone could model this and see if it results in a lower angular momentum for the final planet.

If it helps to visualise it, consider what happens if you spin a bucket of water (you get a vortex), and what happens if you spin a bucket with gravel in it.

#### Re: Suggestion

Why the downvote? Care to explain, rather than merely dismiss?

#### Re: Suggestion

There's only one gas cloud and that's the planetary nebula itself. It will be spinning in the same direction as its star. Accretion of ice crystals occurs first by electrostatic attraction, and then by gravitational attraction. At that point there are millions of planetisimals in an orbit, which eventually collect into planetoids and finally a planet, sweeping its orbit clear of debris. Your hypothesis requires something to separate a cloud out of that homogenous pre-planetary mess and give it rotation, but if there isn't already a planet there to do that, how might it happen?

#### Re: Suggestion

Give me time to finishing typing! ;-)

#### Re: Suggestion

Actually your bucket analogy points towards the opposite, for me at least. Since a planet's spin would be whole bucket with it's contents. The bucket as it rotates doesn't experience much friction towards the outside. The bucket with gravel will have no internal movement and thus no energy lost to friction. The bucket with water on the other hand will have water constantly shifting it's position and thus loose energy to friction as water bumps into water, which I can only assume has to be taken from it's spin.

*Mine is the lab coat. I will have to go and check this.

#### Re: Suggestion

The thing is that this is an idealised model, and since nobody has observed or reproduced it, it remains that - a model. Current models of planetary formation fail to explain a number of empirical observations (such as why the moon has the same isotopic composition as the Earth's crust if it was formed in a collision with a body from an other part of the solar system). Given that computation fluid dynamics as a computationally intensive process, I'd be very surprised if any of the current models take into account turbulence at all. I see no reason why the gravitational collapse of fluids and the gravitational collapse of a collection of discrete solid chunks should be treated as the same.

In terms of how an area of a gas cloud would separate and coalesce into a planet; consider two things - firstly that for a given area of a rotating gas cloud (that around a forming star), the gas closer to the star will be moving faster than the gas moving farther away, and secondly that although largely homogenous, local non-uniformity in a gas cloud will lead to uneven gravitational forces, and regional collapse. Matter will move away from areas of lower density, and towards areas of higher density. Conservation of angular momentum will lead to rotation of this mass in the direction of rotation of the parent gas cloud.

Also, bear in mind that whilst a gas cloud may start off as homogenous, once a star forms at the centre and fusion begins, the solar wind exerts a pressure which sorts the elements by mass, with heavier metallic elements ending up in the inner planets, and lighter elements ending up in the outer planets. Hence rocky planets with metallic cores followed by gas giants and icy bodies.

#### Re: Suggestion

I concede the bucket of gravel analogy is a bit stretched. I was trying to think of something better that models a dispersed field of objects that are not in immediate contact but are gravitationally bound.

The point I was trying to make is that the mechanics of planetary formation for what are essentially solid planets and fluid planets probably differ due to the difference in nature of solids and fluids.

#### Re: Suggestion

"He doesn't like you."

"I'm sorry."

"I don't like you either. You just watch yourself. ..... "

#### Re: Suggestion

Gas giants also collect rocky material in the same fashion as terrestrial planets, so they're subject to the same bombardment by planetismals. Were your model to work, Loyal Commenter, gas giants would end up with the same randomized spin.

Rather, what appears to happen is something like this:

Step 1: interstellar cloud of dust gets upset (due to a passing star, a nearby supernova, losing money on the races, etc.)

Step 2: This disturbance causes some spot in the cloud to get more mass than the rest, meaning more gravity, and a runaway collapse sets in - definitely a physical collapse, possibly emotional.

Step 3: Most of the cloud collapses into a proto-star (example: Sol has 99.9% of the system's mass) with most direction of the cloud's motion heading toward the proto-star. However, some vague currents and drift leftover from the cloud manifest as a common spin direction. As seen in Sol and the debris disk around a number of other stars, the stars, disks, and planets all tend to move in the same direction on roughly the same plane. In the case of Sol, the planets, asteroids, and sun also show a common direction of spin with a few caveats.

Step 4: To get those caveats, outside forces are involved. Small bodies like asteroids tend to have their spins disturbed by, yes, collisions. There's also the YORP effect, and the wiki article on that discusses the spins of asteroid depending on size. Mercury is close enough to Sol for most primordial spin to have been lost to tidal effects. Venus might've had a normal planetary spin until drag and tides in its thick atmosphere slowed it (according to one group of modern theories). Earth probably had a reasonable spin like most planets until it got beaned by Theia, which left it with a very high spin until Luna's tides damped that. Uranus probably started with a typical spin until - per 2011 theory - it took a succession of large impacts.

Step 5: Profit!

Summary: You were correct to note the influence of impacts on spin, but that's not the only means of altering planetary spin and it does apply to gas (ice) giants like Uranus, too.

#### Re: Suggestion

I see no reason why the gravitational collapse of fluids and the gravitational collapse of a collection of discrete solid chunks should be treated as the same.

Can the materials in the inner and outer systems be treated as chunks in the first and only fluids in the second? We're talking about dust and ice crystals, which will behave much the same while they remain small (an approximation to a fluid) but both will then accrete to become chunks. The ice won't become gaseous again until there's enough heat for it to do so (via Kelvin-Helmholtz, perhaps).

and secondly that although largely homogenous, local non-uniformity in a gas cloud will lead to uneven gravitational forces, and regional collapse. Matter will move away from areas of lower density, and towards areas of higher density. Conservation of angular momentum will lead to rotation of this mass in the direction of rotation of the parent gas cloud.

Those areas of higher density becoming planetisimals, which then compete amongst themselves to gather more gas and ice. How likely is it that one will grow large enough to grab the majority of the gas directly (which would gain its angular momentum from a dominant direction) rather than gain mass from collision with other planetisimals (which would have a more scattered distribution of momenta, due to energy loss from breakups).

Also, bear in mind that whilst a gas cloud may start off as homogenous, once a star forms at the centre and fusion begins, the solar wind exerts a pressure which sorts the elements by mass, with heavier metallic elements ending up in the inner planets, and lighter elements ending up in the outer planets. Hence rocky planets with metallic cores followed by gas giants and icy bodies.

Thanks, I'm well aware of that, but I meant homogeneous within an orbit, where you're expecting your spinning gas cloud to form.

#### Re: Suggestion

Again, the point I was trying to make is that planets comprised mostly of gas (estimates for Jupiter's rocky core suggest it is between 4 and 14% of the mass of the planet), will have formed from gas clouds, whereas planets that are mostly rocky (such as Earth, which has a negligible gaseous component) will have formed from the aggregation of solid particles. Simply put, solids bounce, whereas gases do not. What I am suggesting is that the spread of velocities of a group of gravitationally bound planetesimals is likely to be broader than the spread of velocities of gas molecules in a cloud. Accepted that within that gas cloud there will also be planetesimals, and within a field of planetesimals, there will also be gas molecules, but I am considering the main components of the resulting body.

In a field of planetesimals that are all roughly orbiting in the same direction, there is the opportunity for collisions which will speed some bodies up, and slow others down. Those sped up have the chance to escape, whilst those slowed down move inwards towards the centre of the gravitational well. The angular momentum of the escaping bodies is lost from the field as a whole.

In an area of mostly gas, collapse should be much smoother, and more of the original angular momentum is retained.

Bear in mind that I am talking about areas that may originate from regions of a larger accretion disc several AU across, there is going to be a significant differential in rotational speed between the inside and outside of such areas - I propose this as the origin of such angular momentum within the planetary accretion disc. The result of such differentials is easily seen in the atmospheric turbulence of the gas giants - Jupiter's 'Red Spot' being a very good example.

#### Re: Suggestion

In an area of mostly gas, collapse should be much smoother, and more of the original angular momentum is retained.

In an area of mostly gas, collapse would be much slower and more prone to disruption from all those planetisimals in your AUs-wide ring. Look at how Jupiter has kept the asteroid belt from forming into a planet. It would take far, far less to stop an amorphous cloud of gas from starting to spin in its own right, without something already at its centre to hold it together.

#### Re: Suggestion

I don't know who's right, but I reckon this is the best argument I've seen on the Reg for a good while.

#### Re: Suggestion - why the moon has the same isotopic composition as the Earth's crust?

Don't really see the problem myself.

Given the nature of the collision there would have been some pretty thorough mixing of the two bodies thus evening out the isotopic mix if there was any difference. The Earth's core would probably remained more or less intact but the mantle and surface and that of the impactor would have molten and been thrown into orbit like trajectories. The lower stuff fell back to become crust on Earth and the further away stuff remained in orbit to coalesce as the Moon.

#### Re: Suggestion

Not completely accurate. When the star ignites, it gives out a shockwave, which with the gravitational force from the newborn star, does give a angular increase to the speed, depending on where the object, cloud is, will determine whether it is rotational speed, orbital speed, or if it is ejected.

There are formulas out there (NASA and ESU) that can calculate this. Think of a rope, as you spin the rope close it goes faster, as you let out slack, it goes slower, takes more to push it out (ejection velocity), but as you fling some slack out to the end, the rope increases velocity ever so slightly (think flicking with wet towel).

Kind of like this, but think, in space, when you push on an object, you either 1) increase speed/cause momentum or 2) decrease momentum (friction, think Moon-Earth relationship).

http://www.sjsu.edu/faculty/watkins/planetarysweep.htm

#### Question regarding the Big Bang

A question not linked to spin, though I like the idea of a rapidly spinning planet mitigating the effects of high gravity.

The question is w.r.t the Big Bang. It seems logical that when a black hole is formed, the explosive outward force is overrun by the gravitational collapse. At that point the Event Horizon forms, we would effectively get a Big Bang within the formed singularity from the perspective of something inside the black hole.

In essence the Big Bang would be an explosion within a singularity, and thus to an observer in that universe, it would look like their universe is expanding at an accelerated rate, when in fact both expanding and accelerating towards the singularity, essentially shrinking infinitely.

This is consistent with the idea of multiverses, does not need dark matter to explain away what we are seeing, and fits with the curvature of space.

Therefore I assume it is correct to see the Big Bang as an explosion caused by a catastrophic collapse into a singularity?

#### Re: Question regarding the Big Bang

A singularity is a dimensionless point of infinite density, so it's difficult to see how it could be described as an explosion. Our mathematics breaks down at that point, but then our theory of gravitation breaks down long before, when the collapsing mass reaches the quantum scale, so really we don't know (and possibly can't know).

#### Re: Question regarding the Big Bang

a number of physicists, Sir Roger Penrose notably among them, have posited theories of a cyclical universe. Some of those theories have been discredited or possibly even disproven (stupid work internet filter prevents verification), but there is a certain beauty to the idea of such symmetry.

#### Re: Question regarding the Big Bang

a number of physicists, Sir Roger Penrose notably among them, have posited theories of a cyclical universe. Some of those theories have been discredited or possibly even disproven (stupid work internet filter prevents verification), but there is a certain beauty to the idea of such symmetry.

If I recall what I've read on the subject correctly the cyclical universe model has been mostly dismissed by cosmologists. I believe the current theories suggest the universe will eventually end in a big freeze as entropy sets in rather than a big crunch as gravity pulls everything back together.

I've also heard a big tear theory where the fabric of space-time rips under the strain of the accelerating expansion of the universe, but that may have been in a sci-fi novel rather than a scientific journal. I know that one sci-fi series I'm fond of ends with a rather shocking confirmation of that model (nope, not saying which, no spoilers here) but I can't recall if it was also a serious model or not.

Back to the original post, I find the idea that the universe exists in the event horizon of a black hole very interesting. Unfortunately my knowledge of cosmology and black hole mechanics stops far short of being able to know how likely it is.

#### Re: Question regarding the Big Bang

Relative to observer watching a black holder, the event horizon is the singularity, which can never be reached. Relative to someone passing the event horizon, the forces will continue to accelerate you towards the singularity.

Time has changed relative to observer each side of the event horizon. The observer inside the black holder, w.r.t to the cycles of light - how time is measure - is now in a new universe. It is really a no-brainer once you think about it.

#### Re: Question regarding the Big Bang

I am not sure you have understood - I am not in anyway suggesting a Big Bang followed by a Big Crunch.

#### Re: Question regarding the Big Bang

I think you have misunderstood. I am definitely NOT talking about a Big Bang - Big Crunch. I am talking about a Multiverse.

#### Re: Question regarding the Big Bang

I am talking about a Multiverse.

#### Re: Question regarding the Big Bang

"It is really a no-brainer once you think about it."

I see what you did there :-)

#### Spin me another one

Meh!

It's a slow work week, so here we go:

That artificial brain the researchers at Stanford created is pretty impressive and potentially very useful. And that 3-atom-thick wire. And 3D transistors. And study of orbital mechanics brought us GPS and satellite TV, etc. I'm with you there. But how is knowing that a planet around a star 67 l.y. away spins twice as fast as Joopiter after it's been lubricated going to benefit the world? I'm failing to see the big picture. This kind of research sounds more like some kind of a sink for grant money to me.

Disclaimer: I myself sometimes / regularly in my spare time code away at little pet projects that are of no use whatever to another man so I know the entertainment value of such endeavors. But nobody pays me for it.

(Which brings me to the inevitable question: Is there any way that I can turn those megabytes of uselessness into cash in the name of research? Send your answers together with a \$25 processing fee to the address provided.)

#### Re: Spin me another one

@ Sceptic Tank - "turn those megabytes of uselessness into cash in the name of research?"

That's too easy: Link it to some AGW hogwash and watch the \$\$s flow in, probably get a chair at the University of East Anglia's Climate Unit as well.

My consultancy fee for this scam, sorry, plan is a decade long holiday in the Bahamas before they disappear under the rising waves. Pip pip!

#### Re: Spin me another one

So what you are saying essentially, is that it's not worth spending money on finding out how the universe works.

Lucky our ancestors didn't feel the same way, eh?

#### Re: Spin me another one

Lucky our ancestors didn't feel the same way, eh?

Some of them did. Galileo could tell you all about it if he were still around.

But a bit of perspective here. Eventually life on Earth will be wiped out. That's an unavoidable fact. It most likely won't be in our lifetimes, but it will happen. When it does the only hope humanity (and our favored pets, plus - more importantly - the mice) have to survive is to have colonized other planets. In order to do that we need to first learn as much as possible about the rest of the universe. The 8 planets plus several dozen dwarf planets we can study in our solar system aren't going to cut it for filling in our knowledge.

Granted we could just ask the mice, but they're still pretending to be dumb animals rather than a race of super genius computer boffins so they may not answer.

#### Bah!

Beta Pictoris b is obviously the spiritual grave of banking ethics.

#### Carbon Monoxide?

" The team picked out the molecular signal of carbon monoxide in the air and watched how it changed as the planet rotated."

Ooh, carbon monoxide....did they pick up any traffic jams (afternoon commute :)) as well ?

(Better get jammin' out the door...gets me coat..)

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