Astronomers Determine the Length of Day of an Exoplanet

The Bad Astronomer writes: "Astronomers have just announced that the exoplanet Beta Pic b — a 10-Jupiter-mass world 60 light years away — rotates in about 8 hours. Using a high-resolution spectrometer and exploiting the Doppler shift of light seen as the planet spins, they measured its rotation velocity as 28,000 mph. Making reasonable assumptions about the planet's size, that gives the length of its day. This is the first time such a measurement has been achieved for an exoplanet."

But have they determined the length of its week?

[jfdavis668]

I mean, we need to know when to schedule parties.

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[jfdavis668]

What if the week is 8 days, or 9? Are they working on this?

smallprint

[oldhack]

7 hours in the mountain and central tz.

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[jfdavis668]

Only 6 on the west coast.

Big deal

[Mister Liberty]

Day length can amount to half a year on good old Earth.

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[jfdavis668]

It would be measuring whatever is reflecting or generating the light being analyzed. The cloud deck or whatever. The Doppler effect measures velocity, not RPM. The 28,000 is probably the maximum value, so it would be how fast the clouds at the equator are going east. The surface of the Earth rotates at the equator at 1400 mph.

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[cusco]

I still remember when respectable astronomers were phoo-phooing the idea that we would ever be able to detect planets in other stellar systems. This is incredibly cool. I wonder what we'll know about these systems in another 20 years.

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[Urquhardt]

Planet 'A' rotates once and hour. Planet 'B' likewise. But 'A' is the size of Pluto and 'B' is 10 times the size of Jupiter. One really needs to consider two numbers when comparing rotations of planets - either angular velocity or surface linear velocity at the equator AND the size of said planet.

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[jfdavis668]

My car gets 40 rods to the hogshead and that's the way I likes it!

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[Mister Liberty]

I did not understand one word of what you just said, but it sure sounds important.

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[daremonai]

I think he was on Beta Pic b, and the rotation was so fast it made him a little dizzy. He meant to post on Deadspin, but it ended up here instead.

Derived Properties of the Planet

[DanielRavenNest]

You can calculate a lot from this information. From the rotation period and velocity we get a radius of 57,000 km, and an equatorial rotation velocity of 12.5 km/s.

From the mass we get a surface gravity of 389.6 m/s^2 (about 40 g's), but the centrifugal acceleration from rotation is -2.74 m/s^2. Thus the body would not be flattened as much as Jupiter. The density is about 24,500 kg/m^3, higher than Osmium. Iron at the core of a planet is quite compressible, so for a large body such as this, it can give such a high density.

Circular planets, circular reasoning

[Mister Liberty]

Are you sure none of what you just derived wasn't what lead up
to their 28,000 mph in the first place?

On a side note: are universal distances measured in miles?
I think not.

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[BlackPignouf]

Are you sure none of what you just derived wasn't what lead up
to their 28,000 mph in the first place?

It goes further!
From the rotation period, radius and equatorial rotation velocity, we get pi = 3.1578947 and 1 mile = 1.6071429 km

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[Anonymous Coward]

How can you do what you are doing? TFS says they made reasonable assumptions about the planets size to calculate the length of its day. You can't take those results and use them to calculate the exact size of the planet. Shit doesn't work like that. It's like like using 3 as an approximate value for pi, calculating the area of a circle, and then using the calculated area of the circle to figure out pi and declare it to be precisely 3.0000.

At what point is it a star?

[HeckRuler]

Beta Pic b — a 10-Jupiter-mass world

How is that not a star?
I thought Jupiter was half-way to critical mass where it all explodes into a fireball?

Searching...

Nope. I was wrong. Jupiter would need about 75 times more mass before it went nuclear, fused hydrogen at it's core and became a star. A 10-Jupiter mass planet is totally legit.

How does the rotation speed affect gravity?

[uslurper]

How does the rotation affect the gravity of a planet?
If the planet is rotating fast enough, does that reduce the force of gravity, or does the gravity still 'squash' you since it is actually affecting the space around it.

For example, if there was a planet with twice the mass of Earth, but spinning twice as fast, what would it be like to stand on the surface?

Do black holes spin? -or are they 'locked in' because the mass would be impossible to move.