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."

Comments

But have they determined the length of its week?

[jfdavis668]

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

Re:Rotation velocity

[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.

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.

Re:Circular planets, circular reasoning

[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

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.

Re:Rotation velocity

[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.