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## Astronomers Determine the Length of Day of an Exoplanet34

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."
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## Astronomers Determine the Length of Day of an Exoplanet

• #### Derived Properties of the Planet (Score:4, Informative)

on Wednesday April 30, 2014 @03:42PM (#46883719)

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.

• #### At what point is it a star? (Score:5, Informative)

on Wednesday April 30, 2014 @03:49PM (#46883805)

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.

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