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.

smallprint

[oldhack]

7 hours in the mountain and central tz.

Re:smallprint

[jfdavis668]

Only 6 on the west coast.

Big deal

[Mister Liberty]

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

Rotation velocity

[Anonymous Coward]

Rotation velocity of 28,000 mph? Shouldn't rotation velocity be measured in rpm or an equivalent? Where is this 28,000 mph from? 45 latitude? Is the assumption that this is the atmosphere or the surface?

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.

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.

Re:Rotation velocity

[Anonymous Coward]

Since you didn't bother to read the article... this is one of the few exoplanets that can be directly imaged across multiple pixels. By doing spectral analysis of each pixel they can measure the doppler blue-shift of the advancing side and the doppler red-shift of the retreating side (the maximum shifts being at the equator) and the difference tells them how fast those points are advancing and retreating. They don't know the physical size of the planet, but they can estimate it, and from the size guesstimate and the rotational speed they can calculate the orbital period.

Re:Rotation velocity

[Anonymous Coward]

Regardless of how many pixels the planet shows up as on an image, it would take a telescope (or interferometer setup) over 2 km wide to be able to distinguish parts of the planet in the visible band (or over 10 km wide for IR work in the bands they used here). They instead used a simple model taking into account you would be seeing both red blue shifted light at the same time, resulting in a broadening of a spectral line.

Re:Rotation velocity

[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.

Imperifail

[Anonymous Coward]

>they measured its rotation velocity as 28,000 mph

LOL. Sure they did.

Re:Imperifail

[jfdavis668]

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

Re:Imperifail

[Anonymous Coward]

The speed of light is, funnily enough, a velocity. Dopper shifts are also a velocity. You epic failed twice.

Geffen + Oprah + Eillson = Clippers ? NOT

[Anonymous Coward]

Not a hope in Hell getting ownership of the Clippers !

Sterling is for sure an ASS HOLE. But the man is entitled to talk as he wants to his "Bitch" aka 'Girl Friend,' any where and any time.

If a sleeze hears the words, the sleeze has no case in court ! That is just the way IT is. Get over it.

The SLEEZE will pay.

Re:Geffen + Oprah + Eillson = Clippers ? NOT

[Mister Liberty]

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

Re:Geffen + Oprah + Eillson = Clippers ? NOT

[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.

Re:But have they determined the length of its week

[Anonymous Coward]

7 * 8 hours = 56 hours.

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.

Re:Derived Properties of the Planet

[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.

Re:Derived Properties of the Planet

[Anonymous Coward]

One assumption that they probably used is that gas-giants from the mass of Jupiter up to perhaps 80x that mass all have approximately the same diameter. The gas of a larger-mass planet is more compressed, and the size of the planet doesn't change much. Smaller-mass gas-giants may end up as 'puffy', a fair bit larger in diameter than Jupiter. Look up super-Jupiter on that most wonder of all sources, wikipedia.

That's a record! Perhaps.

[Anonymous Coward]

This makes it the fastest rotating planet known to mankind. The previous record holder, Jupiter, takes 9 hours and 50 minutes to rotate at its equator. Of course, given the uncertainty of the measurement, the actual rotation period can be off by a wide margin, so it's too early to declare it a record. The measurement is an impressive achievement nevertheless.

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

[Anonymous Coward]

On a side note: are universal distances measured in miles?

If the scale is close enough, sure.

We've got astronomical units based on how far light reaches in one standard earth-year, we've got units based on the orbit distance between earth and its sun, and (my favorite for odd calculation) the parallax arcsecond (parsec).

What makes you think that a mismeasurement of the distance from the equator to the north pole is in any way a superior unit of measurement than the other arbitrary units? (One of the early attempted definitions was to define 10,000,000 meters as the distance from the north pole to the equator, it did not survive the secondary testing, so now the length is defined as 1,650,763.73 wavelengths of krypton-86 in a vacuum. I like the ".73" at the end, it implies they couldn't find any repeatable aspect of nature that cleanly lined up to a meter with any simple multiplier.)

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:But have they determined the length of its week

[jfdavis668]

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

Re:But have they determined the length of its week

[Anonymous Coward]

A week has no physical meaning. A day, a month (for planets with moons, probably required for intelligent life), and a year, have physical meaning. A week is purely an invention of humans. If an alien race had a concept of a week, it wouldn't be based on anything that we could measure. It would be purely cultural.

Re:But have they determined the length of its week

[Anonymous Coward]

Don't even need to hold your breath waiting for an alien race, enough human cultures have concepts of a week both shorter and longer than the now standard 7 day week.

So, I guess astronomically 100x is 'almost'

[Anonymous Coward]

I also remember being told Jupiter was 'almost' a star. Guess within 100x is close enough, reminds me of in my semiconductor class where I was told 10,000 ~= 0 (we were dealing with semiconductor doping).

Re:So, I guess astronomically 100x is 'almost'

[Anonymous Coward]

Good. Can you send me 'almost' 0 dollars?

Re:So, I guess astronomically 100x is 'almost'

[Anonymous Coward]

How about the HF classes?
"Anything below 1 GHz is DC."

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.

Re:How does the rotation speed affect gravity?

[Anonymous Coward]

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?

Probably wouldn't notice the spinning, only that gravity is twice as strong, assuming that the shape doesn't change much (twice as much gravity would kind of squish rock down a lot, and the spinning of the Earth affects its shape a little)., as otherwise the centrifugal force is less than a percent of the force of gravity at the equator.

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

General relativity allows for spinning black holes, and in the not so distant future some long baseline radio interferometer might be able to confirm this by seeing a difference in brightness on either side of a black hole due to a difference in light going around it with or against rotation.