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trinitree88
2009-Jun-30, 04:10 PM
Astronomy magazine has put out a glossy special on "The Milky way" inside & out. It has a recipe for a slow planet and a small star.
p. 21 Three planets are orbiting Gliese 867d, a 10th magnitude red dwarf 15 light years away. In the article by noted planet hunter Geoff Marcy, there's something awry with the specs.
Planets 1 and 2, Jupiter sized,... orbital periods are 30 and 61 days in a tidal locked resonance. While monitoring them they discovered a third of 7.5 Earth masses with a period of 1.9 days. The following page has a velocity/time graph confirming this....with a velocity of 15 mph(~25km/h)

That's pretty slow, since even at my age I can run faster than that:shifty::doh: With a period of 1.94 days...that's 1.94 days/rev times 24 hrs./day times 15mph= ~ 698 miles for the orbit. Divide by 2 pi and then a half yields ~ 50 miles for the orbital radius around Gliese 867d...so it must be a very small dwarf indeed orbited by a very slow planet...Ooops.:lol: pete

StupendousMan
2009-Jul-01, 12:22 AM
Astronomy magazine has put out a glossy special on "The Milky way" inside & out. It has a recipe for a slow planet and a small star.
p. 21 Three planets are orbiting Gliese 867d, a 10th magnitude red dwarf 15 light years away. In the article by noted planet hunter Geoff Marcy, there's something awry with the specs.
Planets 1 and 2, Jupiter sized,... orbital periods are 30 and 61 days in a tidal locked resonance. While monitoring them they discovered a third of 7.5 Earth masses with a period of 1.9 days. The following page has a velocity/time graph confirming this....with a velocity of 15 mph(~25km/h)

That's pretty slow, since even at my age I can run faster than that:shifty::doh: With a period of 1.94 days...that's 1.94 days/rev times 24 hrs./day times 15mph= ~ 698 miles for the orbit. Divide by 2 pi and then a half yields ~ 50 miles for the orbital radius around Gliese 867d...so it must be a very small dwarf indeed orbited by a very slow planet...Ooops.:lol: pete

The astronomers are measuring the motion of the star, not the planet. The star is moving very slowly, because the center of mass of the star-planet system is only, as you have derived, around 50 miles from the center of the star.

However, the planet is much, much, farther from the center of mass of the system. It will move much more quickly, over a much larger orbital radius. Since they tell you the mass of the planet, and you can estimate the mass of the star from its spectral type, you could figure out the orbital radius of the planet. Why not give it a try and see if it appears more sensible to you?

Ilya
2009-Jul-01, 06:49 PM
I read that special, and I noticed this. It's a typo.

trinitree88
2009-Jul-02, 05:15 PM
I read that special, and I noticed this. It's a typo.

Ilya. Thank you. It was odd that it was repeated on the two pages, they say nothing about the center of mass of the system, and it's clear they're talking about the velocity of the small planet..~7.5 Earth masses, which should be rapid near a stellar mass even if it's Sol/10. I was surprised that it was claimed "walk in the park on Sunday afternoon speed".. :shifty::lol:
Stu was right on my reaction pairs, off on this one...no big deal. pete

the real nitty gritty on the system is here:http://astronomyonline.org/Exoplanets/Gliese876.asp#Abstract

StupendousMan
2009-Jul-02, 11:25 PM
Below is a figure showing residuals in the motion
of the star Gliese 876, after removing the contributions
from the planets "b" and "c". The figure can be found at
http://exoplanets.org/gl876_web/gl876_tech.html

http://exoplanets.org/gl876_web/f12.jpg

The amplitude of this motion of the star is about 7 m/s,
which corresponds to the figure of 15 mph which
trinitree88 mentioned.

The star is moving at about 15 mph due to the gravitational
pull of the planet "d". It's moving much more quickly
due to the forces on it from planets "b" and "c" --
about 300 m/s overall.

trinitree88
2009-Jul-05, 04:23 PM
Below is a figure showing residuals in the motion
of the star Gliese 876, after removing the contributions
from the planets "b" and "c". The figure can be found at
http://exoplanets.org/gl876_web/gl876_tech.html

http://exoplanets.org/gl876_web/f12.jpg

The amplitude of this motion of the star is about 7 m/s,
which corresponds to the figure of 15 mph which
trinitree88 mentioned.

The star is moving at about 15 mph due to the gravitational
pull of the planet "d". It's moving much more quickly
due to the forces on it from planets "b" and "c" --
about 300 m/s overall.

Stupendous. Yep. You're right.It's just that the magazine says it's the motion of the third planet...a typo, twice. pete

Fiery Phoenix
2009-Jul-06, 06:50 AM
What's the mass of Gliese 867d? Any idea? I'd like to do some calculations using Kepler's laws and see if any of this makes sense.

trinitree88
2009-Jul-06, 06:14 PM
What's the mass of Gliese 867d? Any idea? I'd like to do some calculations using Kepler's laws and see if any of this makes sense.

Fiery. Here you go, I inadvertently typo-ed 867d in my thread label. :shifty::lol::doh:pete

see:http://www.extrasolar.net/planettour.asp?PlanetID=296

Fiery Phoenix
2009-Jul-06, 06:43 PM
OK, at a distance of 0.02 AU from a 0.32 Ms main-sequence star, the planet would have an orbital period of 1.8-1.9 days, as mentioned in the original post. However, I'm not sure if I'm getting the correct answer for the orbital velocity. I get something over 42,000 km/h (i.e about 120 km/s). I honestly do not see how a planet with a velocity of only 25 km/h could make a complete orbit of its star in just two days.

I did assume a circular orbit, though. Someone correct me if I'm wrong.

trinitree88
2009-Jul-06, 07:08 PM
OK, at a distance of 0.02 AU from a 0.32 Ms main-sequence star, the planet would have an orbital period of 1.8-1.9 days, as mentioned in the original post. However, I'm not sure if I'm getting the correct answer for the orbital velocity. I get something over 42,000 km/h (i.e about 120 km/s). I honestly do not see how a planet with a velocity of only 25 km/h could make a complete orbit of its star in just two days.

I did assume a circular orbit, though. Someone correct me if I'm wrong.

Fiery,It's the article typo...they meant the star's velocity, not the planet... Your numbers are OK. 120 km/sec times 86,400 sec day, times 1.94 days yields ~ 20,000,000 km. Similarly .02 AU times ~ 92 million miles/AU times 1.6 km/miles gives radius of orbit times ~ 6.28 (2 pi)...=~ 20,000,000 km for the orbit...they match.

Fiery Phoenix
2009-Jul-06, 07:13 PM
Fiery,It's the article typo...they meant the star's velocity, not the planet... Your numbers are OK. 120 km/sec times 86,400 sec day, times 1.94 days yields ~ 20,000,000 km. Similarly .02 AU times ~ 92 million miles/AU times 1.6 km/miles gives radius of orbit times ~ 6.28 (2 pi)...=~ 20,000,000 km for the orbit...they match.

Oh, OK then. I knew there was something wrong about this whole thing. The article itself didn't make too much sense to me. I even double-checked my calculations and still got the same different results.

Thanks for clarifying anyway.

trinitree88
2009-Jul-07, 04:38 PM
Oh, OK then. I knew there was something wrong about this whole thing. The article itself didn't make too much sense to me. I even double-checked my calculations and still got the same different results.

Thanks for clarifying anyway.

Fiery. You're welcome. Few authors make zero typos, and few scientists publish no wrong results. Reading old journals, one finds some real nitwit ideas, since disproven, but some of the latest will end up that way, too.
Nobody goes back and highlights them all in yellow to make your life easier...but the process of always considering whether or not this sentence makes sense becomes a valued trait in itself.....you'll find a bunch of people in the forum who are real good at it. That's science...:shifty::lol: pete