What's so strange? The planets b and c apparently, at closest approach, come to within 0.015 AU of one another. Not being a mathematician or anything else that would help me understand this, I would have thought that such close approaches would have disrupted one or both of those planetary orbits.
Are these orbits kept stable by the 3:1 resonance? And if so, is it possible that such a resonance could be in place in some star system between an eccentrc gas giant and a smaller terrestrial world? If so, then perhaps the habitability rating of eccentric giant systems might go up some what. If not... then I'm wishing for horses....
...John...
The Strange Case of 55 Cancri...
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Topic authorDollan
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The Strange Case of 55 Cancri...
"To make an apple pie from scratch, you must first create the universe..."
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--Carl Sagan
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Michael Kilderry
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Do you mean 55 Cancri b & e? (I think it was those two)
I think that they are still in stable orbits because they lie close enough to their sun that the gravity of the sun still dominates over their's. On the website Extrasolar Visions there is an illustration showing that when the two of them are at closest approach to each other, you can see b's possibly blue disk from e.
Here's a link:
http://www.extrasolar.net/
Why wish for horses? Ponies are smaller and easier to care for.
Michael Kilderry
I think that they are still in stable orbits because they lie close enough to their sun that the gravity of the sun still dominates over their's. On the website Extrasolar Visions there is an illustration showing that when the two of them are at closest approach to each other, you can see b's possibly blue disk from e.
Here's a link:
http://www.extrasolar.net/
Why wish for horses? Ponies are smaller and easier to care for.
Michael Kilderry
My shatters.net posting milestones:
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First addon: The Lera Solar System
- Michael
First post - 11th October 2004
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First addon: The Lera Solar System
- Michael
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Spaceman Spiff
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Greetings,
Well let's first sort out what the 'facts' are. Here's a compilation of data for the four 55 Cnc planets as stated on the Extrasolar Visions website, so we can agree on what is being discussed:
(For a reason I don't have, symaski62 pointed to the questionable 'Oort cloud' of the 55 Cnc system, which is not to do with Dollan's observation.)
It seems at first sight that the orbits of b and c could approach to within 0.013A.U., and c and e to within 0.070A.U. We can only say both pairs are possibly close approachers, because we don't know of the true orientation of these orbits with respect to each other.
Yet, even knowing if the orbits are close does not mean the planets also come that close.
We can get some perspective of all this from our Solar System. Pluto and Neptune have crossing orbits, but are in a 3:2 resonance, are inclined to each other, and are phased (very important) so that they don't approach each other to within (I think? help!) 10A.U. So, this is a stable set-up for millions of years to come. Also, Jupiter and Saturn approach to within 4?? A.U. of each other and that is also rather stable.
For the 55 Cnc system, I don't think planets b and c are in a 3:1 resonance. At a period ratio of 3.40 that's way too far from resonance. This suggests instability, and that's what I agree with Dollan is strange.
However, as Michael Kilderry says, the relative distances between planets and star may also be important. If planets b and c happen never to approach within 0.1A.U of each other, then because planet b is usually 0.115A.U from 55 Cnc, it's very much like when Saturn is 4??A.U. from Jupiter while Jupiter is 5A.U. from the Sun.
However, another really important point is that the Extrasolar Visions site has left out the estimated errors of those measurements. It could turn out that where two planets' orbits appear to come within 0.013A.U. with these current estimates, later more accurate data may show the orbits may approach more than 0.1A.U apart, or that they actually have crossing orbits. Don't take these figures as gospel!
Gliese 876 was the first discovered with a clear orbital resonance between its two planets (2:1), and they are so close you can easily see a) the disk of one planet from the other, and b) the inner planet could eclipse Gliese 876 as seen from the outer planet, if the orbits are suitably phased and co-apsided. The original reason I started playing with Celestia was to check this.
You can find out more about what is known about orbital resonances of extrasolar planet systems in this online Sky and Telescope article:
[url=http://skyandtelescope.com/magazinearchive/article_1373_1.asp]
http://skyandtelescope.com/magazinearch ... 1373_1.asp
[/url]
The article gives some hints about what all this means for orbit stability and hence habitability ratings. I think the problem is that there are orbits with stable configurations that still don't have constant low eccentricities...
Spiff.
Well let's first sort out what the 'facts' are. Here's a compilation of data for the four 55 Cnc planets as stated on the Extrasolar Visions website, so we can agree on what is being discussed:
Code: Select all
Name Period Dist. Min. Max. Ecc. Mass
(days) (A.U.) (A.U.) (A.U.) (Earths)
e 2.808 0.038 0.03 0.04 0.174 14.21
b 14.67 0.115 0.113 0.117 0.0197 249.2
c 49.93 0.24 0.13 0.34 0.44 69.0
d 4,517.4 5.257 3.53 6.97 0.327 1,243.
(For a reason I don't have, symaski62 pointed to the questionable 'Oort cloud' of the 55 Cnc system, which is not to do with Dollan's observation.)
It seems at first sight that the orbits of b and c could approach to within 0.013A.U., and c and e to within 0.070A.U. We can only say both pairs are possibly close approachers, because we don't know of the true orientation of these orbits with respect to each other.
Yet, even knowing if the orbits are close does not mean the planets also come that close.
We can get some perspective of all this from our Solar System. Pluto and Neptune have crossing orbits, but are in a 3:2 resonance, are inclined to each other, and are phased (very important) so that they don't approach each other to within (I think? help!) 10A.U. So, this is a stable set-up for millions of years to come. Also, Jupiter and Saturn approach to within 4?? A.U. of each other and that is also rather stable.
For the 55 Cnc system, I don't think planets b and c are in a 3:1 resonance. At a period ratio of 3.40 that's way too far from resonance. This suggests instability, and that's what I agree with Dollan is strange.
However, as Michael Kilderry says, the relative distances between planets and star may also be important. If planets b and c happen never to approach within 0.1A.U of each other, then because planet b is usually 0.115A.U from 55 Cnc, it's very much like when Saturn is 4??A.U. from Jupiter while Jupiter is 5A.U. from the Sun.
However, another really important point is that the Extrasolar Visions site has left out the estimated errors of those measurements. It could turn out that where two planets' orbits appear to come within 0.013A.U. with these current estimates, later more accurate data may show the orbits may approach more than 0.1A.U apart, or that they actually have crossing orbits. Don't take these figures as gospel!
Gliese 876 was the first discovered with a clear orbital resonance between its two planets (2:1), and they are so close you can easily see a) the disk of one planet from the other, and b) the inner planet could eclipse Gliese 876 as seen from the outer planet, if the orbits are suitably phased and co-apsided. The original reason I started playing with Celestia was to check this.
You can find out more about what is known about orbital resonances of extrasolar planet systems in this online Sky and Telescope article:
[url=http://skyandtelescope.com/magazinearchive/article_1373_1.asp]
http://skyandtelescope.com/magazinearch ... 1373_1.asp
[/url]
The article gives some hints about what all this means for orbit stability and hence habitability ratings. I think the problem is that there are orbits with stable configurations that still don't have constant low eccentricities...
Spiff.