# Orientation of Orbiting Planets

## Main Question or Discussion Point

1. Is it true that when the orbital plane of planets about a star is perpendicular to our line of sight, that we will not detect the star wobble they may cause?
2. How many degrees from perpendicular is star wobble is detectible? For this question, I will accept an answer based on currently detected extrasolar planets but I’m really looking for a theoretical answer.
3. Is it believed that the orbital plane of planets about a star can take on any orientation with respect to our line of sight, and no one has detected a favored orientation in our galaxy?

I’m aware of research on other methods of detecting extrasolar planets which may favor a perpendicular orientation of orbiting planets. I’ve been keeping up with the current detection efforts and successes. I learned that we may soon (within the next 10 years) have the technology to detect earth sized planets. I read a planet as small as 14 earth masses has been found around Mu Arae and another of the same mass around 55 Cancri.

I’m trying to establish the likelihood of not detecting extrasolar planets that may support life about nearby sun like stars during the next 25 years. When would you guess that the first life supporting extrasolar planet will be found? I haven’t a clue!

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Labguy
Chris Russell said:
1. Is it true that when the orbital plane of planets about a star is perpendicular to our line of sight, that we will not detect the star wobble they may cause?
Actually, if the plane is perpendicular, that is when we would detect the most "wobble".

Chris Russell said:
2. How many degrees from perpendicular is star wobble is detectible? For this question, I will accept an answer based on currently detected extrasolar planets but I’m really looking for a theoretical answer.
That would be 90 degrees, that is, anything from perpendicular to edge-on. At perpendicular, all motion would be "proper motion" and the wobble most apparent. At any other angle, all the way down to edge-on, we would (and have) see(n) either a combination of proper motion wobble and spectroscopic red-blue shifting in either the star or even the planet if it is also observable. At a pure edge-on view, we would detect only the red-blue shifting in the spectrum.

Chris Russell said:
3. Is it believed that the orbital plane of planets about a star can take on any orientation with respect to our line of sight, and no one has detected a favored orientation in our galaxy?
Yes. Any angle of orientation would depend on the original angular momentum of the collapsing protostar gaseous material which could be totally random on a galactic scale. That said, since the entire "protogalactic" material would probably have an overall angular momentum to determine the plane of the resulting galaxy, it is more likely that more protostars would also have that same orientation. So, it is more likely that more planets would form at or near edge-on to our line of sight.

Chris Russell said:
I’m aware of research on other methods of detecting extrasolar planets which may favor a perpendicular orientation of orbiting planets.
The "favor factor" is only because this orientation would be easier to detect.

Chris Russell said:
I’ve been keeping up with the current detection efforts and successes. I learned that we may soon (within the next 10 years) have the technology to detect earth sized planets. I read a planet as small as 14 earth masses has been found around Mu Arae and another of the same mass around 55 Cancri.

I’m trying to establish the likelihood of not detecting extrasolar planets that may support life about nearby sun like stars during the next 25 years. When would you guess that the first life supporting extrasolar planet will be found? I haven’t a clue!
No clue here, either....

turbo
Gold Member
Labguy said:
That would be 90 degrees, that is, anything from perpendicular to edge-on. At perpendicular, all motion would be "proper motion" and the wobble most apparent. At any other angle, all the way down to edge-on, we would (and have) see(n) either a combination of proper motion wobble and spectroscopic red-blue shifting in either the star or even the planet if it is also observable. At a pure edge-on view, we would detect only the red-blue shifting in the spectrum.
Hi, Labguy! With a perfect edge-on view wouldn't we see both the MOST red-blue shifting AND a perfect example of almost linear radial wobble?

Labguy
turbo-1 said:
Hi, Labguy! With a perfect edge-on view wouldn't we see both the MOST red-blue shifting AND a perfect example of almost linear radial wobble?
At edge-on, we would see the most of red-blue shifting, but the star would not appear to wobble up/down, like a sign wave. At most the star and planet system would appear to move a bit faster then slower in proper motion (linear radial wobble) around the barycenter, but I think that this would be very hard to detect compared to an obvious up-down motion as seen from a perpendicular point of view.

turbo
Gold Member
Labguy said:
At edge-on, we would see the most of red-blue shifting, but the star would not appear to wobble up/down, like a sign wave. At most the star and planet system would appear to move a bit faster then slower in proper motion (linear radial wobble) around the barycenter, but I think that this would be very hard to detect compared to an obvious up-down motion as seen from a perpendicular point of view.
Let's envision this as a classical xyz axis problem (with z pointing away from us). In a face-on orientation, we would see only xy wobble (an elliptical wobble in a 2-body solution). In an edge-on orientation, we would see the full excursion of xy wobble in a straight line AND the the red/blue shift of the deviation in the Z direction as the source recedes/advances in the Z direction. Cool?

tony873004
Gold Member
This is why exosolar planets' masses are not known, so instead of being expressed in solar masses they are expressed in $$m sin(i)$$.

$$m_1 \sin i_{M\_Sun} =\left( {\frac{m_{2,solar\_mass} }{P_{years} }} \right)^{2/3}\cdot {r}'_{2,AU}$$

SpaceTiger
Staff Emeritus
Gold Member
Labguy said:
At edge-on, we would see the most of red-blue shifting, but the star would not appear to wobble up/down, like a sign wave. At most the star and planet system would appear to move a bit faster then slower in proper motion (linear radial wobble) around the barycenter, but I think that this would be very hard to detect compared to an obvious up-down motion as seen from a perpendicular point of view.
Turbo is right about the edge-on thing, but it turns out that one of the planned space missions (SIM) is expecting to detect the wobble in proper motion.

Janitor
I too have recently read that star motion due to planets can only be detected by current methods when we are reasonably close (angularly speaking) to the orbital plane.

Labguy
Janitor said:
I too have recently read that star motion due to planets can only be detected by current methods when we are reasonably close (angularly speaking) to the orbital plane.
That's true, and it is all spectroscopic and based on the mass of the star only. Not knowing the angle of inclination is why the results you read about can only give a minimum mass estimate for the "planet".

Chronos
Gold Member
System viewed in line, or perpendicular to the ecliptic plane, are the easiest to detect. The ones in between are the toughest.

Labguy
Chronos said:
System viewed in line, or perpendicular to the ecliptic plane, are the easiest to detect. The ones in between are the toughest.
I'm not sure what you mean by "in line".(?) Also, I don't know what the relationship would be to our ecliptic plane. I think it would be just (wherever located) whether we are looking edge-on, face-on (perpindicular) or somewhere in between.

At: http://physicsweb.org/articles/world/14/1/7/1 it states that:
The period and size of a star's wobble encode important information about the planet's mass. Usually it is only possible to determine a lower limit on the mass, because for most systems astronomers cannot measure the tilt of the orbit relative to the line of sight. Assuming that the orbit is edge-on to the line of sight gives the smallest possible value of the planet's mass. This is not as serious a problem as it might appear. If the orbital axes of the planetary systems are oriented randomly in space, there is a natural statistical tendency for us to see many more orbits edge-on rather than face-on.
Also, http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/27/text/ mentions the early (and best) uses of astrometry for more precise planet mass determinations because of the combined use of astrometry and spectroscopy can give the orbital inclination.

Not too specific, but: http://www.chron.com/cs/CDA/ssistory.mpl/space/3098228 was released just 2 hours ago as of this post.

EDIT: The second link (astrometry) does mention that it was (in 2002) as yet unproven, so that does imply that spectroscopy was all we had to rely on before that point, hence the "minimum mass" problem quoted in the first link.

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Based on this discussion and what Michael Lyle describes in his “Extrasolar Planets: Methods of Detection” lecture, see http://brahms.phy.vanderbilt.edu/~rknop/classes/a250/fall2001/ExtSolPlanets/XSPlanets.html , I shall summarize what I’ve learned thus far.

In Michael Lyle’s lecture he says, “Three methods, astrometry, pulsar timing and Doppler spectroscopy, all rely on … stellar wobble.” Having this input, it is not surprising that some think of astrometry and others of Doppler spectroscopy. When I wrote questions 1 and 2, I was thinking of the current Doppler spectroscopy work that has detected most of the extrasolar planets we know about. Based on what I knew of Doppler spectroscopy, I knew that an edge-on orientation would give the best red-blue shifting and concluded this would yield a higher count of planet discoveries when this method is used. Thus my first question was rhetorical and assumed Doppler spectroscopy is the current method of planet detection.

Based on Michael Lyle’s lecture, astrometry has yet to make its first planetary discovery but may yield some important information about the planets discovered via the Doppler method. Using pulsar timing might discover planets but I suspect the chance of finding habitable planets about pulsars is remote. Maybe with advances in photometry and microlensing instrumentation will add to the discoveries. Currently, optical interferometry (SIM) appears to be the most promising new technology. Maybe in the 2006-2007 timeframe we’ll be having some exciting discoveries. Also maybe improvements in Doppler spectroscopy, such as the HARPS project, will bag a few more planets. By the way, we are now up to 136 planets outside our Solar System as of Feb 5, 2005.

I realized my second question was going to be difficult to answer. From what I’ve read, no orbital inclination data exits for the extrasolar planets. Assuming this was true, I wondered theoretically at what point away from an edge on orientation that red-blue shifting becomes to slight to be detectable and thus would make a star’s planets undetectable. Again I assumed Doppler spectroscopy. Anyone have any ideas?

With regard to question 3, Labguy made a few relevant statements: “…since the entire "protogalactic" material would probably have an overall angular momentum to determine the plane of the resulting galaxy, it is more likely that more protostars would also have that same orientation. So, it is more likely that more planets would form at or near edge-on to our line of sight.” Are there any others out there that support this conclusion? He also said, “If the orbital axes of the planetary systems are oriented randomly in space, there is a natural statistical tendency for us to see many more orbits edge-on rather than face-on “. I buy this. And finally, “…mentions the early (and best) uses of astrometry for more precise planet mass determinations because of the combined use of astrometry and spectroscopy can give the orbital inclination “. I would like to see someone accomplish this. Has it been done? Thanks for your input.

Concluding, it appears if SIM is successful by 2007 or so we may see our first earth size extrasolar planet. That’s just 3 years from now. Maybe if we are lucky one or more of them will be in a habitable zone. But this still leaves a lot more work and a lot more to be discovered. I suspect during the next 25 years, based on what I know now, there will be a more than one undetected earth like planet relatively close to us.

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