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Fr. Wayne
2006-Mar-22, 09:42 AM
How do we know if a nearby star is planet-less if we happen to be viewing it from its elliptical pole?

Van Rijn
2006-Mar-22, 10:07 AM
How do we know if a nearby star is planet-less if we happen to be viewing it from its elliptical pole?

Elliptical pole? I'm not quite sure what you mean. Anyway, in answer to your question, we don't. The trick currently is to find planets. Some observation methods are: looking for "wobbles" in the star, direct image or light detection, and looking for periodic dimming in the star.

All methods have issues and limitations. Currently, high mass planets are much easier to find. It is virtually certain that the majority of planets around nearby stars remain unknown.

grant hutchison
2006-Mar-22, 10:22 AM
Elliptical pole? I'm not quite sure what you mean.Ecliptic pole, I guess. If we're viewing a planet at right angles to the plane of its orbit, then we can't detect spectroscopic wobbles in the parent star.

Grant Hutchison

Fr. Wayne
2006-Mar-22, 10:25 AM
Our Sol has an axis of rotation for solar system which we plot as found in Draco for example. When we look at a nearby star, and we happen to be on its axis of rotation, then am I safe to conclude that even Proxima Centauri still could have a planet and still could have a planet in an habitable zone as well?

astromark
2006-Mar-22, 11:25 AM
That is my understanding. Fr Wayne. However a great deal of observation of the closest star to Sol has not yet given up its planets location or orbital path. I understand your point re; our view from above the pole of a star not showing by ocoltation or brightness variances. The motion of Alpha A and B are well studied, The path of Proxima around them is understood. Knowing that any other orbiting body would make a wobble. We haven't seen one yet.
It is still a probability that a planet of Earth like proportions is as close to us as this and we still do not know.

grant hutchison
2006-Mar-22, 11:51 AM
Spectroscopic changes will be zero if the plane of the planet's orbit is at right angles to our line of sight (so the star has no acceleration along our line of sight as a result of its motion around the planet/star barycentre).
But astrometric changes will be maximal if the planet's orbit is at right angles to our line of sight (the star will move in a small circle around the planet/star barycentre).
So for nearby stars we should be able to detect planets no matter what the orbital orientation. For distant stars we're more reliant on spectroscopy, and so would not detect a planet that orbits in the same plane as the sky.

Grant Hutchison

max8166
2006-Mar-22, 11:51 AM
Also two, or more, planets could quite easily cancel out each others wobble so that it is not detectable (yet!)

TriangleMan
2006-Mar-22, 11:53 AM
When we look at a nearby star, and we happen to be on its axis of rotation, then am I safe to conclude that even Proxima Centauri still could have a planet and still could have a planet in an habitable zone as well?
Proxima could have planets around it that astronomers have not detected yet regardless of the axis of rotation. Current techniques for finding exosolar planets work best finding high-mass planets that orbit close to the star. Proxima could have smaller planets that we are not able to detect yet.

Fr. Wayne
2006-Mar-22, 12:37 PM
Astrometric work is crucial since planets have an elliptic orbit which may influence star's wobble due to its distance from major axis. Earth-size planets would seldom be detectable due to their relative "whimpee" mass.

grant hutchison
2006-Mar-22, 01:02 PM
Also two, or more, planets could quite easily cancel out each others wobble so that it is not detectable (yet!)Wouldn't that situation be vanishingly rare, though? It would require a precise balance in mass, orbital period and position in orbit.

Grant Hutchison

antoniseb
2006-Mar-22, 02:29 PM
Wouldn't that situation be vanishingly rare, though? It would require a precise balance in mass, orbital period and position in orbit.


I'd guess that about 8% of the time, a system will be oriented so that we are within 30 degrees of a rotational pole. At that orientation we should be seeing the effects of the wobble diminished to about 14% of what we see looking at it edge on (if I have my rust trig-fu working). That should be enough to make planets take a whole new generation of equipment to see (compared to edge on).

grant hutchison
2006-Mar-22, 02:50 PM
Antoniseb, I was referring to max8166's suggestion that two planets could cancel each other's wobbles when I used the phrase "vanishingly rare".
I completely agree that the closer the system is to being face on to ours, the more massive a planet would have to be to fall within our current spectroscopic detection limits, other things being equal.

Grant Hutchison

Edit: Added "spectroscopic" to last sentence, for accuracy.

antoniseb
2006-Mar-22, 02:56 PM
Sorry, Grant. You are generally a very knowledgeable contributer, and I should have assumed you meant something else, but instead chalked it up to early morning fog or something.

Romanus
2006-Mar-22, 03:07 PM
Ditto, GH; we couldn't spectroscopically detect a pole-on system, but we could still find it with sensitive astrometry. Alas, except for the nearest systems, that kind of precision will have to wait for SIM and GAIA.

Eroica
2006-Mar-22, 05:09 PM
Ditto, GH; we couldn't spectroscopically detect a pole-on system, but we could still find it with sensitive astrometry. Alas, except for the nearest systems, that kind of precision will have to wait for SIM and GAIA. Wasn't that how they first detected Sirius B?


"In the years between 1834 and 1844 ... F. W. Bessel found that Sirius had wavy irregularities in its motion through space, and came to the conclusion that the star had an invisible companion revolving about it in a period of 50 years." - Burnham's Celestial Handbook Of course it would be much more difficult to detect a planet this way.

grant hutchison
2006-Mar-22, 08:57 PM
Just a couple of order-of-magnitude calculations, out of interest.
If we can measure star positions to a milli-arcsecond, we should be able to resolve an astrometric wobble at Proxima of a thousandth of an AU. Since Proxima masses about 100J, that means we could astrometrically detect a Jupiter-mass object as close as 0.1 AU to Proxima.
For Alpha Cen A & B, 10 times more massive but about the same distance, we could detect a Jupiter at 1 AU. For Sirius A, 20 times more massive and twice as distant, we're out to 4 AU. Which, BTW, probably wouldn't be a stable planetary orbit, since Sirius B encroaches to 8 AU, but I'm just doing this for the numbers.

So, as Romanus says, detection of planets by astrometry would be restricted to massive objects around the nearest stars, at present.

Grant Hutchison

Fr. Wayne
2006-Mar-23, 05:11 AM
Just a couple of order-of-magnitude calculations, out of interest.
If we can measure star positions to a milli-arcsecond, we should be able to resolve an astrometric wobble at Proxima of a thousandth of an AU. Since Proxima masses about 100J, that means we could astrometrically detect a Jupiter-mass object as close as 0.1 AU to Proxima.
For Alpha Cen A & B, 10 times more massive but about the same distance, we could detect a Jupiter at 1 AU. For Sirius A, 20 times more massive and twice as distant, we're out to 4 AU. Which, BTW, probably wouldn't be a stable planetary orbit, since Sirius B encroaches to 8 AU, but I'm just doing this for the numbers.

So, as Romanus says, detection of planets by astrometry would be restricted to massive objects around the nearest stars, at present.

Grant Hutchison

I would assume that a Jupiter mass at 1 AU from Proxima (used as an example only, of course) would be a sizeable wobble that would displace the star's position by approx. 1 diameter. If instead a ring of similar mass equally distributed with asteroid size objects within it would be completely undetectable unless our line of sight crossed the ring-orbit. Maybe the spoke mechanism around Saturn can be converted into a radio signature (VLF) that our Radio Interferometry Scopes could then test especially at the "water-hole" frequency directly at our nearest neighbor. This monitoring would require a time span on the safe side of 11 years (using Sol cycle) since I am not familiar enough with the cycle of Saturn's spokes or whether they even generate a polar uni-directional signal. Still thanks for your math which gives me an understanding of our detection limits.

Romanus
2006-Mar-23, 05:39 AM
<<Wasn't that how they first detected Sirius B?>>

Yes; however that astrometric motion was on the order of a few arcseconds. The signal from a Jupiter-mass planet would be many thousands of times smaller.