Exoplanets

Andy yes I think we do agree. I think the thing is that a lot of the discoveries are not by Earth based telescopes, they were in the beginning, but now from space telescopes, such as Kepler, which can be pointed in different direction. This is how for instance Tej the observation takes time by pointing at a star then watching for a long period of time. Andy I agree that there could be many planets we have not seen the transit of yet and of course long period transits would take a long time to see. I can see that any mission to observe them would have to be run over generations.

No one is disagreeing with Mike, Brian. We are pushing for further clarity! You and Mike are our educators!

And the result: A beautifully clearcut Nasa diagram. I see it now. Great stuff, Mike.

That diagram and formula assumes that the direction stars' axis are completely random. So it seems my thought that the galactic plane has an influence on the number of stars whos orbital plane is parallel to the galaxy is not a factor. Could it be though?

The only way to know is to repeat my question, can a star polar axis be determined/detected (before an eclipse or tug is observed)? If so, anyone knows how many have we observed and out of that number, how many lies perpendicular to our line of sight?

Sorry I am a bit slow with my writing, I didnt realise there were three more posts since!

Andy, you sum up the question nicely! I am half expecting the answer to be 42 😉

42

It seems to work for rainbows, Tej!

what proportion of those exoplanets would actually be observable from earth, if the angle of their orbital plane relative to our line of sight is, in fact, random

I refer the honourable gentleman to the reply I gave some moments ago 😉

i.e. it depends on the size of the star and the distance that the exoplanet is away from the star – the probability is given by the formula R(s) / a or d(s) / 2a . Sorry, I can’t think of any other way of expressing this!

can a star polar axis be determined/detected

Yes, kind of... you can use spectroscopic measurements to perform an indirect measure of stellar inclination. This is complex stuff, so I suggest you do your own research... cause otherwise this post will go on forever.

There are a couple methods that I’m aware of. The first uses spectroscopic measurements to allow for an indirect measurement of stellar inclination. To measure this accurately is very time consuming and can take several months of observations. Ordinarily, it is only accurate to within 15-30%. Another method uses spectroscopic asteroseismology... don’t ask me how this works, I’ve no idea... but it is, apparently, quicker, taking only a month for Sun-type stars.

Anyway, it all takes time to do these measurements. There have been some papers that explore if it would be worthwhile to determine the stellar inclination first, before looking for exoplanets, in order to narrow down the search parameters... but, at the moment, I don’t think it’s being done.

Two universes:

First universe: the orbital plane of every exoplanet system is completely random. Viewed from earth, some exosystems will show exoplanetary transits of their stars, others won't, eg, because the angle of the orbital plane is perpendicular to our line of sight of the star.

Second universe: All orbital planes everywhere are exactly edge-on, as seen from planet earth. Therefore all exosystems will show exoplanetary transits of their stars, sooner or later (depending on orbital period).

But the mathematical formula for the probability of observing a transit is exactly the same for both, because the variables are stellar radius and the distance between the star and its exoplanet(s).

What am I missing?????

What am I missing?????

Only one thing.

The assumption of the formula is that the orbits are random. This corresponds to your first universe.

In your second universe, the probability would always be 1, for every planetary system. The formula doesn't apply, because the initial assumption is incorrect.

Ah! Okay, then. My mistake. I thought the formula was telling us what the probability is of observing - at any particular moment in time - a transit in a given edge-on exosystem, in which case the probability should be zero for a lot of systems, and relatively high for a smaller number of edge-on, large-star/small orbital radius/short period systems.

Okay - next question: what would we have to be able to do to observe other stars' Oort clouds? As I understand we haven't observed - or even confirmed - the existence of our own but something recently got me thinking about backlighting... (I promise I'll be less trouble on this question 😉

what would we have to be able to do to observe other stars’ Oort clouds

Travel there? 😉

Seriously, though, I think it would be extremely difficult, if not impossible, to detect anything.

Think about our own (theoretical) Oort Cloud. Let's say it's about 10,000AU from Earth (or 1.5 x 10^15 metres). The surface area of this cloud would be 4 x pi x (1.5 x 10^15 m)^2 = 2.8 x 10^31 m2 .

The mass of the Oort Cloud is not known, but some estimates put it at around 5 times the mass of the Earth or 3 x 10^25 kg.

Density of this region would therefore be around 1.0 x 10-6 kg / m3 . Less than one millionth that of air.

I don't know how this could be visible for any star from our relative distance... but I could be wrong!

Wow. Thanks, Mike. To be honest, it was a much more whimsical question - but I'm astounded at the estimated mass/density. So little material scattered over such an enormous area hardly deserves the name 'cloud', however much the name 'Oort' may be spot-on.

Yes, indeed... and, of course, my density estimate assumes that the 'cloud' is on a single plane... which, of course, it isn't. The Oort Cloud is likely to be thousands of AU thick, so the actual density will be several magnitudes smaller.