[Rebelgecko]

Why is it pointing at the same spot for a year ?

Is it to get a more exhaustive survey single star or can full of stars? Or does that help it find smaller/further/different planets?

And how do they pick where to point at? Is there a way of guessing the likelihood of finding a planet?

[dotancohen]

  > Why is it pointing at the same spot for a year?

The transit method requires observing a dip in the brightness of a star. Actually - three dips. The first dip indicates - but does not prove - the existence of a planet transiting in front of the star. The change in intensity, rate of change of intensity, and duration of the dip all give us information.

The second dip, if roughly identical to the first dip in parameters, gives us the orbital period of the star. So now we wait a second period in order to observe the expected... Third dip, which confirms the planet if it occurs with the same parameters at the expected time.

Though I think that such observations would require at least two years, and up to possibly four years, for stars with orbits of periods similar to our own. I don't believe that a single year is long enough.

[dcminter]

> Though I think that such observations would require at least two years

It is at least two years at least if I'm understanding this⁰ correctly:

Observational concept

Ultra-high precision, long (at least two years), uninterrupted photometric monitoring in the visible band of very large samples of bright (V ≤11-13) stars.

⁰ https://sci.esa.int/documents/33240/36096/1567260308850-PLAT...

[hajola]

I should have been more clear in my original post. AFAIK there are two options on the table - looking at two fields, both 2 years OR looking at one field for 3 years and then doing "step and stare" for the rest of the mission. Step and stare being that they "step" into a new field, "stare" at it for some time, and repeat.

[hajola]

Great questions.

> Is it to get a more exhaustive survey single star or can full of stars?

PLATO will look at 100k+ stars at once. And for most we will be unlucky to see a transit between PLATO and the star. Geometrically it won't align - imagine the star systems being in different angles from us. To bring an analogue - Take a pack of cards and throw them in the air, and take a quick picture while they are sitll in the air - how many cards will be facing the camera exactly with their edge. For us to spot a transit, the planet has to pass between us and the star. If the orbital plane is not parallel to us, we will miss the transit. So that's one of the reasons why it helps to look at bunch of stars with transit method. We expect that about 1% of the orbital planes will be aligned so that we can get meaningful data.

> Or does that help it find smaller/further/different planets?

Imagine you are trying to find Earth from another solar system. The longer you look at our Sun the higher the likelihood that Earth will pass between you and the Sun. And once you get lucky, and the Earth transits between you and the Sun, the brightness of the Sun only dips about 0.01%, so that means that in order to find small planets we have to have sensitive instruments and little noise, so that the dip in brightness can be measured. Furthermore, as the planet passes the transit and continues on its orbit, the perceived brightness of the star will increase, due to the planet reflecting some extra light. Measuring that can gives us some rudimentary information about the atmosphere - e.g. if a small planet reflects a lot of light back, maybe it's covered in clouds or snow.

> And how do they pick where to point at?

There's a whole complicated process to find consensus on where to point. Basically they look at spots that have lots of stars, and they look what type of stars they are. Here the objective is to find planets around Sun-like stars, so they would prioritize fields that have more Sun-like stars.

> Is there a way of guessing the likelihood of finding a planet?

It seems that some stars are more likely to have planets than others.

[TeMPOraL]

Since I have your attention - I figure this is still the best condensed ELI5 explainer of the history and methods used in search for exoplanets, and I keep sending this to anyone remotely interested in the topic:

https://www.youtube.com/watch?v=gai8dMA19Sw

(I also consider it to be the only true, original, canonical rendition of the Alladin song.)

It gets into the transit method around halfway through (at 3:43), and makes it glaringly obvious why this is the way to go, over tracking Doppler shifts. Still, this video is almost 8 years old (and neatly coincided with discovery of additional planets around TRAPPIST-1) - I wonder if there are new methods at play that are not covered here, and of course if the middle part still corresponds to how things are done?

[mcswell]

You said: > We expect that about 1% of the orbital planes will be aligned so that we > can get meaningful data Somewhere below, someone used the figure of 0.01%. I assume they were mistaken, and your 1% number is about right for some "average" star sizes and orbits.

At any rate, that figure depends on the size of the star, and the distance from the star that the planet orbits--the further away, the smaller the chance that their orbital plane would be aligned with our solar system. For a Sun-class star, and a planet inside the habitable zone, what is the %? Am I correct in thinking it would be approximately 0.5/180, where 0.5 degrees is the apparent size of our Sun in the sky, and 180 degrees is of course half a circle (since it doesn't matter whether we're on one side or the opposite side of their star, hence 360/2). Which works out to about 0.14%, right?

[exitb]

How does the 0.01% look in comparison to the natural variability of star brightness, due to cycles, spots etc? Would that be a concern in terms of false positives? And also, given the specific line-up needed for us to see the pass, how likely it is for us to be able to observe the same planet in front of the star in the following years?

[teraflop]

Stars do change their brightness in various other ways, but the light curve of planetary transit has a very characteristic shape. It causes the brightness to dim by a small but constant amount, with a (comparatively) very short and sharp start and end. A transit causes this pattern to occur at precisely regular intervals, and I don't think we know of any phenomena related to a star itself that would imitate the same effect.

Stars' relative positions generally don't change fast enough for the angle from which we observe a transit to change significantly. A transit of HD 20794 d is visible anywhere within a roughly 0.7-degree wide band. But our angular rate of motion with respect to the star HD 20794 is the same as its rate of motion in our sky, about 0.001 degrees per year. So the transit will most likely continue to be observable for decades or centuries to come, depending on exactly how the planet's orbit is aligned.

[SJC_Hacker]

Would it be feasible to place telescopes at other orbital inclinations with respect to the sun in order to spot transits in stars that aren't within Earth's orbital plane ?

[teraflop]

The orbital inclination relative to our sun doesn't really have anything to do with it. In fact, stars that are aligned with Earth's orbit are harder to observe, because they go behind the sun once a year.

Detecting an extrasolar planetary transit requires us to be aligned with the planet's orbit around its star. And since those stars are so far away, you would have to travel an immense distance away from our solar system to appreciably change the relative angle.

HD 20794 is about 20 light-years away from us, so changing our observation angle relative to it by 1 degree would require traveling about 0.35 lightyears. Our fastest-ever interstellar probe, Voyager 1, would take 5000 years to travel that distance.

[hajola]

Thanks for the clarification. You are absolutely right, in my post above I accidentally used the word "parallel" that caused the confusion. It wouldn't even be practically possible to use PLATO to observe them.

Here's a visual if that's helpful to any reader: https://www.researchgate.net/figure/Geometric-Probability-fo... .

[tejtm]

>> Is there a way of guessing the likelihood of finding a planet?

> It seems that some stars are more likely to have planets than others.

to the best of my knowledge it has yet to be proved that any star has no planets.

[hajola]

Probably not the best choice of words from me there. However, there is a positive correlation between a star's metallicity and the number of planets a star has.

[tejtm]

Your words are fine from my point of view, I am just tickled at the though of how hard it is to prove a star has no planets. Even with all stars having planets, it is worthwhile to carefully choose the field to maximize the chance of detection. Best would be if planetary disks aligned themselves in any predictable manner, but not much hope there.

[pwatsonwailes]

Light collection. You want to observe one point for a really long time so you get a really good understanding of where the light is coming from, the properties of that light, and its behavioural patterns.

A lot of the detection is statistics around signals, so the better (read more thorough and coherent) your data (observations of changes in light), the more confidence you can have in your conclusions around what's causing the changes (planets with different atmospheres, different positions, different sizes and compositions etc...).