hajola

Well, we also know where we haven't found signs of life. So from that perspective we have more data.

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.

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.

> How big is the L2 Lagrange point? (i.e., how closely do you need to be for an orbit around L2 to be practical?)

The L2 point doesn't really have size, and even its location isn't stable. It's a mathematical point, and when we say "orbit around L2" then that is not fully true either. The spacecraft are on what's called "halo orbit" - maybe imagine balancing a steel ball (like from a bearing) on a bottle that's sideways, it's probably easier to roll and balance the ball lenghtways of the bottle, than on rolling it sideways. The best analogy I could come up with. You don't want to be too close to the L2 point, as then the orbit would be very short and less stable, think of it as having a smaller bottle - probably harder to balance the steel ball on a smaller bottle than a big one.

> How far away PLATO will be from the James Webb Space Telescope? Probably on the magnitude of hundreds of thousands of kms on average. Interesting question though, hopefully they won't get too close :D

> What's the typical time scale for a transit?

Generally measured in hours, or minutes. For example, if we were observing our system with perfect alignment, Earth's transit would be about 12 hours, Jupiter's transit around 29 hours.

> Also, why use transits instead of the Doppler method?

Quantity. PLATO can observe a sizeable portion of the sky at once, 100k+ of stars. With Doppler method the quantities are smaller + afaik there is a trade-off between number of stars being observed and the velocity we can measure. So to find Earth-like planets around Sun-like stars, we would likely have to go one or a few stars at a time.

> Has this patch of sky been selected based on previous Doppler method star studies?

I am not actively involved anymore. So I am not sure if they have already picked what part of the sky they PLATO is going to be observing. The previous Doppler method (aka as radial-velocity or rv method) star studies play a role, not only because if there's one planet, there might be more, but also because rv gave information about the star. However, keep in mind that this is to find new exoplanets, less to find out more data about existing ones. Rv will definitely be used along side PLATO, to confirm and gather more information about exoplanets that PLATO finds.

For many reasons. 1 - it might be very unique itself 2 - it might be very close to us 3 - it might be orbiting a very interesting star, a star type we didn't expect to have that planet type or so many planets, or so close, or so far, etc 4 - the more exoplanets we discover the more we learn how star systems come to be. the more we know how rare ours is, how it might have formed, etc. It helps us answer age old questions.

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.

As far as I know it won't be affected at all, the project is almost fully funded from the European Space Agency. And it will most likely be launched with the European Ariane rocket.

Yes that's correct.

Yes, it's something that's referred to as pointing stability. The telescope will have star trackers to precisely know it's relative position - basically you make sure that you see the correct stars from where it is placed on the spacecraft. It will use reaction wheels to make tiny correction's to its position. Imagine you are in a computer chair and trying to spin yourself without feet or hands touching anything, just by twisting your body. Reaction wheels work on the same principle. As Earth completes a year around the Sun, the gravitational pull from other solar system bodies is very minor on PLATO. That said, keeping a spacecraft in L2 is not easy - there is nothing to "orbit".

Figuring out the optimal placement of CCDs on Plato's 24(+2) cameras. Due to the way CCDs are fabricated, their properties vary a bit, they are not identical. For example, they can vary how much light they can hold before they become saturated. Given the high cost of fabricating these CCDs, and the fact that for each camera 4 CCDs are used, and all these 4 have to share front-end electronics, it was prudent to optimize their grouping to we maximise the dynamic range we get. More dynamic range means that we can tell more about the planets we find with higher confidence.

While it's possible for conditions for life to emerge or sustain itself to be present beyond the habitable zone (e.g. there's likely a subsurface ocean orbiting the farthest plant from the Sun on Triton), afawk it is more probable that life forms in the habitable zone. That is the only one we have a data point for.