[hajola]

Next year there is a plan to send a space telescope to L2 with the main objective being to search for Earth-like planets around Sun-like stars in the habitable zone.

Like Kepler and TESS telescopes it will use the transit method to find new exoplanets, but unlike any mission before, it's going to look at the same spot in the sky for over a year. Super excited to see what data it brings back to us.

The telescope is called PLATO ( https://en.wikipedia.org/wiki/PLATO_(spacecraft) )

I contributed to the project a few years back, very happy to answer any questions.

[metadat]

L2 as related to space telescopes was a new term to me, and turned out to be utterly fascinating. The Webb orbits the sun and periodically boosts velocity using Earth's gravity:

> The James Webb Space Telescope is not in orbit around the Earth, like the Hubble Space Telescope is – it actually orbits the Sun, 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point or L2.

https://science.nasa.gov/mission/webb/orbit/

[chuckwfinley]

The wiki on lagrangian points also has a bunch of useful info on this stuff. Gravity is absolutely incredible

https://en.m.wikipedia.org/wiki/Lagrange_point

[safety1st]

Lagrange points are fascinating to me and I feel they are underrepresented in science fiction, compared to how the space age ahead of us may play out.

The events of human history on earth have revolved in great part around settling at or controlling strategically advantaged locations, for example any coastline, or a geographic bottleneck for trade and travel (think of Singapore and the Strait of Malacca).

A Lagrange point is the simplest space-based analog to this that I know of, if you want to put something in a fixed location relative to other bodies, the Lagrange points are places where you can do it with the highest fuel economy. Then when operating from that position you will have more energy available to do other things, granting you advantage over competitors who are not at the Lagrange point.

So whether it's science, research, trade, defense etc. there is a compelling reason to locate things at a Lagrange point, and it seems this is already happening as we have science satellites at L1 and L2 and I believe L3 has been talked about. The Lagrange points are not all created equal in terms of distance to their respective bodies, size, energy required to maintain a position etc. All two body systems have them, so for example the Earth and Moon have a set of Lagrange points that are significant to us.

The LPs are what a lot of our space politics and problems may eventually revolve around (quite literally!).

[JumpCrisscross]

> events of human history on earth have revolved in great part around settling at or controlling strategically advantaged locations, for example any coastline, or a geographic bottleneck for trade and travel

Not only that, it's easier to send mass between Lagrange points than it is to send it to them from either of the orbital bodies.

Getting from Earth to L1 or L2, each 1.5mm km away, takes 15 km/s, escape + 12 km/s. (You have to fight both the Sun and the Earth's gravity.) Getting from L1 to L2 takes less than 100 m/s. (L1 to L3, L4 or L5 about 1 to 2 km/s.)

This confers strong defensive first-mover advantages; it's energy-wise easier to hold five than take one from the Earth. (Obviously, it's mass-wise easier from the Earth.)

[floxy]

>1.5mm km

Just curious as to why people in general don't write that as 1.5 Tm.

https://www.nist.gov/pml/owm/metric-si-prefixes

[mjevans]

Relays at L4 and L5 (also described in the linked Wikipedia) would be useful. If both are built it's reasonable for relay stations or satellites stay in contact continuously, if over different paths at different times of the day. The second relay station would also be useful if other objects naturally collected within the regions of stability disrupt communication.

[dhosek]

Lagrange points are a key plot device in Iain M. Banks’s The Algebraist. I did figure out the location of the Dweller wormhole pretty quickly, thanks to thinking about how Lagrange points worked, but it is a great work of science fiction.

[vman81]

Regarding sci-fi; Joshua "Lagrange" Calvert did a fancy maneuver in Peter F. Hamilton's Reality Dysfunction involving a lagrange point that gave him that nickname.

[foobarian]

I always wondered what kind of fun treasure might be hiding in the L4 and L5 points. And what kind of secret missions were already sent there by various countries to look for stuff. There could be whole precious metal asteroids sitting around!

[ahazred8ta]

There's a list of probes and telescopes that have been sent to the L1 and L2 points: https://en.wikipedia.org/wiki/List_of_objects_at_Lagrange_po... -- This includes Nasa's Genesis and Goresat, ESA's SOHO, and ISRO's Aditya.

[d1sxeyes]

It’s arguable that even the stuff we consider to orbit the earth like the moon is really orbiting the sun.

MinutePhysics did a great video on this: https://youtu.be/KBcxuM-qXec?si=VngVwXeRKFPjnh15

[jcgrillo]

To what extent (if any) will this program be impacted if all U.S. federal grant funding is permanently cut? Are there U.S. funded components/researchers involved?

[hajola]

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.

[bane]

All the more reason why humanity needs multiple space programs.

[labster]

I’m sure that will come up next year when they privatize NASA.

[DiscourseFan]

You mean when it gets named as a subdivision of SpaceX?

[yieldcrv]

I love it when the Press Secretary says Doge with a straight face while defending attacks by reporters

[gosub100]

you mean make it like 10x more efficient and effective? I'm game.

[consp]

10x more efficient in extracting wealth for its owner. Everything else is a myth.

[JumpCrisscross]

> you mean make it like 10x more efficient and effective? I'm game

Different missions. SpaceX beats Boeing and Lockheed Martin. It does not have the skillset to build a James Webb or Europa Clipper.

[TeMPOraL]

I wonder, is there any human endeavor other than space exploration (and maybe an occasional particle accelerator) that works like this? Sure, a big factor in the history of scientific progress is its structural resilience against localized political and economical kerfuffles, but that's more of an accident of how discovery and innovation are done - in small increments, achieved near-simultaneously by independent people or groups around the world (only one gets to take the credit, though). Meanwhile, it seems to me that space exploration needs to be organized into a competition to survive and thrive. To make things weirder, it's not about regular market competition - it's about staying in public consciousness, through continuously one-upping each other by Doing Something Impressive, which ends up attracting funding to all agencies each time (and conversely, when things get slow on the impressive achievements front, funding starts to dry out).

(We had one dry spell after Space Shuttles were retired, and IMHO this one could've been fatal to the entire field. Thank $deity for NASA's funding of commercial launch services, and SpaceX surviving 2008 and taking advantage of it to get the Falcon 9 to work and effectively re-light the public interest again.)

I imagine this is a transitional period; we're past the times of Cold War - times when everyone poured ~infinite money into weapons programs and space exploration got to leech some of it off - and we're not yet seeing the bootstrapping of cislunar economy on the horizon. I wonder if there's a more sustainable way of getting through to the other end, because relying on public interest feels rather risky. And, again, I can't think of any other field that is in this weird position.

[somenameforme]

Yes. In the past long-range sea voyages worked exactly in the same way.

They were seen as little more than expensive intellectual curiosities and eccentricities. In fact even once we discovered the New World had Columbus not come back and lied his arse off about riches that existed there only by coincidence (as he'd seen nothing of what he claimed), it's entirely possible that would have been the last journey to the New World for decades if not centuries. And over those decades you'd probably have had more and more of the population believing we never even landed on a New World to begin with.

And it'll be the same in the future. Eventually humanity will become a multiplanetary species and more value will be generated off Earth than on it. And I think we're probably not that far away from such point, but we live at a time when we will happily dump trillions of dollars to fund pointless chaos halfway around the world (that invariably just makes the world less safe for everybody), yet every penny that could take us closer to these species defining events is scrutinized like we're down to our last pennies.

And again - this isn't new. It's been the case for centuries and probably will be the case for the foreseeable future of humanity. It's easy to explain with a tautology - positions of power are held by those attracted to power, and those attracted to power are attracted to power. Once the New World became a means to power, that's when the 'trillions' started pouring in. The same will happen with space.

[elliotec]

What a world it would be if he didn't lie his arse off eh?

[geraldwhen]

Your time scale is so small. There are people alive today that witnessed the moon landing on television.

Galactic timescales are large. Plan for 10,000 years out, not tomorrow.

[TeMPOraL]

> Plan for 10,000 years out, not tomorrow.

I'm happy to. But then, like most people, I'm impatient, so I'll draw a second plan that ensures I get to see at least some of the cool stuff before I die, and then I'll get annoyed when this plan isn't followed.

Galactic timescales are large. Human lifespans are tiny.

[jcgrillo]

excellent, thanks.

[nick3443]

What's the typical time scale for a transit? Also, why use transits instead of the Doppler method? Has this patch of sky been selected based on previous Doppler method star studies? Thanks!

[hajola]

> 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.

[stouset]

> Earth's transit would be about 12 hours, Jupiter's transit around 29 hours

…per year, for Earth; per ~12 years for Jupiter is I think what the GP was asking.

This is extremely dependent on the radii of the inner and outer limits of the the habitable zone for any given star, though, as well as the star’s mass.

[nick3443]

Both are relevant! Thanks!

[DiogenesKynikos]

You can find much less massive planets with the transit method.

The Doppler method relies on the planet pulling on the star to change the star's line-of-sight velocity periodically. Because planets are much less massive than stars, the star doesn't move much. You can only find massive or close-in planets with this method.

The transit method is much more sensitive to small planets like the Earth. It's true that the smaller the planet, the less of the star's light it blocks, so it's still easier to detect large planets than small planets using the transit method. However, it's much easier to detect small changes in a star's apparent brightness than it is to detect small shifts in the star's velocity.

There are a few different viable methods of detecting planets. Each has its strengths and weaknesses, and astronomers use all of them.

[mapt]

Radial velocity surveys require so damn much light, and such a complex precision spectrometer that they're only used on the very largest 8m-10m class telescopes on the ground, shooting in near infrared through the most advanced adaptive optics (or even interferometric modes) in great weather, pointed at a single target for a long period of time (this is a big deal), with a focus on super-Jupiter to Jupiter class objects in tight orbits.

The next generation of 30m class telescopes will be an order of magnitude more capable for the RV method, but even then you're not really going to be able to get fast locks on Earth analogs.

The RV method is vastly superior for detecting the planets we really care about - high confidence nearby Earth analogs. The odds of a transit being in the right plane for us to observe are tiny. But if we want to run a survey like that like it really matters (let's say a Solar system catastrophe hits a thousand years from now and humanity wants interstellar diaspora), we'll be studying the nearest thousand stars with the RV method using significant numbers of 100 meter class telescopes, or perhaps big space based interferometers produced in mass quantities, for decades.

What transit studies like Kepler do is study a small patch of crowded sky (most of the stars being very distant) with the sensitivity for very rare in-plane Earth analogs, in order to get a representative sample. When I was born we couldn't say with any confidence that planets around other stars existed, post Kepler we know that they're common. We can perform these surveys even with the shoestring budgets current governments afford astronomy because even if the odds of successfully detecting a planet that does orbit a distant star are very low, we can watch a million stars at a time.

[kosta777]

Hi, thanks for answering the questions in this thread, it feels like something out of a sci-fi novel. Do you know of any similar projects that a software engineer could contribute to in their free time? Could be of much smaller scale of course.

[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.

[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...).

[glomgril]

Very cool. Got a silly sci-fi question for you. IIUC, with current technology it would take on the order of tens of thousands of years for a vessel to physically travel to the closest known Earth-like planet (correct me if I'm wrong).

So any thoughts on what kinds of hypothetical breakthroughs would be needed to make the trip doable in (say) less than a human lifetime?

And related, what do you think about the plausibility of the [Breakthrough Starshot](https://en.wikipedia.org/wiki/Breakthrough_Starshot) initiative? Aware of any alternative approaches?

[idlewords]

A different stab at this is to ask what it would take to build a telescope that could image some of these Earth-like planets, a project that turns out to be easier (in a very loose sense of that word) than sending cameras there.

The idea is you send a camera very, very far out in the Solar System (hundreds of AU) and then use the Sun's gravity well as your lens. Neat stuff and, unlike the interstellar probes, potentially doable in our lifetime.

https://en.wikipedia.org/wiki/Solar_gravitational_lens

[rocqua]

Normally, diffraction and the effective aperture are what limit optical resolution. How does that work with gravitational lensing? Does the effective aperture become the diameter of the sun?

[idlewords]

I'm too ignorant to answer that, but the technical paper here [https://arxiv.org/pdf/2002.11871] goes into a wealth of detail, and includes an image of Earth as it would appear to such a telescope (before and after post-processing) from 30 parsecs away. The optical properties of the solar gravitational lens are pretty astonishing.

[stevenwoo]

Self replicating automata as described by Von Neumann able to repair and duplicate themselves, and other things like electronic components. ICs keep getting faster (so far) but use smaller and smaller features of silicon and could wear out from metal migration and all components will be under much more cosmic radiation than on earth. This makes a large shield of heavy material on front of vehicle to minimize this effect but that increases the energy/fuel needed. The space shuttle only took maybe week long trips but it had four computers for flight control , three extra in case of failure in different parts of the shuttle along with IIRC a separate backup backup computer in for use as last resort.

[nine_k]

* Research faster interstellar travel, especially using something like a Buzzard engine to utilize interstellar hydrogen as resection mass. Required nuclear fusion power plants / engines and ridiculously strong magnetic fields; both seem attainable.

* Slow down human body metabolism and allow humans to stay asleep at near-freezing temperatures for a long time. If bears and chipmunks can do it, chances are humans could learn it, too.

* Invent sets of machines that can reliably self-replicate, given most basic inputs like minerals, water, and sunlight. Advanced semiconductors are going to be the tricky part.

* Study psychology, sociology, history, game theory, etc, so that the early society that will form on the new planet, isolated from Earth, would avoid at least some of the pitfalls that plagued human history on its home planet.

[ben_w]

> Buzzard

That's a bird, the engine is named after a person and is spelled differently:

https://en.wikipedia.org/wiki/Bussard_ramjet

Also, it won't work unless scaled up to the sort of thing only a Kardashev type II could do — 4000 km diameter — and at that level you've got other options that mean they probably won't:

https://arstechnica.com/science/2022/01/study-1960-ramjet-de...

[JumpCrisscross]

The reason being the interstellar medium is way less dense than we thought in 1960.

[konart]

>Slow down human body metabolism and allow humans to stay asleep at near-freezing temperatures for a long time. If bears and chipmunks can do it, chances are humans could learn it, too.

The thing is - our current bodies can't live in space for long. So either we will have to build new bodies for us somehow or build a ship that can have gravity inside and protection from space outside (and we are talking about very heavy protection here)

In any other case there is no point in slowing down metabolism or whatever. You will die rather soon.

[nine_k]

With a big enough ship, pseudo-gravity can be easily produced by rotation, especially if we expect the crew to spend 95% of time asleep.

[BugsJustFindMe]

Time dilation means that the closer you get to the speed of light the less time you experience passing. So even a 12000 year long journey as seen from earth, if moving fast enough, could feel to the travelers like a much shorter amount of time.

[stevenwoo]

Yes, but practically with todays technology there is no feasible way of getting to a speed where time dilation matters over that distance, we run out of fuel so we need some external power source like a laser or solar wind that have other issues, iirc one only gets to 2x time dilation at 0.9 c. That’s a lot of acceleration.

[nejsjsjsbsb]

We need to think about where we want the knowledge and what knows it. We could use humanoid AIs. We could hatch humans "just in time". Run them in a sim to 18 then release them on their mission. Ethics would need to accept this. Maybe we would be happy slowly expanding across the universe and an decendant talking to 'the aliens'.

I am not totally serious. But you wanna meet aliens? Gotta do something a bit radical.

[stevenwoo]

If you haven't, you should read Accelerando, it's a collection of short stories IIRC that were put into a novel by the author. I didn't want to start with that, but that is in there. :)

[mad0]

If I may suggest another read: Perfect Imperfection by Polish author Jacek Dukaj. It's definitely weirder, than Accelerando, as the book drops you straight into the last parts of evolution curve, but definitely worth reading if you have liked Accelerando.

The story is super weird, but what I found out is that piecing together a picture of a far-future society from this story was very exciting.

[dmd]

My one-sentence review of Accelerando is "VASTLY better than the first couple chapters will make you believe."

[kaashif]

I'm reading Accelerando right now and there's some unnecessary weird sex stuff at the start.

Good book despite that though, some very interesting ideas.

[rocqua]

I imagine fuel isn't that big a problem until you care about being able to decelerate once you arrive.

[galangalalgol]

And we don't have to send people, we should do our job as a Von Neumann probe and send frozen rna to distribute across the surface.

[idlewords]

In space culture this is widely considered a dick move.

[dustingetz]

and in that 10,000 year blink, a civilization progresses from bronze metalworking to digital computers, awaiting our arrival

[AtlasBarfed]

Can't pulse nuclear get there? Or does it require antimatter catalyzed fission?

[fragmede]

What was your contribution?

[hajola]

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.

[ziddoap]

>CCDs

I think this is "Charge-Coupled Device"?

"an electronic sensor that converts light to digital signals through charges generated by bouncing photons on a thin silicon wafer"

Is that correct? Not familiar with the acronym.

[UltraSane]

Yes. In telescopes they use high-end CCDs with really big pixels for better light sensitivity and zero dead pixels.

This is a picture of the CCD array for the Gaia space observatory that used parallax to measure precise distances and slightly less precise angular velocities of billions of objects

http://www.bo.astro.it/~altavilla/FTP/GAIA/IMAGES/The%20comp...

[hajola]

Yes that's correct.

[fragmede]

That's awesome! Are the multiple CCDs because you're taking photos in separate colors or something?

[hajola]

Good question. No, these will essentially be black-white "photos". The amount of light is measured. The reason for so many CCDs is so that the field of view would be as large as possible. A larger field of view enables to look at more stars at once. Given that we will be locked into looking at one spot for a whole year, it ups our chances of spotting something cool if we maximise the number of stars we are looking at.

However they won't be photos of planets really. It will be countless photos of the same stars over and over again, it's just that sometimes they will be slightly less bright than other times. Directly imaging exoplanets is incredibly difficult, but humans have managed it: https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...

[byteknight]

Do they move the telescope over the year to account for movement? How is that calculated? Does this change with being closer to planets and their gravitational pull?

Asked from a total moron.

[hajola]

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".

[UltraSane]

And when the reaction wheels get saturated they have to expend propellant to let them spin down. It is a fascinating mechanism.

[daveguy]

Here is the Wikipedia about Lagrange Points (L2 is one of these): https://en.m.wikipedia.org/wiki/Lagrange_point

The orbital corrections are minimized at L2, because of the relative distance of the moon and other planets vs size. But that is what is accounted for in the corrections.

James Webb Telescope is at Sun-Earth L2.

[diegof79]

Many thanks! Comments like yours are what I love about HN.

How is the spot to analyze during that year of focus determined?

[diegof79]

Sorry, I see this was answered in a previous comment: https://news.ycombinator.com/item?id=42855433

[floxy]

And hopefully some day we'll build a solar-gravitation lens to look at the surface of one of these exoplanets:

https://arxiv.org/abs/1802.08421

...with a very interesting video that someone made on that paper:

Someone also made an interesting Youtube video of the concept:

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

[divbzero]

How far away PLATO will be from the James Webb Space Telescope? How big is the L2 Lagrange point? (i.e., how closely do you need to be for an orbit around L2 to be practical?)

[hajola]

> 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