[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

[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.

[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.

[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.

[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.

[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

[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.

[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.

[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.

[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".

[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