ThatGuyWithALongUsername

What would it take to directly image smaller exoplanets? A discussion...

When do you think we'll be able to do this?  

15 members have voted

  1. 1. When do you think we'll be able to do this?

  2. 2. Where will our first pictures of Proxima B come from?

    • Project Starshot!
    • Fancy Telescopes!
    • Neither!
  3. 3. Which telescopes will be built and capable of imaging these?

    • Ground-based telescope array
    • Giant aragoscope
    • Giant regular telescope
    • Space-based telescope array


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Exoplanet research today is more exciting than ever. We still have plenty of data from Kepler (RIP, Kepler- you will be missed). There are thousands of known exoplanets, and dozens of them are potentially habitable. These have shown a great diversity of planets- all of these worlds seem to differ from each other just as much as  the planets in our own solar system, in a variety of different ways. Unfortunately, however, we do not know what these worlds look like (for the most part). We don't even know what they look like in terms of how we knew Pluto looked like before New Horizons. In most cases, we cannot detect moons,  we cannot detect atmospheres, and we cannot detect mass and composition. In addition, we are currently unable to detect the vast majority of exoplanets- most methods can only detect exoplanets with a high mass, and transiting only shows exoplanets that pass in front of their star in our perspective.

The ultimate dream for exoplanet research would be some kind of telescope able to actually make out gas giant AND terrestrial-sized exoplanets that are relatively close to us, say, within 10 light-years. Not only would this give us a whole lot of new information on known exoplanets, but it could be a very effective way to discover new ones. These images would not necessarily be detailed enough to see surface features- just detailed enough to see major moons, atmospheres, and computer models of surface features, like that one of Pluto using Hubble data. And, after that, how long until we get surface features, if possible at all?

The point of this thread is to discuss the challenges and stuff around this technology, how this technology might work, the people trying to tackle them, and speculation on when we might be able to accomplish this. Which might come first: Project Starshot or fancy telescopes?

Overall, there's a lot of complicated optical challenges I don't understand well enough that causes problems here- most notably, planets are dim, stars are bright. But it would certainly be worth the effort to overcome these challenges- wouldn't it just be amazing to see a whole solar system from this perspective (the Voyager family portrait comes to mind), and would it be even more amazing if one of these dots- just one- was some familiar, pale shade of blue?

On a less philosophical note, check out these guys: https://www.planets.life/ <- This is what I'm talking about right here. Also, would it be possible to get some of this detail from the Thirty Meter Telescope or the Extremely Large Telescope?

Edited by ThatGuyWithALongUsername
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Telescopes today are limited by three things: quantity of light they can collect, atmospheric effects deforming incoming rays, and diffraction.

The quantity of light collected is the easiest to solve: just observe for a longer time; if you can't, get a bigger telescope.

Atmospheric effects are harder. Because the atmosphere is made of "cells" of different temperatures moving about, you get changes in the refractive index of the air; different refractive indices means refraction: light gets deviated and you end up with a blurred signal. There are two big ways to get around this, first adaptive optics which means that your mirror can detect when an incoming ray of light is deviated and deforms itself slightly to correct the deviation, a lot of telescopes already use this, but there's still room for improvement. The other way is to get rid of the atmosphere between you and your target, you do this by sticking your telescope into space.
In some cases (to observe IR and beyond-UV parts of the spectrum) you have to go to space because the atmosphere absorbs these wavelengths entirely.

The final limit is the one imposed by physics: diffraction. Light is a wave (and a particle, but it doesn't matter here), stick it through a thin aperture and you will observe some diffraction. Stick it through a big aperture and there will still be diffraction, you'll just won't be able to see it with the naked eye. Diffraction creates a hard limit on maximal resolution your telescope can get. The Rayleigh criterion gives a definition of the diffraction limit: you want the first maximum of one diffraction pattern to be further than the first minimum of the other diffraction pattern, then you can resolve the two and all is well.
In maths this looks like: θ  = 1.22*λ/D, θ is the maximal angular resolution, λ the wavelength of the incoming light and D the diameter (aperture) of your telescope. For Proxima Centauri b, sitting 4.2 ly away from us and having a radius about equal to that of the Earth, you'd need a 2 km wide telescope to resolve it: that is for it to be larger than one pixel on your screen. PCb is a relatively bright object in the sky (m=11 according to Wikipedia) so collecting a good signal shouldn't be too much of a problem, this means an array of a few smaller telescopes which will have a diameter of over 2 km would do the trick. You'll need to put these into space though, since telescopes on Earth still aren't close to the diffraction limit because of atmospheric effects.

 

Back to the main question: none of the telescopes you mentioned, and AFAIK no planned telescope project, would be able to produce any picture of PCb. Fortunately you don't need high-res pictures to get some information about the atmosphere or composition of a celestial body (we have been doing this for stars for quite a while, and exoplanets for a few years).

Starshot is such a long way from being a real thing that I wouldn't rely on it for pics; on the other hand there is basically zero interest in creating a several kilometres wide telescope array in space, so in the end who knows which will come first?

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15 minutes ago, Gaarst said:

*LONG AND INFORMATIVE POST*

Hmm... so, if someone managed to find a way to mass-produce space telescopes, and shot them into space with a BFR or something... that might work?

I wonder if manufacturing the telescopes on-orbit, given some drastic reduction in materials shipping costs in the medium future, would help make telescopes more precise for cheaper? Again, don't know much about these. After that, they could be transported (ACES might be the better job for this, as they never touch the atmosphere) to a lagrange point or something.

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https://www.centauri-dreams.org/2006/08/18/the-focal-mission-to-the-suns-gravity-lens/

https://www.centauri-dreams.org/2009/11/06/the-gravitational-lens-and-communications/

https://www.centauri-dreams.org/2016/04/25/starshot-and-the-gravitational-lens/

https://www.centauri-dreams.org/2016/04/26/gravitational-lensing-with-planets/

https://en.wikipedia.org/wiki/FOCAL_(spacecraft)

https://arxiv.org/ftp/arxiv/papers/1604/1604.06351.pdf

Here's a handful of links for some interesting info on gravitational lensing telescopes(the comments on the centauri dreams were worth reading, if I remember correctly...it's been awhile since I've been to these pages).  I've read that we might be able to resolve continents on exoplanets using our sun, but there are plenty of problems that come along with the nature of the technique.

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3 hours ago, ThatGuyWithALongUsername said:

I wonder if manufacturing the telescopes on-orbit, given some drastic reduction in materials shipping costs in the medium future, would help make telescopes more precise for cheaper?


The problem isn't shipping costs.  The problem is the billions of dollars worth of infrastructure and manufacturing facilities required on orbit.  (And the R&D program required to develop those things, which isn't itself going to be cheap.)

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Imho, that's a general purpose of the Solar System utilization: to keep building and supporting a swarm of telescopes to get high-res pix of the space around and put every piece of matter into an accountant database.

So, the clear picturing of the exoplanets would be going in parallel with extraterrestrial bases.

None of the existing projects are close to it.

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Love it. A lot of useful info in here.

To all of these, technology and material set a limit. The forty meter E-ELT telescope for example is about the max we can do right now and there had and has to be a lot of research work for all the components. Afaik the E-ELT isn't assumed to have its full resolution power right from the start, some more tech is in development. Good news: the E-ELT, once ready, is assumed to be able to resolve some more of the known planets from their stars.

To image such small and faint things as exoplanets, usually outshone by their stars, in a reasonable time one needs a ludicrous resolution. Which is achieved by three things: aperture, aperture and aperture. This is either a single big platter, or several small dishes apart from each other. A big single one has much more area to collect photons than several small ones so does not need as much observation time. Several small ones are called an interferometer, which is what people are intensely working on right now. It has its own problems, like positioning accuracy, data management and processing, etc. First results in exoplanet interferometry are very nice, see the VLT for example. And hopefully soon the EHT and TMT.

 

One more thing, many small sats probably won't help as much, you need a certain diameter to be able to distinguish clearly between noise from electronics or the background from the real signal you want. Not speaking of positioning and herding them all. The smaller they are, the closer false and real signals are together. If you prolong the exposure time instead because of too small an aperture, you also get many false signals. So, bigger is better. Which brings us back to aperture. And cost. $$$ :rolleyes:

 

Edit: and i am on the progressive side. If the economy doesn't crash and things are finished, there is a good chance that we have something nice (late) in the 20s. I hope ;-) By interferometry, maybe participation of the big ones.

Project Starshot is a papertiger ;-)

 

Edited by Green Baron
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My bet is that Proxima Centauri b is not going to be the first one to be photographed.

Why ?

- Dim central star (so small reflection)

- Close distance to parent star.

 

We'll probably photograph something else first that has a much larger orbital radius and orbits a much brighter central star.

 

Or a planet around a pulsar. You decide.

Edited by YNM
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There are a few directly imaged exoplanets and debris- and planetary disks. But things will surely get much better with the upcoming generation of telescopes.

 

Edited by Green Baron

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As of now, we’re limited to imaging hot, newly formed giant planets orbiting tens or hundreds of AU from their stars. While it’s cool to see planets (and entire solar systems like HR 8799) at their youngest, and it helps with understanding the incredibly complicated process of planet formation, these kinds of planets by themselves are rather boring. Imaging gas giants that have finished forming and are cooled down would be a significant improvement in both the capability of exoplanet detecting technology and the types of planets we can study. The JWST and some other giant upcoming telescopes could be able to directly image Jupiter-like planets at Jupiter-like distances in the near future, if I remember correctly. Even thought they won’t be as easy to detect as glowing 1000+ K Super-Jupiters, there will be a few advantages that’ll make them better to study: 

- The color (and by extension, the cloud composition) could be determined. 

- Because Jupiter-like planets would be cooler than 200 K, heat signatures from volcanic moons or comet impacts would be easier to detect. 

- Tracking the motion of Jupiter-like planets found by radial velocity through direct imaging can help pin down their exact orbits (year length, eccentricity, etc).

- Large planet-sized captured moons or ring systems may be detectable around some of the more massive and furthest orbiting gas giants. 

- Long-term imaging of a giant planet could reveal color variations caused by clouds, storms, rings, and/or moons, depending on the angle of the planet to our line of sight.

I’m not too sure if we’ll have the technology to directly image Earth-sized or Super-Earth exoplanets any time within the next 5 or so years, but I could very easily be wrong. With all the projects aiming at taking a photo of a planet in the Alpha Centauri system, there are good amount of opportunities for equipment powerful enough to detect a rocky planet to be developed. My best guess for when imaging of small planets becomes as mainstream as imaging young giant planets would be 2025 at the absolute earliest and 2040 at the latest. 2030 seems like a good estimate IMO. Before that day comes, at least we’ll have plenty of other cool exoplanet analysis advances to look out for (JWST, infrared radial velocity spectrographs, tons of new TESS planets, CHEOPS, and more).

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1 hour ago, ProtoJeb21 said:

I’m not too sure if we’ll have the technology to directly image Earth-sized or Super-Earth exoplanets any time within the next 5 or so years, but I could very easily be wrong. With all the projects aiming at taking a photo of a planet in the Alpha Centauri system, there are good amount of opportunities for equipment powerful enough to detect a rocky planet to be developed. My best guess for when imaging of small planets becomes as mainstream as imaging young giant planets would be 2025 at the absolute earliest and 2040 at the latest. 2030 seems like a good estimate IMO. 

As optimistic as my OP sounds, I completely agree with this. There is no way that this will happen in the next five years. With some effort, the 2040's sounds like the closest possible time period.

I will note, however, that while the ELT and TMT won't be able to do these, the PLANETS foundation does seem to be working towards this goal. While their first telescopes won't be able to do this, the COLOSSUS telescope, with a 74 meter effective diameter, will be built to work in an large array, which could theoretically be built large enough to accomplish this goal. The atmospheric diffraction is still a problem here, but they seem like they know what they're talking about- besides, there's still three generations of technology demonstrators before this. This could happen in the 2030's or something?

Yeah, there are problems with this, and they're gonna need a lot more funding than one measly kickstarter, but they're trying, and it sounds... possible.

Then again... maybe I am being a bit too optimistic.

Edited by ThatGuyWithALongUsername
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And I can remember the time when it was accepted wisdom that the 200 inch Mount  Palomar telescope was the biggest telescope that it was possible to build!

 

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*offers chair* :cool:

Well, maybe it was in the 40s. E-ELT and TMT is the max that's possible with today's and partly tomorrow morning's tech.

 

Yeah, one telescope alone will not have enough resolution to resolve a small rocky planet around the neighbour stars. It definitely will need interometry. The large single dish aperture makes it possible (or more probable) to collect enough light for a meaningful signal from such a faint object. I didn't make that clear enough.

Edited by Green Baron
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4 hours ago, Green Baron said:

E-ELT and TMT is the max that's possible with today's and partly tomorrow morning's tech.

We'd need something on the order of 100m, I imagine.

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55 minutes ago, YNM said:

We'd need something on the order of 100m, I imagine.

I doubt anyone will be able to get enough funding (or mirror-making materials) to build a 100 m telescope mirror anytime within the next few decades. A project like that would require ludicrous amounts of effort to be pulled off and will probably just gobble up money like the JWST.

I think using giant “typical” telescopes to image Mini-Neptunes and Super-Earths is the wrong approach. Building such a telescope, as I mentioned before, is going to take years, and because of that it could slow down our advancement in the understanding of these types of planets. A space-based starshade telescope may be a better alternative; you don’t have to worry about the annoying distortions from Earth’s atmosphere, and it could be both smaller and cheaper than a 100 meter scope. This is due to the starshade itself, as it can almost entirely block out the light of a target star and allow for a smaller mirror to be used. The reason for the large mirrors is to capture as much light as it can from the target planet, which isn’t much because of the overwhelming brightness of the star. When the light of said star is almost entirely gone, you don’t need a giant mirror to pick out something that has not become easy to spot. 

One problem though: that starshade needs to be miles in front of the telescope and somehow stay perfectly aligned. I wonder how that would impact the price tag of such a mission. Also, I know that a previous starshade space telescope was cancelled before the TESS project started due to it eating up money JWST style, but that doesn’t mean another similar mission won’t be able to find some ground in the next decade. 

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Direct imaging of an exoplanet's surface is no small feat. It's comparable to observing the Apollo LEM on the Moon from the surface of the Earth. To do it at all requires a massive aperture. To do it and get a reasonable amount of detail requires an even bigger one. However there is a way to do it, and cheaply too! No need for massive mirrors or flying out to 550 AU.

2014-cash.jpg

I'm actually surprised Aragoscopes haven't been mentioned yet. Rather than use a mirror as a collecting surface, instead an opaque disk or ring would be deployed in front of the spacecraft. It seems a little counter-intuitive, but since light's wave-like nature causes it to diffract, the brightest part of the shadow is actually in the middle. Place a detector + relevant optics at this spot (the Arago spot), and you've essentially created a  telescope for very cheap. It scales too! According to this study, Aragoscopes from 100m to 1,000m were shown to be feasible, with a 100m Aragoscope being possible with what's usually budgeted for flagship class missions (Though we all know from JWST that those estimates tend to be a little low).

 

But the science it would allow would open up a whole new world of astronomy. A 1km Aragoscope + accompanying Starshade observing in visible light would be able to resolve Jupiter's disk and the Galilean moons from 7 parsecs away. Resolution gets better as you move towards the UV/X-ray end of the spectrum too, so such telescopes could potentially be used to directly image the event horizons of black holes such as Sagittarius A* and the BH in M87. The big issues with this design would be the need for very high-accuracy pointing due to the long focal length and the low contrast created by the obstructing disk. However I think these issues are likely to be solved within the next few decades.

 

Until then we can watch Cody'sLab at least. :)

 

 

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Ok... I have never heard of these before. I am intrigued. I've watched Cody's Lab before, but somehow missed this video. I'll be back in approximately nine minutes and thirty-two seconds...

 

EDIT: huh... interesting. My question now is how one wold put a coronagraph on one of these, exactly? This promises a much cheaper solution, but it could still be equally technologically challenging if you want to use it for exoplanets.

 

EDIT 2: Crazy idea here... what if we didn't have to bring up material for these?

Step 1: find an asteroid

Step 2: Somehow find a way to cut a large, completely circular disc out of an asteroid, or mold asteroid material from an asteroid into a disc

Step 3: Somehow make large circular disc out of asteroid

Step 4: Profit!

 

Actually, wait... could... we use a sphere instead of a disc? (EDIT 3: No, they don't) I think you can see where I'm going with that one- the trick is finding a relatively flat object with no atmosphere. I'm sure some Hubble-style software corrections could help...

 

These suggestions are a bit more far-off, obviously.

Edited by ThatGuyWithALongUsername
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2 hours ago, ProtoJeb21 said:

starshade

Ah, yes, they're only viable in vacuum.

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11 hours ago, YNM said:

Ah, yes, they're only viable in vacuum.

I know, although I think I forgot to specifically mention that in my previous post. 

 

13 hours ago, Nutt007 said:

Direct imaging of an exoplanet's surface is no small feat. It's comparable to observing the Apollo LEM on the Moon from the surface of the Earth. To do it at all requires a massive aperture. To do it and get a reasonable amount of detail requires an even bigger one. However there is a way to do it, and cheaply too! No need for massive mirrors or flying out to 550 AU.

2014-cash.jpg

I'm actually surprised Aragoscopes haven't been mentioned yet. Rather than use a mirror as a collecting surface, instead an opaque disk or ring would be deployed in front of the spacecraft. It seems a little counter-intuitive, but since light's wave-like nature causes it to diffract, the brightest part of the shadow is actually in the middle. Place a detector + relevant optics at this spot (the Arago spot), and you've essentially created a  telescope for very cheap. It scales too! According to this study, Aragoscopes from 100m to 1,000m were shown to be feasible, with a 100m Aragoscope being possible with what's usually budgeted for flagship class missions (Though we all know from JWST that those estimates tend to be a little low).

 

But the science it would allow would open up a whole new world of astronomy. A 1km Aragoscope + accompanying Starshade observing in visible light would be able to resolve Jupiter's disk and the Galilean moons from 7 parsecs away. Resolution gets better as you move towards the UV/X-ray end of the spectrum too, so such telescopes could potentially be used to directly image the event horizons of black holes such as Sagittarius A* and the BH in M87. The big issues with this design would be the need for very high-accuracy pointing due to the long focal length and the low contrast created by the obstructing disk. However I think these issues are likely to be solved within the next few decades.

 

Until then we can watch Cody'sLab at least. :)

 

 

I’ve never heard of an aragoscope before, but that idea intrigues me. If a 1 km scope could see the Galilean Moons from 7 pc away, it may also be able to see any potentially habitable exomoons of GJ 876 b. 

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13 minutes ago, ProtoJeb21 said:

I know, although I think I forgot to specifically mention that in my previous post. 

 

I’ve never heard of an aragoscope before, but that idea intrigues me. If a 1 km scope could see the Galilean Moons from 7 pc away, it may also be able to see any potentially habitable exomoons of GJ 876 b. 

Bingo! I haven't known about this concept for more than 12 hours, and it's already looking incredibly promising.

The trick is, of course, building a 1-kilometer disc or ring in space, plus a starshade.

Actually, could the starshade be smaller and closer to the sensor? Perhaps the starshade could even be combined with the smaller disc preventing light from coming through the center of the ring.

Edited by ThatGuyWithALongUsername
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I have edited the last question on the poll to be a bit more reasonable. Everyone who voted for older options are now voting for something else... you might want to change your answer. Sorry!

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Well, we’re going to get 20 meter plus telescopes in the 20s, so ground based scopes likely will.

A 39.3 meter diameter telescope is expected to achieve first light in 2024, but operation likely will start later. This may be able to do it.

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2 hours ago, ThatGuyWithALongUsername said:

I have edited the last question on the poll to be a bit more reasonable. Everyone who voted for older options are now voting for something else... you might want to change your answer. Sorry!

I can't vote again it seems, but this makes more sense now.

My bet is on ground based arrays, with participation of giant ones, like GMT, E-ELT for the light collection part. The TMT though may be too late for my aspiring time frame. Though they can resume construction now, first light has been shifted to 2027.

I am confident we get an image of the orbit of an earth sized planet (planet >= 1 pixel) - together with a (more reasonable than now) atmospheric analysis if circumstances permit - before 2030. Term ends on the 31.12.2029 at 23:59 UTC.

No surface features of course ...

Edited by Green Baron
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22 hours ago, Green Baron said:

*offers chair* :cool:

Well, maybe it was in the 40s. E-ELT and TMT is the max that's possible with today's and partly tomorrow morning's tech.

As you can probably guess, I am really old!

 

 

 

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