How To See the Surface of Exoplanets (Script) (Patreon)

I’m going to tell you about the craziest proposal for an astrophysics mission that has a good chance of actually happening. A train of spacecraft sailing the sun’s light to a magical point out there in space where the Sun’s own gravity turns it into a gigantic lens. What could such a solar-system-sized telescope do? Pretty much anything. But definitely map the surfaces of alien worlds.

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Thanks to the Kepler mission, we now know that there are billions of extrasolar planets - exoplanets - in our solar system. And we’re learning a ton about them - for example, we’ve figured out that there are 40 billion or so Earth-like planets, at least in terms of size and mass. Of course if we want to find life or actually visit these planets it’d be nice to know a bit more than that. The James Webb Space Telescope is helping - it can detect different molecules in the atmospheres of exoplanets a bit bigger than the Earth. JWST will even take images of some exoplanets. But those images will never reveal anything more than a single dot in orbit around a star. If we find evidence for life, we’re going to want to study it in detail - ideally with images of the planetary surface. And if we ever want to visit one of these, it sure would be nice to know what lies at the end of such a many-decade journey.

But resolving the surface features of an exoplanet seems pretty impossible. There’s an absolute limit to the resolving power of any telescope, and it depends on size. Bigger is always better. When light passes into a telescope, its wave nature interacts with the edges of the aperture, causing diffraction, and that blurs the focus of the mirror or lens. The bigger the aperture, the smaller that blur. A telescope’s diffraction limit is the best possible resolution it can achieve. If you try to create an image of anything smaller than this limit, it will always get blurred to the size of the diffraction limit.

That’s a problem, because planets are pretty small when you’re trying to see them from many light years away. For example, to see a planet 100 light years away as anything more than a dot you’d need a telescope way bigger than New York City.

You might recall these pictures of the black holes in the M31 galaxy and the center of the Milky Way. These were taken by bringing together radio signals from telescopes all across the planet, effectively giving us a planet-sized telescope with a tiny diffraction limit. We are not yet able to repeat this trick with visible light because this requires exact measurement of the arrival time and phase of the electromagnetic wave - which gets harder the shorter the wavelength.

But there is one way to take a direct image of an exoplanet in visible light that could reveal it’s detailed surface features. And that’s by sending a spaceship. Not TO the planet - that would take way too long. In fact, it’s by sending a spaceship in the opposite direction. If you travel directly along the line connecting your favorite exoplanet and the Sun, but away from them both, you’ll reach this spot where light rays from the exoplanet are bent inwards by the Sun’s gravitational field to all come together. Forget about a New York sized telescope - at this spot, we have a star-sized telescope. The result is an amplification of the brightness of the exoplanet by a factor of a trillion, and a magnification of the surface details by a factor of 100 billion.

The technical name for this location of incredible light-converging power is the locus of focus hocus pocus. LFHP. OK, that’s just what it SHOULD be called. For some reasons scientists went with SGLF - solar gravitational lens focal region - missed opportunity if you ask me.

But if we can get a telescope into the “SGLF”, then we could start making detailed desk globes of alien worlds. Let’s talk about how we might actually achieve this. Because the plan is further along than you might think.

Actually, first let’s review gravitational lensing. Einstein’s general theory of relativity tells us that gravity is due to curvature in the fabric of spacetime due to massive objects. But that curvature also bends the path of light. You know what also bends light? Lenses. So a gravitational field can also act like a lens, although admittedly a kind of crappy one.

Regular lenses are designed to bring light from the same point to a single focus point, allowing an image to be formed. Gravitational lenses produce highly distorted images, like these stretched out galaxies seen through the gravitational field of a giant galaxy cluster. If the alignment is close enough, we can see an Einstein Ring, like this galaxy being lensed by an intervening galaxy.

The sun also has a gravitational field that would create an Einstein ring of any distant object - including an exoplanet - as long as you were watching from the correct location. Let’s just imagine that we can find that location. We’d catch something like a trillion times more light from the exoplanet, making it possible to even see the thing. And its surface area would be expanded by a factor of around 100 billion. If only it were possible to remove the distortion we could map that surface in intricate detail. Well, it’s not only possible. It’s kind of easy.

Gravitational lens images look pretty messy. For example, here are some simulations of distant galaxies that have been lensed by a second galaxy much closer to us. The detailed structure is scrambled. And this is what you get when you try to reconstruct the original galaxy. The results are remarkably close to the originals. We can go into the details of this process another time. And we should, because it’s a big part of my own research. But not today. Today we’re looking for aliens.

If it’s possible to reconstruct the image from a messy galaxy lens, it’s completely straightforward to do it with the very clean, well-understood gravitational field of our Sun. All we need to do is get our telescope to the right spot. Unfortunately that spot is pretty far away. Like,10 times the distance of Pluto, or well over 500 times the Earth’s orbital radius. Around 550 “astronomical units” or “AUs” in astronomer-speak.

For comparison, Voyager 1 is our most distant probe. It’s been traveling for 45 years and is now around 150 astronomical units from the Sun. To get a telescope to the SGLF we’d want it to travel a bit faster. The authors of our report outline two possibilities: one is the “flagship” model, in which a single craft with a 1-2 meter telescope is sent to do the work. The second is the string of pearls option, in which many so-called small-sats are sent in a long train, each riding on the light of the sun with a solar sail. That second seems the preferred option, and it’s alway way cooler, so let’s talk about that.

Just as regular sails accelerate a ship by catching the momentum of the wind, solar sails catch the momentum of light - of photons from the Sun. More traditional propulsion methods that carry their own fuel have hard constraints on payload size and acceleration period because they’re weighed down by their own fuel supply. But a solar sailing vessel doesn’t carry fuel, making them great options for long-range missions.

And this isn’t even some sci-fi far-future tech. In 2010 the Japanese space agency sent the IKAROS probe to Venus using a 20 meter solar sail, and plenty more solar sails missions are in the design phase. But getting to 550 astronomical units would be the most ambitious among these projects.

To reach their destination in the working lifetime of at least some of the astronomers and engineers who witness the launch - so a travel time of 25-30 years - our spacecraft need an average speed of more than 100 km/s - several times faster than Voyager. We’d want each craft to be very light - ideally under 100 kg. But with advanced modern materials that seems possible. Even with that mass, the solar sail would need to be enormous - with more surface area than a football stadium. Unfurling and then controlling such a giant sail is very difficult, so the scientists are proposing an advanced solar sail design called the SunVane - multiple controllable sail panels mounted along the narrow structure of the craft. These sails would need to be made of some advanced, low-density metal alloy that’s A) highly reflective, B) has a high melting temperature because, as we’ll see, it actually gets  very close to the Sun, and C) is only a few hundred atoms thick so it doesn’t blow out our mass budget.

Solar sails experience more acceleration the closer they are to the Sun. To reach the speeds we need, our spacecraft can’t start their outward journey from the Earth - they needs to get closer to the Sun first. This is how the proposed mission would play out.

Our spacecraft starts out by launching backwards compared to Earth’s orbital direction, using sails to slow down and sort of tack inwards. They speeds up rapidly plummeting towards the Sun, ideally at around a quarter of Mercury’s orbital radius, assuming we can make the things sufficiently heat resistant. The craft then whip around the Sun and set their sails squarely against that intense up-close solar radiation. That propels the craft on a trajectory that will take them out of the solar system and towards our first image of an exoplanet.

Hitting the right spot is a feat of incredible astro-navigation and maneuvering. The solar gravitational lens focal range is indeed a range. While a lens creates a focal point, the Sun’s gravitational field creates a focal line, starting at 550 astronomical units, and extending indefinitely, with the Einstein ring getting wider and more diffuse the further you go. The column for an Earth-sized exoplanet at 100 LY is only 1.3 km across, so this really is like threading very, very tiny a needle. To hit the right spot the craft will maneuver with tiny ion thrusters. Once in the zone, the craft deploys its telescope. One possibility is that multiple craft will assemble into a larger scope. It may even be possible to repurpose the light sail as a mirror, if we want to get really clever about this.

Once in place, our telescope just needs to point back at the Sun and take an image of the faint einstein ring surrounding it. You might note an issue here. The last thing you ever want to do with a telescope is to point it at the Sun - that’s a great way to fry your camera - and good luck seeing anything next to the Sun’s intense glare. To deal with this our telescope will use a coronagraph - a giant circular mask that’ll block the Sun’s light.

From any one location within the focal column, the newly deployed telescope will see an Einstein ring formed from a single tiny patch on the surface of the planet, only 10km across. In order to see the entire planet it’ll have to move around the focal column, mapping the surface one patch at a time. The ion thrusters come into play here also. On top of this, the entire Einstein ring will be moving due to the motion of the exoplanet but also due to the wobble of the Sun as its tugged by the planets of our solar system. Our telescope is going to execute this shifting pirouette as it races away from the Sun, and that entire dance will have to be performed without any guidance from earth due to the several day light travel time back to Earth.

As you might have noticed, our spacecraft have no way to actually stop. But that’s OK. The long focal line means that an Einstein ring will be visible for years of travel time, slowly expanding outwards as we get further from the Sun. Remember that the scientists called this a string of pearls. That first cluster of craft was the first pearl. Even if that wave doesn’t get it quite right, its data will help the next pearl learn, with improved positioning and observing strategies flowing down the line.

Over time, the image that the train of spacecraft sends back to Earth will get clearer and clearer. It should be possible to achieve a resolution of around 25 km on the surface of an exoplanet 100 LY away. We could map coastlines, see islands and mountain ranges and lakes and ice caps and even vegetation - all of which we may be able to distinguish from their colours. And if we spot bright points of light on the planet’s night side - aka cities - that would be pretty compelling evidence of a technological civilization. And over time we’ll see changes in all of this. That means we can remove cloud cover, track the change from day to night, seasonal and tidal and changes, and even changes due to the activity of life.

Every exoplanet that we want to image requires a new fleet. That sounds like a lot, but remember that we build dedicated spacecraft for each of our solar system’s planets - usually more than one. And these little small-sats are designed to be cheap, so hopefully we’ll eventually be able to do this for many exoplanets. And, actually, for distant galaxies and black holes and literally anything else for which we want extreme resolution imaging.

Maybe this all sounds a bit out there. But all of the technology involved is either existent, or in the development phase. Nothing seems like a dealbreaker. There’s no funded mission yet, but the scientists involved have been funded by NASA’s Innovative Advanced Concepts program - the report we looked at was the phase II - and now they’re funded in phase III. The step after that, hopefully, is for the mission to be picked up by NASA. We’ll let you know how that goes. For more details of the solar gravitational lens mission, check out the amazing youtube channel launchpad astronomy, linked below. It’s crazy to image that within our lifetimes we’ll have mapped in detail the surfaces of distant worlds, brought into focus by our own Sun and its lens of curved spacetime.