Using the Sun to Image Alien Planets

Using the Sun to Image Alien Planets

Using the Sun to Image Alien Planet

Human beings have contemplated whether life exists beyond earth for thousands of years. It's thought that there's a possibility that simple microbial life might exist within our solar system, but not advanced complex life. 

Exoplanets

Since the early 90s, technology has allowed us to locate planets outside of our solar system. They're known as exoplanets and planet-hunting telescopes like Kepler and TESS, have discovered roughly4,000 of them and counting. 

The majority of stars will have planets. So planets are ubiquitous. And because we just reach that sensitivity that is allowing us to detect those exoplanets, this field is evolving very rapidly. And while they're too far away to send a probe to even the closest exoplanets, a team at NASA's Jet Propulsion Laboratory has been developing a method more powerful than any telescope ever built. 

It'll be able to give us a close-up look at planets and other objects thousands of light-years away, find worlds that are similar to Earth, and maybe even discover signs of intelligent life. And it will be an amazing discovery, first of all, to see that we are not alone and how life evolved in that system. So it's an enormous step in our sort of philosophical understanding of who we are and where we go. 

Powerful telescopes, like the Hubble Space Telescope, Spitzer, Herschel, and Keck Observatory have photographed objects very far away. The galaxy called The Sombrero Galaxy is 28 million light-years from Earth. But even though known exoplanets are as close as four light-years, these are the best images we have. All we really get are just these tiny little blobs. 

exoplanets images


They just look like fuzzy, little points of light orbiting some distance from the star. So we don't really see anything that looks like a planet. In fact, if we didn't know any better we might think that it was just a very very faint star, when in fact it's a very very large planet reflecting the starlight off of it. And of the 4,000 discovered exoplanets, only about 50 of them have been imaged. 

So basically it's really really hard to image an exoplanet? It's really hard, but not impossible. And that's why we're aiming to do that, you know, that's why we're trying to make that happen, but you're right, it's very difficult, it's certainly is not beyond our capabilities or won't be beyond our capabilities. 

The majority of exoplanets are found using the transit method, a tiny dip in a star's brightness when a planet passes or transits in front of it. And it's that telltale drop off in a light, especially when it repeats over several cycles that tell us that this is caused by a planet, orbiting the star-making regular transits across its surface. 

Another common method is looking at slight shifts in the motion of a star when there's a planet orbiting it. This is known as the radial velocity or wobble method. The planet exerts gravity on the star. So what happens is that as the planet comes around the star, the star also kind of comes around the system's center of mass. And so the star has to move. 

Although both of these methods allow planetary scientists to make predictions like the basic mass, size, whether it's rocky or gaseous, and distance to its host star, it doesn't allow us to see it. In the best-case scenario, the light can be analyzed with modern telescopes to give hints as to whether the planet is habitable. 

How can we even tell that a planet is habitable just by a fuzzy blob? 

We certainly cannot. So the goal with these telescopes not to actually show us what the planet looks like, but it would allow us to analyze the light coming from the planet. The spectrum of light coming from the planet can be analyzed to reveal the chemical components of its atmosphere, a much easier feat than getting a real-life image. 

That's because, in comparison to a galaxy, exoplanets are dark and tiny. Planets just reflect light from their host star. So even though a planet may only be a few dozen light-years down the road, our chances of seeing it are much less than seeing a Brightstar that's even farther or a bright galaxy that billions of light-years away. 

It's just the nature of the luminosities involved. Even with the upcoming James Webb Space Telescope, which will be the most powerful space telescope ever built, the images won't give you much more than fuzzy blobs. 

What we want is an image where you can see color variations that distinguish oceans, continents, vegetation, and even see lights at night. That can determine not just if it's habitable, but show proof of advanced life. 

But to image an Earth-sized exoplanet, that's a hundred light-years away at a decent quality, 1000 by 1000 pixels, you would need a telescope that is 90,000 kilometers in diameter, seven times larger than Earth. And even if it were made of a super lightweight material, it would still weigh about a trillion kilograms. In other words, it's not possible. 

So what is the solution?

What we need to do instead is rather than think about even building a telescope, we need to think about using nature itself as a kind of telescope. And luckily nature gives us a way to do that. This was discovered by Albert Einstein, and it's the idea that mass or gravity can bend the path of light. 

gravity can bend the path of light


As long as that can happen, then we can, in theory, go to where all those light rays that are being bent by a mass converges, and we can take an image. This is called gravitational lensing. As long as we can choose the right place to put a telescope, we could in principle, look back toward the sun, image the gravitational lens from a distant exoplanet, and for the first time, we would actually be able to see continents. 

We'd be able to see the oceans and map the coastlines of an Earth-sized planet, maybe a hundred light-years away. This is a very different approach from making images with a telescope that we could build here on Earth, but it may in fact be the only way that we'd be able to directly image the surface of another planet. 

It may sound like science fiction, but we've actually observed this phenomenon before. When light from a distant galaxy is behind a massive foreground object, from our perspective on Earth, the mass of that foreground object bends space-time, creating gravity and bending the light around that object. This forms a ring called an Einstein ring. 

To visually demonstrate this, you can use a wineglass to act as the lens or a foreground object and a light bulb that can act as the source. If we the observer looking through the center of the base of the wineglass, we see a ring of light. And in theory, we could use computer software to decode that ring of light back into its original state. 

If you replace the wine glass with the sun to bend the light and send a telescope, now the observer, to the point where the light converges, you could then take images of the ring and put it back together using the software. The sun is massive enough that it will amplify whatever you're pointing it at, by a factor of a hundred billion, just like an astronomically massive telescope. 

That's why a team of scientists led by Slava Turyshev at NASA's Jet Propulsion Laboratory is proposing a solar gravitational lens mission. Flying a small telescope to the solar gravitational lens will allow us to study those objects in very fine details and confirm the presence of life, and study the evolution of life on that exoplanet. 

There are a few challenges though, for this to work, the telescope will need to be positioned past the convergence point at around 650 astronomical units or AU from the Sun. One AU equals the distance from the Earth to the Sun. Neptune is at 30 AU for example. And Voyager 1, the farthest manmade object that was launched in the 70s, is currently at about 150 AU. 

So we have thought about how we can reach those distances, what technology do we have? 

Essentially we realized that using chemical propulsion is very challenging because the fastest velocity we were able to achieve with chemical propulsion was achieved with the NewHorizons mission to Pluto and New Horizons was able to reach roughly three astronomical units per year. With this velocity, it will take us a lot of time to get to the solar gravity lens. 

So, we realized that solar sailing offers a unique alternative. With solar sailing, we are using solar photons, solar light to gain a very significant momentum that will push our sailcraft to very high velocity. Solar sailing has been successfully demonstrated by The Planetary Society, NASA, and the JapaneseAerospace Exploration Agency. 

If done in a way that Slava and his team are proposing, which involves flying close to the Sun, the telescope will reach speeds of roughly 25 AU per year. So with those velocities, we will be able to reach solar gravity lens within I would say 20 to 25 years. 

That's the only way to do this. The next challenge is once you get to 650 AU, there's no slowing down. But this is where they get a lucky break. It doesn't have just a single focal point, it has a focal line. If you fly a spacecraft towards the focal region of the solar gravity lens, we don't have to stop. 

Moving along that focal line will still benefit from that large amplification. Another challenge, which current telescopes also face when imaging exoplanets, is that you have to point the telescope at a very bright object, in this case, the Sun. This creates a glare that needs to somehow be blocked out in order to see the planet. 

We will use a very interesting technique called a coronograph. So the, or starshade, the large instrument that is, has to be flown in front of our telescope in space, and that coronagraph will allow us to block the light from the parent star. 

And suddenly the very faint light from the exoplanet may present itself. So it's a challenging technique because of the brightness mismatch. Fortunately, coronagraphs are being developed and in use on many planet-hunting telescopes, including the upcoming James Webb Space Telescope. 

coronograph


Finally, the telescope will be limited to choosing a single target since changing the angle by just a degree, could mean propelling it billions of kilometers in the lateral direction. Usually, people when talking about solar gravity lens there is a significant limitation because we currently are limited by propulsion capability. 

So we can think that we can fly only towards one target essentially. And so, we think about the solar gravity lens mission, it's like any planetary exploration. If you fly towards a Saturn, we're studying the whole set of satellites, the whole formula of satellites around Saturn with Cassini. If you fly around Jupiter with Galileo, we're studying multiple satellites of Jupiter. 

With the solar gravity lens, flying towards a Trappist we can study every planet in that system. The Trappist system is a star with seven planets orbiting it, three of which are in the theoretical habitable zone. 

The sweet spot for a planet to possess liquid water on its surface and possibly support life. Pointing the telescope at the Trappist system could be a solution for looking at multiple planets since they orbit the same star. Another way they're looking to solve this is to build the telescopes cheaply and send many of them. 

These are just a few of the hurdles that Slava and his team face, but none of them are out of reach. And what we realize is that most of those technologies already exist. Some of them in a very high technology readiness level, some of them are yet to reach a very reasonable technology readiness level, but all of them exist in one form or another. 

Most of this technology is being developed alongside other projects that need to get far out into interstellar space cheaply and in a reasonable amount of time. So ultimately we will transfer those engineering developments to allow us to explore further, more, at a very affordable cost. 

Solar gravity lens is our ultimate goal so far, but looking at this we realize that there are plenty of synergistic efforts within the exploration community and who benefits from them, who benefit from those efforts and we invite our colleagues and friends to join us to look at those technologies, how we can use those technologies in the near term, how those technologies will benefit us in the next, five, 10 years. 

Slava and his team received phase III funding from the NASA InnovativeAdvanced Concepts Program, and are developing methods to reconstruct the image from an Einstein ring, as well as building instrument prototypes. They hope to launch their technology demonstration mission in the next three to five years. 

Photos Credit: Google Images

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