Sometimes in order to support an idea, you first have to discredit alternative, competing ideas that could take resources away from the one you care about. In the scientific community, one of the most devastating ways you can do that is by making the other methods appear to be too expensive to be feasible, or, better yep, prove they wouldn’t work at all due to some fundamental limitation. That is what a recent paper by Dr. Slava Turyshev, the world’s most prominent proponent of a Solar Gravitational Lens (SGL) telescope mission, does. He examines how effective alternative telescope technologies would be at creating a 10x10 pixel map of an exoplanet about 32 light years away. Unsurprisingly, there’s only one that is able to do so without giant leaps and bounds in technology development - the SGL telescope.
To be fair, there are lots of proposed alternatives. Dr. Turyshev breaks them down into three distinct groups - traditional telescopes, indirect reconstruction, and in-situ imaging. Each has fatal flaws that make them infeasible, at least in the coming decades, but it’s worth taking a look at each to see what those flaws are.
Flying a large telescope, such as the proposed 15m Large Ultraviolet Optical Infrared Surveyor (LUVOIR), is the most traditional method of all. Other upcoming missions, such as Nancy Grace Roman and HabEx would fall into this category too. However, none come close to the spatial resolution required to have a 10 x 10 pixel image of the surface of a planet. In fact, its resolution is 10,000 times too low to do so, according to Dr. Turyshev’s calculations.
Fraser interviews Dr. Slava Turyshev, the paper's author.Another limiting factor of these large traditional telescopes is their “photon budget”. Since there are relatively few photons that would come from an Earth analogue 32 light years away, and many, many more that would come from other surrounding light sources (which in this case would be considered “noise”). Dr. Turyshev calculates that, to get a reasonable statistical certainty that the photons being analyzed were actually coming from the planet, it would take 1900 years of observational time to map a 10x10 pixel grid with LUVOIR.
Some technologies, such as starshades, do help with that somewhat. However, they still suffer from the resolution issue and are on the order of hundreds of years rather than thousands, and require advanced coordination algorithms that haven’t yet been developed.
Interferometers suffer from the same coordination issues. Getting down to the resolutions required for the 10x10 pixel maps requires baselines of 130 km, well beyond our current capabilities of interferometers in space, and requiring a significant step forward in coordination of the dozens of telescopes that would be required. Even if we were able to overcome that, it would still require thousands of years to properly resolve the planet, making this solution infeasible.
Getting to the SGL will require some new propulsion techniques, as Fraser discusses here.Ground based telescopes, such as the Extremely Large Telescope, still fail the resolution test. They are capable of resolving images that are about 2000 times too coarse for the 10x10 map. In ELT’s case, the time requirement is even worth, taking an estimated 41,000 years to resolve 100 pixels.
So what about indirect methods? Using normal exoplanet detection methods, such as light curves or transiting could work. However, inverting light curves would only give a 1D equivalent - not the same as the 2D reconstruction required for this test. Transiting is more promising, but would require multiple transits, stringing the time frame needed out to potentially thousands of years depending on the speed with which the target planet orbits its star.
We could try to catch an occultation using either a natural Kuiper Belt Object (KBO) or even an artificial one intentionally sent far from the planet for that purpose. However, KBOs are too sporadic to be potentially useful for this purpose, and artificial occulters suffer from the same problem as starshades - they are extremely hard to coordinate, especially at the distances required for this method to work.
Fraser discusses the idea of the SGL a bit more.That leaves one more alternative - sending a probe toward the planet itself. Some proposed methods, like breakthrough starshot, can get speeds up to a significant percentage of light speed. However, they suffer from a set of different problems. At those speeds, a probe would only have a few minutes with the planet, and would have to send whatever data it collected in those few minutes back to Earth from potentially dozens of light years away. Currently there is no known communication system capable of doing that while also being light enough to transport to another star, and it would take a long time to develop one.
That leaves the SGL mission as the most reasonable path to take. We’ve reported on it in detail elsewhere, but it’s safe to say the technologies are already there, and scientists know what needs to be done - it’s just a matter of funding and development at this point. Given the current budget consolidation NASA (where Dr. Turyshev works) is currently suffering through, it’s unlikely that we would see any mission like the SGL any time soon. But it’s good to know it’s the best option if, one day, we do decide to take a stab at directly imaging an potentially habitable exoplanet.
Learn More:
S. Turyshev - Direct High-Resolution Imaging of Earth-Like Exoplanets
UT - The Sun's Natural Gravitational Lensing is More Powerful Than You Thought
UT - A Solar Gravitational Lens Will be Humanity's Most Powerful Telescope. What are its Best Targets?
UT - A Mission to Reach the Solar Gravitational Lens in 30 Years
