This mission concept is the top recommendation in a new report compiled by members of the National Academies of Sciences, Engineering, and Medicine. The academy was tasked by Congress to come up with the best strategy for studying and exploring exoplanets, worlds that are located outside the Solar System. And after gathering input from experts in the field, the National Academies came up with seven recommendations, with the telescope at the top of the list.
It’s an aspirational request, given the harsh realities that NASA has faced while trying to build its next big space observatory, the James Webb Space Telescope, or JWST. It was originally envisioned to cost $1 billion, and to launch in 2007. This past June, NASA conceded that the entire project would run $9.66 billion, and the telescope wouldn’t launch until 2021 at the earliest. And the type of telescope that this report recommends would require new technologies that haven’t been tested out in space yet, which might make the vehicle even more complex and more expensive than JWST.
However, when creating this report, the authors didn’t think about the potential financial restraints. “We were specifically not to consider costs or schedule or any of those sorts of things,” Scott Gaudi, an astronomy professor at Ohio State University who authored the report and is also working on mission concept for a space telescope called HabEx, tells The Verge. “This is simply a consensus on what the exoplanet community feels are the most important things that need to be done to answer the science goals we have.”
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| Scott Gaudi |
Those goals are relatively simple. The first agenda the report outlines is that we need to understand how planets form. When stars are born, they usually form surrounded by a swirling disc of gas and dust, and then individual planets coalesce within these discs. So how do these discs go from simple gas to full-fledged planets?
The second goal is the one that most everyone on Earth wants to find out, too: are there other Earth-like planets out there that could host life? And that’s why the new telescope could be so important. Taking direct images of planets could tell us a lot about what they look like and what’s in their atmospheres. The gas that surrounds a planet may hold important clues about what’s on the surface below. For instance, gases associated with life, like oxygen and methane, would be a big sign that biological organisms are present on a distant world. “We could get a census of the atmospheric composition of a large sample of Earth-like planets,” Laura Kreidberg, an exoplanet astronomer at Harvard University, tells The Verge.
These days, astronomers mostly find exoplanets through clever, yet indirect, methods. Perhaps the most common way to observe distant worlds is to watch as they pass in front of their parent stars — what’s known as a transit. These crossings slightly block light from the star, causing a small dip in brightness that can be measured from Earth. NASA’s TESS spacecraft, already in orbit around Earth, looks for planets this way, as will the future James Webb Space Telescope. Another method is to observe how a planet’s gravity affects its host star. Even though they’re small, planets can cause their stars to slightly wobble — a sign of their presence.
Directly imaging an exoplanet, however, is a monumental task, especially the ones the size of Earth. These planets are super close to their parent stars, and get drowned out by nearby starlight. For instance, a distant, potentially Earth-like planet is about 10 billion times fainter than the star it orbits around. So in order to image these planets directly, scientists will need to equip future space telescopes with special tools that block or decrease a star’s light.
Fortunately, astronomers have been working on just that. One concept is known as a coronagraph, an optical instrument that can be attached to telescopes to suppress the light from a star with mirrors, making it easier to see planets that are lurking nearby. Telescopes on Earth and in space have been outfitted with coronagraphs to help study the Sun, but such a high-contrast instrument to block out light from distant stars hasn’t been fully tested out in space yet. NASA currently plans to demonstrate this technology for the first time on one of its upcoming space telescopes, WFIRST — a move that the National Academies highly recommends.
But there’s another option besides a coronagraph. Astronomers have been eager to try out something known as a starshade. It’s a large flower-shaped structure that would fly out in front of telescope, blocking the light from a star by casting a shadow. “None of the starlight ends up in your telescope, but the light from the planet would enter into your telescope,” says Gaudi.
The problem, though, is that this technology is quite complicated. A starshade would have to fly around 100,000 kilometers away from a telescope in space — very precisely — to properly block the light from a distant star. And like the coronagraph, the starshade is not a fully mature technology yet. Additionally, many of the proposed telescopes that astronomers would like to build to directly image exoplanets would be huge and weighty. For instance, one concept under study at the moment, called LUVOIR, would have a mirror much bigger than that of JWST.
Any direct-imaging telescope projects are going to cost billions of dollars to launch, and that may be difficult for Congress to swallow, especially given the delays and cost overruns of JWST. Meanwhile, the Trump administration has already proposed canceling the WFIRST telescope, which would help test out the coronagraph technology, citing cost overruns on the project. But exoplanet astronomers say they are optimistic that a big project of this nature could happen some day. “I think it’s an ambitious but achievable goal, compared to what we’ve already done, such as landing the first man on the Moon,” says Kreidberg. “This is something that’s within our grasp.”
Gaudi also understands the challenges associated with getting something like this done. But the report was merely meant to reflect what the exoplanet community hopes to achieve. Soon scientists will come together to prioritize which kinds of missions NASA should fund, as part of a huge effort known as the decadal survey. That report will consider financial realities when recommending what missions to pursue.
But Gaudi hopes that lawmakers may be able to see the significance of an ambitious mission like this. “These missions are not out of the box in terms of our fiscal capabilities,” he says. “It’s just something we have to choose to do. We think the time is very ripe for going and doing this now and we could have the answer to this very profound question in the next generation.”
