11.09.2023

In early August, scientists and engineers gathered in a small auditorium at Caltech to discuss how to build the first space telescope capable of detecting life on planets like Earth. The proposed mission concept, called the Habitable Worlds Observatory (HWO), would be the next powerful astrophysics observatory after NASA's James Webb Space Telescope (JWST). It would have the ability to study stars, galaxies, and a host of other cosmic objects, including planets outside our solar system, which are known as exoplanets. Though finding life on exoplanets maybe be a long shot, the Caltech workshop aimed to assess the state of technology needed by HWO to search for life elsewhere.

"Before we can design the mission, we need to develop the key technologies as much as possible," says Dimitri Mawet, a member of the Technical Assessment Group (TAG) for HWO, the David Morrisroe Professor of Astronomy, and a senior research scientist at the Jet Propulsion Laboratory (JPL), which is managed by Caltech for NASA. "We are in a phase of technology maturation. The idea is to further advance the technologies that will enable the Habitable Worlds Observatory to deliver its revolutionary science while minimizing the risks of cost overruns down the line."

First proposed as part the National Academy of Sciences' Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020), a 10-year roadmap that outlines goals for the astronomy community, HWO would launch in the late 2030s or early 2040s. The mission's observing time would be divided between general astrophysics and exoplanet studies.

"The Decadal Survey recommended this mission as its top priority because of the transformational capabilities it would have for astrophysics, together with its ability to understand entire solar systems outside of our own," says Fiona Harrison, one of two chairs of the Astro2020 decadal report and the Harold A. Rosen Professor of Physics at Caltech, as well as the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy.

The space telescope's ability to characterize the atmospheres of exoplanets, and therefore look for signatures that could indicate life, depends on technologies that block the glare from a distant star. There are two main ways of blocking the star's light: a small mask internal to the telescope, known as a coronagraph, and a large mask external to the telescope, known as a starshade. In space, starshades would unfurl into a giant sunflower-shaped structure, as seen in this animation.

In both cases, the light of stars is blocked so that faint starlight reflecting off a nearby planet is revealed. The process is similar to holding your hand up to block the sun while snapping a picture of your smiling friends. By directly capturing the light of a planet, researchers can then use other instruments called spectrometers to scrutinize that light in search of the chemical signatures. If any life is present on a planet orbiting a distant star, then the collective inhales and exhales of that life might be detectable in the form of biosignatures.

"We estimate there are as many as several billion Earth-size planets in the habitable zone in our galaxy alone," says Nick Siegler, the chief technologist of NASA's Exoplanet Exploration Program at JPL. The habitable zone is the region around a star where temperatures are suitable for liquid water. "We want to probe the atmospheres of these exoplanets to look for oxygen, methane, water vapor, and other chemicals that could signal the presence of life. We aren't going to see little green men but rather spectral signatures of these key chemicals, or what we call biosignatures."

According to Siegler, NASA has decided to focus on the coronagraph route for the HWO concept, building on recent investments in NASA's Nancy Grace Roman Space Telescope, which will utilize an advanced coronagraph for imaging gas-giant exoplanets. (Caltech's IPAC is home to the Roman Science Support Center). Today, coronagraphs are in use on several other telescopes, including the orbiting JWST, Hubble, and ground-based observatories.

Mawet has developed coronagraphs for use in instruments at the W. M. Keck Observatory atop Maunakea, a mountain on the Big Island of Hawai'i. The most recent version, known as a vortex coronagraph, was invented by Mawet and resides inside the Keck Planet Imager and Characterizer (KPIC), an instrument that allows researchers to directly image and study the thermal emissions of young and warm gas-giant exoplanets. The coronagraph cancels out a star's light to the point where the instrument can take pictures of planets that are about a million times fainter than their stars. That allows researchers to characterize the atmospheres, orbits, and spins of young gas-giant exoplanets in detail, helping to answer questions about the formation and evolution of other solar systems.

But directly imaging a twin Earth planet-where life as we know it is most likely to flourish-will take a massive refinement of current technologies. Planets like Earth that orbit sun-like stars in the habitable zone are easily lost in the glare of their stars. Our own sun, for example, outshines the light of Earth by 10 billion times. For a coronagraph to achieve this level of starlight suppression, researchers will have to push their technologies to the limit. "As we get closer and closer to this required level of starlight suppression, the challenges become exponentially harder," Mawet says.

The Caltech workshop participants discussed a coronagraph technique that involves controlling light waves with an ultraprecise deformable mirror inside the instrument. While coronagraphs can block out much of a star's light, stray light can still make its way into the final image, appearing as speckles. By using thousands of actuators that push and pull on the reflective surface of the deformable mirror, researchers can cancel the blobs of residual starlight.

The upcoming Nancy Grace Roman Space Telescope will be the first to utilize this type of coronagraph, which is referred to as "active" because its mirror will be actively deformed. After more tests at JPL, the Roman coronagraph will ultimately be integrated into the final telescope at NASA's Goddard Space Flight Center and launched into space no later than 2027. The Roman Coronagraph Instrument will enable astronomers to image exoplanets possibly up to a billion times fainter than their stars. This includes both mature and young gas giants as well as disks of debris left over from the planet-formation process.

"The Roman Coronagraph Instrument is NASA's next step along the path to finding life outside our solar system," says Vanessa Bailey, the instrument technologist for Roman's coronagraph at JPL. "The performance gap between today's telescopes and the Habitable Worlds Observatory is too large to bridge all at once. The purpose of the Roman Coronagraph Instrument is to be that intermediate steppingstone. It will demonstrate several of the necessary technologies, including coronagraph masks and deformable mirrors, at levels of performance never before achieved outside the lab."

The quest to directly image an Earth twin around a sun-like star will mean pushing the technology behind Roman's coronagraph even further. "We need to be able to deform the mirrors to a picometer-level of precision," Mawet explains. "We will need to suppress the starlight by another factor of roughly 100 compared to Roman's coronagraph. The workshop helped guide us in figuring out where the gaps are in our technology, and where we need to do more development in the coming decade."

Other topics of conversation at the workshop included the best kind of primary mirror for use with the coronagraph, mirror coatings, dealing with damage to the mirrors from micrometeoroids, deformable mirror technologies, as well as detectors and advanced tools for integrated modeling and design. Engineers also provided a status update on the starshade and its technological readiness.

Meanwhile, as technology drives ahead, other scientists have their eyes on the stars in search of Earth-like planets that the HWO would image. More than 5,500 exoplanets have been discovered so far, but none of them are truly Earth-like. Planet-hunting tools, such as the new Caltech-led Keck Planet Finder (KPF) at the Keck Observatory, have become better equipped to find planets by looking for the tugs they exert on their stars as they orbit around. Heavier planets exert more of a tug, as do planets that orbit closer to their stars. KPF was designed to find Earth-size planets in the habitable zones of small red stars (the habitable zones for red stars are closer in). With additional refinements over the next several years, KPF may be able to detect Earth twins.

By the time HWO would launch in the late 2030s or early 2040s, scientists hope to have a catalog of at least 25 Earth-like planets to explore.

Despite the long road ahead, the scientists at the workshop eagerly discussed these challenges with their colleagues who had traveled to Pasadena from around the country. JPL director Laurie Leshin (MS '89, PhD '95) gave a pep talk at the start of the meeting. "It's an exciting and daunting challenge," she said. "But that's what we all live for. We don't do it alone. We do it in collaboration."

Quelle: SD