In Search of Habitable Worlds

In 1990, NASA’s Voyager 1 spacecraft put our home in a new context as a tiny “pale blue dot” in a vast expanse of space (shown above). Humanity has long sought to identify other Earth-like planets elsewhere in our Milky Way galaxy. Astronomers and engineers have fine-tuned instruments known as coronagraphs, which are designed to block the glaring light of nearby stars to search for planets orbiting them.

Researchers have identified and examined thousands of distant planets with an array of instruments and techniques, but have not yet verified if any are habitable. The concept for the next astrophysics flagship mission, the Habitable Worlds Observatory, would have a mirror large enough to find and study at least 25 potentially habitable worlds around other stars. It will be the first space telescope specifically engineered to identify habitable, Earth-like planets next to relatively bright stars like our Sun with a coronagraph and examine them for evidence of life.

Planning for a mission like this has long been underway at the institute’s Russell B. Makidon Optics Lab, where complex experiments can run around the clock. Two postdoctoral researchers in the lab, Raphaël Pourcelot and Sarah Steiger, join the lab’s director, Rémi Soummer, to explain their work along with their 2023 successes in contributing to the designs for an advanced coronagraph.

What is the lab set up to do?

Raphaël Pourcelot: To set the scene, it’s helpful to think of an eclipse, since the Moon was the first coronagraph that mankind used. When the Moon lines up with the Sun, blocking most of it, we can see the Sun’s corona, which is the outermost part of its atmosphere. We reproduce this effect on our lab’s testbeds, except we use a small laser in place of a star. We block the laser’s light with miniature coronagraphs, again and again, to ultimately achieve a very dark zone around the star to observe simulated small, Earth-like planets.

What challenges are you trying to address in your technology demonstrations?

Sarah Steiger: We are simulating what it’s like to directly image an Earth-like planet, which is a daunting challenge. The techniques we are developing aim to targets stars that are more than a billion times brighter than the planets. These planets are only emitting a handful of photons an hour. If you are able to block the light of its star, you also have to be careful not to block light from the planet.

Pourcelot: For many years, we ran our experiments with a single wavelength of light, or monochromatic light, to confirm how every facet of our testbed worked. In 2023, we switched to broadband light to expand the amount of light we could observe. This introduced new challenges. Coronagraphs respond differently to different wavelengths of light, which means the dark zone around the star might not be as dark for all of them. We updated the algorithm that interprets the light that passes through the coronagraph on our testbed to respond to these changes.

What are the benefits and challenges of observing a wider range of light?

Steiger: Each wavelength of light coming from a planet contains different information. If you can only observe in a narrow range, you might not be able to tell if the planet contains key elements and molecules that support habitability, like oxygen, carbon dioxide, and water. The more wavelengths you can observe, the more molecular and chemical information you have about the planets you are targeting.

Why is it so difficult to observe small, Earth-like planets?

Pourcelot: Small planets are faint and therefore more difficult to detect. It is also more difficult to observe planets that are closer to their stars, because those instruments need to be designed not only to block more of the star’s light, but only the star’s light. We can create a dark zone around the star, but there is a tradeoff. When we block starlight, we must also be sure not to block light from planets that closely orbit a star.

Tell us about the coronagraphs you tested in 2023.

Steiger: Our team produced two very successful masks, the main components that block starlight in the coronagraph, because both have advantages. One creates a dark zone all around the star, so you could potentially detect a planet anywhere. The cost to this approach is that you can’t target planets that tightly orbit the star.

Another postdoc in our lab, Emiel Por, led the work for a mask that only blocks light on one side of the star in the shape of a semi-circle—and showed that we can get very close to the star, creating a dark zone where Earth-like planets might be orbiting, but only on that side. Demonstrating these two complementary coronagraph masks is a major accomplishment.

The lab has long had international partnerships. How have they recently expanded?

Rémi Soummer: For a few years now, we’ve been able to operate the lab 24/7, which has immense benefits. We’ve supported independent work of doctoral students and postdoctoral researchers, wherever they are based, since our testbeds can be operated remotely. We recently made the lab available to NASA-funded research projects, including one led at NASA Goddard Space Flight Center and another at MIT with contributions from Rensselaer Polytechnic Institute, Caltech, and NASA Jet Propulsion Laboratory. In 2023, we formalized our relationship with several French institutions under the umbrella of a CNRS International Research Program, which is exciting, because it will encourage additional collaborations, including co-advising PhD students and postdoctoral researchers, and site visits.

What’s coming up for your work in 2024?

Soummer: Currently, we use a miniscule seven-millimeter (quarter inch) segmented mirror on our testbed as a stand-in for a telescope, which limits what we can do. We were granted NASA funding to build a more realistic 15-centimeter (six-inch) miniature telescope model with a segmented mirror. That means our model will be physically and scientifically more relevant, and allow us to put better constraints on the technology we’re testing and developing.