Swiss-led group again proposes revolutionary new method for direct imaging of exo-Earths

Almost every two weeks these days, there’s a new media rattle about a newly discovered potentially habitable planet orbiting a nearby star. The problem is that most of these exoplanet detections are indirect, and researchers end up speculating about their mass, their composition, their atmospheric composition, and whether they could ever host life.

But a group of planetary scientists and astrophysicists led by an ETH Zurich professor have largely revived an idea for a flotilla of optically linked, free-flying mid-infrared telescopes that could find previously undiscovered Earth-mass planets in their star’s habitable zone. From there, the telescopes would use a technique known as nulling interferometry to electronically cancel the host star’s light, revealing the planet’s mid-infrared thermal emission.

This would allow the team to characterize planetary systems and detect atmospheric biosignatures from hundreds of nearby extrasolar planets.

Known as LIFE (The Large Interferometer For Exoplanets), the concept is very similar to two missions from the 1990s, NASA’s Terrestrial Planet Finder (TPF-1) mission and the European Space Agency’s (ESA) Darwin mission. NASA and ESA eventually scrapped both of them for the most part because they were just too sophisticated at the time.

Twenty years later, LIFE is trying to capitalize on technological advances that would make the project possible, at a comparatively lower cost than earlier NASA and ESA proposals. Last month the team made a presentation at the EPSC in Granada, Spain, which was actually a pitch to generate interest in the project. And the LIFE team will discuss its proposal at an international interferometry conference in Pasadena late next month.

NASA’s original plan for its TPF mission would have used four 3.5-meter telescopes operating entirely in the mid-infrared. From there, four free-flying spacecraft, spread over distances between 75 meters and one kilometer, would relay their data to a fifth spacecraft, which would beam them back to Earth.

The problem was that the technology to successfully operate such free-flying spacecraft with the precision required to achieve the goals of those missions was proving too difficult. And at that point, the planetary science community had yet to collect data from missions like NASA’s Kepler telescope, which discovered thousands of new extrasolar planets orbiting other Sun-like stars.

The LIFE team admittedly used the previous mission outlines as a starting point for their new proposals. But they believe that they are now ready to start developing new technologies that would lead to a full-fledged mission already by the end of the next decade.

The team’s current mission concept consists of four free-flying collector spacecraft and a fifth spacecraft that acts as a beam combiner and also relays the data back to Earth, Sascha Quanz, Associate Professor of Physics in the Planetary Habitability Group at ETH Zurich, who Principal investigator of the LIFE concept, he told me in his office just outside the city. The size of the collector spacecraft’s main mirrors is thought to be in the range of 2 to 3.5 meters, he says.

Ultimately, the final aperture size will also depend on the overall throughput of the system (i.e. photons landing on the detector, says Quanz. The better the overall throughput, the smaller the primary mirrors on the collector spacecraft could be, he says.

Unlike any other planet-hunting method in use today, LIFE would allow for a direct mapping of the system in its entirety and the ability to focus on a specific planet within a system.

Quanz says the project is now being funded by ETH Zurich, but the big bucks will have to come from the big space agencies once their lab work is complete in about three to four years.

The life mission itself, says Quanz, would take at least six years, divided into an approximately 2.5-year search phase to discover new planetary systems, and then 3.5 years for in-depth characterization of a subset of those newly discovered planets.

The LIFE team hopes that the technological advances needed to position the spacecraft can achieve something called interferometric nulling. As ESA notes, when light from a distant star strikes two optically linked telescopes, the beam from a telescope is delayed by half a wavelength.

This means, says ESA, that when the beams are combined, the wavelength peaks of one telescope align with the wavelength troughs of the other telescope and are so canceled that no starlight is left behind. Light from a potential planet orbiting the star, in turn, enters the telescopes at an angle, and when the photon beams combine, ESA finds that the planet’s light is amplified, not quenched.

At the time of the NASA and ESA initiatives, this was considered too technologically challenging to be performed on free-flying spacecraft in space. But Quanz insists that even ESA and NASA have not adequately considered sensitivity. The number of photons you’ll get from the planet is really small, but you’re really interested in mid-infrared thermal emissions, he says.

In other words, LIFE will lock to the thermal emission of the planet’s atmosphere itself, rather than the star’s light reflected off the planet’s atmosphere.

But achieving this will still be no small matter.

To detect methane, ozone (a natural byproduct of oxygen), and nitrous oxide, we need to suppress starlight, says Quanz. But we also need to make sure the entire setup is sensitive enough to detect just a few photons from the planet per hour per square meter, he says. This is not off-the-shelf technology, says Quanz.

What about NASA’s Webb Telescope? Can it detect Earth-mass planets directly?

Webb doesn’t have the spatial resolution to get close enough to the stars to look for small terrestrial planets in the habitable zone, Quanz says. That’s why a mission like LIFE is needed, he says.

There have been big leaps in optical photonic chips in the past two decades, since NASA and ESA have been pursuing this type of interferometer, says Quanz. This, in turn, could allow the mission to use a much less massive payload for LIFE’s central beam combiner and data collector, he says.

If some of the bulk optics could be replaced with photons just a few microns in size, one would have less payload and could launch at lower cost and be less prone to failure, says Quanz. LIFE’s beam combination spacecraft could actually be shrunk to the size of a shoebox, he says.