These days, almost every two weeks, there’s a new piece of news 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, composition, atmospheric compositions, and whether they could ever harbor life.
But a group of planetary scientists and astrophysicists, led by an ETH Zurich professor, have largely resurrected the idea of a flotilla of optically-linked, free-flying medium-infrared telescopes that could find Earth-mass planets up to here unknown in the habitable zone of their star. From there, using a technique known as nulling interferometry, telescopes would electronically cancel light from the parent star so that the planet’s thermal emission would be detectable in the mid-infrared.
This would allow the team to characterize planetary systems and detect atmospheric biosignatures of hundreds of nearby extrasolar planets.
The concept, known as LIFE (The Large Interferometer For Exoplanets) is very similar to two missions from the 1990s, NASA’s Terrestrial Planet Finder (TPF-1) mission and the European Space Agency’s Darwin mission ( ESA). Both NASA and ESA eventually dropped both largely because at the time they were simply too technically difficult.
Twenty years later, LIFE is trying to capitalize on the technological advances that would make the project possible and at a comparatively lower cost than earlier NASA and ESA proposals. Last month at EPSC in Granada, Spain, the team gave a presentation that was actually a pitch to generate interest in the project. And the LIFE team will discuss their 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 that would operate entirely in the mid-infrared. From there, four free-flying spacecraft spread over distances between 75 meters and 1 kilometer were to relay their data to a fifth spacecraft which would transmit it back to Earth.
The problem was that the technology to successfully operate these spacecraft in free flight with the precision needed to achieve the objectives of these missions proved too difficult. And at that time, the planetary science community had yet to harvest data from missions like NASA’s Kepler telescope that detected thousands of new extrasolar planets circling other sun-like stars.
The LIFE team certainly used the first sketches of the mission as a starting point for its new proposals. But they believe they are now ready to begin development of new technologies that would lead to a full-fledged mission as early as the end of the next decade.
The team’s current mission concept is four free-flight collector spacecraft and a fifth spacecraft acting as a beam combiner that also relays data back to Earth, Sascha Quanz, an associate professor of physics in the d planetary habitability at ETH Zurich, principal investigator of the LIFE concept, told me in his office just outside of town. The size of the collector spacecraft’s primary mirrors is believed to be on the order of 2 to 3.5 meters, he says.
The final aperture size will also depend on the overall throughput of the system (in other words, photons that end up on the detector, says Quanz. The better the overall throughput, the smaller the collector spacecraft’s primary mirrors can be. , he says.
Unlike all other planet-hunting methods used today, LIFE would allow direct imaging of the entire system and the ability to zero in on any given planet within a system.
Quanz says the project is now funded by ETH ZURICH, but the bulk of the money will have to come from major space agencies once their lab work is complete in about three to four years.
The life mission itself, says Quanz, would involve a minimum of six years divided into a research phase of about 2.5 years to detect new planetary systems, then 3.5 years for the in-depth characterization of a sub -all of these newly detected planets.
The LIFE team hopes that the technological advances needed to position the spacecraft will achieve what is called interferometric cancellation. As the ESA notes, when light from a distant star strikes two optically linked telescopes, one telescope’s beam is delayed by half a wavelength.
This means, according to the ESA, that when the rays are brought together, the wavelength peaks of one telescope align with the wavelength troughs of the other and are therefore canceled out, leaving no light. stellar. Light from a potential planet orbiting the star, in turn, enters telescopes at an angle and when the photon beams are combined, the ESA notes, the light from the planet is enhanced rather than cancelled.
At the time of the NASA and ESA initiatives, this was deemed too technologically challenging to be achieved with free-flying spacecraft in space. But Quanz insists that the ESA and NASA also failed to take sensitivity sufficiently into account. The number of photons you’re going to receive from the planet is very small, but it’s really the mid-infrared thermal emission that you’re interested in, he says.
In other words, LIFE will focus on the thermal emission from the planet’s atmosphere itself, rather than the starlight reflected from the planet’s atmosphere.
But getting there won’t be easy.
To detect methane, ozone (a natural byproduct of oxygen), and nitrous oxide, we need to suppress starlight, Quanz explains. But we also need to make sure the whole setup is sensitive enough to detect just a few photons from the planet per hour, per square meter, he says. it’s not off-the-shelf technology, says Quanz.
What about NASA’s Webb Telescope? can it directly detect Earth-mass planets?
Webb doesn’t have the spatial resolution to get close enough to stars to probe smaller terrestrial planets in the habitable zone, Quanz says. This is why a mission like LIFE is necessary, he says.
In the two decades since NASA and ESA researched this type of interferometer, optical photonic chips have made great strides, Quanz says. 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 of just a few microns, you’d have less payload and could launch more cheaply and be less prone to failure, Quanz says. In fact, LIFE’s beam-combination spacecraft could be shrunk to the size of a shoebox, he says.
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