Universe Today
What do you do when the observation you need comes round once a year? Well preparation is key but this is the awkward truth about finding another Earth. When a planet crosses the face of its star, a sliver of starlight passes through its atmosphere on the way to us, and the gases there absorb their own particular colours. Read the spectrum carefully and you can list what the air is made of; oxygen, ozone and water vapour, the things that would make you sit up.
For a genuine Earth type planet orbiting a star like the Sun, that signal amounts to roughly one part per million. It’s a very faint fingerprint on a very bright light. Astronomers have a standard answer to faint signals, which is to collect the same measurement repeatedly until the noise averages itself away. It works beautifully for the hot Jupiters which whip round their stars every few days and hand you a fresh transit most weeks. But an Earth at an Earth like distance from its star, transits once per orbit, and an Earth like orbit takes a year. Wait a decade and you have ten attempts. Stacking is no longer the plan since each single transit has to be good enough on its own.
One transmission spectrum with each point showing how much of a given wavelength the planet's atmosphere absorbed as it crossed its star, and the hump is carbon dioxide. WASP-39 b is a hot, puffy giant that transits every four days an Earth twin offers a signal far fainter, once a year (Credit : NASA, ESA, CSA, and L. Hustak (STScI). Science: The JWST Transiting Exoplanet Community Early Release Science Team)
Which leaves aperture. You need to catch enough photons in those few hours, and that means a telescope around thirty metres across, in space. JWST's mirror is six and a half metres, it took decades, and it had to unfold itself on arrival in orbit.
Jian Ge and colleagues at Shanghai Astronomical Observatory, working with collaborators in China and Spain, have proposed the alternative. Their concept, Life 2.0, is nine hundred, one metre telescopes flying as a distributed array. Each carries a miniature spectrograph and a very low noise detector; each observes the transit independently, each is calibrated on its own, and the spectra are combined afterwards. The result collects light like a thirty metre aperture without anybody having to build one.
The appeal is that identical detectors can be manufactured. A single vast mirror is a bespoke, slow build process and terrifying if it gets dropped. Nine hundred identical one metre units are a production line, with lightweight silicon carbide mirrors and CMOS detectors. The waveguide spectrographs are not hypothetical either since prototypes have already reached 40 to 66 per cent throughput.
WASP-39 b, as an artist imagines it. A Saturn mass world orbiting a Sun like star 700 light years away, with an atmosphere Webb has already read in detail. The hard part is doing the same for something the size and temperature of home (Credit: NASA, ESA, CSA, Joseph Olmsted (STScI))
I should say plainly that this comes from a concept paper presented at a conference, not a mission plan. What I like is that the authors refuse to gloss over the difficulties; detector systematics, instrument stability, and the maddening variability of the stars themselves are named as problems to be solved with the techniques the JWST era has taught us, rather than assumed away.
The PLATO and China's Earth 2.0 missions are being built to find the candidate worlds. Somebody has to be ready to look at them properly and maybe this proposal is the way to do just that.
Source - Life 2.0: A Scalable Distributed Space-Telescope Array for Biosignature Spectroscopy
