(above: A depiction of a 16-petal silicon starshade for New Worlds attached to its mounting.)
Webster Cash is among the vanguard on the frontier of distant solar systems. In February 2008, NASA awarded a team lead by Cash, a CU professor of astrophysical and planetary science, a $1 million grant to study the cost and feasibility of New Worlds Observer, a proposed space-based observatory designed to detect Earth-like planets (and, potentially, evidence of life) in distant solar systems.
It is hard to see Earth-like planets that are, say, 30 light years away. As Cash notes, an Earth-like planet is 10 billion times fainter than its parent star. From afar, we can see only the stars, whose light completely obscures the planets near them.
New Worlds’ proposed solution to that problem is to launch a 4-meter telescope with a 50-meter “starshade.” The starshade and telescope would move in tandem about a million miles from Earth. There, the daisy-shaped starshade would align itself in between the telescope and target stars. The starshade would be about 50,000 miles from the telescope.
Thus able to see the planets unobscured by their stars, the New Worlds Observer would be able to analyze the spectra of light on the planets. Spectroscopy would allow scientists to determine the makeup of the distant planets’ atmospheres.
Spectroscopic analyses would reveal, for instance, if a distant planet’s atmosphere contained abundant free molecular oxygen, as Earth’s does. That element is, Cash says, “as far as we know, only present in abundance because of the presence of life.”
Astronomers have inferred the existence of more than 200 planets within 30 light years of Earth.
NASA’s picking New Worlds for further study is a positive sign, a “clean win,” says Cash. The resulting study will help the space agency decide which missions to fund in the next decade.
Every 10 years, American astronomers and astrophysicists help the National Academy of Sciences assess future research in their fields. Their work is designed to ensure accurate cost and technical estimates for the Astronomy and Astrophysics Decadal Survey. That survey guides funding decisions made by NASA and the National Science Foundation.
If the decadal survey picks New Worlds as a top priority, the funding prospects for the $3.3 billion project would rise.
Cash believes exoplanet exploration, and his system for undertaking it, have a “good shot” of being realized.
New Worlds, he says, is “the equivalent of getting in a rocket ship and going to Alpha Centauri,” which is the closest star system to us. But remote sensing is much more practical than space travel, given that Alpha Centauri is 4.37 light years away.
Sitting in his office, a model of a daisy-shaped starshade resting on his desk, the generally tranquil Cash gets animated as he discusses how New Worlds would function. Given the time it would take to move the telescope and starshade into alignment with individual stars, Cash figures the team would survey two to three solar systems a month.
As data from an individual solar system pour in, the team would know within a few hours whether they were observing Earth-like planets, Cash says. “If it’s an uninteresting system, we’ll take a picture and go to the next one.”
In the event of finding an Earth-like planet, New Worlds would linger for a couple of weeks, doing spectroscopy and other analyses. “When we see that first planet with oxygen, there will be immediate controversy,” Cash predicts.
As he observes, when the discovery is big, the standard of proof grows accordingly. There is the risk of getting a “false positive,” data showing evidence of microbial or higher life that is, in fact, not there. Conversely, there’s a chance of the “false negative,” gathering data that show no indication of life, when in fact life exists.
At the same time, Cash acknowledges that extraterrestrial life may well be composed differently from life on Earth, and would therefore manifest itself via spectroscopy differently. On Earth, he says, atmospheric oxygen indicates disequilibrium; without life, oxygen would be bound up in other compounds, not atmospherically abundant.
Another disequilibrium on a distant planet might be evidence of life, he says.