Exoplanet studies deemed “a great threshold in the human history of space exploration”
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A year ago, the MacArthur Foundation described MIT astrophysicst, planetary researcher, and new MacArthur fellow Sara Seager as a “visionary scientist” who “is finding new celestial frontiers and fueling curiosity about life in worlds beyond our reach.” Now that she has published the commentary “The future of spectroscopic life detection on exoplanets” in the Proceedings of the National Academy of Sciences, a few well-known publications have begun telling the world about her vision. She summarizes it in the first and last sentences of her commentary’s abstract:
The discovery and characterization of exoplanets have the potential to offer the world one of the most impactful findings ever in the history of astronomy—the identification of life beyond Earth. Life can be inferred by the presence of atmospheric biosignature gases—gases produced by life that can accumulate to detectable levels in an exoplanet atmosphere. Detection will be made by remote sensing by sophisticated space telescopes. The conviction that biosignature gases will actually be detected in the future is moderated by lessons learned from the dozens of exoplanet atmospheres studied in last decade, namely the difficulty in robustly identifying molecules, the possible interference of clouds, and the permanent limitations from a spectrum of spatially unresolved and globally mixed gases without direct surface observations. The vision for the path to assess the presence of life beyond Earth is being established.
On 23 August, the Wall Street Journal published “An astrophysicist in search of E.T.: Sara Seager of MIT thinks we could be able to detect life on other planets in just 20 years.” That interview article sought to capture the grandness of the vision, especially in its closing line: “Twenty years may sound like a long time to wait, she says, ‘but not considering the thousands of years we’ve been waiting.’” Other publications—the Telegraph and the Guardian in the UK, the Huffington Post, the Delhi Daily News in India, Space.com—have quoted extensively from Seager's commentary itself to convey the grandness.
The commentary opens with this: “For thousands of years, people have wondered, ‘Are we alone?’” Seager explains that exoplanets have become increasingly studied, then adds:
Our own galaxy has 100 billion stars, and our universe has upwards of 100 billion galaxies—making the chance for life elsewhere seem inevitable based on sheer probability. We can say with certainty that, for the first time in human history, we are finally on the verge of being able to search for signs of life beyond our solar system around the nearest hundreds of stars.
She outlines near-term, intermediate, and distant-future research steps to be taken, and calls accordingly for the necessary research tools. Studies in the next several years will rely on the Transiting Exoplanet Survey Satellite mission, she says, scheduled for launch in 2017, and on the James Webb Space Telescope, scheduled to launch in 2018. Later, small space telescopes for direct imaging will be needed. “Simply put,” she writes, “we need to take pictures of potentially habitable exoplanets.” She offers a much-quoted comparison: “The challenge of direct imaging of an Earth analog is similar to the search for a firefly in the glare of a searchlight when the firefly and searchlight are 2500 miles distant.”
She describes two direct-imaging techniques. The first is the internal coronagraph, “where specialized optics are placed inside a space telescope to block out the parent starlight and reveal the presence of any orbiting exoplanets.” The second—the starshade and telescope system somewhat dramatically illustrated in a NASA animation—has drawn particular media attention. She puts it this way:
A starshade (also called an external occulter) is a spacecraft with a carefully shaped screen flown in formation with a telescope. The starshade size and shape and the starshade-telescope separation are designed so that the starshade casts a very dark and highly controlled equivalent of a shadow, where the light from the star is suppressed while leaving the planet’s reflected light unaffected; only the exoplanet light enters the telescope. Most designs feature a starshade tens of meters in diameter and separated from the telescope by tens of thousands of kilometers.
For the far future, Seager sees a large space-based telescope to search 1000 Sun-like stars. Only “with a large pool of Earth-like planets,” she declares, can “we gain a probabilistic confidence of the existence of biosignature gases by mitigating the inevitability of false positives.”
Her closing has been particularly widely excerpted. She writes:
We stand on a great threshold in the human history of space exploration. On the one side of this threshold, we know with certainty that planets orbiting stars other than the Sun exist and are common. NASA’s Kepler Space Telescope has found that approximately one in five Sun-like stars should host an Earth-sized exoplanet in the star’s habitable zone. On the other side of this great threshold is the robust identification of Earth-like exoplanets with habitable conditions and signs of life inferred by the detection of biosignature gases in exoplanetary atmospheres. If life is prevalent in our neighborhood of the galaxy, it is within our reach to be the first generation in human history to finally cross this threshold and learn if there is life of any kind beyond Earth.
It might be important to note, though, that not all public discussion of Seager’s work and vision exudes enthusiasm. A commentary from the intelligent-design-defending Discovery Institute charges that NASA seeks to justify its budget by overstating the case for habitable planets and extraterrestrial life. It begins by disputing Seager.
Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA's history program, and is a science writer at a particle-accelerator laboratory.