I had the opportunity to review Hidden in the Heaven's by Jason Steffen for Nature Astronomy. Here's the link to the review, as published in the journal, or you can just read below.
It’s a mild March night in Florida. A group of scientists and engineers has gathered at Cape Canaveral to watch the launch of the Kepler Space Telescope, the first instrument ever built that can find planets like our own. Some of the onlookers, like the mission’s principal investigator, Bill Borucki, have devoted decades of their life to the project, while others – including this book’s author Jason Steffen – have just been hired in anticipation of a torrent of data, assuming the launch does not go awry. Tensions are high. A few weeks prior, another NASA mission crashed shortly after launch. Will Kepler survive its launch? Will the instruments perform to spec? Are there any other Earths to be found?
In Hidden in the Heavens, Jason Steffen tells the tale of the Kepler mission: from its conception in the early 1980s, through its long road to approval in 2001, to the drama of its launch in 2009, to its bounty of early science results, to its heartbreaking demise in 2014, to its miraculous second life as the K2 mission through 2018. During its lifetime, Kepler detected some 5000 planets. Those of us who worked on it were blessed to be part of a revolution in astronomy. The book is, at turns, a popular science survey of exoplanet discovery, an oral history of the Kepler mission, and Steffen’s own personal memoir.
Steffen sets the stage with a breezy journey through several hundred years of Solar System astronomy up to the first discoveries of extrasolar planets in the mid-90s. His writing is entertaining and snarky. On the centuries-old debate about what constitutes a planet and Pluto’s infamous demotion, Steffen states: “many people were saddened by this turn of events, feeling sorry for Pluto and its loss of stature. I was not among them. I didn’t really care. And to be honest, Pluto probably didn’t care either.” Indeed.
Kepler was first conceived in the early 1980s by Bill Borucki, before a single extrasolar planet had been found. His idea was to find planets by monitoring the brightness of a large number of stars and detecting periodic dimmings that occur when planets transit (cross in front of) their stars. It was an audacious proposal, well ahead of its time. The indefatigable Borucki led a small cadre of true believers who developed Kepler through the 80s and 90s. Their proposals to NASA were declined four times before they were approved in 2001.
That same year, Steffen began his PhD at the University of Washington. He initially intended to study black holes, but a twist of fate led him to study how interactions between planets could lead to transit-timing variations, or TTVs. At the time, TTVs were a niche topic that Steffen pivoted away from after graduate school. But with Kepler slated to launch in 2009, Steffen’s transit analysis skills were suddenly in high demand. As he puts it, “I was given the opportunity of a lifetime to work with the best colleagues, in the most exciting scientific field, on the hottest mission in the world.”
Kepler’s planet haul exceeded everyone’s wildest expectations. For myself, and countless others I’m sure, just viewing the raw data had the feeling of a divine revelation. From its Earth-trailing orbit, Kepler was returning data 100x better what we could get from the ground. Even before the fits were performed or the papers written, the magnitude of what we were seeing was clear. Previously, astronomers had gone through great lengths to detect Jupiter-size planets from the ground; now Kepler was delivering Earth-size planets by the hundreds!
The insular management structure that had carried the project through its long road to launch could not, at first, keep pace with the torrent of results. Steffen gives a first-hand account of a junior researcher trying to build his scientific career within a strict hierarchy where data was siloed off from the public and even within the collaboration. Current graduate students accustomed to today's culture of public datasets and open-source codes will be shocked to learn just how secretive the early days of the Kepler mission were. Roughly a year after launch, a revolt of sorts led to a freer exchange of information and more opportunities were given to junior scientists.
With the bottlenecks to publication loosened, a firehose of papers ensued. Steffen gives a recapitulation of the sheer diversity of planets and planetary systems revealed by Kepler: gas-rich sub-Neptunes and rocky super-Earths; compact multi-planet systems with as many as five planets within the orbit of Mercury; scorched planets with orbits of only a few hours; planets the density of Styrofoam; and circumbinary planets orbiting multiple stars.
And yes, Kepler succeeded in its goal of finding a handful of Earth-size planets in the habitable zones of Sun-like stars. Through this precious sample, we measured the frequency of such planets in the Milky Way Galaxy. There is still an error bar on this number, but it’s in the tens of percent, which means tens of billions of such planets in our Galaxy alone. If the emergence of life is a lottery, there are many rolls of the dice.
For all its successes, Kepler’s life was tragically cut short. Even though the spacecraft had enough fuel to operate for eight to ten years, the health of the spacecraft began to seriously deteriorate three years into the mission. Steffen recounts Kepler’s final months in terms that evoke the loss of a beloved family member: prayers that the spacecraft would somehow reverse its inexorable decline, the “pit in [his] stomach” upon reading the mission manager’s declaration of death, and all the stages of grief.
The consequence of this loss cannot be overstated. Kepler had just begun to probe the population of true Earth analogs. Another four years would have added many more valuable Earth-like planets to the census and firmed up our understanding of how common these planets are.
But Kepler wasn’t done yet. Readers will see how some clever engineering gave Kepler a second life as the “K2” mission. Kepler could continue taking data, but only for 90-day increments and only of stars in the ecliptic plane. While not ideal, many new scientific discoveries poured in from K2 including the first planets around newborn stars, planets around white dwarfs, and planets across different galactic sightlines. This second phase finally concluded in 2018 when the last of its onboard fuel ran out.
The field of extrasolar planets has enjoyed a remarkable period of growth over the last 30 years, but the Kepler mission stands out as a true revolution. It completely changed our understanding of what planets are out there and how they form.
Hidden in the Heavens spans several genres, and different readers will take different things from this book. People generally interested in astronomy will get an engaging introduction to extrasolar planets. Aspiring astronomers will appreciate the human drama of scientific research: how a good idea and decades of persistence can reshape how we understand the universe, how competition and cooperation can play out within a scientific collaboration, and how a junior scientist can make extraordinary contributions if they are bold enough to seize an opportunity. And for those of us who were lucky enough to work on Kepler, it’s a fond trip down memory lane, a reminder of how exhilarating science can be.
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