When Berkeley graduate student Erik Petigura and fellow researchers released analyses of Kepler spacecraft data late last year, it made headlines around the world. No surprise there: The findings led to the conclusion that the number of inhabitable Earth-like planets in the galaxy could top 40 billion. The Milky Way seemed somehow transformed from a cold and glittering panoply of massive fusion reactors churning in the void into something cozier—a kind of galactic farm pond, teeming with the ET-equivalent of rotifers and ciliates.
Life changed for Petigura, the lead author of that paper published in the November issue of the Proceedings of the Academy of Sciences; he became something of a celebrity. But reflecting on it now, he observes that the research also raised some disturbing questions, specifically in regard to the Drake Equation—the formula devised by radio astronomer Frank Drake in 1962 to determine the number of advanced extraterrestrial civilizations in our home galaxy.
In simple terms, the equation uses several linked factors to arrive at a reasonable estimate for advanced extraterrestrial societies: the average rate of star formation, the percentage of stars that have planets, the average number of planets per star capable of supporting life, the fraction of potential life-supporting planets that develop life, the number of life-supporting planets that develop intelligent life and ultimately civilizations, the fraction of civilizations that can transmit signals into space, and the length of time that such advanced civilizations actually transmit signals.
The research by Petigura and his colleagues did a lot to answer the second and third questions implied by the Drake Equation: How many stars have planets, and how many of those planets are capable of supporting life? It now seems certain that many of the galaxy’s stars have Earth-size planets in the habitable “Goldilocks Zone”—that area of potential orbits where temperatures are just right. That said, then, where is everybody? Where are the ET tourist shuttles from the Pleiades cluster? Why aren’t our radio telescope arrays buzzing with the alien analog of soap operas and reality shows from the millions of G-type stars surrounding us? It feels like nobody’s out there.
“I have to say that our work raised profound questions, and they keep me up at night,” ruminates Erik Petigura. “Though it now seems very likely there are a huge number of planets that are similar to Earth in the sense that they are small and orbit what we consider a habitable zone, we still aren’t at all certain about the necessary parameters for life.”
“I have to say that our work raised profound questions, and they keep me up at night,” ruminates Petigura. “Though it now seems very likely there are a huge number of planets that are similar to Earth in the sense that they are small and orbit what we consider a habitable zone, we still aren’t at all certain about the necessary parameters for life. We think rocky surfaces are good for life. And while (life-sustaining) planets have to mass to a certain size to attract enough gas for an atmosphere, they are probably relatively small. And they are probably lukewarm—if they’re too hot, complex biomolecules can’t form and function, and if they’re too cold, there probably won’t be enough energy available to sustain life processes.”
Well, many if not most of the planets postulated by Petigura’s work probably meet those criteria, right? He agrees they do. But the leap from mere life to intelligent life could be large, he adds. Intelligent life, in fact, may require a very narrow and specific set of cosmic circumstances.
“We saw when Comet Shoemaker-Levy 9 slammed into Jupiter (in 1994) that giant gas planets can provide a significant degree of protection to small, rocky planets in closer solar orbits,” says Petigura. “They can sweep up comets and asteroids before they collide with the planets in the inhabitable zone. So maybe you need some gas giants in outer orbits to provide breathing room between major collisions, to give intelligent life the time and scope it needs to develop.”
Also: Earth has a single large moon, caused when a Mars-size body slammed into our planet early in its history. Such collisions—and resultant moons—may be rare. A paper published in the journal Icarus by a team of researchers from the University of Zurich estimated that fewer than 10 percent of exoplanets may have such moons. But they could be a sine qua non for intelligent life, Petigura observes.
“Earth’s big moon stabilizes the planet’s orbital axis, and thus stabilizes its climate,” Petigura says. “Without the moon, there would be massive climatic shifts on Earth, and that in itself might be enough to stymie the formation of intelligent life.”
Further—getting back to the Goldilocks metaphor—Earth seems to have just the right amount of water to provide a nice mix of seas and dry land.
“If it had just a little less water, there’d be no oceans,” says Petigura. “There’d be water vapor, but no free liquid. Maybe you’d have some microorganisms, but that’s about it. And if there was a little more, you’d basically have a planet covered with water, except for Mount Everest and maybe a few other peaks. There could certainly be abundant life on such a planet, of course, but it’s probably unlikely a technologically advanced civilization could develop.”
But if sentient and thinking beings are out there, Petigura wants to know. With Berkeley graduate student Hong Chen, he is actively engaged in a SETI (search for extraterrestrial intelligence) investigation, using the National Radio Astronomy Observatory telescope in Green Bank, West Virginia. The project builds on Petigura’s earlier work, concentrating on stars with known planetary systems.
“Kepler has already identified more than 4,000 planets, so that’s allowing us to really refine our searches,” Petigura explains. “Until Kepler, SETI searches were kind of scattergun in their approach—they focused on stars or clusters of stars, without any knowledge of what kind of systems they supported. Now we can focus on the planets more than the stars.”
With their own search, Petigura and Chen are hoping to eavesdrop on advanced civilizations that may have colonized more than one planet.
“We’re concentrating on systems that have multiple planets with orbital planes that are edge-on with Earth’s orbital plane,” Petigura says. “Those kinds of alignments should produce the clearest transmissions, as received from Earth. We’ve already logged 24 hours of active searching, we’re starting to analyze the data, and we have more time booked on the telescope. We’re pretty excited.”