The reason is the James Webb Space Telescope, aka the Next Generation Space Telescope, the successor to the Hubble that NASA has been thinking about since the mid-1990s. Originally, the Webb was supposed to have a light-gathering mirror eight meters, or more than twenty-six feet, across. That proved too expensive and too hard to launch, so the mirror was eventually downsized to six and a half meters. That’s still pretty big: the Hubble’s mirror, by comparison, is less than two meters across.
The original launch date was supposed to be 2007, but that was never really firm. The actual launch, once NASA started funding the project seriously, was set for 2015. In the fall of 2010, however, an independent review panel requested by Congress reported that the Webb project was badly behind schedule and over budget. Under the best of assumptions, there would be no launch until 2018 at the earliest, and the money it would take to do that would come at the expense of other projects. “This is NASA’s Hurricane Katrina,” said Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington, to the New York Times. It will, he said, “leave nothing but devastation in the astrophysics division budget.”
This could actually work to TPF’s advantage, however, according to Kasting, “because the Europeans will probably fly their Euclid satellite, which does a lot of the same science as WFIRST. Many of us think it’s a waste to do both WFIRST and Euclid.” If Euclid flies late in the current decade, and if WFIRST gets pushed back by Webb’s budget problems, maybe WFIRST will get canceled. “So that could help us. If WFIRST went away then maybe the next flagship could be TPF. Thinking optimistically,” he added.
But if Alan Boss is right, there’s also a realistic possibility that the Webb telescope could take money not just from future missions, but that it could also hurt Kepler. Originally, NASA agreed to a four-year Kepler Mission, with the possibility, but no guarantee, that the agency would spring for another four years, in what would be called an extended mission. Like the Webb, Kepler went over budget, albeit far less drastically, so the original mission was cut to three and a half years. “We’re up for senior review on the extended mission in February 2012,” Natalie Batalha said, “and I’m really worried.” She was even more upset when, in the spring of 2011, a House committee voted to cancel the Webb entirely. “JWST is just a double-whammy. The whole community has sacrificed to fund it. Everyone was unhappy at how much it was costing, but we knew how valuable it could be. And now you have Congress talking about canceling it.”
In the end, the Webb wasn’t canceled. Barbara Mikulski, the Maryland senator who runs the Senate Appropriations Committee, is a big supporter of the telescope, at least partly because its headquarters, like that of the Hubble, is in Baltimore. The Goddard Space Flight Center, from which the Webb will be controlled, is in Greenbelt, Maryland. She put the Webb back in the Senate’s version of the NASA budget, and that’s the version that survived. It’s a good thing for astronomy, if not for planet-hunting in particular: The Webb will be one of the most powerful astronomical instruments ever built, able to peer all the way back into the Dark Ages shortly after the Big Bang, when the stars first began to form. But the Webb will be useful for exoplanetology too: Its infrared-sensitive detectors will be able to analyze the reflected light from hot Jupiters and hot Neptunes to see what their atmospheres are made of—much like what Dave Charbonneau and others have been doing with the Spitzer and Hubble space telescopes, only more effectively. You could even fly an occulter along with the Webb, making it an ad hoc version of TPF.
Like the Spitzer and the Hubble, however, the Webb is a general-purpose telescope, where Kepler has an extremely narrow mission. Any planet-hunting duties would have to compete for Webb observing time with all of the other science the telescope is capable of doing. “I’m worried,” said Batalha, “that Kepler will make this amazing catalog of planets and there won’t be anything to follow it up with—that we’ll end up waiting decades and decades to explore these worlds we’ve found.”
That might well be true of the planets Kepler identifies in any case. As everyone knew from the beginning, most of the Kepler stars were too faint to follow up transit detections with radial-velocity measurements. For Earth-size planets in the habitable zone, there was pretty much no way it could happen. And if you did manage to confirm such a planet through transit-timing variations, say, you’d still need a TPF or a Webb-plus-starshade to have a hope of studying it.
For many young exoplaneteers in grad school or doing postdocs or holding junior faculty appointments, the litany of canceled and postponed space missions is clearly discouraging. Still, a few missions, less expensive and less ambitious but still potentially exciting, continue to move forward. One of them is being cooked up in the Green Building at MIT—the same place Sara Seager works. It’s a high-rise, the tallest building on the campus, topped with two white, spherical radar domes that have been there, looking down on the Charles River Basin, at least since I was in college back in the early 1970s. (As a sophomore, I had a job driving the motorboat for the freshman crew coach. On cold November afternoons, the sight of those domes, still lit by a Sun that had fallen below the horizon from where I was sitting, was a reminder that it would be at least an hour before I could get back upriver to the warm boathouse and stop shivering.)
I heard about the mission from Josh Winn, an affable young assistant professor who began his astronomical career, much like Dave Charbonneau and Sara Seager, in cosmology. As an undergraduate at Princeton, he worked on gravitational lensing, trying to measure the size of the universe by looking at the flickering of quasars. The idea is that when the gravity of a nearby galaxy distorts the light of a distant quasar, it can create a multiple image—what looks like two or three or even four quasars where there’s actually just one. The light paths the images follow to our eyes vary slightly in length, so when the actual quasar flickers, the flickering shows first in one image, then in another. This time delay (plus a bit of calculating) tells you how far away the quasar really is. “When I talk to people about lensing,” Winn said, “they listen politely, but mostly their eyes glaze over.” Now that he’s part of the search for life on other planets, he said, “they get it right away. I don’t have to explain why it’s important.”
This isn’t the main reason Winn became an exoplaneteer, but as he went through grad school at MIT he felt the urge to do something a little more practical. He tried medical physics, but it didn’t click with his personality, so he returned to lensing, and also did some work in condensed matter physics. After grad school, he did a stint as a science journalist, writing for the Economist for a year, and he considered abandoning science in favor of writing. In the end, he took a postdoc at Harvard, where he continued to work on cosmology. But just in the years since Seager and Charbonneau had departed, Harvard had become a hotbed of exoplanetology.
“There was all this excitement in the air about exoplanets,” said Winn. “The prospect was just emerging that we could study their atmospheres, and there were all of these other intriguing physics problems posed by multiple planet systems and close-in planets.” Back at Princeton, meanwhile, his old mentor, Ed Turner, along with many other senior astronomers, had turned into exoplaneteers as well. “It seems to me,” he said, “that the early days of lensing in the 1980s had that same feeling of excitement and newness as exoplanets. But exoplanets have more staying power because of the quest for life.”
So now, among many other duties, Winn was serving as the project scientist for a space telescope project called TESS, the Transiting Exoplanet Survey Satellite. “It would be a successor to Kepler,” he explained, “but looking across the whole sky rather than at one narrow area.” The trade-off would be that TESS would gaze at each of the two million stars on its list for a much shorter time—months, not years. It couldn’t find a Mirror Earth, with an orbital period as long a year. The biggest thing in TESS’s favor is that if JWST or even TPF finally goes into operation, it will have a nice, juicy set of planets to look at: The stars in t
he TESS catalog are much brighter, on average, than the ones Kepler is looking at. That makes radial-velocity follow-up with a telescope like the Keck much easier. It also makes it easier to study the light passing through or bouncing off the planets’ atmospheres, so exoplaneteers can study their compositions. And ultimately, if future telescopes can image the planets directly, the fact that TESS planets are closer to Earth will make those observations easier as well. It is, said Winn, “the natural next thing to do.”
It’s also far cheaper than a billion-dollar flagship mission. Kepler was a Discovery-class mission, limited to a budget of no more than $300 million. TESS was being proposed as a Small Explorer mission—a SMEX, in NASA’s acronym-happy universe—which had to come in at under $200 million. The project’s principal investigator, George Ricker, also at MIT, had first submitted a proposal to the agency in 2009, but it didn’t make the cut. The team resubmitted at its next opportunity, however, in early 2011, along with twenty-two others; the following September it was selected as one of five that would get $1 million for an eleven-month “concept study.” The best two of these would go on to launch, as early as 2016. If TESS loses out on the final round, the team might want to call Bill Borucki in as a motivational speaker.
Chapter 14
HOW MANY EARTHS?
As the Kepler team tried to remind reporters every time their satellite found a new planet, finding planets wasn’t the goal—not individual planets, anyway. The goal was to determine how many stars, on average, have a Mirror Earth orbiting around them, a planet of about Earth’s size, located in the star’s habitable zone. If the percentage is high in the Kepler sample, that boosts the odds that there will be Mirror Earths close to us. If it’s low, you need a flagship mission after all, which can look farther out into the Milky Way.
It all depends on a number exoplaneteers have begun, over the last few years, to call ηEarth (that’s the Greek letter eta, so the term is pronounced either “eta Earth” or, more commonly, to make it clear that the word Earth is written as subscript, “eta-sub-Earth”). It’s the fraction of Sun-like stars that have a Mirror Earth orbiting them—or that’s one definition. “There are actually many different definitions,” Andrew Howard, a postdoc working with Geoff Marcy, told me during my Berkeley visit. “That one is just the narrowest.”
Sometimes, Howard explained, people use the term to mean Earth-mass planets around Sun-like stars, without saying anything about the planets’ sizes. Sometimes, as with Kepler, they mean Earth-size, whatever the mass. Sometimes the definition is expanded to include M-dwarfs, not just Sun-like stars. Ultimately, it doesn’t matter all that much. The point is to figure out how hard it will someday be to focus in on a Mirror Earth and search for evidence of life. If eta-sub-Earth is around 10 percent, Jim Kasting says, and you’re talking only about Sun-like stars, that means you should expect to find just three Mirror Earths within the nearest fifteen parsecs, or about fifty light-years—about three hundred trillion miles in all directions. “I’m actually optimistic,” he said, “that eta-sub-Earth is going to end up higher than 10 percent. I think it’s going to be more like 20 percent to 40 percent, somewhere in there. But we will have to wait for the Kepler folks to tell us.”
He said this in the knowledge that Kepler was up and working and beaming down information faster than the team at Ames and their collaborators elsewhere could process it. None of this had been certain back in 2007, when NASA officials came to Geoff Marcy urging him to write a proposal to come up with a preliminary number for eta-sub-Earth from the ground. “The goal for that project,” Marcy said, “was always very clear.” He would use the Keck II telescope, which NASA had helped fund with the idea of finding planets, to survey 166 nearby Sun-like stars, looking for radial-velocity wobbles. “Same old same old,” he said, “except we would be looking at very high precision, and doing repeated measurements at very high cadence.”
“High cadence” means they took measurements frequently, to delineate the curve of back-and-forth motion as accurately as possible. With Kepler, the cadence is extremely high—the satellite measures the brightness of all 150,000 stars in its field of view once every thirty minutes, and a small subset of 512 stars once a minute, the latter mostly to look for transit-timing variations. With this project, said Marcy, which he called the Eta-Sub-Earth Survey, the cadence would be one measurement, or sometimes two, every night, which is pretty fast for a survey that has to go from one star to the next to the next. “The stars were chosen blindly,” he said. “We selected them without knowledge about the planets that might or might not be around them.” Choosing stars where you know planets exist is a cheat. It makes the survey nonrandom. Even choosing stars you think are more likely to have planets would be a cheat—by picking stars high in metallicity, for example, which Debra Fischer had shown to be especially fertile planet-hunting territory.
“We knew we wouldn’t be able to find planets exactly the mass of the Earth,” he said. “Our technique can’t do that, even for the closest-in ones. You get very, very close, but you can’t find planets that are Earth mass or smaller, and certainly not out at one AU.” An AU, or astronomical unit, is the distance Earth lies from the Sun, or about ninety-three million miles. A Mirror Earth around a Sun-like star has one Earth mass and orbits one AU out. In our solar system, Venus orbits at a little over .7 AU. Mars is at a hair more than 1.5 AU. Pluto, with a highly elliptical orbit, varies from just under 30 to nearly 50. “So the goal,” continued Marcy, “is to measure the fraction of stars that have very small planets in close-in orbits where our technique is very strong.” There were a handful of others doing similar projects, he said. “The Swiss team is doing the best. They’re doing very good work, and they’ve found more than we have.”
By the fall of 2010 when we spoke, the project had been under way for more than three years, and Andrew Howard had just written up the results to date in a paper that was about to come out in Science. “When all is said and done,” said Marcy, “cutting right to the bottom line, we surveyed the planet inventory from those larger than Jupiter all the way down to the smallest we could detect, which was three Earth masses, and found that there’s an ever-increasing number of planets toward lower and lower masses, down to the smallest. The funny thing about this result is that for me, this is like a lifelong dream. It was just fifteen years ago that finding a Jupiter, any old Jupiter, was amazing. Here we have the distribution of planets down to three Earth masses. It’s completely unbelievable that we have come this far.”
There was just one thing that worried him. “As exciting as this discovery and the new paper is, the theory of planet formation, albeit still adolescent at best, makes a distinction between the formation of rocky planets like Earth on the one hand, and the formation of Neptune and Uranus, which are mostly not made of rock, on the other.” Neptune and Uranus are roughly 50 percent water; the rest is gas. “They are literally water planets,” said Marcy, “with a rocky core of five to ten Earth masses.” In our solar system, there’s a huge gap between Neptune and Uranus, at seventeen and fifteen Earth masses (which simply means seventeen and fifteen times the mass of the Earth), and Earth, at one. Somewhere in that gap, there would presumably be a transition. Above a certain size, you form a water world. Below, you form a rocky planet. Nobody currently knows where that transition lies, but with Kepler, and more crudely, with the Eta-Sub-Earth Survey, you can start to fill in the gap and see. “We already have masses and radii from lots of planets,” said Marcy, “and I can tell you that a whole lot of them down to just a few Earth masses are fluffy. They are low density. They are reminiscent of Uranus and Neptune.”
“So this then goes back to our wonderful Howard et al paper,” he continued, “which on the one hand is a dream come true but on the other hand offers pause for some sobering thoughts.” The good news, he said, is that the number of planets is rising inexorably higher and higher as you go toward less massive planets, all the way down to three Earth masses. This is a good sign that
with just a little more work the exoplaneteers will find lots and lots of planets with the mass of Earth. Presumably, said Marcy, “the Earths would have a rocky surface with continents and plate tectonics and maybe oceans and lakes—the vision that comes out of science fiction movies.”
But the final verdict, he continued, was not yet in. “I’m a little bit worried,” he said, “and from the Kepler data, I have reason to worry. From the theoretical side and from the observational side, we haven’t answered the question yet about whether Earth-like planets are common.” Planets the size of Earth, he was saying, might well not be Earth-like in any other way. He also didn’t have any strong evidence to support this worry, but he said, “I have some weak evidence.”
This evidence came from the theorists who try to create virtual solar systems with computer simulations. The planets with lots of gas—Jupiter, Saturn, Uranus, Neptune—form quickly, in the first few million years, because after that the gas left over from the original collapsing interstellar cloud that formed the solar system disperses. “What’s left over,” said Marcy, “is dust, dust particles, maybe some pebbles of up to a half inch in size, and according to theory it takes another hundred million years roughly, maybe fifty million years, nobody knows for sure, to form the Earths.” But those Earths turn out to be hard to form. The theorists let the leftover material whirl around and collide and stick together in their simulations, and, said Marcy, “you end up with Marses with no trouble, but you don’t make Earths very easily.” Earth, he pointed out, is nearly ten times more massive than Mars, and it’s hard to gather enough material together to create one—in the simulations, anyway.
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