Kepler’s Dilemma: Not Enough Time

From all accounts, NASA's Kepler spacecraft has been an unabashed success since its launch 2½ years ago. In February, mission scientists announced that they'd found an astounding 1,235 candidate planets in just the first four months' observations. This cache brims with multiple-planet systems: 115 doubles, 45 triples, and 10 with at least four.

An artist's impression of the Kepler spacecraft, which is essentially a wide-field photometer fed by a 38-inch (0.95-cm) telescope.
NASA / Ames Research Center
Moreover, beyond its planet-hunting prowess, Kepler has returned paradigm-changing data on the nature of stars themselves, heralding what one researcher calls a "golden age for stellar physics." An estimated 500 astronomers are poring through the spacecraft's ultraprecise light curves, churning out roughly one new research paper per week on everything from a detailed census of stellar sizes and masses to using stars' subtle, rhythmic pulsations to probe their interiors.

But project managers have become quietly concerned that Kepler's top priority — finding true analogs of Earth circling Sunlike stars in temperate, life-friendly orbits — can't be achieved by the time the spacecraft completes its planned 3½-year-long mission just 16 months from now.

Kepler has been staring at a patch of sky about 10° across near the Cygnus-Lyra border, continually recording the brightness of more than 145,000 stars. It's designed to detect the tiny drop in a star's light as a planet passes in front of it. Spotting big planets in this way is a snap for the orbiting telescope's 36-inch (95-cm) aperture and supersensitive detector array.

In February 2011, Kepler scientists announced that the spacecraft had identified more than 1,200 candidate planets. But the fraction of Earth-size worlds was far below theorists' expectations.
NASA / Kepler / Wendy Stenzel
However, it's much more challenging to pick out the intermittent blips from Earth-size bodies, which create brightness drops of less than 0.01%. "We've always believed that there are more small planets than big ones," comments Sara Seager, an MIT exoplanet specialist. Yet among Kepler's 1,235 announced candidates, only 68 of them (fewer than 6%) are Earth-size — a tally far short of the anticipated windfall.

So are other Earths truly rare, or is something amiss with the spacecraft? The answer likely neither of these. Instead, it appears that (recasting Shakespeare), "The fault lies not in ourselves, but in our stars."

The first stirrings of a problem appeared early in the mission. "We weren't getting anywhere near the precision we should be," explains principal investigator William Borucki (NASA-Ames Research Center). Yet an end-to-end check showed that the spacecraft itself was behaving as expected: its pointing was rock solid (10 times better than the Hubble Space Telescope's stability) and electronic noise very low.

It soon became apparent that the problem lay with the targeted stars themselves. Kepler scientists had assumed that stars like the Sun would behave, well, just as the Sun does. Up-and-down churning of gas in the solar photosphere causes the Sun's brightness to vary by about 10 parts per million (10 ppm) over time scales of a few hours.

Kepler scientists expected solar-type stars to fluctuate in brightness by only about 10 ppm, as the Sun does. But as this plot of 12th-magnitude stars shows, they were surprised to find that the fluctuations are much greater.
R. Gilliland & others / Astrophysical Journal
But brightness fluctuations from the stars being watched by Kepler are greater than that — averaging about 20 ppm for 12th-magnitude dwarfs. This added noise makes their light curves messier and identifying transits by small bodies all the more difficult. "They're much more variable than the Sun," Borucki admits. "It was a big surprise to us."

The team estimates that a planet like ours, passing in front of a star like ours, should yield a brightness drop of about 85 ppm. "That's going to be a fairly subtle signature in the data," explains Ron Gilliland (Space Telescope Science Institute). Stars with lower masses than the Sun are much more common, but they're also smaller and dimmer. Unless they happen to be relatively close by, less of their light reaches Kepler's detectors and noisy detections become an even bigger problem.

The main reason Kepler has identified any Earth-size candidates is that a planet in a tight orbit will cross its star's face many times in just a few months. So, eventually, the recorded number of transits grows and a distinct repetitive pattern emerges even from small planets in the noisier-than-expected brightness measurements.

But these close-in worlds are far too hot to be habitable. Kepler's raison d'être is to identify true analogs of Earth — small planets orbiting far enough from their stars to maintain nurturing temperatures. Mission scientists have known all along that these prized candidates would take the most time to spot, both because they cross in front of their host stars only about once per year (lasting at most 13 hours for an Earth-and-Sun clone) and because their much larger orbits have only a 1-in-200 chance of appearing exactly edge-on from our perspective.

If stars were no "noisier" than the Sun, then Kepler should take less than 6 months to identify an Earth-size world circling a solar-type star in a 10-day-long orbit — and less than 4 years to spot one in a 300-day-long orbit. But stars are noisier than the Sun, so more time is needed to make such detections.
D. Koch & others / SPIE
That's why Kepler is staring at so many stars, and that's why the basic mission is 3½ years long. Over that stretch a candidate planet circling roughly 100 million miles from its star (as we do) will complete three transits — the minimum to be considered statistically robust. But throw in the added difficulty of noisy stars, and the chance of finding true Earth analogs around Sunlike stars plummets.

More Time Needed

There is a way around this unexpected setback: the longer Kepler can stare, the better its odds of identifying small worlds. According to a comprehensive analysis just concluded by Gilliland and 16 colleagues, the spacecraft would need to amass at least 7 or 8 years of observations — double the planned mission length — to identify all the Earths passing in front of solar-type stars in the spacecraft's field of view.

Ordinarily, getting NASA managers to extend Kepler operations for a few more years would be nearly automatic. After all, even though the total mission cost is roughly $600 million, it would take no more than $17 million annually to keep it going. The spacecraft is healthy, with enough consumables aboard to last through most of this decade.

But these are turbulent times for the space agency. Substantial cost and schedule overruns by the James Webb Space Telescope threaten to gobble up an ever-greater fraction of NASA's space-science budget. Worse, within the past month Congressional budgeteers have proposed not only to cancel JWST outright but to slash more than $1.9 billion from the Obama administration's NASA budget for fiscal year 2012.

William Borucki, Kepler's lead scientist, has championed the idea of detecting planets around other stars by precisely recording their transits since the 1980s.
NASA / Kepler
Borucki remains hopeful that an extension will be approved. "I think this is the greatest mission that NASA has ever flown," he exults. "I can't imagine any other mission being more important." Borucki's bravado is understandable, given that he's championed Kepler and its cause for more than 25 years. Still, plenty of others agree with him.

"We certainly need an extended mission," says Geoff Marcy (University of California, Berkeley). "Kepler offers the clear ability to answer a profound question for humanity: are Earth-size, rocky planets common or rare in the Milky Way galaxy?"

Jon Morse, director of NASA's astrophysics division, understands all this but doesn't have "a crystal ball that can tell us what the outcome is going to be." For now, despite all the Congressional posturing, he's assuming that Kepler will get the $19 million requested for the upcoming fiscal year. That's enough for the project to collect observations through November 2012 and to analyze them for a year thereafter. But Morse acknowledges that greater forces are in play. "One of the hardest things we do is to plan our next year's activities in uncertain times."

To get Kepler's mission extended, first Morse and his staff must affirm that it's worth doing (a decision is due within the next two weeks). Then next spring a formal request will go to a scientific review committee, where, he says, "missions that project the highest science return in the future have priority."

Since Kepler can't complete its most basic objective without an extension, it figures to be high on the review panel's approval list. But if NASA money doesn't materialize, some in the exoplanet community feel private funding should be found. "Kepler is the political cornerstone of the exoplanet search," says Seager. "There's no other option."

Meanwhile, the next batch of planet candidates won't be announced until next June. Until then, the spacecraft's discoveries will keep researchers hopping. "Remarkably, planets of near-Earth-size are emerging," says Marcy, "and we are working day and night to confirm and validate them as bona-fide planets."

11 thoughts on “Kepler’s Dilemma: Not Enough Time

  1. Anthony Barreiro

    Extending the Kepler mission seems like a no-brainer. The craft is already in orbit, mechanically and optically sound, with enough coolant and fuel to last for years. All that’s needed is common sense here on Earth. Unfortunately given the current political debacle in Washington DC, common sense, historical perspective, and public responsibility seem to be in short supply these days, at least among those representatives who got elected with the goal of dismantling the government. They’re doing a great job.

  2. Robert

    "But these are turbulent times for the space agency. Substantial cost and schedule overruns by the James Webb Space Telescope threaten to gobble up an ever-greater fraction of NASA’s space-science budget."

    This is the problem with such large projects. They eat up most of the money and cut into other programs.

  3. Andrew Planet

    Why not extend funding to it being derived from an international source? The ceasing of funding for the Space Shuttle program has led to a dependance on supranational rocket science for payload transport to the International Space Station and elsewhere. Why not foster a supranational economy that will have a greater amount of interested parties therefore funding, be they govenmental or private, to invest in Space Research? A world government within a world economy lessens the strain on domestic national economies already extremely under pressure to not pay up like in the USA.

  4. Roger

    Like supranational funding, another option is voluntary funding via the Internet. Instead of taxing all for the mission, those who believe can contribute. The $17 Mil might be scaled down, too. Make it tax deductible: we all win!

  5. crite40

    Well I’m NOT from the USA and you might be amazed to see the situation from an outsiders point of view.
    This is just another symptom of what ails the USA.
    Cutting off a $600 million project to save less than $20 million a year!!!
    I note the "newspeak" about "supranational" orbital access.
    What you really mean is Russian!
    Not only that but with spacecraft designed when I was in my 20′s and 30′s and I am 71 years old.
    IMO the only thing that can save the US from going the same way as Rome is some form of proportional representation.
    It looks as though that will be the only way to shake up the
    staus quo in Washington and get some common sense back into both science and politics

  6. Al Dutcher

    It seems that there are a lots of space / science minded rich people
    wholl pony up 200K$ for a sub orbital joy ride.
    Can we make a direct appeal to these people for science funding?
    Maybe this could work too.

    I’m way behind the Dems plan to tax those who can spare it.
    The Republicans are out of there minds!
    Its a pity that we cant keep politics out of science.
    Its way too important to lose.
    Ancient Rome? Indeed!

  7. Peter Nelson

    The James Webb vs Kepler fund dilema is analogous to earth based telescope funding. Is it better to have one 100m telescope or 1000 1m telescopes?

  8. Oliver Taylor

    I am much more upset about this than I am about the possibility of canceling JWST. JWST is, fittingly, proportionally as challenging as HST was. Yes, HST is a watershed, but it was literally Hades getting built. JWST is having the same problems, proportionally and the costs (not just in money) are proportional. I’ve said before that I wouldn’t cry over losing JWST. But to sacrifice such a prolific data-producing instrument as Kepler is even more tragic, IMO.

    Clear, DARK skies.

    Oliver Taylor

  9. James Greenfield

    Let’s see, $170 x 100,000 contributors = $17 million. We could do it, if we wanted to. NASA can accept private funds,can’t it? Or contributors could work through the universities whose people partly staff such research.

  10. coolstar

    It’s been quite clear since las February’s data release that Kepler was finding a LOT fewer small, earth sized planets than expected. The noise level reported here was released months ago in very specialized publications; it’s good to see the Kepler team finally going public with this information.
    I agree that extending Kepler’s mission is an incredible no-brainer. $17M really is pocket change, even in today’s funding environment. Still, NASA has stupidly killed several other missions that required even LESS money to keep going, WISE being the most recent. So, I’m keeping my fingers crossed…
    Dr. Borucki is right about Kepler’s value, it has actually resurrected American work in stellar astrophysics, which has been languishing badly for two decades. An extended mission will back the NON exoplanet work even more valuable.

  11. Russ Wallace

    My company, CivicSponsor, can help with funding initiatives like these.

    We’re custom-built for governments to allow citizens and businesses to directly fund the projects they want to see completed outside of the tax system; in fact, you get a tax-deduction for what you give. And we bring corporate dollars to help where possible, making sure the large dollar amounts needed by public projects are achieved.

    We’re launching in November, but check us out at civicsponsor.org. And anyone who wants to help fund an extension of the Kepler mission should reach out to me, the CEO, directly at russ [at] civicsponsor.org.

    Would love to see this get funded and to help.

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