Ever since the realization that our sun was a star amidst uncountable others in the universe, astronomers and philosophers have speculated about the possibility of extrasolar planets, or “exoplanets” — planets that orbit other stars — and whether or not they could harbor life or intelligence. The modern era has given us tools to allow us to answer that question, and the answer turns out to be, yes, at least in terms of the existence of exoplanets, though there is still no evidence of extraterrestrial life or intelligence.
The initial, and vast majority of such discoveries were made not by directly viewing them with powerful telescopes (though there have been some of those), but by indirect methods that require inference. The first such discovery was confirmed in the early 1990s, when telescopes observed a slight regular wobble in a distant star, that was likely caused by a very large planet (perhaps like Jupiter) pulling it from side to side as the massive body orbited it. But a more powerful means of detection has been by observing so-called transits -- when a planet passes between us and its star and slightly, momentarily dims its light. Though we can’t see the planet itself because there is no light coming from its night side, its size can be calculated by measuring the degree to which the starlight is dimmed by it.
So promising was the technique in fact that, five years ago, in March 2009, after multiple failed attempts to get it funded, NASA finally launched a space telescope specifically designed to look for such events. It was named after the astronomer Johannes Kepler, who first described planetary orbits as ellipses, a result which helped Isaac Newton come up with his universal law of gravitation.
Unlike the Hubble Space Telescope, which is in low earth orbit, Kepler is in a heliocentric (or sun-centered) orbit, far from earth, to prevent it from being dazzled by earthlight and moonlight as it gazes deep into the galaxy. In order to stare at a star well enough to see transits, it has to have very precise pointing capability, akin to “pinpointing a soccer ball in Central Park as seen from San Francisco.” To do this smoothly, it has four gyroscopic devices called reaction wheels. By rotating the axes of these small spinning flywheels, the attitude of the spacecraft can be moved and held in a very precise controlled manner.
For the past half decade Kepler has been observing and sending back data, but in the summer of 2012, a little over three years into its mission, it started to run into problems when one of its reaction wheels started to fail, with friction buildup in the bearing, slowing the wheel. Last May, almost a year ago, another one started to display similar symptoms. With too much friction in half of its redundant devices, it was no longer able to point sufficiently accurately to perform its primary mission of star staring. Last August, NASA engineers despaired of fixing it, and were searching for ways to repurpose it for other missions.
But last fall, some engineers at Ball Aerospace, its prime contractor, came up with a clever fix. Our own nearby star puts out so-called “radiation pressure,” resulting from photons of sunlight bouncing off spacecraft surfaces. They figured out how to maintain an attitude almost as precisely as with the reaction wheels by changing the angle of the telescope’s solar arrays to vary the resulting force on the vehicle. While it’s not as good as when originally launched, it is still returning data, and may now continue to do so until its successor, the Transiting Exoplanet Survey Satellite (TESS), is launched in 2017. A few weeks ago, in fact, by using a new more efficient technique to verify the discoveries, NASA reported a doubling of the number of detected planets, and potentially quadruple the number of earth-sized ones, and it may be that as many as one in five stars have such bodies.
Ideally, it would be nice to look at the exoplanets directly, something that the Webb Space Telescope, named after the NASA administrator who launched the Apollo program, will be able to do. But that is not its primary mission, which is to carry on from the Hubble and look far enough into the universe to see its birth. Moreover, it is far behind its original schedule, and is a very high-risk program, because it will have to deploy multiple mirrors very precisely, millions of miles from earth where no one will be able to fix it as we have with Hubble multiple times, if something goes wrong.
There is also a great deal of concern that its budget growth, which could continue into the future, could eat up all available funding for other planetary science in an austere budget environment for NASA. However, it is likely to survive as long as one of its chief proponents, Senator Barbara Mikulski (D-Maryland), remains the chair of the budget committee that appropriates funds for the space agency. Goddard Space Flight Center, where the program is based, is in her state.
While exoplanets per se now appear to be abundant, it takes more than just being the right size to support life as we know it. It also has to be in what is called the “habitable zone,” that Goldilocks region that is close enough to the sun to provide adequate energy and warmth, but not so close as to boil off literally vital volatile substances such as water. So the most exciting discovery of Kepler’s career was made this week, when scientists who had been poring over its voluminous results announced an earth-sized rocky planet in a habitable zone, with an atmosphere. Named Kepler-186f, after its “discoverer,” living there wouldn’t be exactly earth like, though. Its sun, unlike our yellow one, is a dwarf, and more red. But a related result is that this isn’t just a one-in-a-million shot. Characterized as a “tip of the iceberg” discovery, it in fact implies that there are billions of them in our own galaxy alone, never mind our local group or the rest of the universe.
Of course, the real problem remains. The “twin” is about 500 light-years away. In a galaxy 100,000 light-years across, that makes it a relatively close neighbor. But in terms of our current propulsion technology, it might as well be on the other side of the universe. Leaving aside the fact that we are seemingly unable to even get Americans into low earth orbit on American vehicles, we have nothing approaching a means of traveling anywhere near the speed of light. And even if we could approach that velocity (a theoretical upper limit absent some new physics), it would take half a millennium in earth years to get to it (though it wouldn’t be that long for someone on the starship). So while it has exciting philosophical implications on whether or not we are alone in the universe, don’t pack your bags yet.