NASA’s Kepler space telescope just had its first birthday. Launched on March 6, 2009, this satellite is the first telescope capable of detecting planets as small as Earth orbiting other stars. While it hasn’t found any of those yet, its first year was not without its successes. Kepler has already found five new extrasolar planets, and on the strength of these early results, scientists are hoping for many more detections as the mission continues. The Kepler mission has two and a half years left, with the possibility of an extension up to six years, and jubilant NASA scientists are estimating that it may turn up hundreds of Earth analogs before it’s finished.
Since the first planets orbiting other stars (exoplanets) were discovered in the 1980′s, the field has come a long way. Already several hundred have been discovered- in fact, they are being found so quickly that we are hesitant to give an exact number, for fear that it may be outdated by the time this article gets posted.
Unfortunately, the early discoveries of exoplanets were limited by the type of method that was used. Popularly called the “wobble method,” this technique took advantage of a simple fact: if there is a very large planet orbiting a star, the gravity of that planet causes the star to move. As the big planet swings around in its orbit, the star wobbles back and forth. If you have instruments of sufficient sensitivity, you can measure this motion and deduce the presence of the planet.
There is only one problem with this method. Only extremely big planets have enough gravity to cause a star to wobble. Smaller planets like Earth just don’t have enough pull to move a star to any measurable degree. Because of this, they are invisible to the wobble method.
This limitation has dictated the kind of planets that could be found. To date, they have all been enormous worlds with powerful gravity. Some of them are many times larger than Jupiter, and orbit very close to their primary stars. Scientists have invented a new category into which these bodies fall: “hot Jupiters.” At the other extreme, they have also found some huge ice worlds. These monstrous planets were certainly interesting from a scientific point of view, but we knew that we weren’t really seeing a representative sample. We knew that there were probably smaller planets out there, some of them possibly similar to Earth, but this method just couldn’t show them.
Obviously, we needed a new detection method, something that wasn’t so dependent on size. To answer this need, the transit method was invented. This method uses a curious fact that was noticed by astronomers looking at certain stars. When these stars were observed continuously over a long period of time, their total output of light would occasionally drop for a little while, then jump back up to its former level. If you waited long enough, this dimming would happen again and again, and it would always be exactly the same; the light would decrease by the same percentage and remain at that level for the same amount of time.
The explanation was clear. There was a planet orbiting that star, and its path happened to pass through our line of sight (in other words, the planet was “transiting” the star). When it came between our telescope and the star, of course this caused a slight decrease in the amount of light reaching us. Theoretically, if we could get a precise enough measurement, we should be able to tell the size of the planet. The recurring pattern would tell us the orbital speed of the planet, and from that, we could calculate the planet’s distance from the star. Measuring the brightness of the star would tell us how hot it was, and that fact, plus our knowledge of the planet’s distance from the star, could give us an idea of how hot the planet was. Knowing that, we would also know whether water could exist there in liquid form.
What luck! Suddenly we had a single measurement that could tell us how big a planet was, how long its year was, how hot it was, and therefore, whether it could have liquid water. In the world of science, it rarely works out so neatly.
Unfortunately, this method also had its limitations. There were two big ones here, one that could be remedied by technology, the other inescapable. The first one was the fact that such sensitive measurements could not be performed from the Earth’s surface. Wind currents in the upper atmosphere have a blurring and dimming effect on starlight, and the kind of precision necessary for these measurements would not be possible. To use this method to its best advantage, the observations would have to be made from a space-based telescope. The other limitation has to do with the nature of the method itself. Obviously, it can only work if the planet’s path happened to pass between our telescope and the primary star. If that didn’t happen, we would never know the planet was there.
On the other hand, the big problem that we had with the wobble method, the fact that it’s only capable of detecting very large planets, would not be an issue with the transit method. Theoretically, if your instruments were sensitive enough, you could detect even very small planets with this method.
So now we have two methods for finding planets, and they each have their strengths and their limitations. We will continue to use both of these methods in the future; they can both tell us things, and by using both, perhaps we can get a more complete view of the universe.
However, the transit method has one thing that the wobble method doesn’t have: the ability to find planets like Earth. It can tell us how big they are and how hot they are- two crucial factors in determining whether they can support something that matches our Terrestrial definition of life.
Of course, there could be some exotic lifeform out there that drinks molten iron for breakfast, and thinks a hot Jupiter is the ideal place to live. If we explore enough planets, we may eventually find such extremophiles. But if we are looking for worlds like ours, with lifeforms that bear at least some basic similarity to ourselves, these are the planets where we’ll find it.
And if we are looking for places we can colonize someday, this is the method that will show them to us. Only with the transit method can we find little pebbles like Earth, where we can live on a long-term basis.
NASA built the Kepler space telescope with all of this in mind. The strategy was to put a telescope in orbit around Earth, train it on a certain segment of the sky, and just keep it there for a long time. The satellite would adjust its own orientation so that it always pointed at that one spot, and over the course of several years, hopefully it would detect the periodic dimming of stars that signals the presence of a planet. With no atmosphere to blur the image, it was hoped that Kepler would be able to detect planets down to half the size of Earth.
Well, it’s a year on now, and Kepler still hasn’t found any Earth twins- but it has found five larger planets, which have been named Kepler 4b, 5b, 6b, 7b, and 8b. Kepler 4b, the least massive of the planets found by Kepler, is about the same size and density as Neptune- but there the similarity ends; Kepler 4b is a hot world, blasted by 800,000 times as much solar radiation as Neptune gets. Kepler 5b is two times Jupiter’s size. It orbits so close to its primary star that iron is liquid there. Kepler 6b is .7 of Jupiter’s size, and it, too, circles very close to its star, whizzing around it in only 3.23 Earth days. Kepler 7b has an unusually low density, about the same as styrofoam. Although it is about one and a half times the size of Jupiter, it has less than half its mass. Rounding out the ensemble, Kepler 8b is another searing fire world, with a year that only lasts three and a half Earth days and an average temperature of more than 1700 degrees Kelvin.
Don’t worry, the smaller planets will be found later. These big ones are orbiting close to their primaries, moving very fast, so they passed into our line of sight soon after we started looking. The Earth-like planets will be farther from their primaries, therefore moving more slowly, and may take longer to move into our line of sight. This is the reason why Kepler is kept pointed at the same part of the sky for its entire mission.
NASA scientists are elated at these preliminary results, and are optimistic about finding Earth-like planets later. Of course, Kepler will miss some planets because their orbits do not cross its line of sight, but it will be surveying an unusually large segment of the sky filled with many stars. With so many chances, it is hoped that it will find many Earth analogs.
Kepler is a mission for the far future. We are a long way from visiting other planetary systems, and colonization is just a distant dream. But these dreams will come true eventually, and to make that happen, the groundwork must be done now. If we can survive long enough, we will find these Earth-like worlds and visit them. When we do, it will be the result of the work done by Kepler.
Sources:
“Kepler: a Search for Habitable Planets- Mission Overview” at NASA website: nasa.gov/mission_pages/kepler/overview/index.html
NASA News: “NASA’s Kepler Mission Celebrates One Year in Space” at NASA website: nasa.gov/mission_pages/kepler/news/one_year_anniv.html
Alexander, Amir: “Planetary News 2009: First Light for Kepler” at website of the Planetary Society: planetary.org/news/2009/0417_First_Light_for_Kepler.html
“Planet Quest- Exoplanet Exploration: Kepler” at website of the Jet Propulsion Laboratory, California Institute of Technology: planetquest.jpl.nasa.gov/missions/keplerMission.cfm
Cowen, Ron: “Kepler Space Telescope Finds its First Extrasolar Planets,” Science News magazine, January 30, 2010: sciencenews.org/view/generic/id/52465/title/Kepler_space_telescope_finds_its_first_extrasolar_planets
Kepler mission page at NASA website: kepler.nasa.gov/
Kepler in Brief: “NASA’s First Mission Capable of Finding Earth-size and Smaller Planets Around Other Stars” at NASA website: kepler.nasa.gov/Mission/QuickGuide/
NASA News: “NASA’s Kepler Space Telescope Finds its First Five Exoplanets” at NASA website: nasa.gov/mission_pages/kepler/news/kepler-5-exoplanets.html
Keep your eyes wide open for ET! Elliot…