Good Morning! I’m here to tell you about the profusion of planets discovered by NASA’s Kepler Mission!
Have you ever laid back on the cool green grass on a warm summer night gazing up at a slice of heaven and wondered, &quot;Are there other worlds up there, unseen and uncounted against the rich black velvet sky? Are those flickering pinpoints up there campfires for other beings to warm their toes with or roast their marshmallows?&quot; Humanity has been asking this question for at least two thousand years since the ancient Greeks. Today we stand on the threshold of knowing the answer to this age-old question, thanks to NASA's Kepler spacecraft. I'd like to share the story of this remarkable machine and the daunting data challenges we face in executing our mission. As you’ll learn, this is no easy task.
Kepler is NASA’s first mission capable of finding Earth-size planets orbiting sun-like stars. The question we are asking is: What fraction of stars in our galaxy host potential habitable planets similar to the Earth? Kepler watches the stars in its FOV looking for instances where the planet transits or crosses the face of its star, blocking a small fraction of light. Of course, it can’t see the planets directly – the stars are way too far away for that. Kepler measures the brightness of each target star watching for small dips corresponding to planetary transits.
How hard is it to find transiting planets? This is an image of the Sun, with a Jupiter-size planet transiting it. Jupiter is 10 times smaller than the sun, so it causes a 1% drop in brightness. But Earth is 10 times smaller than Jupiter, so it causes a 1% of a 1% drop in brightness during transit: that’s just 100 parts per million! We are looking for teeny tiny changes in the brightness of our target stars. We can’t make these measurements from the ground. The atmosphere causes too much twinkle. So we have to go to space.
Very few planets will be seen transiting their star: The planet’s orbital plane has to be edge-on from our point of view. There’s only a 0.5% chance that one of our galactic neighbors will see Earth transiting the Sun. So we have to look at many thousands of stars to ensure that we can find some planets. Here is Kepler’s field of view: a piece of sky the size of 2 dips of the Big Dipper. This about the size of your palm held at arm’s length. The FOV is nestled under the left wing of Cygnus the Swan as he flies along the galactic plane. You can find Kepler’s field of view is near the Summer triangle formed by the stars Altair in the constellation Aquila, Deneb in Cygnus, and Vega in the constellation Lyra.
Kepler vaulted into the sky on March 7 2009 during an absolutely magnificent launch on one of the last Delta II rockets. It was breathtaking to behold. One month later we released the dust cover, allowing Kepler to open his eyes for the very first time. The image on the right is our first light image, consisting of over 96 million pixels collected by 42 CCDs, similar to those you find in digital cameras and cell phones. There are 4.5 million stars in the FOV, and we’ve chosen the best 150,000 stars for the purpose of finding Earth-size planets. This is an enormous field of view. The gaps between the modules are the size of the full moon. Let’s zoom in on one sensor. Yes, Dave, it really is full of stars!
The Kepler spacecraft is about the size of a minibus and has a .95-m aperture and a 1.4-m primary mirror. It has sufficient fuel for its thrusters to last for at least 3.5 years and as long as 10 years. It follows behind earth in a slightly larger and longer orbit pointing at the same place on the sky all the time without interference from the Sun or the Earth or the Moon : you never know when a star is going to wink at us as a planet transits its disk. At the heart of the spacecraft is the focal plane or camera. It has 95 million pixels and allows us to monitor the brightness of 150K stars simultaneously. It’s the largest camera ever built and flown by NASA: it’s about a foot across as you can see in the image on the right.
I’d like to catch you up on the progress we’re making with Kepler. At the time we launched about 70 transiting planets had been identified. In this chart you can see the orbital period on the x-axis and the planet size on the y-axis. Most of these are as large or larger than the planet Jupiter, and are in very short period orbits of 4 days or less. That’s because most of these planets have been identified from the ground where, for the most part, you can only discover transiting planets that are large and that are in very short orbits.
Kepler released its first 305 candidate planets based on the first 44 days of data. As you can see, Kepler’s contiguous measurements and extremely high precision are allowing us to find much smaller planets than previously possible, and to push the orbital periods out to beyond 40 days. Indeed, most of the planets are between the size of Earth and Neptune.
In February we released another 90 days of data and announced the discovery of over 1200 planetary candidates. You can see that we are pushing the planet sizes down well below Earth-size and are reaching out to much longer orbital periods. 17% of the stars that harbor planets have multiple transiting planets telling us that nature loves to make flat planetary systems.
We can use Kepler’s laws together with the temperature and size of the host stars to estimate the equilibrium temperature of each of our candidates as shown here. The green region denotes the habitable zone, that range of orbital distances for which liquid water can pool on the surface of a rocky planet. A total of 54 of our candidates lie in the HZ.
Now I’d like to introduce you to our most prolific star: the supercalifagilisticexpialidocious Kepler-11, which has not one, two or merely three planets,but a total of 6 (so far), all of which transit the star. This is a very compact system: the orbits of the 5 innermost planets are smaller than the orbit of Mercury, and the outermost lies within the orbit of Venus. This is an extremely flat system. If it were squeezed down to the size of an old 10” LP record, all of the planets would stay within the vinyl as they orbit the star.
Kepler is collecting a lot of data: we have over 150,000 planetary target stars. We collect and store 6 million pixels every half hour and downlink about ~40 GB of data each month. Over the lifetime of the mission we’ll have collected over 40 billion flux measurements of our target stars. We have big processing challenges, as I’ll show you. Instrumental effects are large compared to transits of small, rocky planets. The observational noise is not white and it isn’t stationary. It takes a lot of statistical tests to search for transit signatures over: 17 million per star per search! This problem gets bigger over time: the computations for the search scale with the square of the number of data points.
Whenever you build a new instrument that is an order of magnitude better than anything before, you’ve just invented the world’s most sensitive thermometer – and Kepler is no exception. Here are the light curves for two stars. The red curves are the raw data. The blue curves are corrected for instrumental systematics, such as safe modes, which introduce thermal transients that change the shape of the telescope and its focus. It’s hard to imagine, but we can easily see a change in the distance between the primary mirror and the CCDs of 0.1 microns out of 1.4 meters! We also had to adjust the pointing a few times during early science operations. The sensors are sometimes damaged by cosmic rays, which can permanently affect the sensitivity of the affected pixels. We have to identify and correct these various systematic effects so that we can find the small transit signatures in the data.
Searching for transits is a lot like looking for needles in a haystack. And this is one big bale of hay, and we get new bales every month and the data just keep on stacking up.
I’d like to illustrate the problem by looking at the Sun’s brightness over time. Here are almost 4 years of data from SOHO which monitors the flux from the Sun. As you can see, the solar constant isn’t! The wiggles you see are from sunspots, like you see in the movie at left, crossing the face of the Sun. How big is the transit of an Earth-size planet across the Sun? It’s the size of the this image of Earth. As you can see, the signatures from small rocky planets are smaller than spot signatures. We can find Earth-size transits because they are relatively short: they last from a few hours to as long as half a day. The Sun rotates once every 27 days, so it takes two weeks for one of these spots to cross the face of the Sun. There are four Earth-sized transits buried in this data. Can you see them? I can’t, but our adaptive wavelet-based matched filter can!
Here’s how it works: We separate the timeseries into different bandpasses using a wavelet filterbank and characterize the noise power in each band. We whiten the noise in each channel and then test the significance of transits at each point in time. You can see the peaks in the blue data, which has the Earth-size transits, compared to the red data, which do not.
Then we fold the data over trial periods from half a day to the duration of the mission. If we choose the correct orbital period and phase, the transits all line up and we get a large detection statistic. I’ve plotted the results of searching through the data on the last slide and have plotted the maximum statistic for each trial orbital period. The red data don’t have transits. The blue data do. Where is our Earth? There it is in a 320 day period orbit with a whopping signal of over 10 sigma!
As you can see, Kepler is revolutionizing the field of exoplanets. We’ve discovered over 1200planetary candidates, and believe that at least 80% of them are real planets. That means that we’ve more than doubled the number of known exoplanets in just over two years. Moreover, we’ve identified over 170 stars that have multiple transiting planets, demonstrating the flatness and compactness of planetary systems.
A Profusion of Exoplanets: Key Science Results from the Kepler Mission <ul><li>Jon M. Jenkins </li></ul><ul><li>SETI Institute/NASA Ames Research Center </li></ul><ul><li>Thursday September 22, 2011 </li></ul>STScI SAO
Do there exist many worlds or is there but a single one? This is one of the most noble and exalted questions in the study of Nature — Saint Albertus Magnus 1206-1280 Scholar, Patron Saint of Scientists Credit: Carter Roberts ... the ways by which men arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves. — Johannes Kepler 1571-1630
The Kepler Mission <ul><li>What fraction of sun-like stars in our galaxy host potentially habitable Earth-size planets? </li></ul>
How Hard is it to Find Good Planets? Jupiter: Jupiter 1% area of the Sun (1/100) Earth or Venus 0.01% area of the Sun (1/10,000)
Kepler : Big Data, Big Challenges <ul><li>Big Data: </li></ul><ul><li>150,000 target stars </li></ul><ul><li>6x10 6 pixels collected and stored per ½ hour </li></ul><ul><li>~40 GB downlinked each month </li></ul><ul><li>>40×10 9 points in the time series over 3.5 years </li></ul><ul><li>Big Processing Challenges </li></ul><ul><li>Instrument effects are large compared to signal of interest </li></ul><ul><li>Observational noise is non-white and non-stationary </li></ul><ul><li>~17×10 6 independent tests per star for planetary signatures [O(N 2 )] </li></ul>
Conclusions <ul><li>Kepler has found well over 1000 planetary candidates </li></ul><ul><li>Kepler has doubled the number of known planets orbiting other stars in our galaxy </li></ul><ul><li>We’re finding that small planets are more common than large planets </li></ul><ul><li>We’ve found a planet similar to Tatooine orbiting two stars </li></ul><ul><li>We find that multiple planet systems are quite common </li></ul><ul><li>Each day we are getting closer and closer to finding an Earth-Sun analog </li></ul>