This view of Saturn was taken from Voyager 1 on October 30, 1980 from a distance of 11 million miles. A very careful look reveals three moons: Tethys (outer left) and Enceladus (inner left) appear at the left below the rings, and Mimas is the tiny white dot above the rings at the upper right of the picture. Note the shadow of the rings on the planet and the shadowing of the rings by the planet. As always, click on the image to enlarge and use the back arrow to return to the blog.
If you are not inspired by a planet hunt beyond the limited realm of our solar system, this particular blog posting may not be for you.
At the beginning of February, prior to and during the startling and welcome upheavals in the Mideast, there was another event — a far out event I might say — that caught my rapt attention. The NY Times reported that the spacecraft Kepler had discovered an additional 1235 exoplanets candidates (those orbiting stars other than our own) to add to the already known 500 or so. Of course, I have a very warm interest in the news of spacecraft missions being run from JPL (Jet Propulsion Labs, the NASA facility responsible for all unmanned space missions) where I spent lengthy periods of exciting time working on many missions such as Viking, Voyager, and Galileo. My work was designing the computer and associated hardware needed to fulfill the missions. My old buddies at JPL are now either retired or dead, but I continue to get great pleasure in following the work of the current creative scientists and engineers.
A few years back when first reading the proposal for the Kepler mission I was fascinated by its courage and chutzpah. The spacecraft is not in an orbit around Earth but is trailing in a sun-centered orbit so it can keep its telescopic eye constantly in view of a tiny patch of the star field within our Milky Way galaxy that is never obscured by the bulk of our planet. It is a planet hunter. Because cosmologists were uncertain how commonly planets might appear among any group of stars, the hunting ground had to be dense with stars to provide a better chance of noting planets. The area selected has over 100,000 stars, most of which are similar to our own sun. The Kepler computer has the precise coordinates of each of these stars and, amazingly, its major task is to sample the faint light emanating from each of these stars every 30 minutes! It registers the brightness of each star and sends this measurement down to the anxiously waiting computer system on Earth. Its whole Raison d'être is to determine if a planet is crossing in front of the distant star and has therefore slightly obscured the light and created a small dimming. The instruments are incredibly sensitive. They could easily detect a mosquito crossing in front of an automobile headlight a mile away. This detection technique is called the transit method.
A very large planet orbiting close to its star will create an easily detected shadow while a small planet orbiting some distance away may cause a barely detectable dimming. Note also, that a transit can occur only if the orbit of the planet is edgewise to our point of view. This is problematic depending on the tilt of the orbit. For example, if some curious aliens were looking our way there would be only a 0.5% chance that they would see our orbit edgewise and observe a transit as our Earth passed before our sun. A Jupiter-sized planet orbiting so close to its star that it makes a swift orbit in four days has a 10% chance of having a favorable tilt. So, Kepler will miss the vast majority of planets due to their unfavorable orbit tilt. This is why the mission needed to examine a patch of the galaxy with over 100,000 stars. That huge number was necessary for the telescope to get any results at all. From the planets we do detect we can infer that there exists an additional abundance whose detection awaits different search techniques.
The mission scientists and engineers (and those funding such programs) require a good dose of patience and this is why. Since our own orbital time is one year, a far off observer might have to wait a year to detect a single transit. If a planet is whisking around its star in mere days only a small dose of patience is needed. To confirm the data and to determine the orbital time, at least three or four successive transits are needed. Long orbit periods will result in other missed planets for the mission. In our own solar system the “years” for all of our planets vary. As Kepler’s third law predicts, planets further from our sun have increasingly long “years”. Here are a few, e.g. Mercury 88 days; Mars 1.9 years; Saturn 29.5 years; Pluto 248 years.
I was pleased to find the media reporting on the Kepler mission but am often put off by their hasty statements that the majority of planets found are giants orbiting in close to their stars and having speedy orbits. The media then generally leave the impression that this is characteristic of the whole planetary population. From what I have covered above, it is clear that there is a strong detection bias to the hunt. Large planets create shadows that are easier to see during transit, and short orbit times are bound to show up more frequently than orbits of a year or more. The basic mission is scheduled for only 3.5 years with a chance for an extension if all goes well. Imagine observing, through binoculars, a large enclosure walled by a six-foot stone wall. You observe a number of people whose heads protrude above the wall. A flawed conclusion would be that all those behind the wall are six feet tall or more. There is a strong detection bias towards large planets with speedy orbits, but that does not preclude the existence of other, more earth-size planets, with earth-length orbits. In fact, though it is early in the mission life, five of the candidate planets are rocky, smaller planets more like our Earth than many of the large, gas planets also found.
The astronomer Kepler certainly deserves the honor of having this mission named after him. In the early 1600s he worked for years poring over the astronomical data collected by the wealthy Danish nobleman Tycho Brahe who, in spite of publications of Copernicus, still believed in an Earth-centered system. Kepler, who never had any idea why the planets moved as they do, through repeated trial and error attempts, finally arrived at an explanatory scheme that fit all the planetary data he had at his disposal. He was able to predict all planetary paths. His years of effort were summarized in his three laws of planetary motion: planets moved in ellipses (not circles), their orbital speed was related to their position in their orbit, and their distance from the sun determined the period (time) of their orbit. Fifty years later in 1680 the genius of Newton explained all planetary motion based on gravity and confirmed Kepler’s laws. The Kepler mission computers use his laws to determine the orbital parameters of its candidate planets. Incidentally, Kepler and Shakespeare were contemporaries. So, Merwin and Bernice each have their familiars in this age.
I have already mentioned that the Kepler spacecraft computer has the coordinates of each of the stars it is sampling in its ephemeris (star table), and I would be remiss in not mentioning the Babylonians who were the master astronomers of the ancient world and have left us with a rich legacy. The Babylonians used a base 60 number system as opposed to our base 10 system. Their stellar observations were recorded in their number system from about 3000 BC to 500 BC but were so comprehensive and accurate that their data was handed down to succeeding civilizations in its original form. You might ask, what has this to do with me? We use their base 60 system every day. We measure latitude and longitude in degrees with 60 minutes to a degree and 60 seconds to a minute. The 360 degrees to a full circle is also Babylonian. Their star tables also used degrees, minutes, and seconds and all astronomers since their day, including Kepler and his mission, do likewise. Of course, the Babylonians, recognizing the intimate relationship between the Earth’s rotation and time, measured time in hours, minutes, and seconds as we all still do.
Finally, the large number of exoplanets already found further confirms my belief that many are out there teeming with life.
Good stargazing and love to all,
Merwin
Wow!
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