Dear Friends,
I’ve noticed an increase in this list’s number of subscribers, so welcome to all of you who are new! The revolutions spreading across the Middle East make clear that this is a momentous time in human history. But political change is not the only major change that is currently happening. On the scientific front, the Kepler mission has for the first time provided strong evidence that other Earth-size, and possibly Earth-like, planets exist in the universe. So I will devote most of this e-mail to this momentous discovery, along with a few other space and math news items. Here are the contents for this e-mail:
1. Understanding the Kepler Mission
2. Latest Kepler Discoveries
3. Is Pluto as large as Eris?
4. 2,000+ year old astrology story makes headlines!
5. Math: A Tool for Understanding the Budget Debate
6. Teachers: Please contact me if you’d like to review my next children’s book
Best wishes,
Jeff
1. Understanding the Kepler Mission
In the 13th century, Saint Albertus Magnus wrote:
Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of nature.
If we assume that by “world” he meant worlds like Earth, then Kepler is the mission that was built to answer his question. Launched in 2009, Kepler searches for planets by looking for transits in which an orbiting planet passes in front of its star, causing a tiny drop in the star’s brightness. Only a small fraction of stars with planets will produce transits that we can see from Earth, because a transit can occur only if the planet’s orbit happens to be oriented just right. For this reason, Kepler essentially “stares” at more than 100,000 stars, in hopes of catching the occasional transit from systems oriented the right way. Kepler is sensitive enough to detect transits by planets as small or smaller than Earth, which is why it is the mission to answer the question posed by Saint Albertus Magnus.
For Classroom Discussion:
* What is a transit? Do we ever see transits in our own solar system? (Answer: A transit is when a planet passes in front of its star as seen from our vantage point on Earth. We see occasional transits of Mercury and Venus; the next Venus transit will be in 2012. We cannot see transits of any other planets in our solar system, since they orbit beyond us and therefore never pass in front of the Sun from our vantage point.)
* How must another planetary system be oriented to produce transits that would be visible to us on Earth? (Answer: The planetary orbits must be aligned so that we see them edge-on from Earth, which is the only way that the planets will pass in front of their star as we view them. If a planet’s orbit is inclined to our line-of-sight, then the planet will never pass in front of its star.)
* How often does a transit occur for a particular planet? (Answer: A planet with the right orbital orientation will pass in front of its star once with each orbit. Therefore, if we want to observe, say, three transits to be sure we have detected a planet, we must wait for at least three orbits. For a planet in an Earth-like orbit, that would mean waiting 3 years — and Kepler has been up for less than two years. Fortunately, many planets have smaller and faster orbits, which is why Kepler has already succeeded in detecting many planets.)
* Can Kepler actually see a detected planet? (Answer: No. Kepler is detecting only a dip in the star’s brightness during a transit, and cannot actually see the planet that is causing the dip. Nevertheless, this dip can tell us a lot about the planet. In fact, with the aid of a few other measurements, we can usually determine the planet’s size, mass, and density, as well as its orbital period and how far it orbits from its star. This is enough information to allow us to make inferences about the planet’s nature with at least moderate confidence.)
* Discuss the quotation from Saint Albertus Magnus. Do you agree with his claim that this question is so important? Defend your opinion.
2. Latest Kepler Discoveries
Now we come to the amazing new results. For some background: Based on our theory of how stars and planets form, astronomers have for decades assumed that planets should be common around other stars. But until recently, we had no way to actually check. The first detection of a planet around another ordinary star came in 1995 (planets orbiting a neutron star were found a few years earlier). Since then, the discoveries have kept coming fairly rapidly (using ground-based telescopes and different search techniques than Kepler), and by the beginning of this year the number of known extrasolar planets (planets around other stars) had reached about 500. Then, on February 2, the Kepler mission announced their results from just their first few months of data. These results are framed as planet “candidates” rather than confirmed detections, because it will take additional observations to be sure that the candidates are real. Nevertheless, the results are astonishing:
- 1,235 planet candidates
- 68 of them approximately Earth-size of smaller
- 54 of them (including both Earth-size and larger) orbiting in the “habitable zone” of their star, which is the range of orbital distances within which a suitable planet or moon could have liquid water on its surface
- and 5 of them both Earth-size and in the habitable zone.
In other words, Kepler almost overnight tripled the number of known planets and provided strong evidence that Earth-size planets exist, with some of them in the habitable zone of their stars. This does not guarantee that any of the planets are actually Earth-like, but it sure ups the odds…
For Classroom Discussion:
* Do all stars have the same size habitable zone? (Answer: No; it depends on the star’s luminosity. Brighter stars have habitable zones located farther away from them, and dimmers stars have habitable zones closer in. Remembering that Kepler has not yet observed long enough to detect planets in year-long orbits like Earth, you will see that the planets in habitable zones must be orbiting small, dim stars for which the habitable zone is much closer in than it is for our Sun. That way, the orbital periods are much shorter in the habitable zone, which is why Kepler was able to detect transits from these planets in just a few months of observations.)
* Can Kepler tell us how common Earth-size planets are? (Answer: Yes, through statistics. Although Kepler can detect only planets with “just right” orbital orientations and monitors only a small fraction of all the stars in our galaxy, we can use its results to build statistics from which we can extrapolate to the full population of stars in the Milky Way Galaxy. The statistics will get better as more observations from Kepler come in, but already it is becoming clear that Earth-size planets must be at least moderately common.)
3. Is Pluto as large as Eris?
On a less momentous note, you’re probably aware of how the 2005 discovery of Eris led to the demotion of Pluto to being a “dwarf planet” for a brief summary of what happened and what it means, see my article Should We Bulldoze Pluto? Now, new data suggest that Eris may not be quite as large as had been thought. I don’t think this is likely to make a significant change in the Pluto debate, but if you want to know more, read this article from the New York Times. Keep in mind, however, that the data are difficult to interpret, and many scientists, including Eris discoverer Mike Brown, still suspect that Eris is the larger object.
For Classroom Discussion:
* Do you think the precise size of Eris matters to the debate over the definition of “planet”? Why or why not?
4. A 2000+-year-old astrology story makes headlines
About a month ago, astrology was suddenly in the headlines with the “news” that the astrological signs have been shifting with time, so that your true “sun sign” is not what your horoscope says it is. Personally, I like this story, because it makes people question astrology… but this was hardly news. The changing of the sun signs is due to the gradual precession of Earth’s axis, which has been recognized by both astronomers and astrologers for more than 2,000 years. The only real news is that these days, old news can be new again if it spreads virally on the internet. If you are interested in learning more about precession and the issue of the validity of astrology, I’ve posted here a pair of excerpts from my astronomy textbook (The Cosmic Perspective, 6th Edition) in a single file, crossing out everything except the parts relevant to precession and validity. You might note in particular the box that explains the issue with sun signs.
For Classroom Discussion:
* Read the excerpt about precession. Does it affect anything observable in the sky besides the zodiac signs? (Answer: It also changes the location of north relative to the stars, which means our current North Star — Polaris — will not always be a good marker of due north.)
* Read the excerpt about astrology, and discuss why scientists say that astrology lacks scientific validity.
5. Math: A Tool for understanding the budget debate
If you watch any news, you can’t miss the debate going on between Congress and the Administration about how to trim the federal budget. What is less commonly realized is that nearly all of the debate has been focusing on only a small portion of the budget — because “biggies” such as social security, medicare, and interest on the national debt are still considered off-limits by most politicians. I hope to explain this further in a book I’ve been working on about the importance of math, but for now I’ll just point you to one of the best graphics I’ve ever seen about the national budget. Notice what happens as you scroll over different chunks of the budget map. No matter what your political persuasion, I think you’ll find it illuminating.
6. Teachers: Please contact me if you’d like to review my next children’s book
I am working on a new children’s book, and I’m hoping to find some teachers who might be willing to read the draft with their class and send me feedback on the reactions. If you would like to do this, simply reply and let me know.
Thanks so much for your time!