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The Webfooted Astronomer - February 2001

 

Minutes: Out of This World Banquet!

By Greg Donohue

THE SAS Awards Banquet for 2001 was truly out of this world, in more ways than one! First, the food was out of this world (at least my prime rib was terrific, and so were the couple of bites of salmon I stole while my wife wasn't looking). The company was out of this world, with some 80 people in attendance—lots of stimulating conversation and interaction. The awards and door prizes were also out of this world. And finally, John Armstrong, this year's speaker, gave a presentation that was truly out of this world on a topic that was literally out of this world!

Hot Jupiter! What a Great Lecture

After dinner, Jerry West introduced our speaker, John Armstrong. John is a Ph.D. candidate at the University of Washington Astronomy Department and member of the first class of academically trained astrobiologists. John's presentation, entitled "Extrasolar Planets: What We've Learned from the First Fifty" was both informative and entertaining.

Though we have long assumed that planets may exist around other stars, this was only conjecture until 5 years ago. And the planets we have found around other stars (called extrasolar planets, or exoplanets, for short) are very different from those in our own solar system.

We cannot "see" extrasolar planets due to their small size and the overwhelming brilliance of their stars. So we search for them indirectly by detecting the "wobble" they produce in their parent star as both star and planet co-rotate about a common center of mass (the same way we detect binary stars). The displacement of stars due to planets is too small to be detected by current astrometric techniques. So we must use the Doppler effect, where motion of an object toward or away from us causes changes in the frequency of light waves. In the 1980s, we could only detect Doppler shifts equivalent to displacements of several hundred kilometers per second—good enough to detect spectroscopic binary stars, but not planets. But current technology allows us to detect movement as slight as 3 meters/second (fast walking speed).

Detection via the Doppler effect suffers from one serious drawback: it only detects radial motion (motion directly toward or away from us). This means that the inclination of an extrasolar planet's orbit relative to us affects our measurements. The result is that we can only say what the minimum mass of an extrasolar planet is. If the orbit is inclined very little, then the mass of the extrasolar planet is close to this minimum. But if the inclination is large, then the mass can grow arbitrarily large.

Fortunately one exoplanet, detected around the star HD209458, also transited in front of the star, causing about a 1% drop in its light curve. The time it takes for the light curve to drop from maximum to minimum gives us the planet's diameter. The duration of the dip discloses its orbital inclination. Thus we can calculate the size, mass, and even the average density of the planet unambiguously. For this one planet, the quantities are: 0.63 Jupiter masses; 1.6 Jupiter radii; density 0.2g/cc (Saturn's density is about 0.7g/cc).

Using the Doppler technique, detection of the first extrasolar planet, 51 Peg B, was announced by Mayor and Queloz in late 1995. Roughly the mass of Jupiter, 51 Peg B's orbital period is only 4 days, placing it 10 times closer to it star than Mercury is to the Sun. Since then, 50 extrasolar planets have been found, all of them ranging from about Jupiter size up to 14 times the mass of Jupiter. Many are extremely close to their parent star, and those farther out have highly elliptical orbits. These close-in gas giants—dubbed "hot Jupiters" – blew away our model of planetary formation. Terrestrial planets, composed of less volatile materials, are expected to form close to the star, while gas giants, comprised of more volatile materials, should only be able to form in the colder regions much farther from the star.

So we need a new model that accounts both for the makeup of our own solar system, as well as those systems containing these "hot Jupiters." Though there is still much debate, current theory and computer modeling suggests the following scenario. Gas giants do form farther out. But spiral density waves in the remaining protoplanetary disk material, through which these planets orbit, creates a torque that causes them to migrate in toward the star.

Why do these planets not continue to spiral in and collide with the star? Some suggest that they actually do, with many cycles of planets forming and crashing into the star. Only those forming toward the end, when very little protoplanetary material remains, survive and don't spiral into their stars. However, protoplanetary disks are thought to last for only 10-100 million years. Under current models, there is simply not enough time for these multiple cycles of planetary formation to occur. Another suggestion is that the magnetic field of a star clears out the protoplanetary material in its immediate vicinity. Once the gas giants reaches this empty region it stops spiraling in, just prior to being gobbled up. Why then did not the gas giants in our solar system end up close to our Sun? Many questions remain unanswered.

So what's next in extrasolar planet hunting? Well, obviously we would like to know if there are earthlike planets out there! This is far more difficult, due to their much smaller size. But plans are underway. The Kepler 1-meter class scope, is scheduled to launch in 2005. It will look for transits of earth size planets across their stars, where the dip in the light curves are 10,000 times fainter than the 1% dips caused by hot Jupiters! But the program still expects to find signatures of 640 earth size planets out of 100,000 stars surveyed. The SIM (Space Interferometry Mission), scheduled for June 2006, would use 1 microarcsecond resolution to detect Jupiter/Saturn size planets via astrometric techniques, giving us the actual masses of the planets, not just a minimum value. Nulling Interferometry projects, relying on as-yet-unproven technology, would perform actual spectral analysis of the atmospheres of planets, looking for signs of the chemicals necessary for life.

Following what can only be described as a lively question and answer period, John wrapped up by giving a very succinct answer to the question posed in his lecture's title – "Extrasolar planets: What have we from the first fifty?" We've learned that we need to know a lot more!

And the award goes to ….

After John Armstrong's excellent presentation, Randy Johnson presented this year's awards and prizes – his last official act as president of the SAS for 2000.

The crew of the Webfooted Astronomer newsletter team were recognized for their hard work and dedication throughout the past year. Newsletter editor Laurie Moloney introduced and thanked them: Joanne Green and Pat Lewis, circulation managers; John Waters, production assistant; and Sheri Feld, producer of the online version of the newsletter.

Bill Bruener and Paul Ham were recognized for their faithfulness and dedication to the sidewalk astronomy star parties held each month—an important outreach to the community. George Melendez received the "Celestial Pathfinder" award for working with the Department of Natural Resources to help us gain access to Tiger Mountain for star parties. Peter Hirtle gained the "Master Mirror Maker" award for his leadership of the Telescope Makers group. Ed Barnes was named "Kamp Kookie King" for keeping attendance high at the general meetings by bribing the members with great cookies and coffee every month. And finally, Bruce Weertman and Karl Schroeder shared the "Raiders of the Lost Dark" award for their excellent foresight and leadership in regaining our dark skies in cooperation with the IDA (International Dark-sky Association). Congratulations and thanks to all of this year's "Stars of the SAS"!

Booty call

Next was the booty call, as in "spoils or plunder." The evening wrapped up with the long-awaited lottery drawing for the SAS door prizes, of which there were more than 30! (For those of you keeping score at home, given the attendance of around 80, the odds of winning a door prize were close to 40%!) We are grateful to those who generously donated door prizes, including: Captain's Nautical Supply (beautiful 6-inch Dobsonian Telescope, many books and other prizes); Anacortes Telescope and Wild Bird (17mm Nagler eyepiece and several gift certificates); Meade (8.8um UWA eyepiece); Orion (eyepiece/accessories case); Televue (8mm Plossl eyepiece); and Tasco (star pointers and this year's Grand Prize, an 80mm "goto" telescope).

Thanks to Jerry West and all those who had a hand in making this year's banquet a great success. It really was OUT OF THIS WORLD!

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