This month’s meeting will be on Friday, April 1, at 8:00PM in room 215 of the Aerospace Engineering Building.  Riders from the Montgomery area are welcome to meet at the home of Russell Whigham, 518 Seminole Dr., and carpool over to Auburn.  Plan to be ready to leave for Auburn at 7:00PM. 

Our star party this month will be the better of the two nights of Friday/Saturday, April 8/9, at Cliff Hill’s farm. 

April 1, AAS Meeting (No Foolin') 
April 3, Begin Daylight Saving Time
April 6-9, 2005, Mid-South Star Gaze
April 8 Partial Solar Eclipse  Begin,4:35; Max 5:10; End 6:00PM CDT
April 8/9, AAS Star Party, Cliff Hill’s farm 
April 16, Astronomy Day, W.A. Gayle Planetarium 
May 6-May 8, 2005, Georgia Sky View, Indian Springs GA 
May 6, AAS Meeting 
May 14th, Combined AAS Star Party / Forest Preserve Star Gaze 

One of the most dramatic breakthroughs in astronomy in the past decade has been the discovery of planets around other stars.  In every case, the presence of the planet is inferred by the motion they impart on their parent star; there is a periodic Doppler shift in the spectrum of the star as it moves in reflex to its planet.  But now, astronomers have used the Spitzer Space Telescope to detect photons from the planet directly.
There are over 160 planets that have been discovered to date.  A small fraction of them have orbits oriented such that they pass periodically in front of their parent star, as seen by us.  When this happens, they block a small fraction (typically 1%) of the light of the star while they pass in front.  The question some clever astronomers considered is: can we see the effect when the planet is hidden by the star?

Luckily, they had taken Astro 203, so they could reason as follows:  from the Doppler shift data, I know the period of the orbit.  Knowing the mass of the parent star (given its spectrum, the temperature can be inferred, and knowing it is a main sequence star, the mass follows), I can therefore determine the radius of the orbit from Kepler’s Third Law.  Given the radius of the orbit, and the temperature of the star, I can infer the temperature of the planet.  From the fraction by which the light diminishes during the planetary transit, I know the planet’s radius.  So if I know the planet’s radius and its temperature, I can get its luminosity.

When they went through these considerations for a particular transiting planet known as TrES-1 (that stands for ‘Trans-Atlantic Exoplanet Survey’, in case it wasn’t obvious), they concluded that the planet in this case it roughly the size of Jupiter, in a 3-day orbit at 0.04 Astronomical Units (i.e., very close to its parent star!); it is therefore quite hot, with a predicted surface temperature of 1000 K.  Such a planet has a black-body spectrum that peaks at about 3 microns, i.e., in the infrared part of the spectrum.  The parent star, with a surface temperature of perhaps 4000 K, is far from its peak in the infrared, so the contrast between planet and star should be maximum at that wavelength.  So they observed the planet-star system with the Spitzer Space Telescope, with its sensitivity to infrared radiation.  The telescope certainly doesn’t have the resolving power to distinguish the planet from the star, but it when the planet disappears behind the star, the total light should decrease by just a smidgen.

This is exactly the effect seen.  David Charbonneau and colleagues took a picture of the star with the Spitzer Space Telescope every 13 seconds for six hours, centered on the predicted period of 2.5 hours when the planet should be behind the star.  They saw a diminution of light exactly when expected.  The effect is tiny.  Even though the planet is at its peak of the blackbody spectrum, and the star is far from its peak, taking the light of the planet away decreases the total brightness at 4 microns by only 0.06%!  But this was just the amount expected, given the numbers above.

Pretty cool!  But what does it really tell us?  Well, this is the first time that photons from a planet have been unambiguously been seen directly (as opposed simply to measuring the effect of the planet on its parent star).  Astronomers are eager to learn about the physical properties of these planets, starting with such basics as their mass, size, temperature, and albedo (i.e., reflectivity).  These measurements confirm estimates of their size and temperature, and show that the albedo is actually quite similar to that of Jupiter.  The evidence that these objects look something like Jupiter gets stronger every day.

On a completely different subject, check out one of the most astonishing astronomical images I’ve seen in a long time:  http://photojournal.jpl.nasa.gov/jpeg/PIA06606.jpg

This was taken by the Cassini spacecraft orbiting Saturn; it was taken from an orientation with the rings exactly edge-on.  Notice the moons Dione and Enceladus in the ring plane.  Most striking is the shadow cast by the rings on the Northern hemisphere of the planet.  Breathtaking!
If you want to see more pictures from Cassini, look at:
http://www.nasa.gov/mission_pages/cassini/main/index.html

[The above article was sent to me from a cousin who lives near Princeton and took Dr. Strauss’ continuing education astronomy course.  So, there are advantages to living in light-pollution. ;-)  Thanks, Ed.] 

Hope to see everyone at the meeting and star party,

Russell