What I Forgot to Mention Just Before

[aesmael]

(What do you mean I could just edit the previous entry? I'm sure I would have heard of such a feature)

While I was in Borders the other day I picked up a copy of Astronomy magazine, not available in most places yet (Borders got it in through air freight, or so says the sticker, which would explain why it was so expensive), which may be the mag my lecturer recommended we check out.  It is a special feature on extrasolar planets with a fold-out poster and everything Interesting contrast in progress and expectations from some of the older papers I have been looking at - three years ago we knew of only half as many planets and none were so small as the ones with all the buzz in this issue. Future missions had not been pushed back so far and some that will not go up for years were expected to have launched by now, or in the near future.

According to this magazine there has been at least one planet discovered so far from lensing (though I am still not entirely clear on circumstances for detection [for shame!]) and possibly one directly detected orbiting a brown dwarf (hm, sounds a little like Hephzibah).

But now, to the pointmobile!

Radial Velocity Method (treated briefly in this source, since it gets so much publicity elsewhere) -
This is the method which works by studying a star's spectrum to see if it undergoes a periodic red/blue shift

• Convection currents in stellar surfaces are likely to limit precision to +/- 1m/sec (referred to Marcy & Butler 1998)
• The semiamplitude of the motion Earth induces in Sol during our orbit is 0.1m/sec, so forget about trying to find similar worlds with this method (still going off the same paper as before, so this was their primary concern

Astrometry -
This is the method of finding planets by tracking the motion they induce in their stars visually rather than spectroscopically

• Limits of detection from Earth's surface are likely set by the atmosphere
• Palomar Test Interferometer has (as of 2003) short-term precision of 250microarcseconds over five minute exposures
• Precision of 70 microarcsec over an hour is expected to be eventually achievable
• The Keck Interferometer is expected to reach a resolution of 20 microarcseconds
• Stellar Interferometry Mission spacecraft expected to achieve 1 microarcsecond for narrow-angle work (I must confess I had misapprehended the nature of SIM - I have only just realised it has long been intended to be an astrometry mission)
• Earth-induced oscillation from a distance of 10 parsecs is 0.3 microarcseconds (this is semiamplitude again)

With Limitations:

• Earth-induced semiamplitude is 500km or 0.03% of solar diameter
• Sunspot areas are observed up to 1% solar diamater so they can move the apparent centre of light as much as 0.5%, or 7,000km
• Faculae cause even larger brightness changes

Occultations/Transits -
Simple enough to explain; if a planet passes between us and its star it will block some light and we may detect this

• May be limited by photometric variability of the stars in question
• Jupiter-sized planets occlude ~1% of starlight
• Earth-sized planets occlude ~0.005% of starlight
• Precision required to detect this is ~ 1 part in 10⁵, this is not much more than the Sun's intrinsic fluctuation over the expected duration of a transit