View Full Version : Ep. 182: Astrometry

2010-Apr-30, 03:40 AM
Astronomers have been cataloging star positions for thousands of years, from the first calculations made by Hipparchus, to the more recent star catalogs made by the spacecraft named after him. This is astrometry; another way to find our place in the Universe.http://feeds.feedburner.com/~r/astronomycast/~4/aLNw79XaZXA

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Terry Gush
2010-May-02, 03:40 AM
Why have so few exo-planets been discovered with Astrometry? Is it a lack of suitable space based instruments or perhaps the long observation times required? ...

If direct imaging of exo-planets is possible, you might have expected Astrometry of the parent star to have thrown up more results by now.

2010-May-03, 07:52 AM
Difficulty in the method it seems.

Absolutely zero planets have been discovered so far with astrometry. It isn't for a lack of trying, but rather perhaps a lack of resources. With planets that have had astrometric confirmation, there was justification. "Eps Eri" has a planet candidate, let's measure it's astrometric motion and see if it's a planet." This took HST's FGS. "HD LOLOL is your average star. Let's stare at it with HST FGS to see if we can detect a planet." does not seem like an adequate use of HST resources, simply because you can conduct the search from the ground with Radial Velocity. From Keck, ~5% of sun-like stars host readily detectable Jovian planets. Spending HST resources on a 5% gamble doesn't seem very practical.

So for now, astrometry isn't much good for anything than follow-up. But in the future, astrometry will become essential because it does not have the inclination degeneracy that radial velocity has, or the sensitivity to stellar activity.

Now, reality aside,

Van de Kamp ruined the field of astrometry by his bunk claims of planets at Barnard's Star and Lalande 21185. This is known as the "Curse of Van de Kamp."
Based on the examples of Barnard's Star, Lalande 21185, and VB10 b,

The first rule of planet discovery by astrometry: there are no planets discovered by astrometry.
The second rule of planet discovery by astrometry: there ARE NO PLANETS DISCOVERED BY ASTROMETRY.
The third rule of planet discovery by astrometry: if someone says "let me try to confirm that planet" or points a telescope in that general direction, the planet's existence is over.
The fourth rule of planet discovery by astrometry: only two teams of astronomers are needed.
The fifth rule of planet discovery by astrometry: one team to make the discovery, one team to disprove it.
The sixth rule of planet discovery by astrometry: no gas giants, no terrestrials.
The seventh rule of planet discovery by astrometry: planet disproof will take as long as it has to.
The eighth rule of planet discovery by astrometry: if this is the first planet discovered by astrometry, IT WILL BE DISPROVEN.
(credit: Lazarus)