StarDate 4/28/14: Distant Planets

FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M.

STARDATE 4/28/2014: The two most distant planets that are easily visible to the unaided eye bracket the sky late this evening. Jupiter — the brightest object in the sky at that time — is in the west around 10 or 11, with golden Saturn low in the east-southeast.

Both planets are easy to see even though they're quite far from the Sun. But planets at similar distances from other stars have been much more challenging to find. Of the more than a thousand confirmed planets in other star systems, only a few percent are as far from their stars as Jupiter or Saturn are from the Sun.

That doesn't mean that distant planets aren't common — only that they're difficult to find.

One technique looks for a planet to cross in front of its star, briefly making the star look dimmer. This technique works best for close-in planets. For one thing, a close planet is more likely to pass in front of its star as seen from Earth. And it takes many years for a distant planet to make enough crossings for astronomers to confirm it.

The other major planet-finding technique looks for a slight "wobble" in the star's light caused by a planet's gravitational pull. But distant planets produce a smaller wobble than close ones, and it also takes a long time to detect their influence.

But astronomers at McDonald Observatory have been using this technique to watch hundreds of stars for many years — long enough to detect several planets in distant orbits. More about that tomorrow.

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