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Making New Friends
by Bruce Moomaw
Cameron Park - June 5, 2000 - All in all, although there are obviously great ranges of uncertainty in the various planetological phenomena that we've talked about, it seems quite possible to me that Peter Ward and Donalr Brownlee are correct in saying that -- although simple, durable one-celled life may be very common in the universe -- at most one nearby star out of several hundred may have any planet capable of evolving metazoan life, let alone intelligence.

But given the vast number of stars within a few thousand light-years of us, it still seems very possible that our technology may allow us to identify some such stars -- and in any case, we badly need to know much more about all the different types of planets around other stars in order to better understand just what the odds for complex life on them really are.

There was, predictably, a fair amount of Conference discussion of the new planet-detection techniques that are now becoming available, and seem certain to vastly improve our observations of extasolar planets in the next few years.

Until this year, the only successful technique available to us was "Doppler radial-velocity measurements".

Any planet orbiting a star naturally tends to drag the star around with it in a slight wobble.

This wobble is very small given the tiny mass of all planets relative to their stars (Jupiter has only 1/1000 the mass of the Sun), and Earth-based telescopes aren't sharp-eyed enough to directly see it visually -- but since the planet tends to drag the star alternately toward and away from our own Earth, the very sensitive spectrometers that we now have can detect the resulting very slight Doppler shifts in the frequency of the star's light spectrum, and for five years now they've been sensitive enough to detect Jupiter-mass planets closely orbiting fairly bright stars.

By timing the period of the Doppler changes, we can determine the planet's orbital period (its "year") -- and by measuring irregularities in the shape of the rhythmic change, we can also measure just how eccentric the planet's orbit is.

This technique, as I say, requires very sensitive Doppler measurements -- capable of detecting stellar velocity changes of only a few dozen meters per second.

The smaller the planet, the smaller the Doppler shift -- and so it was only last month that we were able to confirm the existence of the first two Saturn-sized extrasolar planets ("hot Jupiters").

But the sensitivity of our Doppler measurements is increasing rapidly; one Conference poster by D.J. Erskine described a newly developed instrument capable of sensing a Doppler shift of only 1 meter/sec.

The general feeling is that within a year, we'll be able to confidently detect Uranus-sized extasolar planets if they are close to their suns.

But detecting planets as tiny as Earth (only 1/300 the mass of Jupiter) using this technique is, of course, far harder.

And quite apart from mass sensitivity, this technique has two serious problems.

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